JPH087478B2 - Image forming method and apparatus thereof - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming method and apparatus for forming an image of two systems by using an electrostatic latent image, and more particularly, to an image forming apparatus on a latent image carrier. A new image forming method and a new image forming method based on a type in which latent images of two systems formed are sequentially developed and then collectively transferred to a transfer medium, in other words, a type capable of printing a plurality of types of images in one image forming cycle. Regarding the device.

[Prior Art] Conventionally, as an image forming method of this type, Japanese Patent Application Laid-Open No. 48-37148
Or those described in Japanese Patent Laid-Open No. 55-137538 are known.

In the former type, the photoreceptor is uniformly charged, and by exposure based on a signal corresponding to each color (for example, red and black), a red latent image portion having a low surface potential, a black latent image portion having a high surface potential, and an intermediate portion between them. , A three-value latent image consisting of the background latent image part is formed, and the red latent image part having a low surface potential is subjected to reversal development by applying a developing bias lower than that of the background latent image part and a black latent image part having a high surface potential. Is applied with a developing bias higher than that of the background latent image part to perform normal development to form two types of toner images, a reverse image and a normal image, on the same surface, and a two-color image with no toner adhered to the background part is obtained. It was done like this.

In the latter case, the surface of the photoconductor is uniformly charged, then a negative image is projected, and then reversal development is performed with a toner having the same polarity as that of the photoconductor on the static elimination portion of the photoconductor where light is projected. A positive image is projected onto the surface of the photoconductor to eliminate static electricity from the portion other than the positive image projection portion of the residual charge portion on the surface of the photoconductor, and normal development is performed on the residual charge portion of the positive projection portion with toner having a polarity different from that of the photoconductor. , Positive two types of toner images are formed on the same surface of the photoconductor,
After the polarities of two types of negative and positive toner images are aligned, they are simultaneously transferred to a transfer medium.

[Problems to be Solved by the Invention] By the way, in the former type among the image forming methods of this kind, the surface potential of the photoconductor must be divided into three in advance to form a latent image, and Since the charging potential of an ordinary photoconductor has an upper limit, it is not possible to secure a large latent image potential contrast between the red latent image portion and the black latent image portion.
There is a problem that the image density tends to be insufficient for both black images. In the latter type, the latent image potential contrast can be sufficiently secured, but the ready-made toner image is liable to be disturbed in the second developing step, and the second-stage development process is performed. In addition to the problem that the color toners are mixed with each other, color development is likely to occur, and various problems occur that it is not suitable for reproducing an area image (so-called solid image).

The inventors of the present application have made earnest studies focusing on the above problems, and as a result, while effectively avoiding the disturbance of the existing toner image and the color mixing phenomenon of the toner, sufficient image density is obtained regardless of whether the image is a line image or a solid image. The present invention has devised an image forming method and an apparatus therefor capable of excellent image reproduction.

[Means for Solving the Problem] That is, the inventors of the present application technically analyze the problems (disturbance of a ready-made toner image, color mixing phenomenon, and improper reproduction of a solid image) particularly in the latter type, and I reached a conclusion like this.

More specifically, in the above-described image forming method, since charge is removed from a portion other than the positive image projecting portion of the residual charge portion on the surface of the photoconductor, as shown in FIG. , The surface potential VT1 of the first toner image T1 and the potential VH2 of the background portion H2 other than the second positive image projecting portion Z2 are substantially the same, if anything, only the charge of the toner of the first toner image T1 The surface potential VT1 becomes slightly higher in absolute value than the background potential VH2.

At this time, the potential of the latent image portion carrying the first toner image T1 is originally low, the electrostatic holding force of the first toner image T1 is weak, and moreover, the peripheral portion of the first toner image T1 and the photoreceptor Q.
As shown in FIG. 2 (b), the electrostatic attraction force between the surface and the surface is due to the electrostatic field S0 toward the direction around the first toner image T1.
F0 works slightly. Therefore, the toner particles forming the first toner image T1 are easily disturbed due to a slight fluctuation of the electrostatic field or the like, and particularly, the toner particles around the toner particles forming the first toner image T1 are easily disturbed.

Further, the surface portion potential VT1 of the above-mentioned first toner image T1
2 is higher than the background portion potential VH2 as shown in FIG. 2 (c), and a fringe electric field Sf1 is formed at the edge of the first toner image T1 which is larger than the other portions. It Therefore, in the second developing step, when the normal development is performed on the second latent image portion, the phenomenon that the second toner is easily electrostatically attracted to the edge of the first toner image T1 may occur. Occur.

Further, considering the case where a solid image is to be obtained in the second developing step, focusing on the latent image potential distribution at that time, as shown in FIG. 2 (C), the potential with respect to the background portion H2 is increased. Since the image part (second positive image projection part) Z2 that secures the contrast is formed, inevitably,
At the edge of the image portion Z2, a fringe electric field Sf2 that is larger than other portions is formed, and when the developer is dropped, the second toner is electrostatically adsorbed based on the fringe electric field Sf2. Conceivable. Therefore, when the image part is a solid image, the image part Z2 with the fringe electric field Sf2 is
While the second toner adheres to the edge of, the image area Z2
The second toner does not enter the central portion of the image, and a phenomenon of poor image quality occurs, such as a second toner image T2 (see FIG. 2A) that is missing.

As a means for preventing such a phenomenon, it is possible to adopt a developing bias method such as a magnetic brush developing method in the second developing step. This is because, as shown in FIGS. 2C and 2D, by applying an appropriate developing bias VB2 to the developing electrode D, static electricity generated between the second positive image projecting portion Z2 and the developing electrode D is generated. The second toner (negative polarity in this example) is positively attracted to the second positive image projection unit Z2 side based on the electric field Sa to form the second toner image T2. The electrostatic opening Sb generated between the first toner image T1 and the toner image T1 presses the first toner image T1 with the electrostatic attraction force F.

If such a system is adopted, the turbulence of the first toner image T1 can be suppressed to a small extent by the existence of the electrostatic attraction force F, and the second toner is moved toward the first toner image T1 side. Since the electrostatic attraction force of the second toner hardly acts, the toner color mixing phenomenon that the second toner mixes with the first toner image T1 is suppressed, and further, regarding the image reproduction of the solid image, Since the electrostatic attraction force F ′ toward the second image portion Z2 acts on the entire area of the second image portion Z2, even if the second image portion Z2 is a solid image, the second toner is It is certainly possible to obtain a good second toner image T2 that is electrostatically attracted to the entire area of the image portion Z2 and has no voids.

However, as described above, as long as the electrostatic field S0 toward the periphery of the first toner image T1 acts even a little, the disturbance of the first toner image T1 cannot be reliably prevented, and the second toner image T1 cannot be prevented. As shown in FIG. 6C, the fringe electric field generated at the edge of the first toner image T1 is also set in the step of setting the developing bias VB2 and performing the normal development on the second image portion Z2.
Sf1 may approach or exceed the developing bias VB2 level, and surely prevent the toner color mixing phenomenon in which the second toner is mixed in the first toner image T1 portion corresponding to the fringe electric field Sf1 portion. I came to the conclusion that I could not.

Under such circumstances, the present inventors have focused on the mutual relationship between the potential contrast around the first toner image T1 and the developing bias VB2 in the second toner image forming step, and The present invention has been found that can reliably prevent color mixing of toner.

That is, the present invention, as shown in FIG.
A first latent image corresponding to the first image is formed on the latent image carrier, and one of normal development and reversal development is performed on the first latent image with the first toner charged to one polarity. A first toner image forming step A for forming a first toner image using
A second latent image corresponding to the second image is formed on the latent image carrier that carries the first toner image without changing the polarity of the first toner image, and the other is different from the first toner polarity. Second toner image forming step B in which the second toner is formed by applying either the normal development or the reverse development to the second latent image with the second bias charged with the second toner in the developing bias applying method. And a transfer processing step C for collectively transferring both toner images to a transfer medium. In the second toner image forming step C, a developer comprising a predetermined color toner and a magnetic carrier having a density of 4.0 g / cm ³ or less. Using the magnetic brush developing method used, the developing bias VB2 is set as an intermediate potential between the background portion potential VH2 of the second latent image and the image portion potential of the second latent image, and the second toner image Surface potential VT1, second latent image background potential V
H2 and development bias VB2 are | VT1-VB2 |> | VH2-VB2 |
In addition, the image forming method is defined such that | VT1-VB2 |> | VT1-VH2 |.

In such an image forming method, the two-system toner images do not necessarily have to be formed with different color toners, and may include those formed with the same color toners.
As the developing process performed in each of the toner image forming steps A and B, any one may be applied as long as one is regular development and the other is reverse development, but the reverse development is performed in the first toner image forming step A. By performing the normal development in the second toner image forming step B, it is possible to secure a sufficiently large contrast between each image portion potential and the background portion potential, and to form an image with sufficient density better. It will be possible.

Further, an apparatus for realizing the above-mentioned image forming method is, for example, as shown in FIG. 1 (b), a latent image carrier 1
And a first latent image forming means 2 for forming a first latent image on the latent image carrier 1 corresponding to a first image to be subjected to either normal development or reversal development. First developing means 3 for forming the first toner image by developing the first latent image with the first toner charged to the polarity, and the latent image without changing the polarity of the first toner image. A second latent image corresponding to the other of the regular development and the reversal development is formed on the carrier 1, and a second latent image corresponding to the second image is formed. A second latent image forming means 4 having an intermediate potential with respect to the surface portion potential VT1 as a background portion potential VH2 of the second latent image,
A magnetic brush developing method is used in which a developer consisting of a predetermined color toner and a magnetic carrier having a density of 4.0 g / cm ³ or less is used, and the background potential VH2 of the second latent image and the image of the second latent image are used. A developing bias VB2 set to an intermediate potential with respect to the partial potential is applied, and the second latent image is developed with the second toner charged to the other polarity different from the first toner polarity to develop the second latent image. A second developing means 5 for forming a toner image and a transfer processing means 6 for collectively transferring both toner images to a transfer medium 7 are provided.

In such technical means, the latent image carrier 1 may be appropriately selected as a photoconductor, a dielectric or the like as long as it can form a latent image by the latent image forming means 2 and 4. The specific configuration may be drum-shaped or belt-shaped.

Also, for the first and second latent image forming means 2 and 4,
If a latent image having a predetermined level of potential can be formed on the latent image carrier 1, the latent image carrier 1 is charged in advance, and a portion corresponding to an image or a non-image portion is exposed to light or ions. It is possible to appropriately change the design such as one for adjusting to a predetermined level, or one for forming a latent image at a predetermined level with ions without precharging the latent charge carrier 1. In this case, in order to form a latent image with light, a so-called optical imaging system using a mirror or a lens system, an optical writing means such as a laser diode, a light emitting diode array, a crystal shutter array, or a fluorescent display element array may be used. In order to form a latent image with ions, a discharge stylus such as a multi-stylus or ion flow modulation may be used as appropriate.

Further, when the second latent image is formed by the second latent image forming means 4, an intermediate portion between the surface portion potential of the first toner image and the image portion potential of the second latent image is formed. It is necessary to set the background portion potential. In this case, the potential difference between the surface portion potential VT1 of the first toner image and the background portion potential VH2 is a range that produces an electrostatic field capable of forming a holding force sufficient to prevent the toner particles of the first toner image from scattering. It is preferable to set in. Specifically, although it depends on the charging potential of the latent image carrier 1, it is preferable to set it to 300 V or less,
It is preferably set to 30 to 200V. Further, in the so-called negative and positive image forming method in which reversal development is first performed and then regular development is performed as an image forming step, a latent image carrier is used when forming the background portion of the second latent image. In the so-called positive or negative image forming method in which the exposure amount to the photoconductor is adjusted, regular development is first performed, and then reversal development is performed,
When the background portion of the second latent image is formed, the recharging amount with respect to the photoconductor as the latent image carrier is adjusted.
When a dielectric is used as the latent image carrier,
The background portion is formed by controlling the amount of ions from the discharge head in each image forming method.

Further, for the first and second developing means 3 and 5, the developer to be used, as long as they are developed normally or by reversal with toners of different polarities corresponding to the first and second electrostatic latent images. ,
The developing method and the like may be appropriately selected, but at least the second developing means 5 needs to adopt the bias developing method. From the viewpoint of developing efficiency, the magnetic brush developing method is applied. Preferably. Further, each developing means 3 and 5 may be provided as long as it has one developing function part, but for example, it is designed to provide a large number of developing function parts of different colors and selectively switch each function part. It doesn't matter.

Then, in designing the second developing means 5, the potential difference between the developing bias VB2 and the surface potential VT1 of the first toner image can be prevented from being disturbed due to the floating of the first toner image. It is preferable to set the range to 250 V or higher. Further, with respect to the developing bias VB2, a range in which toner is not fogged on the background portion and the toner image density is sufficiently secured for the second image portion. And the background potential VH2
It is preferable to set it to about + 100V.

Further, when the magnetic brush developing method is adopted as the second developing means 5, the rubbing force of the magnetic brush of the second developing means 5 is applied to the first toner image already developed by the first developing means 3. Therefore, there is a problem that the first toner image is disturbed, the first toner image is scraped, and the scraped toner particles are mixed in the developing unit 5.

To avoid such a situation, for example, a low density of a predetermined color toner and a magnetic carrier having a density of 4.0 g / cm ³ or less is used so as to weaken the rubbing force of the magnetic brush on the first toner image. It is preferable to use the two-component developer of. In this case, as the magnetic carrier, a porous carrier, a ferrite carrier, a carrier in which magnetic powder is dispersed in a resin binder or the like can be appropriately selected, but a carrier in which magnetic powder is dispersed in a resin binder is It is preferable in that the density can be easily adjusted by the content of the magnetic powder.

Then, in order to obtain a sufficient toner image density when the two-component magnetic brush developing method is used, it is necessary to determine the amount of the developer existing in the developing nip area within an appropriate range.
The amount of the developer is the distance between the developer layer regulating member and the developing roll, that is, the trimming gap (TG), for example, the distance between the surface of the drum-shaped latent image carrier 1 and the developing roll, that is, the drum.・ Depending on the roll space (DRS) and the developer moving speed, if the developer moving speed is constant, the above TG / DRS value is set within the range of 0.6 to 1.2 to obtain high developing efficiency. You can

More specifically, if the value of TG / DRS is lower than 0.6, the amount of developer is too small to obtain sufficient developing efficiency,
The image becomes a low density image. If the value of TG / DRS is higher than 1.2, the amount of developer becomes too large, and the toner once adhering to the magnetic brush is adsorbed again to the magnetic brush, resulting in a low density image. Therefore, in order to obtain a sufficient image density, it is necessary to set the value of TG / DRS of the developing means 5 within the above-mentioned range of 0.6 to 1.2.

However, when a toner of a predetermined color and a magnetic carrier having a density of more than 4.0 g / cm ³ are used as the developer to be used, the rubbing force of the magnetic brush of the developing means 5 becomes too large and the first toner already formed. The image is disturbed and scraped off. When using such a two-component developer, reduce the amount of developer in the developing nip area (that is, TG / DRS to 0.
By setting the value smaller than 6, it is possible to prevent the disturbance and scraping of the first toner image, but it is difficult to obtain a satisfactory second toner image density. However, when a developer composed of a toner of a predetermined color and a magnetic carrier having a density of 4.0 g / cm ³ or less is used as the developer, the developing efficiency is high.
Even if the TG / DRS value (0.6 to 1.2) is set, the above-mentioned disturbance and scraping of the first toner image is effectively avoided, and the second toner with high density is not disturbed without disturbing the first toner image. It is possible to obtain a toner image.

Further, when the magnetic brush developing method is adopted as the developing means 5, as another means for weakening the rubbing force of the magnetic brush, a magnet roll is fixedly arranged in the non-magnetic rotating sleeve with respect to the second developing means 5. It is possible to adjust the magnetic brush of the developer in the developing nip area to be soft by disposing a repellent magnetic pole corresponding to the developing nip area on the magnet roll. Further, as another means, a developer carrier in which a magnet roll is rotatably arranged in a non-magnetic fixed sleeve is provided with respect to the second developing device 5, and the rotational movement speed of the latent image carrier 1 is set to V. _P , the moving speed of the developer on the developer carrier
If the V _DEVE, 0.5 satisfies _{<V DEVE / V P ≦ 2.0} , include be adjusted to reduce the impact force of the magnetic brush of the developer developing performance is not jeopardized.

Further, the transfer processing means 6 may be appropriately changed in design such as an electrostatic transfer method or a heat transfer method as long as it can collectively transfer the first toner image and the second toner image onto the transfer medium 7. However, it is preferable to use the electrostatic transfer method. In the case of adopting this electrostatic transfer system, at least after performing a pretreatment for making the polarities of the first and second toner images uniform, the transfer medium 7 is charged to a polarity different from the polarities of the toner images, 7 must be designed to electrostatically attract both toner images. In this case, from the viewpoint of effectively suppressing the transfer of so-called fog toner attached to the background portion of the surface of the latent image carrier 1 to the transfer medium 7, for example, the image area toner and the fog toner are charged to different polarities. Then
It is preferable that only the toner in the image area is transferred to the transfer medium 7 by the transfer means.

[Operation] According to the technical means as described above, first, in the first toner image forming step A, for example, as shown in FIG. After the first latent image Z1 corresponding to the first image to be subjected to the reversal development is formed, the first latent image Z1 is reversely developed by, for example, the first toner charged with a positive polarity to develop the first latent image Z1. One toner image T1 is formed.

Then, in the second toner image forming step B, the third
As shown in FIG. 2B, after the second latent image Z2 corresponding to the second image to be the target of the regular development is formed on the latent image carrier 1, the second latent image Z2 is negatively charged. The second latent image Z2 is normally developed with toner to form a second toner image T2.

At this time, the background portion potential VH2 of the second latent image Z2 is the surface portion potential VT1 of the first toner image T1 (during the second toner image forming step) and the image portion potential VZ2 of the second latent image Z2. Is set to an intermediate potential between the above and the surface potential of the first toner image T1.
Since VT1 is lower than the background portion potential VH2, this first toner image T1 portion becomes a kind of potential well with respect to the surroundings. On the other hand, the developing bias VB2 is applied to the developing electrode of the second developing means 5 facing the latent image carrier 1.

In this state, as shown in FIGS. 3B and 3C, the developing bias VB2 is set to be larger than the background portion potential VH2 of the second latent image Z2, so that the second toner having the negative polarity is The second toner image T2 is electrostatically attracted to the second latent image Z2 side without adhering to the first toner image T1 portion and the background portion H2 of the second latent image Z2 by the action of the electrostatic field Sa. To form. On the other hand, under the condition that the developing bias VB2 is applied, the potential difference ΔVm between the first toner image T1 and the developing bias VB2 becomes larger than that of the surroundings, as shown in FIGS. 3B and 3C. Therefore, the electrostatic field Sm at the portion corresponding to the first toner image T1 becomes larger than the electrostatic field Sb at the other background portion H2, and the electrostatic attraction force Fm that presses the first toner image T1 accordingly.
In addition, in the peripheral portion of the first toner image T1, an electrostatic field Sn is formed in the arrow direction based on the potential difference ΔVn between the background portion H2 and the first toner image T1. An electrostatic attraction force Fn that holds and restrains in the horizontal direction is generated. Therefore, the first toner image T1 is firmly held on the latent image carrier 1 by the electrostatic attraction forces Fm and Fn, and the first toner image T1 is changed by the electrostatic field change or the magnetic brush. Even if an external force is applied to the first toner image T1, the first toner image T1 becomes difficult to be disturbed.

In the second toner image forming step B, the electric field distributions corresponding to the first toner image T1 and the second latent image Z2 are as shown in FIG. 3 (d), and the first toner image T1 The fringe electric field Sf1 generated at the edge of is formed in the direction away from the developing bias VB2 level. Therefore, the fringe electric field Sf1 will not be placed close to or exceed the developing bias VB2 level, and the fringe electric field Sf1 portion will not be developed with the second toner in the second developing step. Toner color mixture development in which the second toner is mixed in the first toner image T1 is effectively avoided.

Further, in the second toner image forming step B, since the developing method using the developing bias VB2 is adopted,
As shown in FIG. 6C, even if the second latent image Z2 is a solid image, the second toner is electrostatically attracted to the entire area of the second latent image Z2 based on the electrostatic field Sa. ′ Will be electrostatically adsorbed, and a good second toner image with no voids
It is possible to obtain T2.

Since the first toner and the second toner have different polarities, the second toner may come into contact with the first toner image or the first toner may be mixed in the second developer. Even so, both toners repel each other, and the situation in which both toners coexist is effectively avoided.

After that, in the transfer processing step C, both toner images T1 and T2 on the latent image carrier 1 are simultaneously transferred to the transfer medium 7, and a plurality of types of image printing can be performed in one image forming cycle. It will be.

In such an image forming process, the first latent image Z
If a sufficient potential contrast between the first and second latent images Z2 is ensured, a toner image with sufficient density can be obtained.

Contrary to the image forming process described above, the first latent image Z1 is normally developed in the first toner image forming process A, and the second latent image Z2 is reversely developed in the second toner image forming process B. In the second toner image forming step, the potentials of the first toner image T1 (negative polarity in this example), the second latent image Z2 and the background portion H2 thereof, and the developing bias VB2 are the third.
The first toner image is set in the relationship as shown in FIG.
The T1 site becomes a kind of mountain of electric potential with respect to the surroundings.

At this time, as shown in FIG. 3 (f), the potential difference ΔVm between the first toner image T1 and the developing bias VB2 becomes larger than that of the surroundings, and therefore, in the portion corresponding to the first toner image T1. The electrostatic field Sm becomes larger than the electrostatic field S at the other background portion H2, and the electrostatic attraction force Fm that presses the first toner image T1 of negative polarity becomes larger by that amount. In the peripheral portion, an electrostatic field Sn is formed in the direction of the arrow based on the potential difference ΔVn between the background portion H2 and the electrostatic force that holds and restrains the negative first toner image T1 in the horizontal direction. An electric attraction force Fn is generated. Therefore, similar to the image forming process described above, the first toner image T1 is set so as not to be disturbed.

In the second toner image forming step B, a fringe electric field (not shown) generated at the edge of the first toner image T1 is arranged close to the developing bias VB2 level, as in the image forming step described above. Since the fringe electric field portion is not developed with the second toner in the second developing step, the color mixture phenomenon of the toner is prevented and the developing bias method is adopted. Therefore, even if the second latent image Z2 is a solid image, the second toner is electrostatically adsorbed to the entire area of the second latent image Z2, which is a good image with no voids. It is possible to obtain the second toner image T2. Since both toners have different polarities, the second toner may come into contact with the first toner image,
Even if the first toner tries to mix in the second developer,
Both toners repel each other, and the situation where both toners coexist is effectively avoided.

[Embodiment] The present invention will be described in detail below based on an embodiment shown in the accompanying drawings.

Embodiment 1 FIG. 4 shows an image forming method according to the present invention applied to a two-color printer.

In the figure, 10 is a positive charging type photosensitive drum as a latent image carrier having a photoconductive layer 10a on the periphery, and 11 is the photosensitive drum 10
Charging corotron for precharging, 12 is a first LED array for forming a first latent image, 13 is a magnetic brush type first developing device using black toner which is positively charged, 14
Is a second LED array for forming a second latent image, 15 is a magnetic brush type second developing device using red toner which is negatively charged, and 16 is on the photosensitive drum 10 before the transfer step. The pre-transfer corotron that aligns the charging polarities of the toner images, 17 charges the recording sheet 18 to the opposite polarity to the polarities of the toner images that have been aligned by the pre-transfer corotron 16, and electrostatically charges each color toner image on the recording sheet 18. A transfer corotron for transfer, 19 is a destaticizing corotron for separating the recording sheet 18 from the photosensitive drum 10 after the transfer process, and 20 is a photosensitive drum before the cleaning process.
A charge eliminating corotron for removing the residual charge of 10 and the residual toner, 21 a cleaner for removing the residual toner on the photosensitive drum 10, and 22 a photosensitive drum 10 before the next image forming cycle.
A charge eliminating lamp for completely removing the residual charge on the upper side, 23 a sheet supply tray accommodating the recording sheet 18, 24 a fixing device for fixing the toner image on the recording sheet 18 after the transfer process, 25 a recording sheet It is a guide plate that defines 18 transport paths.

Next, the image forming operation of the two-color printer according to this embodiment will be described.

Now, as an example of a recorded image, as shown in the upper part of FIG. 5, an image having a black image portion GB and a red image portion GR on a white background W will be taken as an example.

First, the photosensitive drum 10 is uniformly positively charged by the charging corotron 11 (FIG. 5 (a)).

Next, the portion corresponding to the black image portion GB of the photosensitive drum 10 is negatively exposed by the first LED array 12. At this time, the first latent image Z1 corresponding to the black image portion GB of the photosensitive drum 10 has the potential
The charge is removed by VZ1, and the white background W of the photosensitive drum 10 and the background portion H1 corresponding to the red image portion GR are held at the initial charging potential VH1 (FIG. 5 (b)).

Thereafter, the developing bias VB1 of the first developing device 13 is set between the potential VZ1 of the first latent image Z1 and the initial charging potential VH1, and the black color positively charged by the first developing device 13 is set. The first latent image Z1 is reversely developed with toner to form a first toner image T1 (FIG. 5 (c)).

Further, the second LED array 14 positively exposes a portion of the photosensitive drum 10 corresponding to the red image portion GR. At this time, the second latent image Z2 corresponding to the red image GR of the photosensitive drum 10 is held at the potential VZ2 substantially equal to the initial charging potential VH1, and the background portion H2 other than the second latent image Z2 is One toner image
The charge is removed so that the potential VH2 is higher than the surface potential VT1 of T1 (FIG. 5 (d)).

After that, the developing bias VB2 of the second developing device 15 is set between the potential VZ2 of the second latent image Z2 and the background portion potential VH2, and the red toner negatively charged by the second developing device 15 is set. Then, the second latent image Z2 is normally developed to form a second toner image T2 (FIG. 5 (e)).

At this stage, two color toner images T1, T2 are formed on the photosensitive drum 10.
Are formed, and these toner images T1 and T2 are
For example, after the corotron before transfer 16 is aligned to have a negative polarity (FIG. 5 (f)), the recording sheet is transferred by the transfer corotron 17.
It is transferred to the 18 side at once. Then, the recording sheet 18 after transfer
Passes through the fixing device 24, so that the toner image of each color is fixed on the recording sheet 18.

At this time, the image on the recording sheet 18 was hardly disturbed, and it was confirmed that the image quality was good.

In such an operation process, in order to obtain a good two-color image without disturbing the first toner image T1, the condition for the second toner image forming step is | VT1-VB2 |> | VH2-VB2 |. (1) | VT1-VB2 |> | VT1-VH2 | ... We confirmed that it is necessary to satisfy (2) based on the results of the following experiments.

Experiment Under the same conditions of the first toner image forming process, each toner image is formed by appropriately changing the parameters of the second toner image forming process, the disturbance of the first toner image T1 and the image density based on each toner image T1, T2. Was measured.

In this case, the toner image to be measured is a line image of 300 μm extending in the axial direction (X) and the circumferential direction (Y) of the photosensitive drum 10, and the disturbance is the line image line of the monochromatic mode. The line width reproducibility indicating the ratio of the line width of the reproduced toner image T1 when the width is set to 1 and the roughness indicating the degree of disorder dimension at the edge portion of the reproduced toner image T1 are used.

Also, during the experiment, ☆ Photosensitive drum ・ Se (selenium) type photoconductor (positive charging type) ・ Drum diameter 200mm ☆ Process speed ・ 160mm / sec ☆ First developer ・ Two-component system (positive charging black toner) ) Carrier Ferrite carrier having an average particle size of 100 μm Black toner 92 parts of styrene-n-butyl methacrylate copolymer and 8 parts of carbon black # 4000 (manufactured by Mitsubishi Kasei Co.) and charge control agent (Bontron P-51, Orient) (Manufactured by Kagaku Co., Ltd.), melted and kneaded, and then finely pulverized to an average particle size of 12 μm, and the carrier is positively charged.

☆ Second developer ・ Two-component system (negatively charged black toner) ・ Carrier Styrene-n-butyl methacrylate copolymer (35 parts) and magnetite (65 parts) are mixed, melt-kneaded, and then finely pulverized into a magnetic powder dispersion type. The average particle size is 30 μm and the density is 2.2 g / cm ³ .

Red toner: 92 parts of styrene-n-butylmethacrylate copolymer, 8 parts of red pigment resource cult (BASF), and 2 parts of a charge control agent (E-84, manufactured by Orient Chemical Co., Ltd.) were mixed and melt-kneaded. It is pulverized to an average particle size of 12 μm and is negatively charged with respect to the carrier.

☆ Each parameter of the first developing device ・ Trimming gap (TG) 0.6mm ・ Drum ・ Roll ・ Space (DRS) [Space between photosensitive drum and developing roll] 0.8mm ・ Magnet set angle (MSA) [Developing nip Angle of the main pole set position of the magnetic pole with respect to the area] + 5 ° ・ Development sleeve diameter and rotation speed 50mm, 480mm / sec ・ Development sleeve rotation direction WITH mode (forward direction with photosensitive drum) ・ Developer conveyance amount 120mg / cm ²・ Main pole type and magnetic force carrying pole, 750 gauss ☆ Each parameter of the second developing device ・ TG 0.6mm ・ DRS 0.8mm ・ MSA -5 ° ・ Development sleeve diameter and rotation speed 50mm, 220mm / sec ・ Rotating direction of developing sleeve WITH mode (forward direction with photosensitive drum) ・ Conveyance amount of developer 60mg / cm ²・ Type of main pole and magnetic force Repulsion magnetic pole (those of the same polarity are arranged adjacent to each other), 122
0 gauss * Voltage applied to corotron before transfer DC-5.0KV * Voltage applied to transfer corotron AC 400Hz, V _pp 8.5KV, DC + 2.5KV are common.

Further, as shown in FIG. 6 (a), in forming the first toner image, in forming the first toner image, the potential VZ1 of the first latent image Z1 is set to 200 (V ), The first latent image Z1
Background potential VH1 of 800 (V), first developing bias VB1
Is fixed at 650 (V), the first toner image is formed, and the second toner image is formed after 0.7 seconds have elapsed.
As shown in FIG. 6B, the surface potential VT2 of the second toner image, the second developing bias VB2, the background potential VH2 of the second latent image Z2, and the exposure at the time of forming the first latent image. The exposure amount E2 at the time of forming the second latent image when the amount E1 was set to 1 was appropriately changed, and six experimental examples 1 to 6 shown in Table 1 below were selected. When forming the second toner image, the potential V of the first and second latent images Z1 and Z2 is
Z1, VZ2 and the surface potential VT1 of the first toner image T1 are reduced by dark attenuation by 160 (V), 700 (V) and 190 (V).
Fixed to.

Then, the results of the respective image characteristics of the respective experimental examples are shown in Table 2 below.

In Table 2, in assigning a grade G to the image characteristics of each of Experimental Examples 1 to 3, the criteria shown in FIG. 7 were used. That is, when evaluating the image disturbance, based on the empirical rule that if the line width reproduction is less than 1.30 and the roughness is less than 15 μm, almost no image disturbance can be detected,
With this range as a good range, grades G of 1 to 0 were set according to the degree of goodness, and for those other than the good range, grades G of 1.5, 2, 3, and 4 were set according to the degree of failure.

According to this, as shown in FIG. 8, the experimental examples or (indicated by P1 to P3 in the figure) are in the good range within grade 1, and the experimental examples or (P4 to P6 in the figure) exceed the grade 1. It is understood that it is in the defective range.

Further, when the degree of color mixture of the second toner with respect to the first toner image was examined and observed, the toner color mixture phenomenon was not observed in Experimental Examples 1 to 3, and was slightly observed in the Experimental Example. It was confirmed that the color mixture of was visible.

In addition, in this experimental example, each experiment was performed on the line image, but when the image formation was performed on the solid image, a good image without a hollow image was obtained.

Embodiment 2 FIG. 9 shows an image forming method according to the present invention applied to a two-color color copying machine.

In the figure, reference numeral 30 denotes a negative charging type photosensitive drum as a latent image carrier having a photoconductive layer 30a on the periphery, and 31 denotes the photosensitive drum 30.
, 32 is an LED array for forming a first latent image, 33 is an exposure lamp 33a for irradiating the original 35 on the platen 34, and light reflected from the original 35 is the photosensitive drum 3
Optical imaging for forming a second latent image consisting of a plurality of mirror 33b groups for forming an optical path leading to a predetermined portion of 0 and an imaging lens 33c for forming a light image from the original 35 on a predetermined portion of the photosensitive drum 30. 36 is a magnetic brush type first developing device using a negatively charged black toner, 37 is a magnetic brush type second developing device using a positively charged red toner, and 38 is a transfer process Pre-transfer corotron that makes the charging polarity of the toner image on the photosensitive drum 30 uniform, 39 is a transfer corotron that transfers each color toner image on the photosensitive drum 30 to the copy sheet 40, and 41 is the copy sheet 40 after the transfer process. A destaticizing corotron for peeling off from the photosensitive drum 30, 42 is a destaticizing corotron for removing the residual charge and the residual toner charge on the photosensitive drum 30 before the cleaning step, 43 is a cleaner for removing the residual toner on the photosensitive drum 30, and 44 is the next. Copy cycle The photosensitive drum in
An electric discharge lamp for completely removing the electric charge on 30; 45, a sheet feeding tray for storing the copy sheet 40; 46, a fixing device for fixing the toner image on the copy sheet 40 after the transfer process; and 47, a copy after the fixing process. A discharge tray for discharging and storing the completed copy sheet 40, and 48 is a sheet conveying system for transmitting the copy sheet 40 in the sheet feeding tray 45 to a transfer portion at a predetermined timing and for conveying it to a discharge tray 47 via a fixing device 46. Is.

In this embodiment, as shown in FIG. 10, the second developing device 37 includes a developing roll 52, an agitator 53 for agitating the developer, and agitated developer g in the housing 51 to the developing roll 52. And a mixing plate that returns the developer g scraped by the trimming bar 55 to the agitator 53 side.
56 is stored.

The developing roll 52 is provided with a fixed sleeve 57 made of a non-magnetic material and a large number of transport magnetic poles 58a, 58b mounted on the periphery thereof, and is rotatably disposed at a predetermined speed in the fixed sleeve 57. It consists of a magnet roll 58. In this case, the rotation speed of the photosensitive drum 30 is V _P ,
When the moving speed of the developer g on 52 is V _DEVE , the condition of 0.5 ≦ V _DEVE / V _P ≦ 2.0 is satisfied based on the experimental result described later.

The basic structure of the first developing device 36 is substantially the same as that of the second developing device 37, but unlike the second developing device 37,
A rotary sleeve 59 and a magnet roll 60 around which a large number of carrier magnetic poles 60a and 60b are mounted and which are fixedly arranged in the rotary sleeve 59 are shown.

Next, the operation of the two-color color copying machine according to this embodiment will be described.

First, the charging corotron 31 is used to negatively charge the photosensitive drum.
30 is uniformly charged (FIG. 11 (a)), and then the LED array 32 irradiates light according to image information to form a first negative latent image Z1 on the photosensitive drum 30 (11th). Figure (b)). After that, in a state where an appropriate developing bias VB1 is applied to the developing roller 52 of the first developing device 36, the first negative latent image Z1 is developed with a negative black toner to form a first toner image T1. It is formed (Fig. 11 (c)). Then, the second positive latent image Z2 corresponding to the image of the original document 35 on the photosensitive drum 30 by the optical imaging system 33 (the absolute value of the potential VH2 of the background portion H2 is the surface portion of the first toner image T1. (Above the absolute value of the electric potential VT1) (FIG. 11 (d)), an appropriate developing bias VB2 is applied to the developing roller 52 of the second developing device 37, and The positive latent image Z2 is developed with positive polarity red toner to form a second toner image T2 (FIG. 11 (e)). After this, each toner image T1, T2 on the photosensitive drum 30
After aligning the polarities of with the pre-transfer corotron 38, each toner image T1, T2 is transferred to the transfer sheet 40 by the transfer corotron 39,
Each toner image T1, T2 is printed on the copy sheet 40 after a predetermined fixing process.
To settle.

In such an operation process, as shown in FIG. 12 (b), when the rotating sleeve 57 'and the fixed magnet roll 58' are used as the developing roll 52 in the second developing step, the rotating sleeve 57 'is used. The developer g (carrier gc, toner gt) group which is in a spiked state above collapses from the solid line state to the dotted line state, and then rises again to the state indicated by the one-dot chain line, while performing a so-called scooping movement, Since the rotating sleeve 57 'moves in the moving direction K, the rubbing force between the developer g and the photosensitive drum 30 becomes relatively large. However, in this embodiment, the second developing step is performed. As shown in FIG. 12 (a), by moving the magnet roll 58 in the direction of arrow U1, the developer g (carrier gc, toner gt) groups in the spiked state on the fixed sleeve 27 are rotated respectively. Predetermined speed in the direction of arrow U2 To revolve V _DEVE, rubbing force between the developing agent g group and the photosensitive drum 30 is kept small, scraping of the first toner image T1 is effectively prevented.

In order to confirm such an operation process, the first developing device
With each parameter of 36 being uniquely fixed, an experiment was performed to measure the disturbance of the first toner image by changing the rotation speed and the number of poles of the magnet roll 58 among the parameters of the second developing device 37. It was

In conducting the experiment, ☆ Photosensitive drum ・ Negatively charged organic semiconductor system ・ Movement speed: 100 mm / sec ☆ First developer ・ Two-component system (negatively charged black toner) For example, iron powder with polymethylmethacrylate copolymer 95 parts by weight of coated carrier having an average particle size of 100 μm and styrene-n-butyl methacrylate copolymer (copolymerization ratio 8
0:20) A mixture of 93 parts by weight and 7 parts by weight of carbon black and 5 parts by weight of a toner having an average particle size of 11 μm.

☆ Second developer ・ Two-component system (positively charged red toner) Styrene-n-butylmethacrylate copolymer (density
1.1g / cm ³ ) and cubic magnetite (density 4.8g / cm ³ )
Is mixed at a ratio of 35/65, melt-kneaded, and then finely pulverized. 90 parts by weight of a carrier having a density of 2.2 g / cm ³ and an average particle size of 30 μm, and a styrene-butyl methacrylate copolymer are graft-polymerized with a low molecular weight polyolefin. 92 parts by weight of the resin thus obtained and 8 parts by weight of red-based content: Resorskart (trade name, BASF) are melt-kneaded and finely pulverized to obtain an average particle size of 9.8.
A mixture of 10 parts by weight of toner of μm.

☆ Potential condition First negative latent image Z1: -60V Background of first negative latent image Z1: -600V First developing bias VB1: -400V Second positive latent image Z2: -580V Second positive latent image Background part of image Z2: −200V Second developing bias VB2: −300V.

☆ Each parameter of the first developing device ・ Trimming gap: 0.6mm ・ Drum roll space: 0.8mm ・ Magnet set angle: + 5 ° ・ Development sleeve diameter: 50mm ・ Magnet roll configuration: Asymmetric 6 poles ・ Main pole Magnetic force: 750 gauss * Each parameter of the second developing device (magnet rotation type) -Trimming gap: 0.6 mm-Drum roll space: 1.0 mm-Developing sleeve diameter: 50 mm-Main pole magnetic force: 800 gauss did.

Under this condition, the number of poles of the second developing device 37 is symmetrical 8, 10 and 12 poles, and the rotation number of the magnet roll 58 is 5, 10,
Change to 15, 20, 25, 30 (rps).

Then, when the first toner image is a horizontal line image having a line width of 250 μm and the ratio of the line width after passing the second developing step to the line width before passing is within 1.1, ◎, also in the range of 1.1 to 1.2. The results are shown in Table 3 below when the value is within the range of ◯ and the other cases are within the range of x.

Table 4 below shows the experimental conditions by the speed ratio between the developer moving speed V _DEVE and the rotational moving speed V _P of the photosensitive drum 1.
become that way. In Table 4, assuming that the magnet roll diameter is D (mm), the number of poles is N, the number of revolutions of the magnet roll is Rm (rps), and the spike length of the developer is l (mm), V _DEVE = It is approximated by (πD × NlRm) / (πD-Nl) [mm / sec.], But the effective spike length l is about 1 mm,
Assuming πD >> Nl, V _DEVE = NRm is approximated.

Therefore, according to Tables 3 and 4, the developer moving speed V
_If the speed ratio between _DEVE and the rotational movement speed V _P of the photosensitive drum is m, the line width _enlargement ratio of the first toner image can be kept within 1.2 as an allowable disturbance of the first toner image. 0.5 ≦
m must be set so as to satisfy the range of m ≦ 2.0. In order to keep the line width thickening ratio of the first toner image within 1.1, the range of 0.8 ≦ m ≦ 1.5 should be satisfied. It is understood that m must be set.

Third Embodiment FIG. 13 is a diagram in which the image forming method according to the present invention is applied to a two-color printer.

In the figure, 70 is a positive charging type photosensitive drum (Se in this embodiment) serving as a latent image carrier having a photoconductive layer 70a on the periphery.
System), 71 is a charging corotron, 72 is a first LED array for forming a first latent image, 73 is a magnetic brush type first developing device using negative black toner, and 74 is a photosensitive drum. A recharging scorotron as a recharging device for recharging 70, 75 a second LED array for forming a second latent image, and 76 a magnetic brush type second development using positive red toner. Reference numeral 77 is a simultaneous exposure charging corotron, 78 is a transfer corotron, 79 is a roll-shaped recording sheet, 80 is a guide roll for the recording sheet 79, 81 is a static elimination corotron, 82 is a cleaner, and 83 is a static elimination lamp.

In this example, the exposure simultaneous charging corotron 77
While uniformly exposing the photoconductive layer 70a of the photosensitive drum 70,
The discharge process is performed by applying an AC voltage on which a DC voltage having the same polarity as the charging polarity of the photoconductive layer 70a is applied, and the discharge characteristic is as shown in FIG. 14, for example.

In FIG. 14, the vertical axis represents the inflow current I to the surface of the photoconductive layer due to the discharge treatment, and the horizontal axis represents the surface potential VPR of the photoconductive layer 70a.
V0 means the surface potential of the photoconductive layer 70a when I = O, respectively. When the discharge process is performed, the potential V0 is set to be a little higher in absolute value than the background potential.

Next, the operation of the two-color printer according to this embodiment will be described.

First, the photoconductive layer 70a of the photosensitive drum 70 rotating in the direction of the arrow is uniformly charged to +1300 (V) by the charging corotron 71 (FIG. 15 (a)).

Then, light irradiation (positive exposure) corresponding to the first image is performed by the first LED array 72 to form a first positive latent image Z1 corresponding to the first image on the photoconductive layer 70a (fifteenth). Figure (b)). The potential VZ1 of the first positive latent image Z1 after exposure is +120
The potential VH1 of the background portion H1 was +650 (V).

After that, regular development is performed by the first developing device 73 using the negative polarity black toner under the developing bias VB1 + 800 (V) to form the first toner image T1 (FIG. 15 (b)). .
The symbol T1 'means the first fog toner attached to the background portion.

Furthermore, the recharged scorotron 74 again causes the photoconductive layer 70a to
Is charged, and the potential VT1 of the first toner image T1 is +600 (V),
The background potential VH2 was set to +500 (V) (Fig. 15 (c))
After that, light irradiation (negative exposure) corresponding to the second image is performed by the second LED array 75 to form a second negative latent image Z2 corresponding to the second image (FIG. 15 (d)). . The potential VZ2 of the second negative latent image Z2 after exposure was +100 (V).

After that, reversal development is performed by the second developing device 76 using the positive red toner under the developing bias VB2 + 350 (V) to form the second toner image T2 (FIG. 15 (d)). .
The symbol T2 'means the second fog toner attached to the background portion.

Next, the photoconductive layer 70a is exposed by the exposure simultaneous charging corotron 77.
Was subjected to a discharge treatment under uniform exposure. In this case, the uniform exposure allows the photoconductive layer 70a in the background portion without the toner images T1 and T2 to be formed.
However, since the toner blocks light in the toner images T1 and T2, the photoconductive layer 70a is not rendered conductive, and the surface potential of the toner images T1 and T2 is lower than the background potential. And is kept high (Fig. 15 (e)). Further, the discharge treatment was performed by applying an AC voltage on which a positive DC voltage having the same polarity as the charging polarity of the photoconductive layer 70a was superimposed. At this time, when V0 is set to a value (about 50 (V)) which is slightly higher in absolute value than the background portion potential, the first and second toner images T1 and T2, which are image portions, have negative polarity. As a result of being charged, the fog toners T1 'and T2' in the background portion are positively charged (Fig. 15 (f)).

Then, transfer was performed by a transfer corotron 78 to which a DC voltage having a polarity opposite to that of the toner in the image area was applied. As a result, only the toner images T1 and T2 having the negative polarity are transferred to the recording sheet 79, so that a good red-black two-color image without fog was obtained.

In this embodiment, when the DC component of the voltage applied to the exposure simultaneous charging corotron 77 is configured to be variable, V0 can be changed according to the change of the potential due to environmental changes, etc. Therefore, it is possible to always obtain a two-color image of good quality without being affected by environmental changes.

Fourth Embodiment FIG. 16 shows a two-color printer to which the present invention is applied. The basic structure is substantially the same as that of the third embodiment, but unlike the third embodiment, the recharging corotron 74 is not used. Instead of the simultaneous exposure charging corotron 77, a pre-transfer exposure lamp 91 and a pre-transfer charging corotron 92 that are functionally separated are used. The same components as those in the third embodiment are designated by the same reference numerals as those in the third embodiment, and detailed description thereof will be omitted here.

In this embodiment, the first LED array 72 performs negative exposure corresponding to the first image in the first latent image forming process, and the second LED array 72 in the second latent image forming process.
The array 75 is adapted to provide a positive exposure corresponding to the second image. Further, the first developing device 73 carries a positive polarity black toner, and the second developing device 76 carries a negative polarity red toner.

Next, the operation of the two-color printer according to this embodiment will be described.

First, the charging corotron 71 uniformly charges the photoconductive layer 70a of the photosensitive drum 70 to +1000 (V) (FIG. 17 (a)).

Next, light irradiation (negative exposure) corresponding to the first image is performed by the first LED array 72, and a first negative latent image Z1 corresponding to the first image is formed on the photoconductive layer 70a (the seventeenth image). Figure (b)). The potential VZ1 of the first negative latent image Z1 after exposure is +250
(V), and the potential VH1 of the background portion H1 was +900 (V).

After that, reversal development is performed by the first developing device 73 using the positive black toner under the developing bias VB1 + 750 (V) to form the first toner image T1 (FIG. 17 (b)). .
The symbol T1 'means the first fog toner attached to the background portion.

Then, light irradiation (positive exposure) corresponding to the second image is performed by the second LED array 75 to form a second positive latent image Z2 corresponding to the second image (FIG. 17 (c)). ). The potential VZ2 of the second positive latent image Z2 after exposure is +800 (V), the potential VH2 of the background portion is 300 (V), and the surface portion potential VT of the first toner image T1.
1 was 200 (V).

After that, regular development is performed by the second developing device 76 using the negative polarity red toner under the developing bias VB2 + 450 (V) to form the second toner image T2 (FIG. 17 (c)). .
The symbol T2 'means the second fog toner attached to the background portion.

Next, the photoconductive layer 70a is uniformly exposed by the pre-transfer exposure lamp 91 (FIG. 17 (d)), and then the pre-transfer charging corotron.
At 92, the photoconductive layer 70a was discharged. In this case, the first and second toner images T1 and T2, which are image portions, are negatively charged on the basis of the action substantially similar to that of the third embodiment.
The fog toners T1 'and T2' in the background portion are now positively charged (Fig. 17 (e)).

Then, transfer was performed by a transfer corotron 78 to which a DC voltage having a polarity opposite to that of the toner in the image area was applied. As a result, only the toner images T1 and T2 having the negative polarity are transferred to the recording sheet 79, so that a good red / black two-color image without fog was obtained.

EFFECTS OF THE INVENTION As described above, according to the image forming method and the apparatus thereof according to claims 1 to 9, in addition to the basic effect that a plurality of images can be formed in one image forming cycle, It produces the effect like.

That is, firstly, in the second toner image forming step,
By devising the relationship between the potential contrast around the first toner image and the developing bias, an image disturbance prevention force was applied to the existing toner image, so that fluctuations in the electrostatic field and impact force of the magnetic brush etc. Even if it acts on the toner image, it is possible to more reliably prevent the disturbance of the existing toner image.

Secondly, in the second toner image forming step, the potential contrast around the first toner image and the developing bias are set within a range in which development on the first toner image portion is avoided, so that the second toner is already formed. It is possible to effectively prevent the color mixing phenomenon in which the second toner is mixed with the existing toner image without being developed on the toner image.

Thirdly, at least in the second toner image forming step, since development by the developing bias system is performed, electrostatic attraction force for toner attraction is generated over the entire latent image area of the latent image carrier. Therefore, even if the reproduced image is a solid image, the toner can be surely electrostatically attracted to the entire latent image portion, and a good toner image can be obtained regardless of whether it is a line image or a solid image. .

Fourthly, each latent image is formed by utilizing the entire charged potential area of the latent image carrier, so that the latent image potential contrast can be sufficiently secured, and the image density is insufficient accordingly. It is possible to effectively prevent a situation that is likely to occur.

Fifth, since toners of different polarities are used to form two-system toner images, in the second toner image forming step, the second toner comes into contact with the existing toner image, Even if the toner is mixed in the second developing means, the two will repel each other, and the phenomenon that the two are mixed in the coexisting state can be effectively avoided.

Sixth, since the magnetic brush developing method is used in the second toner image forming step, the tip position of the magnetic brush can be used as a substantial bias electrode on the developing means side, and the latent image bearing member is correspondingly provided. And the bias electrode are arranged close to each other, so that the electrostatic field intensity for inducing toner can be set to be large, and the developing operation based on the developing bias can be made more stable.

Seventh, by devising the developer itself to be used,
Since the rubbing force of the magnetic brush on the ready-made toner image can be reduced, under conditions of high development efficiency,
Disturbance and scraping of the existing toner image can be reliably prevented.

According to the image forming method of claims 2 and 3,
When a photoconductor is used as the latent image carrier, two-system image formation can be efficiently realized. In particular, according to the second aspect, if the photoconductor is initially charged. Since the first latent image and the second latent image contrast can be secured sufficiently large without recharging during the process, it is possible to easily form an image with sufficient density.

Further, according to the image forming apparatus of the fifth aspect, the magnetic brush restraining force on the developer carrying member in the second developing means is weakened in the developing nip region based on the magnetic field of the repelling magnetic pole. The rubbing force of the magnetic brush on the latent image carrier in (1) is suppressed, and the disturbance of the first toner image can be more reliably prevented by that amount.

Further, according to the image forming apparatus of the sixth aspect, the second developing means is devised so that the rubbing force of the magnetic brush against the latent image carrier is suppressed within a range where the developing ability is maintained. It is possible to more reliably prevent the disturbance of the first toner image without impairing the formation condition of the second toner image.

Further, according to the image forming apparatus of the seventh to ninth aspects, it is possible to efficiently transfer the toner images of different polarities to the transfer medium by using the electrostatic transfer method. In this case, if the invention according to claim 7 is applied, it is possible to change the polarities of the toner in the image area and the fog toner in the background area, and transfer only the image area toner to the transfer medium. It is possible to form a good image which does not exist, and particularly when the charging means according to claim 9 is applied, the polarities of the toner in the image portion and the fog toner in the background portion can be effectively changed.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 (a) is an explanatory view showing steps of an image forming method according to the present invention, and FIG. 1 (b) is an explanatory view showing a schematic configuration of an image forming apparatus according to the present invention. 2 (a) to 2 (d) are explanatory views showing the technical principle for completing the present invention, and FIG. 3 (a) is the first toner of the image forming method according to the present invention to which the negative / positive developing method is applied. The explanatory view showing the image forming step, the same (b) is an explanatory view showing the second toner image forming step, and the same (c) shows the electric field acting state around the first toner image in the second toner image forming step. Explanatory drawing, (d) is an explanatory view showing the electric field contrast around the first toner image, and (e) shows the second toner image forming step of the image forming method according to the present invention to which the positive / negative developing method is applied. Explanatory drawing, (f) shows the vicinity of the first toner image in the second toner image forming step 4 is an explanatory view showing an electric field acting state of FIG. 4, FIG. 4 is an explanatory view showing a first embodiment of a two-color color printer to which the image forming method according to the present invention is applied, and FIGS. 5 (a) to 5 (f) are the first embodiment. FIG. 6 (a) is an explanatory view showing the image forming process of FIG.
(B) is an explanatory diagram of potential parameters in Experimental Examples-, FIG. 7 is an explanatory diagram showing criteria for grading the image characteristics of Experimental Examples-, and FIG. 8 is VH2- of Experimental Examples-. FIG. 9 is a graph showing the relationship between VT1 and grade, FIG. 9 is an explanatory view showing Embodiment 2 of a two-color color copying machine to which the image forming method according to the present invention is applied, and FIG. 10 is a specific example of the developing device in the embodiment. 11A to 11E are explanatory views showing the image forming process of the second embodiment.
FIG. 12 (a) is a schematic diagram showing the developing operation of the second developing device according to the second embodiment, FIG. 12 (b) is a schematic diagram showing the developing operation of another type developing device, and FIG. Example 3 of a two-color printer to which the image forming method according to the invention is applied, FIG. 14 is an explanatory diagram showing the characteristics of an exposure simultaneous charging corotron used in Example 3, and FIGS. (F) is an explanatory view showing the image forming process of Embodiment 3, FIG. 16 is an explanatory view showing Embodiment 4 of a two-color color printer to which the image forming method according to the present invention is applied, and FIGS. (E) is an explanatory view showing an image forming process of Example 4. [Explanation of Codes] 1 ... Latent image carrier 2 ... First latent image forming means 3 ... First developing means 4 ... Second latent image forming means 5 ... Second developing means 6 ... … Transfer processing means 7 …… Transfer medium

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Ken Inagawa, 2274 Hongo, Ebina City, Ebina, Kanagawa Prefecture Fuji Xerox Co., Ltd.Ebina Business Office (72) Koji Adachi, 2274 Hongo, Ebina City, Kanagawa Prefecture Fuji Xerox Co., Ltd.Ebina Co., Ltd. On-site (72) Inventor Nobumasa Furuya 2274 Hongo, Ebina-shi, Kanagawa Fuji Xerox Co., Ltd. On-site Ebina (56) References JP 61-219964 (JP, A) JP 57-2047 (JP, A) ) JP-A-63-41874 (JP, A)

Claims (9)

[Claims] 1. A first latent image corresponding to a first image is formed on a latent image carrier, and normal development and reversal are performed on the first latent image with a first toner charged to one polarity. A first toner image forming step of forming a first toner image using either one of development, and a second toner image forming step on the latent image carrier carrying the first toner image without changing the polarity of the first toner image. A second latent image corresponding to the first latent image is formed, and either the normal development or the reversal development is performed on the second latent image with the second toner charged to the other polarity different from the first toner polarity. The second toner image forming step of forming the second toner image by using the other in the developing bias applying method, and the transfer processing step of collectively transferring both toner images to the transfer medium, the second toner image forming step, developer predetermined color toner and density composed of a 4.0 g / cm ³ or less of magnetic carrier By using the magnetic brush developing method used, the developing bias VB2 is set as an intermediate potential between the background portion potential VH2 of the second latent image and the image portion potential of the second latent image, and the surface of the second toner image. The partial potential VT1, the background potential VH2 of the second latent image, and the developing bias VB2 are set so that | VT1-VB2 |> | VH2-VB2 | and | VT1-VB2 |> | VT1-VH2 | A characteristic image forming method. 2. The method according to claim 1, wherein after uniformly charging the surface of the photoconductor as a latent image carrier, a first negative exposure of irradiating the first image area with light is carried out, The exposed portion is reversely developed with the first toner charged to the same polarity as the charging characteristic of the photoconductor to form a first toner image, and then the first toner image is formed on the background portion area other than the second image portion. A second positive exposure is performed in which light is emitted at a level weaker than the exposure level, and the unexposed portion is normally developed with the second toner charged to the opposite polarity to the charging characteristics of the photoconductor to form a second toner image, An image forming method characterized in that both toner images are collectively transferred onto a transfer medium. 3. The method according to claim 1, wherein after uniformly charging the surface of the photoconductor as a latent image carrier, a first positive exposure is carried out by irradiating a region other than the first image portion with light. , The unexposed portion is normally developed with the first toner charged to the opposite polarity to the charging characteristic of the photoconductor to form a first toner image, and then the first toner image is formed without changing the polarity. After the photoconductor is recharged to a level lower in absolute value than the toner image potential of No. 2, the second negative exposure of irradiating light to the second image area is performed, and the charge is charged to the same polarity as the charge characteristic of the photoconductor. An image forming method characterized in that an exposed portion is reversely developed with a second toner to form a second toner image, and both toner images are collectively transferred onto a transfer medium. 4. A latent image carrier and a first latent image forming a first latent image on the latent image carrier corresponding to a first image to be subjected to either normal development or reversal development. Image forming means, first developing means for developing the first latent image with the first toner charged to one polarity to form a first toner image, and polarity of the first toner image The second latent image corresponding to the second image to be subjected to either the normal development or the reversal development on the latent image carrier without changing A second latent image forming means for setting an intermediate potential between the surface potential VT1 of one toner image and the background potential VH2 of the second latent image, and a predetermined color toner and a magnetic density of 4.0 g / cm ³ or less. A magnetic brush development method using a developer consisting of a carrier is used, and the potential between the background potential VH2 of the second latent image and the image potential of the second latent image is intermediate. A developing bias VB2 set to a potential is applied, and the second latent image is developed with the second toner charged to the other polarity different from the first toner polarity to form a second toner image. An image forming apparatus including a second developing unit and a transfer processing unit that collectively transfers both toner images to a transfer medium. 5. The second developing means according to claim 4, wherein the second developing means has a developer carrier for forming a magnetic brush in which a magnet roll is fixedly arranged in a non-magnetic rotating sleeve, and the magnet roll is provided. An image forming apparatus characterized in that a repulsive magnetic pole is arranged corresponding to a developing nip region. 6. The latent image according to claim 4, wherein the second developing means has a developer carrier for forming a magnetic brush in which a magnet roll is rotatably arranged in a non-magnetic fixed sleeve. When the rotational speed of the carrier is V _P and the speed of the developer on the developer carrier is V _DEVE , 0.5 ≦ V _DEVE / V _P ≦ 2.0
An image forming apparatus satisfying the above conditions. 7. The transfer processing means according to claim 4, wherein the transfer pretreatment means aligns the polarities of the first and second toner images, and the polarity of the toner images aligned by the transfer pretreatment means. An image forming apparatus comprising: a transfer unit that charges a transfer medium with opposite polarities and electrostatically transfers both toner images to the transfer medium. 8. The pre-transfer processing means according to claim 7, wherein the transfer pretreatment means uniformly exposes the surface of the photosensitive member as a latent image carrier, and the exposure under or by the exposure means. An image forming apparatus comprising: a charging unit that performs a discharging process later. 9. The image forming apparatus according to claim 8, wherein the charging means applies an AC voltage on which a DC component having the same polarity as the charging polarity of the latent image carrier is superimposed. .