JP3110953B2 - Image forming device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming apparatus such as a copying machine or a printer, and more particularly, to a latent image carrier by repeating an image forming process including charging, image exposing, and developing on the surface a plurality of times. The present invention relates to a multicolor image forming apparatus for forming a multicolor image on an image forming medium and then transferring the multicolor image to a transfer material at a time.

[0002]

2. Description of the Related Art Conventionally, a plurality of toner images of different colors are previously formed on an electrophotographic photosensitive member by repeating charging, exposing, and developing a plurality of times, and a toner image is collectively transferred to a transfer material to obtain a color image. Many multicolor image forming apparatuses have been proposed. Such a multi-color image forming apparatus can provide a small-sized color copying apparatus with a reduced area occupied by a transfer drum and the like, and can perform development of each color on the same latent image.
It is characterized by the fact that there is no misalignment seen in other types of color copiers.

By the way, this type of multicolor image forming apparatus is
In some cases, color superposition is performed on the photoreceptor, in which case the previously developed toner image is recharged, and then the area on which the color superposition is to be developed is irradiated with light to erase the charge on the photoreceptor. There must be. Therefore, the toner to be developed first needs to have good light transmittance, and therefore, usually, a chromatic color toner is used instead of a black toner.

In recent years, a developing means using a chromatic toner is a one-component developing method using only a non-magnetic chromatic toner instead of a two-component developing method for the purpose of further reducing the size and cost of the apparatus. The developing sleeve as a developer carrier and the photoreceptor as a latent image forming medium are brought into a non-contact state so that a direct current +
An AC bias is applied to form a toner image on the photoconductor. Also, by using a non-magnetic toner in addition to the final-stage developing means, it is possible to make the saturation recharge amount of the toner at the time of recharging higher than that of the magnetic toner, and as a result, the reflection power with the photoconductor is increased. Further, there is an additional effect that the toner of the toner image on the photosensitive member at the preceding stage is less likely to be mixed into the developing device at the subsequent stage.

[0005]

However, when a multi-color image is formed with the above configuration, the following problems may occur.

When a low-density solid image is developed in the preceding image formation, the toner of the toner image is mixed into the later developing means in the later developing region, and the toner image is deteriorated.

[0007] If a large number of low-density solid images are further developed, toner scattering occurs, which causes contamination of the recharger in the subsequent stage to cause uneven recharging. As a result, even in the subsequent image formation, And image deterioration such as density unevenness is likely to occur.

[0008] This phenomenon is caused by a conventional analog exposure method or a digital image exposure method using a laser beam of a semiconductor laser as an image exposure.
The problem occurred more remarkably when a method of controlling the laser emission time in proportion to the magnitude of an image signal value for each arbitrary pixel (hereinafter referred to as a PWM method) was used. When the cause was examined in detail, it was found that this was caused by the potential contrast in the developing region and the amount of charge of the toner flying from the developing sleeve to the photosensitive member accordingly.

FIG. 10 shows a potential model of a latent image of a low-density portion and a high-density portion for each pixel when the PWM method is used. From this figure, it can be seen that the potential contrast of the latent image in the low-density part is smaller than the contrast in the high-density part due to the on-off response of the laser emission and the shape of the laser spot.

FIG. 6 shows the reflection density of the toner to be developed with respect to the potential contrast of the latent image when the non-magnetic one-component chromatic toner is developed with the developing sleeve and the photoreceptor in non-contact and in an alternating electric field. 5 is a graph showing the charge amount of the toner at that time.

As can be seen from this graph, the charge amount of the toner in the low density area is considerably lower than in the high density area. This is because the toner flight in the non-magnetic one-component non-contact alternating electric field development depends on the potential contrast between the photoconductor and the developing sleeve and the amount of toner charge. This is because the toner having a high charge amount on the developing sleeve does not fly due to too much mirroring power with the developing sleeve, and as a result, only the toner having a low charge amount flies on the photoconductor.

FIG. 11 is a graph showing the recharging characteristics of the toner on the photosensitive member. This graph shows that the charge amount of the toner before recharging is directly reflected on the charge amount after recharging.

The above-mentioned problem has occurred for such a reason.

Accordingly, an object of the present invention is to provide a non-magnetic one-component non-contact alternating electric field developing device as a developing device other than the last developing device, even if a low-density solid image is to be formed, the image can be transferred to a subsequent developing device. It is an object of the present invention to provide a multicolor image forming apparatus which does not cause contamination of a recharger in a subsequent stage due to toner mixing or toner scattering.

[0015]

The above object is achieved by a multicolor image forming apparatus according to the present invention. In summary, the present invention forms an electrostatic latent image on an initially charged latent image forming medium by erasing a charge corresponding to an image signal by image exposure means, and forming the electrostatic latent image. After forming the toner image by the developing means, the surface of the latent image forming medium on which the toner image is formed is recharged, and the next image exposure means responds to the next image signal on the recharged latent image forming medium. A multicolor image is formed on a latent image forming medium by repeating a process of forming an electrostatic latent image by erasing part of the charge and a process of forming the electrostatic latent image into a toner image by the next developing means at least once or more. And an image forming apparatus that collectively transfers the toner onto a transfer material, wherein the developing unit using a chromatic toner among the plurality of developing units is a developer carrying a one-component developer composed of only a non-magnetic chromatic toner. A carrier is disposed with a gap from the latent image forming medium.
And a latent image is formed into a toner image by forming an alternating electric field in the gap, and the final image exposing unit of the image exposing unit emits light having a pulse width corresponding to a multi-level digital image signal. Irradiating the latent image forming medium, and the other image exposure means irradiating the latent image forming medium with light corresponding to a binary digital image signal at least when the image density to be output is low. An image forming apparatus.

It is preferable that the resolution of the final image exposing means of the plurality of image exposing means is larger than the resolution of the other image exposing means.

It is preferable that the exposure means other than the final one of the plurality of image exposure means is a binary digital image signal obtained by binarizing a multi-valued digital image signal. .

The final developing means of the plurality of developing means may be a latent image forming medium including a developer carrier having a magnetic field generating means inside a one-component developer comprising only a magnetic black toner. It is preferable that the toner image is formed by disposing a gap and a gap, and forming an alternating electric field in the gap.

The alternating electric field formed between the developer carrying member of the developing means other than the final developing means and the latent image forming medium among the plurality of developing means causes the developer to move to the latent image forming medium side. Is preferably larger than the peak value for returning the developer from the latent image forming medium to the developer carrier.

[0020]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The multicolor image forming apparatus according to the present invention will be described below in more detail with reference to the drawings.

Embodiment 1 A first embodiment of the multicolor image forming apparatus of the present invention will be described with reference to FIG. The operation principle of the multicolor image forming apparatus according to the present embodiment will be described below by taking a two-color image forming apparatus as an example.

In FIG. 1, the two-color image forming apparatus has a latent image forming medium 1. This latent image forming medium 1 is, for example, a so-called xerographic photosensitive member that forms an electrostatic latent image by the Carlson process, and an insulating layer on the surface on which an electrostatic latent image is formed by the NP process described in JP-A-42-23910. A photoreceptor, an insulator that forms an electrostatic latent image by an electrostatic recording method, an insulator that forms an electrostatic latent image by a transfer method,
A member that forms an electrostatic latent image (including a potential latent image) by another method can be used.

Around the photosensitive member 1 serving as a latent image forming medium, a first charging device 2, a first developing device 6, a second charging device 3, a second developing device 7, and a transfer device 8 are arranged along the rotation direction. Are arranged.

Photoconductor 1 uniformly charged by first charging device 2
The surface is irradiated with a first image exposure 4 by a first laser scanner (not shown) whose light emission is controlled in accordance with the first image signal, to form a first electrostatic latent image on the surface of the photoconductor 1. The electrostatic latent image is developed by the first developing device 6 to obtain a first toner image.

The surface of the photoconductor 1 on which the first toner image has been formed is uniformly charged again by the second charging device 3. The surface of the photoreceptor 1 after the second uniform charging is irradiated with a second image exposure 5 by a second laser scanner (not shown), the light emission of which is controlled in accordance with the second image signal, to the second surface. Is formed. The electrostatic latent image is developed by the second developing device 7 to obtain a second toner image.

After the first toner image and the second toner image sequentially formed on the surface of the photosensitive member 1 are collectively transferred to the transfer material 9 by the transfer device 8, the toner image is transferred to the transfer material 9 by a fixing device (not shown). And discharged outside the machine.

Here, the first developing device 6 and the toner used in the first developing device will be described. The toner used in the first developing device is a non-magnetic chromatic toner composed of, for example, red, which has a good light transmittance and a high saturation charge amount, for example.

As shown in FIG. 2, the first developing device 6 applies a developing sleeve 16 as a developer carrier to the opening of the developing container 6 ', and applies the toner in the developing container 6' to the developing sleeve 16. A toner supply roller 10 made of sponge rubber rotatably brought into contact with the developing sleeve to peel off undeveloped toner on the developing sleeve; and a toner supplied onto the developing sleeve 16 by the toner supply roller 10. The elastic regulating member 13 is formed of a flexible member such as urethane rubber, which regulates the toner to a thin layer and frictionally charges the toner at the contact portion.

In the developing system of the first developing device, a one-component developer consisting only of the toner is formed on the developing sleeve 16 in a thin layer, and the photosensitive member 1 and the developing sleeve 16 are disposed with a gap therebetween. An alternating electric field is formed there to cause the toner to fly on the photoconductor 1.

In the present embodiment, the second developing device 7, which is the last stage developing means, arranges the photoconductor 1 and the developing sleeve 16 in a non-contact manner so as not to disturb the first toner image on the photoconductor 1. In addition, black toner that does not require light transmittance is used as the toner. In this embodiment, the developing method uses a one-component toner having magnetism, and the photosensitive member 1 and the developing sleeve 1 are used.
A so-called jumping development method is adopted, in which an alternating electric field is formed between the photoconductors 6 and toner adheres to the photoconductor 1.

As shown in FIG. 3, the second developing device 7 is provided with a developing sleeve 15 at the opening of the developing container 7 ', and the toner supply roller 12 shown in the first developing device 6 described above. And a magnetic blade 13 that uses a magnetic field of a magnet 14 included in the developing sleeve 15 to reduce the thickness of the toner on the developing sleeve 15. In the present embodiment, the toner supply roller 12 is
A single-cell foam rubber made of, for example, a silicone rubber sponge is used, which prevents the toner from being overloaded by light pressure contact and has high durability without clogging of the toner.

In the present embodiment, the surface of the developing sleeve 15 has a pseudo-mirror surface which is not subjected to a surface roughening treatment using abrasive grains or the like, so that the toner layer on the developing sleeve can be made thinner and higher. The charge amount can be increased.

The structure of the first and second developing devices is not limited to the structure of the present embodiment. The first developing device is capable of applying chromatic non-magnetic toner to the photosensitive member in a non-contact manner and under an oscillating electric field. Any configuration may be used as long as it is a flying system. In addition, the second developing device, which is the last stage developing means, may use any configuration as long as it uses a black toner and develops without contact with the photoconductor.

Next, the first and second image exposure means 4 and 5 will be described in detail. Both the first and second image exposures are performed by scanning and irradiating a laser beam of a semiconductor laser, which emits light corresponding to a digital image signal, onto the surface of the photosensitive member with a polygon scanner (laser scanner).

The second (final stage) image signal is a multivalued digital image signal from 0 to 255 proportional to the image density to be output, and has a resolution of 400 dpi in this embodiment. Then, the second (final stage) semiconductor laser for image exposure emits PWM light as shown in FIG. 4 within the period of the fundamental frequency corresponding to the resolution of 400 dpi. Therefore, a high-resolution and high-gradation image can be formed.

The first image exposure means emits light corresponding to a binary digital image signal at least when the image density to be output is low, and outputs the binary or multi-valued image when the image density is high. But it is possible. Here, the low density means that, for example, the image signal level of a certain portion of the document is detected, and the level, the developing density shown from the photosensitive characteristics shown in FIG. 5 and the developing characteristics shown in FIG. 80% or less.

As can be seen from the relationship between the development characteristics and the charge amount of the toner image shown in FIG. 6, the charge amount is reduced at about 80% or less.

In this embodiment, the dither method is used for the gradation expression of the image of the low density portion.

FIG. 7 shows latent image potential models of the low-density portion and the high-density portion in the above-described configuration. By performing the binary digital exposure, the potential contrast of each pixel to be developed can be sufficiently reduced at the same level as the high-density portion. The toner of a high charge amount can fly to the photoreceptor in the portion, and as a result, it is possible to prevent image deterioration caused by color mixing and contamination of the recharger.

If the first image exposure means includes a binarizing device for converting a multi-valued digital image signal into a binary digital image signal, the above-described effect can be obtained even when the first image signal is multi-valued. Is obtained. In particular, if binarization is performed by an error diffusion type binarization apparatus, a binary image can be formed without deteriorating the gradation of a multi-valued image signal. Not only is the image quality high, but it can be obtained as a high-gradation image.

Although the configuration of the present invention has been described as a two-color image forming apparatus, it is obvious that the present invention can be applied to full-color image formation. PWM method with black toner,
In the former stage, chromatic magnetic toners of three colors of yellow, magenta and cyan are developed under a non-contact alternating electric field, and at least a low density part of each image exposure may be binary digital exposure.

Experimental Example Next, an example of an experimental example in this embodiment will be described.

First, the photosensitive member was an OPC drum having a diameter of 80 mm and the peripheral speed was 240 mm / s. Each of the first and second charging devices 2 and 3 is a corona charger with a grid. First, the surface of the photoreceptor is uniformly charged to -600 V by the first charging device 2, and then the first image exposure 4 is performed by irradiating a semiconductor laser, thereby substantially erasing the charge at the portion (pixel) to be developed. The voltage was lowered to -100V.

The resolution was set to 400 dpi, a low density part was applied by a binary dither method, and a high density part was applied by a multi-valued PWM method. The first developing means uses a red non-magnetic toner having a diameter of 8 μm, and a red toner is brought into contact with a SUS sleeve 16 having a diameter of 20 mm by contacting a supply roller 10 made of a urethane rubber blade or a silicon foam foam. A thin coating was applied on the sleeve. 240mm sleeve speed
/ S, the gap between the sleeve and the drum is 300 μm,
V _pp = 1200 when the developing bias is -450 V DC
V, a rectangular wave having a frequency of 1800 Hz was superimposed and developed. At this time, the charge amount of the toner flying on the drum is -30.
μC / g.

Next, the drum was recharged by the recharger 3 to -850V. At this time, the charge amount of the toner developed on the drum was -60 [mu] C / g. Thereafter, the second image exposure 5 is performed by irradiating a semiconductor laser, and the potential of a portion (pixel) to be developed is set to -250.
V. The resolution was 400 dpi, and the gradation reproduction was a PWM method.

Next, the second latent image is developed by second developing means. Here, the toner is applied to the SUS sleeve 16 having a diameter of 20 mm by rotatably abutting the supply roller 12 made of silicon single-foam rubber to the SUS sleeve 16 having a diameter of 6 μm. did.
Thereafter, the magnetic layer 13 is used to reduce the thickness, the peripheral speed of the sleeve is set to 360 mm / s, and the gap between the sleeve and the drum is set to 300 μm.
m, and the developing bias is V _pp = 1 when the DC component is -450 V.
A rectangular wave having a voltage of 200 V and a frequency of 200 Hz was superimposed and developed.

Thereafter, the two-color image on the drum was collectively transferred to the transfer material by the transfer charger 8, and was fixed by the fixing device.

In this image forming process, even after the first toner image has produced several thousand low density solid images having a reflection density of 0.7, the first toner does not enter the second developing means. Also, no wire contamination of the recharger due to the scattering of the first toner occurred, and a good image could be maintained.

Embodiment 2 In Embodiment 2 of the multicolor image forming apparatus according to the present invention,
In addition to the configuration of the first embodiment, by further reducing the resolution of the exposure unit other than the final image exposure unit to the resolution of the final exposure unit, it is possible to further enhance the color mixing prevention effect.

FIG. 8 shows 200 dpi and 400 dpi.
As can be understood from a latent image potential model when a photoconductor is irradiated with a laser at a resolution of, a reduction in the resolution allows the potential contrast to be obtained up to the end of the pixel, and as a result, a toner having a high charge amount Becomes easier to fly.

It is to be noted that gradation is deteriorated by lowering the resolution and forming a binary digital image signal.
If an error diffusion method or the like is used as an image processing method, it is possible to reproduce gradations as inferior to the PWM method.

Embodiment 3 Embodiment 3 of the multicolor image forming apparatus according to the present invention is Embodiment 1.
In addition to the above configuration, there is a characteristic in the waveform of the developing bias used in the developing means other than the final developing means.

In other words, as shown in FIG. 9, in order to increase the development contrast as much as possible without causing dielectric breakdown between the photoreceptor and the developing sleeve, the peak value in the direction in which the toner for the development bias AC is caused to fly to the photoreceptor is determined. It is larger than the peak value in the reverse direction. With such a waveform, the electric field intensity on the flying side can be increased, and as a result, the toner with a high charge amount can fly more efficiently.

In this embodiment, when the ratio of the peak value in the toner flying direction to the peak value in the opposite direction is 7: 3, image formation is performed. The amount ranges from −30 μC / g to −35 in the low concentration part.
μC / g.

[0055]

As is apparent from the above description, in the image forming apparatus according to the present invention, the developing means using the chromatic toner among the plurality of developing means is a one-component developer comprising only the non-magnetic chromatic toner. Is disposed with a gap from the latent image forming medium, and an alternating electric field is formed in the gap to form a latent image into a toner image. Image exposure means irradiates the latent image forming medium with light having a pulse width corresponding to a multi-valued digital image signal, the other image exposure means at least when the image density to be output is low density, binary By irradiating the latent image forming medium with light corresponding to a digital image signal, a non-magnetic one-component non-contact alternating electric field developing device that can be reduced in size and cost as a developing device other than the final developing device is used. ,
Even if a low-density solid image is formed, no toner is mixed into a later-stage developing device, and contamination of a later-stage recharger due to scattering of toner does not occur, so that a high-quality image can be obtained. The size of the apparatus main body can be reduced.

[Brief description of the drawings]

FIG. 1 is a schematic configuration diagram showing a first embodiment of a multicolor image forming apparatus according to the present invention.

FIG. 2 is a schematic configuration diagram illustrating a developing device using a chromatic toner of the multicolor image forming apparatus of FIG. 1;

FIG. 3 is a schematic configuration diagram illustrating a developing device using a black toner in the multicolor image forming apparatus of FIG. 1;

FIG. 4 is a graph showing a state of PWM light emission of the image exposure apparatus.

FIG. 5 is a graph showing the photosensitive characteristics of the photosensitive member.

FIG. 6 is a graph showing development characteristics in a developing device using a chromatic toner.

FIG. 7 is a graph showing a latent image potential model of an image carrier at the time of binary image exposure.

FIG. 8 is a graph showing a latent image potential model on an image carrier at the time of image exposure at 200 dpi and 400 dpi.

FIG. 9 is a graph showing a developing bias waveform and a potential contrast in a developing device using a chromatic toner.

FIG. 10 is a graph showing a latent image potential model on a photoconductor when a PWM method is used.

FIG. 11 is a graph showing the recharging characteristics of the toner on the photoconductor.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 photoconductor (latent image forming medium) 2 first charging device 3 second charging device 4 first image exposing device 5 second image exposing device 6 first developing device 7 second developing device 8 transfer device 9 transfer material 15, 16 Developing sleeve (developer carrier)

────────────────────────────────────────────────── ─── Continued on the front page (51) Int.Cl. ⁷ Identification code FI G03G 15/04 G03G 15/04 111 111 15/08 503A 15/043 15/04 120 15/08 503 Int.Cl. ⁷ , DB name) G03G 13/01 G03G 15/01-15/01 117 G03G 15/04-15/04 120 B41J 2/44 H04N 1/46

Claims (5)

(57) [Claims] 1. An electrostatic latent image is formed on an initially charged latent image forming medium by erasing a charge corresponding to an image signal by an image exposing means, and the electrostatic latent image is formed by a developing means. After the toner image is formed, a process of recharging the surface of the latent image forming medium on which the toner image is formed, and charging the portion corresponding to the next image signal by the next image exposure means on the recharged latent image forming medium Forming a multicolor image on the latent image forming medium by repeating at least once a process of forming an electrostatic latent image by erasing the image and a process of forming the electrostatic latent image into a toner image by the next developing means. In an image forming apparatus that performs a batch transfer onto a transfer material, the developing unit using a chromatic toner among the plurality of developing units includes a developer carrying member carrying a one-component developer including only a non-magnetic chromatic toner. Disposed with a gap with the latent image forming medium, and A latent image is formed into a toner image by forming an alternating electric field in the gap, and the final image exposing unit of the image exposing unit emits light having a pulse width corresponding to a multi-level digital image signal to the latent image. Image forming means for irradiating the latent image forming medium with light corresponding to a binary digital image signal when at least the image density to be output is low, apparatus. 2. The image forming apparatus according to claim 1, wherein a resolution of a final image exposing unit among the plurality of image exposing units is larger than resolutions of other image exposing units. 3. An exposure unit other than the final image exposure unit out of the plurality of image exposure units is a binary digital image signal obtained by binarizing a multivalued digital image signal. The image forming apparatus according to claim 1, wherein: 4. The latent image forming apparatus according to claim 1, wherein a final developing unit of said plurality of developing units carries a one-component developer comprising only a black toner having magnetism, and a developer carrier having a magnetic field generating unit therein. 2. The image forming apparatus according to claim 1, wherein the image forming apparatus is provided with a gap with a forming medium, and forms an alternating electric field in the gap to form a toner image. 5. An alternating electric field formed between a developer carrier of a developing unit other than a final one of the plurality of developing units and the latent image forming medium moves the developer toward the latent image forming medium. 2. The image forming apparatus according to claim 1, wherein the peak value on the side where the developer is moved is larger than the peak value returning the developer from the latent image forming medium to the developer carrier.