JP3170901B2 - Color image forming equipment - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a color image forming apparatus for forming a color image by forming a plurality of toner images on an electrostatic latent image carrier by repeating an electrostatic latent image forming step and a developing step a plurality of times. The present invention relates to an image forming apparatus.

[0002]

2. Description of the Related Art Heretofore, as a typical color image forming apparatus, an apparatus using an electrophotographic system is exemplified. In a color image forming apparatus using such an electrophotographic method, a method of repeating each of charging, exposure, development, and transfer steps a plurality of times is common. That is, the method of charging, exposing, developing, and transferring is repeated four times in total for the number of colors of yellow, magenta, cyan, and black toners. However, in such a method, a toner image is transferred onto a transfer material every time development of each color is completed. Therefore, a means for transporting the transfer material is required, and there is a problem that the apparatus itself becomes large.
For this reason, a method has been proposed in which a plurality of toner images are superimposed and developed on the same electrostatic latent image carrier and developed so that the transfer process can be performed only once to reduce the size of the apparatus.

In such a system, development is repeatedly performed from the toner image formed on the electrostatic latent image carrier, and therefore, the toner image already formed on the latent image carrier is placed at a subsequent stage. The toner image is disturbed by mechanical contact of the developer (image disorder), or a part of the toner image formed on the latent image carrier is scraped and mixed into a subsequent developing device. (Color mixing).

In order to avoid such a problem, there are a method in which the electrostatic latent image carrier and the developer are brought into a non-contact state and development is performed by applying an AC electric field, and a method in which development is performed by applying only a DC electric field. Are known. However, in view of the image quality of the line image, particularly, the reproducibility of the fine line, the former method of performing the development by applying the AC electric field is considered to be advantageous. Further, as a developer used in such a method, there are a two-component developer composed of a toner and a carrier, and a one-component developer composed of only a toner.
It is more advantageous to use a two-component developer in consideration of the ease of toner conveyance and the like.

According to this method, the problem of image disturbance and color mixing caused by the mechanical contact of the subsequent developer with the toner image formed on the latent image carrier can be avoided. The occurrence of image disturbance and color mixing due to the electric field of the AC component is a new problem. That is, in the case of the non-contact developing method, although the subsequent developer does not mechanically come into contact with the toner image already formed on the latent image carrier, the toner is not transferred onto the latent image carrier by the AC electric field. When flying back and forth, the toner image already formed on the latent image carrier is slightly disturbed compared to mechanical contact, or the toner image formed on the latent image carrier is There is a problem in that the developer is scraped off little by little and mixed into the subsequent developing device, causing color mixing of the developer.

[0006] To deal with such a problem, for example,
An image forming method disclosed in JP-A-2304 (Japanese Patent Application Laid-Open No. 60-129765) has already been proposed. The image forming method disclosed in the present application is configured such that a step of forming a latent image on an image carrier and a step of developing the latent image using a developer composed of a plurality of components are repeated a plurality of times. an image forming method for forming an image on, at each development step, the amplitude of the AC component of the developing bias V _AC (V), the frequency f (H
z), where d (mm) is the gap between the image carrier and the developer carrying member for carrying the developer, 0.2 ≦ V _AC / (df ·) ・ (V _AC / d) -1500 ｝ /F≦1.0.

In the image forming method according to this proposal, a method of forming an image by performing development by superimposing an AC component on a developing bias has been studied. It has been found that there is an area where a high-quality image can be obtained without causing disturbance or color mixture.

[0008]

However, the above-mentioned prior art has the following problems. That is, in the case of the image forming method according to the above-described proposal, the image bias and the color mixture are dealt with by the developing bias condition.
Actually, even if it can be an initial improvement measure, if the image is continuously formed for a long period of time, the toner is inevitably affected by the AC component of the developing bias, and there is a problem that image disturbance and color mixing occur. ing. This point is based on the above-mentioned
No. 04 (Japanese Patent Application Laid-Open No. 60-129765) states, "In order to develop a subsequent toner image at a constant density on a photoconductive drum sequentially without destroying the toner image formed on the photoconductive drum, As the development is repeated, a toner having a larger charge amount is sequentially used, the amplitude of the electric field intensity of the AC component of the developing bias is sequentially reduced, and the frequency of the AC component of the developing bias is sequentially increased. It is more preferable to arbitrarily combine them. "It is not possible to effectively prevent image disturbance and color mixing simply by setting the developing bias conditions to predetermined ones.

[0009]

SUMMARY OF THE INVENTION Therefore, the present invention
It was made to solve the above-mentioned problems of the prior art,
The purpose is to provide a highly reliable color image forming apparatus that does not cause image disturbance or color mixing for a long time, while being a small apparatus that forms a plurality of toner images on an electrostatic latent image carrier. Is to do.

[0010]

In order to solve the above-mentioned problems, the present invention provides a method for forming an electrostatic latent image on an electrostatic latent image carrier by repeating the forming and developing steps a plurality of times. In a color image forming apparatus for forming a plurality of toner images on an image carrier, a two-component developer including a toner and a carrier is brought into non-contact with the electrostatic latent image carrier by at least developing means of the second and subsequent stages. Using a developing method in which the toner is caused to fly by an AC electric field while holding, the charging property of the developer of the developing means is set to be lower as the latter stage, and the frequency of the AC component of the developing bias is later. It is configured to be set so as to become lower as the value becomes.

It is desirable that the particle size of the carrier used for the developer of the developing means is small. In non-contact development, it is necessary to reduce the distance between the developer carrier and the latent image carrier due to the need to increase the electric field strength of the latent image, and the distance is 0.2 mm to 0.8 mm. General. Therefore, the developer layer formed on the developer carrying member needs to be formed as a thin layer having a thickness of 0.1 mm to 0.6 mm. When a thin developer layer is formed, the developer layer becomes coarse and full of gaps, and image quality unevenness in printing occurs. On the other hand, if the carrier is too small, the magnetic binding force acting on the carrier is weakened, and the carrier is developed. Therefore, the average particle size of the carrier is 20 to 70 μm, preferably 30 to 70 μm.
50 μm is desirable. As such a carrier, iron powder or metal powder such as ferrite powder is used, but if the residual magnetization of the carrier is large, the developer remains in a chain even when the developer is separated from the developer carrier, Ferrite powder which can control the residual magnetization to be small is more advantageous because the mixing property in the developing device is reduced. In addition, there is a polymer carrier obtained by dispersing magnetic powder in a polymer. This is a carrier suitable for the present invention because there is an advantage that the magnetic properties of the carrier can be easily controlled by the content of the magnetic powder. It can be said that.

At this time, the charge amount of the toner is too high. If the charge amount is too high, the adhesion of the toner to the carrier becomes too high, and the phenomenon that the toner is not developed this time occurs. On the other hand, if the charge amount is too low, the adhesion of the toner to the carrier becomes too weak, and a toner cloud due to free toner is generated, which causes a problem of fogging in printing. Therefore, if the charge amount of the toner in the developer is specified from the viewpoint of developing the toner, the absolute value is 5 to 50 μC / g, preferably 10 to 4 μC / g.
In the color image forming apparatus according to the present invention, the order of use of the developer to be used should be such that the charge amount of the toner falls within this range, in ascending order. Good.

[0013]

In the color image forming apparatus according to the present invention, the electrostatic latent image forming step and the developing step are repeated a plurality of times to form a plurality of toner images on the electrostatic latent image carrier. In the developing process, a two-component developer composed of a toner and a carrier is held in a non-contact state with respect to the electrostatic latent image carrier, and the toner is caused to fly by an AC electric field. The toner image formed on the body is basically prevented from being mechanically disturbed in a subsequent development process. Further, by setting the chargeability of the developer after at least the second time to be lower in the later stage and by setting the frequency of the AC component of the developing bias to be lower in the later stage, the AC of the developing bias can be reduced. The occurrence of image disturbance and color mixing due to the electric field of the component can be suppressed.

The function of the charging property of the developer and the frequency of the AC component of the developing bias will be described below.

In a developing method in which the developer is held in a non-contact state with respect to the electrostatic latent image carrier and the toner in the developer is caused to fly by an AC electric field, the developer is already developed on the electrostatic latent image carrier. A return electric field acts on the toner in the direction of returning the toner to the developing sleeve of the developing device due to the AC component of the developing bias. At this time, it is generally considered that the larger the charge amount of the toner already developed on the electrostatic latent image carrier, the greater the Coulomb force due to the return electric field, and the more easily the toner is scraped off from the electrostatic latent image carrier. However, in actuality, as the charge amount of the toner already developed on the electrostatic latent image carrier increases, the adhesion of the toner to the electrostatic latent image carrier increases due to the action of the mirror image electric field. Conversely, it becomes difficult to scrape off the electrostatic latent image carrier.

As for the frequency of the AC component of the developing bias, the higher the frequency is, the more the toner having a large charge amount is selectively developed. This is due to the moving speed of the toner to be developed. In other words, the toner having a small charge amount has a small Coulomb force acting on the toner due to the AC component of the developing bias due to the small charge amount, so that the toner moving speed is reduced. Therefore, the return electric field of the AC component acts until the toner reaches the latent image carrier, and the toner is hardly developed.
On the other hand, a toner having a large charge amount has a large Coulomb force acting on the toner by an amount corresponding to the large charge amount, and has a high toner moving speed. For this reason, the toner reaches the latent image carrier before the return electric field of the AC component acts, and the toner is easily developed. Therefore, even if the same developer is used, as the frequency increases, the toner having a large charge amount is selectively developed on the latent image carrier as shown in FIG.

Furthermore, the higher the charge amount of the highly charged developer, that is, the amount of toner contained in the developer, the more the development can be performed up to a high frequency region. Conversely, when the charge amount of the low-charged developer, that is, the toner contained in the developer is low, the toner cannot follow the development in the high frequency region, and the toner development amount may be insufficient ( (See FIG. 4).

Therefore, to summarize the effects of the charging property of the developer and the frequency of the AC component of the developing bias, a toner having a high charge amount is used in the former developing unit, and the frequency of the AC component of the developing bias is set higher. Thus, the toner having a large charge amount can be made to adhere to the latent image carrier in the preceding developing unit. As a result, the adhesion of the toner to the latent image carrier is increased, and the occurrence of image disturbance and color mixing due to the electric field of the AC component of the developing bias in the subsequent developing step can be effectively suppressed.

[0019]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below based on the illustrated embodiment.

FIG. 2 shows an embodiment of the color image forming apparatus according to the present invention.

In the color image forming apparatus shown in FIG.
Reference numeral 2 denotes a photosensitive drum. As the photosensitive drum 2, for example, an organic photosensitive member (O
PC). The photosensitive drum 2 is driven to rotate in a direction indicated by an arrow at a predetermined rotational speed (for example, a peripheral speed of 160 mm / s) by a driving unit (not shown). The photosensitive drum 2
Is uniformly charged to a potential of, for example, -700 V by the scorotron 1 and then, for example, an LED of 400 dpi
The first image is exposed by the first exposure means 3 composed of the first color image forming apparatus, and a first-color electrostatic latent image is formed. At this time, the first image exposure is performed on the image portion, and the charge on the surface of the photosensitive drum 2 corresponding to the image portion is removed. This first electrostatic latent image is subjected to so-called reversal development by the first color black developing device 4, and a first color black toner image is formed on the photosensitive drum 2. Subsequently, the surface of the photosensitive drum 2
Second exposure means 5 composed of, for example, 400 dpi LED
Exposes the second image to form a second color electrostatic latent image. The second electrostatic latent image is developed by a developing device selected from a plurality of color developing devices 6, 7, and 8.
At this time, the second image exposure is performed on the background portion of the image, and so-called regular development is performed on the image portion where the charge remains.

The color developer 6 uses a yellow developer, the color developer 7 uses a magenta developer, and the color developer 8 uses a cyan developer. At this time, since black is subjected to reverse development and color is subjected to regular development, the charging polarity of the toner is black, while black has a negative polarity,
The colors are positive and different. Therefore, the two-color toner image of black and color formed on the photosensitive drum 2 is
After the polarities are adjusted by the pre-transfer corotron 9, the toner is transferred onto the transfer material 12 by charging the transfer corotron 10. Then, the two color toner images of black and color transferred onto the transfer material 12 are fixed on the transfer material 12 by a fixing device (not shown), and a color image is formed. After the transfer step is completed, the toner remaining on the photosensitive drum 2 is removed by the cleaner 11 to prepare for the next image forming step.

As shown in FIG. 1, the black developing unit 4 and the color developing units 6, 7, and 8 have a surface inside a housing 14 for accommodating a two-component developer 13 composed of a toner and a carrier. A cylindrical development carrier 15 that transports the developer 13 by adsorbing and rotating the developer 13
A developer regulating member 16 that regulates the amount of the developer 13 attached to the surface of the development carrier 15, and agitating and transporting the developer 13 by rotating the developer carrier 15,
And a screw auger 17 for supplying the pressure.

The developer carrier 15 has a built-in magnet roll 18 which is fixed so as not to rotate, and has a developing sleeve 19 which is rotatably instructed around its periphery. The developing sleeve 19 has, for example, a diameter of 20 mm.
mm, the distance between the developing sleeve 19 and the photosensitive drum 2 is set to 0.6 mm, the thickness of the developer 13 layer on the developing sleeve 19 is set to 0.4 mm, and the peripheral speed is set to 400 mm / s. One that performs development without contacting the photosensitive drum 2 is used. This is a configuration that is a prerequisite for preventing the toner image that has been developed on the photosensitive drum 2 first during the overlap development from being disturbed in the subsequent development process.

Hereinafter, the developing conditions will be described in detail.

In this embodiment, in order to form 13 thin layers of the developer having a thickness of 0.4 mm on the developing sleeve 19,
A magnetic powder-dispersed polymer carrier having an average particle size of 40 μm and a saturation magnetization of 50 emu / g is used.

The black developer carrier is composed of 30% by weight of a styrene-acrylic copolymer, 68% by weight of magnetite, and 2% of nigrosine 2 in order to charge the black toner to a negative polarity.
wt% is kneaded and pulverized to form a positively chargeable carrier.
The carrier of the color developer is a styrene-acryl copolymer 3 for charging the color toner to a positive polarity.
0wt%, magnetite 68wt%, hydrophobic silica 2w
t% is kneaded and pulverized to provide a negatively charged carrier.

On the other hand, the average particle diameter of each toner was 10 μm. The black toner was composed of 80% by weight of a styrene-acryl copolymer and 10 parts of a styrene-butadiene rubber.
After mixing and pulverizing 10 wt% of carbon black and 10 wt% of carbon black to obtain colored particles having an average particle diameter of 10 μm, hydrophobic silica was externally added to the colored particles at 5 wt% to obtain a negatively chargeable toner. The yellow toner is obtained by kneading and pulverizing 90% by weight of a polyester and 10% by weight of a benzine derivative, and has an average particle diameter of 10 μm.
Colored particles. Magenta toner is polyester 9
0 wt% and 10 wt% of polytungstophosphoric acid were kneaded and pulverized to obtain colored particles having an average particle size of 10 μm. Sian Toner is polyester 90% by weight, copper phthalocyanine 10
wt% was kneaded and pulverized to obtain colored particles having an average particle size of 10 μm.

For these color toners, titanium oxide is added to the colored particles, and for yellow, 5 wt.
%, Magenta for 3 wt% and Sian for 1 wt% to obtain positively chargeable toners having different chargeability. When the charge amount Q of the toner was measured for these three types of color developers, Q ₂ = 20 μC / g for the yellow toner and Q ₃ = 15 μC for the magenta toner.
/ G, Sian Toner was Q ₄ = 10 μC / g.
The charge amount of the toner of the black developer is Q ₁ = −25 μC /
g.

With the above arrangement, the color image forming apparatus according to this embodiment forms a color image as follows.

That is, the conditions of the color image forming process will be described with reference to FIG.

First, as shown in FIG. 3A, the surface of the photosensitive drum 2 is V ₀ = −700 by the scorotron 1.
It is uniformly charged to a potential of V. Next, as shown in FIG. 3B, the potential of the image portion is reduced by -6 by the first image exposure means 3.
The first image exposure is performed so as to reduce the voltage to 0 V, an electrostatic latent image of the first color is formed on the surface of the photosensitive drum 2, and the electrostatic latent image is developed by the black developing device 4 using black toner. . At this time, the developing sleeve 19 of the black developing device 4, -600 V as a DC component, frequency F ₁ 3.5kH as an AC component
A developing bias having a DC component and an AC component of Z, V _PP = 1.5 kV is applied.

Thereafter, as shown in FIGS. 3C to 3E, a second image exposure is performed by the second image exposure means 5, and a second color electrostatic latent image is formed on the surface of the photosensitive drum 2. Form. At this time, the latent image potential of the second color is determined depending on how many times the second-color electrostatic latent image is subjected to the overlap development.

That is, in this embodiment, in addition to the step of performing the development once for the second electrostatic latent image, the second electrostatic latent image is continuously developed a plurality of times. It is configured. When the second electrostatic latent image is developed only once, the second electrostatic latent image is subjected to a plurality of color developing units 6, 7,.
8, and in the case of two-time development, the second electrostatic latent image is developed by any one of the plurality of color developing devices 6, 7, and 8. In the case of three developments, the second electrostatic latent image is developed by operating all of the plurality of color developing units 6, 7, and 8 to perform development. It has become.

Here, as shown in FIGS. 3 (c) to 3 (e), the value of the latent image potential of the second color is V ₁ when the image is developed once,
V ₂ when developed twice and V ₃ when developed three times.

[0036] When the one-time development, as shown in FIG. 3 (c), the second image exposure by exposing only the background portion of the second color latent image, the potential V ₁ of the image portion only dark decay fraction V _It becomes slightly lower than ₀ , and V ₁ = −680 V. However, in the case of development twice, as shown in FIG.
Exposure is performed not only on the background portion of the color latent image but also on the image portion itself of the second color latent image, and the potential V _{2 of the} image portion is set to V lower than V _1.
2 = −580V. When developing three times,
As shown in FIG. 3E, by further increasing the exposure intensity of the second-color latent image, the potential V ₃ of the image portion is set to V V lower than V _2.
3 = −480 V

The reason why the image portion potential in the second image exposure is controlled in accordance with the number of times of superposition development is to keep the amount of developed toner constant regardless of the number of times of superposition development. That is, three times when the image portion potential V ₃ of the development equal to the image portion potential V ₁ of the once developed, the resulting yellow,
The image in which the three colors of magenta and Sian are mixed has a thin line slightly thicker than the black single-color image of the first color. This is for the following reason. Each of the color developing units 6, 7, and 8 is set so that a toner image having a predetermined density can be obtained by independent development. Therefore, when developing the same electrostatic latent image in a superimposed manner, if the potential difference between the electrostatic latent image and the developing sleeve is the same for each of the developing devices 6, 7, and 8, each developing device 6,
A sufficient amount of toner to obtain a predetermined density by one development of Nos. 7 and 8 adheres to the photosensitive drum 2. If development is performed by another developing device from above, more toner than necessary will adhere to the photosensitive drum 2. In the case of performing the superposition development three times, the amount of adhered toner is further increased, and the tendency of the thin line to become thicker becomes more remarkable. From such a point, by controlling the potential of the latent image in accordance with the number of times of the superposition development, the amount of toner of each developing device adhered to the same electrostatic latent image can be reduced, and the thickening of the thin line can be prevented. I can do it.

At the time of exposing the second color image, FIG.
As shown in (c) ~ (e), as the first color background portion potential V _B of the second color latent image than the potential of the black toner image is both high level, the so-called well-type with respect to the black toner The binding force caused by the electric field of the potential is applied to more effectively prevent the black toner image from being deteriorated in the color developing process. At this time, the potential of the black toner image of the first color is-
100 V, 2 color latent image background portion potential V _B of a -150 V.

Further, each of the color developing units 6, 7, 8 has
-250V in common in the developing device as the DC component, as the AC component, the frequency F ₂ 3.0kH the yellow developing device 6
Z, V _PP = 1.5 kV, magenta developing device 7 has frequency F ₃ 2.0 kHz, V _PP = 1.5 kV, and Sian developing device 8 has frequency F ₄ 1.0 kHz, V _PP = 1.5 kV.
Is applied.

Further, when the charge amount of the toner image developed on the photosensitive drum 2 under such conditions was measured,
Yellow was 16 μC / g, magenta was 10 μC / g, and Sian was 4 μC / g. The charge amount of the toner of the black developer was −18 μC / g.

In the above embodiment, two colors of black and a desired color were actually printed. The print modes are "black + red (yellow + magenta)", "black + blue (magenta + sian)", "black + green (yellow + sian)", "black + black (yellow + magenta + sian)".
The repetition of the printing in this order was defined as one cycle, and this cycle was performed for 100,000 cycles. When the change in image quality was observed, it was confirmed that a stable image was always obtained. By the way, "black + black (yellow + magenta + Sian)"
The mode is aimed at obtaining a black image having a different color tone due to a difference in the color mixture ratio of the three color toners of (yellow + magenta + syan).

As described above, in the color image forming apparatus according to the first embodiment, a plurality of toner images are formed on the photosensitive drum 2 by repeating the electrostatic latent image forming step and the developing step a plurality of times. , At least the second and subsequent developing units 6
By using a developing method in which a two-component developer composed of a toner and a carrier is held in a non-contact state with respect to the electrostatic latent image carrier and the toner is caused to fly by an AC electric field, The toner image formed on the photosensitive drum 2 is basically prevented from being mechanically disturbed in a subsequent development process. Further, by setting the charging property of the developer 13 to be lower as the latter stage and setting the frequency of the AC component of the developing bias to be lower as the latter stage, the electric field of the AC component of the developing bias is reduced. The occurrence of image disturbance and color mixing is suppressed.

That is, the charge amount of the toner already developed on the photosensitive drum 2 is increased by setting the chargeability of the developer 13 to be lower as the latter stage. Therefore, the toner that has been developed earlier by the action of the mirror image electric field has a higher adhesive force of the toner on the photosensitive drum 2, so that the toner is less likely to be scraped off from the photosensitive drum 2.

Further, by setting the frequency of the AC component of the developing bias to be lower as it goes to the later stage, it becomes possible to selectively develop the toner having a larger charge amount in the earlier stage of the developing device. Therefore, the adhesion of the toner to the photoreceptor drum 2 is higher for the toner developed first by the action of the mirror image electric field.
By the synergistic action with the control of the charge amount, it becomes more difficult to be scraped off from the photosensitive drum 2. As a result, the adhesive force of the toner on the photosensitive drum 2 is increased, so that the occurrence of image disturbance and color mixing due to the electric field of the AC component of the developing bias in the subsequent developing step can be effectively suppressed, and the image can be formed for a long time. It is possible to provide a highly reliable color image forming apparatus that does not cause disturbance or color mixing.

Comparative Example 1 In the first embodiment, only the AC component of the developing bias of the color developing device was changed, and yellow had a frequency of 1.0 kHz, V _PP = 1.5 kV, and magenta had a frequency of 2.0 kHz. , V _PP = 1.5 kV, and Sian applied a developing bias of a frequency of 3.0 kHz and V _PP = 1.5 kV to actually form a color image. As a result of printing, the magenta and Sian developers were used. Color mixing has occurred. Observation of these developers with an optical microscope confirmed that yellow toner was present in the magenta and cyan developers. Therefore, it is clear that the color mixture of these developers occurs when the previously developed toner is scraped off in the next development step. When the charge amount of the toner image developed on the photosensitive drum 2 at this time was measured, yellow was 7 μC / g, magenta was 10 μC / g,
Sian was 7 μC / g. Therefore, the reason for the occurrence of color mixing is that the amount of charge of the yellow toner previously developed on the photosensitive drum 2 is lower than that of the first embodiment, and the adhesion of the yellow toner to the photosensitive drum 2 is low. It is considered that the reason for this was that the toner was easily scraped off during the superposition development because of the small size. FIG. 4 shows how the chargeability of the developer and the charge amount of the developing toner on the photosensitive drum 2 change depending on the frequency of the AC bias at this time.

Second Embodiment FIGS. 5 and 6 show a second embodiment of the present invention. FIG. 5 is a schematic view of a color image forming apparatus used in the second embodiment, and FIG. It is a process figure of a 2nd example.

In the color image forming apparatus shown in FIG.
The photoreceptor belt 21 uniformly charged by the first scorotron 20 is exposed by the first
An electrostatic latent image of a color is formed. The exposure at this time is performed on the image portion to remove the electric charge of the image portion. This electrostatic latent image is subjected to so-called reversal development by the first color black developing device 23, and a first color black toner image is formed on the photoreceptor belt 21. Subsequently, the photoreceptor belt 21 is exposed by the second exposure unit 24 to form a second-color electrostatic latent image, which is developed by the blue developing unit 25. The exposure at this time is performed on the image portion and the background portion. The exposure is performed so that the image portion potential is lower than the background portion potential, and reversal development is performed on the image portion. Further, the photoreceptor belt 21 is
The image portion and the background portion are exposed by the exposure means 26 in the same manner as the second exposure to form a third color latent image, and the third green developing device 27 performs reversal development. Then, the fourth exposure unit 2
At 8, the image portion and the background portion are similarly exposed to form a latent image of the fourth color, and the fourth red developing device 29 performs reversal development.

In this manner, black, blue,
The four color toner images of green and red are formed, and the four color toner images of black, blue, green and red are transferred onto the transfer material 31 by the transfer corotron 30. After the transfer, the toner remaining on the photosensitive belt 21 is removed by the cleaner brush 32.
In the figure, 33, 34, and 35 are driving rolls for driving the photoreceptor belt 21.

In this embodiment, the photosensitive belt 21
Uses an OPC with a total length of 900 mm and a peripheral speed of 200 mm
/ S, and the charging potential was -900V. Exposure means 22, 2
As 4, 26 and 28, 300 dpi LEDs were used. The diameter of the developing sleeve of each of the developing units 23, 25, 27, and 29 used is 30 mm, the interval between the developing sleeve and the photosensitive belt 21 is 0.5 mm, and the thickness of the developer layer on the sleeve is 0.35 mm. , The peripheral speed of the developing sleeve is 600
mm / s, and development is performed in a state where the developer does not contact the photoreceptor belt 21. This is to prevent the toner image previously developed on the photoreceptor belt 21 from being disturbed in the subsequent development process during the overlap development.

Hereinafter, the developing conditions will be described in detail.

Here, as in the case of the first embodiment, a magnetic powder-dispersed polymer carrier having an average particle size of 40 μm and a saturation magnetization of 50 emu / g was used.

The carrier used for each developer is common, and 30% by weight of a styrene-acrylic copolymer, 68% by weight of magnetite, and 2% by weight of nigrosine are kneaded and pulverized to form a positively charged carrier. .

The average particle diameter of each toner was 10 μm. The black toner was composed of 80% by weight of a styrene-acryl copolymer, 10% by weight of a styrene-butadiene rubber,
10 wt% of carbon black is kneaded and pulverized, and the average particle size is 1
After forming colored particles of 0 μm, 8 wt% of hydrophobic silica was externally added to the colored particles to obtain a negatively chargeable toner. Blue toner 90% polyester, 7% copper phthalocyanine
%, And 3 wt% of polytungstophosphoric acid were kneaded and pulverized to obtain colored particles having an average particle diameter of 10 μm. Hydrophobic silica was externally added to the colored particles at 7 wt% to obtain a negatively chargeable toner. Green toner 90% polyester, 6w benzine derivative
After mixing and pulverizing 4% by weight of t% and copper phthalocyanine to obtain colored particles having an average particle diameter of 10 μm, 6% by weight of hydrophobic silica was externally added to the colored particles to obtain a negatively chargeable toner. Red toner is polyester 90wt%, benzine derivative 4wt
%, And 6 wt% of polytungstophosphoric acid were kneaded and pulverized to obtain colored particles having an average particle diameter of 10 μm. Hydrophobic silica was externally added to the colored particles by 5 wt% to obtain a negatively chargeable toner. When the charge amount of the toner was measured for these four types of developers, black was -25 μC / g, blue was -20 μC / g, green was -15 μC / g, and red was -10 μC / g.

Next, the process conditions will be described with reference to FIG. First, as shown in FIG. 6A, the photosensitive belt 21 is charged to V ₀ = −900V. Next, FIG.
As shown in FIG. 8, the latent image potential V _I1 of the first color is reduced by −7 by exposure.
The voltage is lowered to 00V, and the black toner is developed by a black developing device.
At this time, the DC component is -850 V as a DC component and the frequency is 3.5 kHz and V _PP = 1.5 k as an AC component.
A developing bias having a DC component and an AC component of V is applied. After that, as shown in FIG. 6C, the second color exposure reduces the image potential V _I2 of the second color to −500 V and the background potential V ₂ to −700 V, and develops the blue toner by the blue developing device. At this time, the blue developing device
650 V, AC component, frequency 3.0 kHz, V
A developing bias having a DC component and an AC component of _PP = 1.5 kV is applied. Then, after that, FIG.
As shown in the figure, the image portion potential V _I3 of the third color is obtained by the exposure of the third color.
The -300 V, performs development of green toner by green developing device off the background portion potential V ₂ to -500 V. At this time, a developing bias having a DC component of -450 V as a DC component and a frequency component of 2.0 kHz and V _PP = 1.5 kV as an AC component is applied to the green developing device. Thereafter, as shown in FIG. 6E, the fourth color image portion potential V _{I4 is set} to −100 V and the background portion potential V _{2 is set} to −30 by the fourth color exposure.
The voltage is lowered to 0 V, and red toner is developed by a red developing device.

At this time, a red component is supplied to the red developing device as a DC component.
250V, AC component, frequency 1.0kHz, V
A developing bias having a DC component and an AC component of _PP = 1.5 kV is applied. Under these conditions, when the charge amount of the toner image developed on the photoreceptor belt 21 was measured, black was −18 μC / g, blue was −16 μC / g, green was −10 μC / g, and red was −4 μC. / G.

Thus, when printing in four colors of black, blue, green, and red was performed, a stable image was obtained even after 100,000 sheets were printed, and no occurrence of color mixing was observed.

COMPARATIVE EXAMPLE 2 In the second embodiment, only the AC component of the developing bias of the color developing device was changed, and blue had a frequency of 1.0 kHz.
Z, V _PP = 1.5 kV, green is frequency 2.0 kHz, V
_PP = 1.5 kV, red is frequency 3.0 kHz, V _PP =
When a developing bias of 1.5 kV was applied, color mixing occurred in the green and red developers as printing was performed. When these developers were observed with an optical microscope, it was confirmed that blue toner was present in the green and red developers. Therefore, it is clear that the color mixture of these developers occurs when the previously developed toner is scraped off in the next development step.
When the charge amount of the toner image developed on the photoreceptor belt at this time was measured, blue was −7 μC / g, and green was −10 μC.
C / g and red were −7 μC / g. Therefore, the reason for the occurrence of color mixing is that the amount of charge of the blue toner developed on the photoconductor belt is lower than that of the second embodiment, as in Comparative Example 1, and the blue toner is applied to the photoconductor belt. It is considered that the reason for this is that the toner has been easily scraped off during the overlap development due to the small adhesive force.

In the second embodiment, the case where the image exposure step and the developing step are repeated without recharging the photoreceptor belt has been described. However, the same applies to the case where the photoreceptor is recharged. can do. That is, even when recharging is performed, the higher the charge amount of the original toner, the higher the charge amount after recharging. Therefore, the present invention is effective even in the case of recharging.

[0059]

The present invention has the above-described structure and operation. By increasing the amount of charge of the developing toner to increase the adhesion of the developing toner to the latent image carrier, color mixing of the toner and image deterioration can be prevented. Occurrence can be suppressed, and a print of stable image quality can always be obtained.

[Brief description of the drawings]

FIG. 1 is a configuration diagram illustrating a developing device in an embodiment of an image forming apparatus according to the present invention.

FIG. 2 is a configuration diagram showing a first embodiment of the image forming apparatus according to the present invention.

FIGS. 3A to 3E are graphs showing potentials of an image forming process in the embodiment, respectively.

FIG. 4 is a graph showing the relationship between the frequency of a developing bias and the amount of toner charge.

FIG. 5 is a configuration diagram showing a second embodiment of the image forming apparatus according to the present invention.

FIGS. 6A to 6E are graphs respectively showing potentials of an image forming process in the embodiment.

FIG. 7 is a graph showing the relationship between the frequency of the developing bias and the amount of toner charge.

[Description of Signs] 4 Black developing device, 6, 7, 8 color developing device, 13 developer, 15 developing carrier, 19 developing sleeve, Q ₁ , Q
₂ , Q ₃ , Q ₄ Toner charge, F ₁ , F ₂ , F ₃ ,
F ₄ frequency of the AC component of the developing bias.

──────────────────────────────────────────────────続 き Continuation of the front page (72) Inventor Taku Aoshima 2274 Hongo, Ebina-shi, Kanagawa Fuji Xerox Co., Ltd. (56) References JP-A-60-159766 (JP, A) JP-A-1-222275 ( JP, A) JP-A-2-167577 (JP, A) JP-A-61-209466 (JP, A) JP-A-60-154271 (JP, A) JP-A-61-174556 (JP, A) Hei 4-86750 (JP, A) (58) Field surveyed (Int. Cl. ⁷ , DB name) G03G 15/01-15/01 117 G03G 15/06-15/06 102 G03G 15/09-15 / 09 101 G03G 15/08-15/08 115

Claims (1)

(57) [Claims] 1. A color image forming apparatus for forming a plurality of toner images on an electrostatic latent image carrier by repeating a process of forming an electrostatic latent image on an electrostatic latent image carrier and a developing process a plurality of times. Wherein a developing system in which a two-component developer composed of a toner and a carrier is held in a non-contact state with respect to the electrostatic latent image carrier in at least a second-stage developing means and the toner flies by an AC electric field is adopted. Wherein the chargeability of the developer of the developing means is set to be lower as the latter stage, and the frequency of the AC component of the developing bias is set to be lower as the latter stage. Forming equipment.