JPH06118764A - Color image forming device - Google Patents

Abstract

PURPOSE:To provide a highly reliable color image forming device which is free of the disturbance of an image and color mixture over a long period of time, although the device is a small one which forms a plurality of toner images on an electrostatic latent image carrier. CONSTITUTION:As a development means for at least second or later stage, the color image forming device uses the development means which employs a development system in which two-component developer 13 composed of toner and carrier is held without being in contact with the electrostatic latent image carrier 15 and the toner is caused to fly by means of an AC electric field. The electrification of the developer of the development means is set so as to decrease as the stage develops and the frequency of the development bias AC element is also set so as to decrease as the stage develops.

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 method for forming a color image by repeating a plurality of electrostatic latent image forming steps and a developing step a plurality of times to form a plurality of toner images on an electrostatic latent image carrier. The present invention relates to an image forming apparatus.

[0002]

2. Description of the Related Art Conventionally, as a typical color image forming apparatus, there is one using an electrophotographic system. In a color image forming apparatus using such an electrophotographic system, a system in which each process of charging, exposing, developing and transferring is repeated a plurality of times is general. That is, it is a system in which the steps of charging, exposing, developing, and transferring are repeated four times in total for the number of colors of yellow, magenta, cyan, and black toners. However, in such a method, since the toner image is transferred onto the transfer material each time the development of each color is completed, a means for transporting the transfer material is required and the apparatus itself becomes large.
In view of the above, a method has been proposed in which a plurality of toner images are superposed on the same electrostatic latent image carrier and developed, and the transfer process is completed in a single step to downsize the apparatus.

In such a system, since development is repeatedly performed on the toner image formed on the electrostatic latent image bearing member, the toner image already formed on the latent image bearing member is formed in the latter stage. The toner image is disturbed (mechanical disturbance) due to mechanical contact of the developer, or a part of the toner image formed on the latent image carrier is scraped and mixed into the developing device in the subsequent stage. However, there is a problem that the color of (color mixture) changes.

In order to avoid such a problem, there are a method in which the electrostatic latent image bearing member and the developer are not in contact with each other and a developing method is performed by applying an AC electric field, or a method in which only a DC electric field is applied. Are known. However, from the viewpoint of the image quality of a line image, particularly the reproducibility of a fine line, the former method of developing by applying an AC electric field is considered to be advantageous. Further, as a developer used in such a system, there are a two-component developer composed of a toner and a carrier, and a one-component developer consisting of only the toner.
It is more advantageous to use a two-component developer in consideration of ease of toner transport and the like.

According to such a system, the problem of image disturbance and color mixture caused by the mechanical contact of the developer on the latent image carrier with the developer in the subsequent stage can be avoided, but the developing bias Image distortion and color mixture due to the electric field of the AC component become new problems. That is, in the case of this non-contact developing method, the toner on the latent image bearing member is not mechanically contacted with the toner image already formed on the latent image bearing member by the AC electric field. When reciprocating, the toner image already formed on the latent image bearing member is disturbed slightly compared to mechanical contact, or the toner image formed on the latent image bearing member is disturbed. There is a problem that the developer is scraped off little by little and mixed into the developing device in the subsequent stage to cause color mixing of the developer.

To solve this problem, for example, Japanese Patent Publication No. 3
-304 (Japanese Patent Laid-Open No. 60-129764) has already been proposed. In the image forming method disclosed in this application, the step of forming a latent image on the image carrier and the step of developing the latent image using a developer composed of a plurality of components are repeated a plurality of times to obtain the image carrier. In the image forming method of forming an image on the upper surface, the amplitude of the AC component of the developing bias is V _AC (V) and the frequency is f (H
z), where d (mm) is the gap between the image carrier and the developer transporter that transports the developer, 0.2 ≦ V _AC / (d · f) {(V _AC / d) -1500 } /F≦1.0.

In the image forming method according to this proposal, as a result of research on a method of forming an image by superimposing an AC component on a developing bias and developing the image, an image is formed depending on how to select the developing conditions such as the AC bias and the frequency. It was found that there is a region where a high-quality image can be obtained without causing the disturbance and 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-mentioned proposal, although the image disturbance and the color mixture are dealt with by the developing bias condition,
Although it may actually be an initial improvement measure, if an image is continuously formed for a long period of time, the toner is inevitably affected by the AC component of the developing bias, which causes image disturbance and color mixture. ing. Regarding this point, the Japanese Patent Publication No. 3-23
No. 04 (JP-A-60-129764), "To develop subsequent toner images at a constant density on the photosensitive drum sequentially without destroying the toner image formed on the photosensitive drum, As the development is repeated, toner having a large charge amount is sequentially used. The amplitude of the electric field strength of the AC component of the developing bias is gradually decreased. The frequency of the AC component of the developing bias is gradually increased. It is more preferable to combine them arbitrarily. ”, And simply setting the developing bias condition to a predetermined value cannot effectively prevent image disturbance and color mixture.

[0009]

Therefore, the present invention is
It was made to solve the above-mentioned problems of the prior art,
The object of the present invention is to provide a highly reliable color image forming apparatus that does not cause image distortion or color mixture for a long period of time, although it is a small apparatus that forms a plurality of toner images on an electrostatic latent image carrier. To do.

[0010]

SUMMARY OF THE INVENTION In order to solve the above-mentioned problems, the present invention repeats the process of forming an electrostatic latent image on an electrostatic latent image carrier and the developing process a plurality of times to obtain the electrostatic latent image. In a color image forming apparatus for forming a plurality of toner images on an image carrier, a two-component developer composed of toner and carrier is brought into non-contact with the electrostatic latent image carrier in at least the second and subsequent developing means. A developing method that holds and flies the toner by an AC electric field is used, and the charging property of the developer of the developing means is set so as to become lower in the latter stage, and the frequency of the AC component of the developing bias is set in the latter stage. It is configured so that it becomes lower as becomes.

The particle size of the carrier used for the developer of the developing means is preferably small. In the non-contact development, since it is necessary to increase the electric field strength of the latent image, it is necessary to narrow the gap between the developer bearing member and the latent image bearing member, and the gap is 0.2 mm to 0.8 mm. It is common. Therefore, the developer layer formed on the developer carrying member needs to be a thin layer of 0.1 mm to 0.6 mm, and such a large carrier having a normal average particle diameter of 100 μm is used. When a thin developer layer is formed, it becomes a coarse developer layer with gaps, which causes uneven image quality in printing. If the carrier is too small, the magnetic restraining 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, an iron powder or a metal powder such as a ferrite powder is used, but if the residual magnetization of the carrier is large, the developer remains connected in a chain even when the developer is separated from the developer carrier, Since the mixing property in the developing device is lowered, the ferrite powder, which can control the residual magnetization to be small, is more advantageous. Further, there is a polymer carrier in which magnetic powder is dispersed in a polymer, but this is a carrier suitable for the present invention because it has an advantage that the magnetic characteristics of the carrier can be easily controlled by the content of the magnetic powder. Can be said.

Regarding the amount of electric charge of the toner at this time, if the amount of electric charge is too high, the adhesion of the toner to the carrier becomes too high and the toner is not developed this time. Further, if the charge amount is too low, the adhesive force of the toner to the carrier becomes too weak and a toner cloud is generated due to the free toner, causing fog 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
It is desirable to be in the range of 0 μC / g, and in the color image forming apparatus according to the present invention, regarding the order of use of the developers to be used, those in which the charge amount of the toner falls within this range should be used in descending 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, and at least the second and subsequent times are performed. In the developing process, a two-component developer consisting of toner and carrier is held in a non-contact state with the electrostatic latent image carrier, and a developing method in which the toner is ejected by an alternating electric field is used to carry the electrostatic latent image carrier first. Basically, the toner image formed on the body is mechanically prevented from being disturbed in the subsequent developing process. Further, by setting the charging property of the developer at least after the second time to be lower in the latter stage, and setting the frequency of the AC component of the developing bias to be lower in the latter stage, the AC of the developing bias is changed. It is possible to suppress image distortion and color mixture due to the electric field of the components.

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

In the developing system in which the developer is held in a non-contact state with the electrostatic latent image bearing member and the toner in the developer is ejected by an alternating electric field, it is already developed on the electrostatic latent image bearing member. The return electric field in the direction of returning the toner to the developing sleeve of the developing device acts on the toner by 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 is, the larger the Coulomb force due to the returning electric field is, and the toner is more likely to be scraped from the electrostatic latent image carrier. However, in reality, the larger the charge amount of the toner already developed on the electrostatic latent image carrier is, the stronger the adhesion of the toner on the electrostatic latent image carrier is due to the action of the mirror image electric field. Conversely, it becomes difficult for the latent electrostatic image bearing member to be scraped off.

Regarding the frequency of the AC component of the developing bias, the higher the frequency, the more selectively the toner having a large amount of electrified charge is developed. This is due to the moving speed of the toner to be developed. That is, 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 moving speed of the toner becomes slow. Therefore, the return electric field of the AC component acts before the toner reaches the latent image carrier, and the toner is hard to be developed.
On the other hand, a toner having a large amount of electric charge has a large Coulomb force acting on the toner and an increased moving speed of the toner due to the large amount of the electric charge. Therefore, 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 is increased, the toner having a large charge amount is selectively developed on the latent image carrier as shown in FIG.

Further, the higher the charge amount of the highly charged developer, that is, the toner contained in the developer, the more the development can be performed in the high frequency region. On the contrary, 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 development amount of the toner may be insufficient ( (See FIG. 4).

Therefore, the action of the charging property of the developer and the frequency of the AC component of the developing bias can be summarized as follows. Toner having a high charge amount is used in the developing device in the preceding stage, and the frequency of the AC component of the developing bias is set high. As a result, the toner having a large charge amount can be attached to the latent image carrier in the developing device in the preceding stage. As a result, the adhesive force of the toner on the latent image carrier is increased, and it is possible to effectively suppress the occurrence of image disturbance and color mixture due to the electric field of the AC component of the developing bias in the subsequent developing process.

[0019]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below with reference to illustrated embodiments.

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 photoconductor drum. As the photoconductor drum 2, for example, an organic photoconductor (O
PC) is used. The photoconductor drum 2 is driven to rotate in a direction of an arrow at a predetermined rotation speed (for example, a peripheral speed of 160 mm / s) by a driving unit (not shown). The photoconductor drum 2
The surface of the LED is uniformly charged by the scorotron 1 to a potential of −700 V, for example, and then the LED of 400 dpi, for example.
The first image is exposed by the first exposure unit 3 composed of to form the electrostatic latent image of the first color. At this time, the first image exposure is performed on the image portion, and the electric charge in the area corresponding to the image portion on the surface of the photosensitive drum 2 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. Then, the surface of the photosensitive drum 2 is
For example, the second exposure unit 5 including an LED of 400 dpi
Thus, the second image is exposed to form a second-color electrostatic latent image. This second electrostatic latent image is developed by the developing device selected from the plurality of color developing devices 6, 7, 8.
At this time, the second image exposure is performed on the background portion of the image, and so-called normal development is performed on the image portion where the electric charge remains.

As for the developing agent, a yellow developing agent is used in the color developing device 6, a magenta developing agent is used in the color developing device 7, and a cyan color developing agent is used in the color developing device 8. At this time, since black is subjected to reversal development and color is subjected to regular development, the charging polarity of the toner is negative while black is negative.
The colors are positive and different from each other. Therefore, the two color toner images of black and color formed on the photoconductor drum 2 are
After the polarities are aligned by the pre-transfer corotron 9, they are transferred onto the transfer material 12 by charging the transfer corotron 10. Then, the black and color toner images transferred onto the transfer material 12 are fixed on the transfer material 12 by a fixing device (not shown) to form a color image. 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.

By the way, as shown in FIG. 1, the black developing device 4 and the color developing devices 6, 7 and 8 are provided on the surface of a housing 14 which contains a two-component developer 13 composed of toner and carrier. A cylindrical development carrier 15 that adsorbs the developer 13 and conveys the developer 13 by rotating it.
And a developer regulating member 16 that regulates the amount of the developer 13 adhering to the surface of the developing carrier 15, and the developer 13 is rotated and agitates and conveys the developer 13 to the developer carrier 15.
And a screw auger 17 for supplying

The developer carrying member 15 contains a magnet roll 18 fixed so as not to rotate, and has a developing sleeve 19 which is rotatably instructed around the magnet roll 18. The developing sleeve 19 has, for example, a diameter of 20.
mm, the distance between the developing sleeve 19 and the photosensitive drum 2 is 0.6 mm, the thickness of the developer 13 layer on the developing sleeve 19 is 0.4 mm, and the peripheral speed is 400 mm / s. What develops without contacting the photosensitive drum 2 is used. This is a precondition for preventing the toner image previously developed on the photosensitive drum 2 during the overlap development from being disturbed in the subsequent development process.

The developing conditions will be described in detail below.

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

The carrier of the black developer is 30 wt% of styrene-acryl copolymer, 68 wt% of magnetite and 2 nigrosine in order to negatively charge the black toner.
Wt% is kneaded and pulverized to obtain a carrier having a positive charging property.
The carrier of the color developer is a styrene-acrylic copolymer 3 in order to charge the color toner to the positive polarity.
0wt%, magnetite 68wt%, hydrophobic silica 2w
t% is kneaded and pulverized to obtain a negatively charged carrier.

On the other hand, regarding the toner, the average particle diameter of each toner is 10 μm. Black toner is styrene-acrylic copolymer 80 wt%, styrene-butadiene rubber 10
After wt% and carbon black 10 wt% were kneaded and pulverized to give colored particles having an average particle size of 10 μm, 5 wt% of hydrophobic silica was externally added to the colored particles to obtain a negatively chargeable toner. For the yellow toner, 90 wt% polyester and 10 wt% benzine derivative are kneaded and pulverized to obtain an average particle diameter of 10 μm.
And colored particles of. Magenta toner is polyester 9
0 wt% and polytungstophosphoric acid 10 wt% were kneaded and pulverized to obtain colored particles having an average particle size of 10 μm. Sian Toner is 90 wt% polyester, 10 copper phthalocyanine
wt% was kneaded and pulverized to obtain colored particles having an average particle diameter of 10 μm.

For these color toners, titanium oxide is used for the colored particles, and 5 wt% for yellow.
%, 3 wt% for magenta, and 1 wt% for cyan, 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, the yellow toner had Q ₂ = 20 μC / g and the magenta toner had Q ₃ = 15 μC.
/ G, and Cyan toner had Q ₄ = 10 μC / g.
The charge amount of the black developer toner is Q ₁ = −25 μC /
It was g.

With the above structure, 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 V potential. Next, as shown in FIG. 3B, the potential of the image portion is set to −6 by the first image exposure means 3.
The first image exposure is performed so as to reduce the voltage to 0V, 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 has a DC component of −600 V and an AC component of frequency F _{1 of} 3.5 kHz.
A developing bias having a DC component and an AC component of Z, V _PP = 1.5 kV is applied.

After that, as shown in FIGS. 3C to 3E, the second image exposure means 5 performs the second image exposure to form an electrostatic latent image of the second color on the surface of the photosensitive drum 2. Form. At this time, the potential of the latent image of the second color is determined by how many times development is performed on the electrostatic latent image of the second color.

That is, in this embodiment, in addition to the step of developing the second electrostatic latent image once, 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 developed by a plurality of color developing devices 6, 7,
In the case of developing twice with any one of the developing devices selected from among the plurality of color developing devices 6, 7 and 8, any one of the plurality of color developing devices selected from the plurality of color developing devices 6, 7 and 8 is used. Continuous development is performed by one developing device, and in the case of developing three times, the second electrostatic latent image is developed by operating all the developing devices of the plurality of color developing devices 6, 7, and 8 to superimpose it. It is like this.

Here, as shown in FIGS. 3C to 3E, the value of the latent image potential of the second color is V _{1 at} the time of one-time development,
The second development is represented by V ₂ , and the third development is represented by V ₃ .

At the time of one-time development, as shown in FIG. 3C, only the background portion of the latent image of the second color is exposed by the second image exposure, and the potential V _{1 of the} image portion is V by the dark decay amount. _It becomes slightly lower than ₀ , and V ₁ = −680V. However, in the case of the double development, as shown in FIG.
Not only the background portion of the color latent image, but also the image portion itself of the second color latent image is exposed, and the potential V _{2 of the} image portion is lower than V ₁ by V.
₂ = -580V. When developing 3 times,
As shown in FIG. 3E, by increasing the exposure intensity of the second-color latent image, the potential V ₃ of the image portion becomes V lower than V _2.
3 = -480V.

As described above, the potential of the image portion of the second image exposure is controlled according to the number of times of repeated development so that the amount of toner to be developed is always constant regardless of the number of times of repeated development. That is, when the image part potential V ₃ of _three- time development is made equal to the image part potential V ₁ of one-time development, yellow is obtained,
The image in which the three colors of magenta and cyan are mixed has a slightly thicker thin line than the black monochromatic 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, in the case of developing on the same electrostatic latent image in an overlapping manner, if the potential difference between the electrostatic latent image and the developing sleeve is set to be the same for each developing device 6, 7, 8, each developing device 6,
A toner amount such that a predetermined density is obtained by the single development of 7 and 8 adheres to the photosensitive drum 2. If development is further performed from above on another developing device, more toner than necessary will adhere to the photosensitive drum 2. In the case of performing the three-time development, the toner adhesion amount is further increased, and the thin line tends to be thicker and more prominent. From this point of view, by controlling the potential of the latent image according to the number of times of overlapping development, it is possible to reduce the amount of toner of each developing device that adheres to the same electrostatic latent image, and prevent thickening of thin lines. You can do it.

Further, at the time of image exposure of the second color, FIG.
As shown in (c) to (e), the background portion potential V _B of the second color latent image is set to a level higher than the potential of the first color black toner image, and the so-called well type is used for black toner. The black toner image is more effectively prevented from being deteriorated in the color developing process by applying a restraining force by the electric field of the potential. At this time, the potential of the first color black toner image is −
The background potential V _B of 100 V and the second color latent image is −150 V.

Further, each of the color developing units 6, 7, and 8 has
As a DC component, -250V is common to each developing device, and as an AC component, a frequency F _{2 of} 3.0 kH is applied to 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
Developing bias is applied.

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

In the above example, two colors, black and the 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)"
It was confirmed that a stable image was always obtained by observing the change in image quality by repeating the printing in the order of 1 cycle as 1 cycle and performing this cycle for 100,000 cycles. By the way, "black + black (yellow + magenta + cyan)"
The mode is intended to obtain a black image having different color tones due to the difference in the color mixture ratio of the three color toners (yellow + magenta + cyan).

As described above, in the color image forming apparatus according to the first embodiment, 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 photosensitive drum 2. , At least the second and subsequent developing units 6,
By using a developing method in which a two-component developer composed of toner and carrier is held in a non-contact state with the electrostatic latent image carrier and the toner is jetted by an alternating electric field, The toner image formed on the photoconductor drum 2 is basically prevented from being mechanically disturbed in the subsequent developing process. Further, by setting the charging property of the developer 13 to be lower in the latter stage and the frequency of the AC component of the developing bias to be lower in the latter stage, the electric field of the AC component of the developing bias is changed. It suppresses image distortion and color mixing.

That is, the charge amount of the toner already developed on the photoconductor drum 2 is increased by setting the chargeability of the developer 13 to be lower as the latter stage. Therefore, the adhesion force of the toner onto the photosensitive drum 2 is higher for the toner previously developed by the action of the mirror image electric field, so that the toner is less likely to be scraped from the photosensitive drum 2.

Moreover, by setting the frequency of the AC component of the developing bias to be lower as it goes to the latter stage, it becomes possible to selectively develop the toner having a larger charge amount in the developing unit at the former stage. Therefore, the adhesive force of the toner on the photoconductor drum 2 is higher for the toner previously developed by the action of the mirror image electric field.
Due to the synergistic action with the charge amount control, it becomes more difficult to scrape the photosensitive drum 2 from above. As a result, the adhesive force of the toner on the photosensitive drum 2 is increased, and it is possible to effectively suppress the occurrence of image disturbance and color mixture due to the electric field of the AC component of the developing bias in the subsequent developing process, and thus to suppress the image for a long period of time. It is possible to provide a highly reliable color image forming apparatus in which the disturbance of colors and the color mixture do not occur.

Comparative Example 1 In the first embodiment, only the AC component of the developing bias of the color developing device is changed, the frequency is 1.0 kHZ for yellow, V _PP = 1.5 kV, and the frequency is 2.0 kHZ for magenta. , V _PP = 1.5 kV, Sian has a frequency of 3.0 kHZ, and a development bias of V _PP = 1.5 kV was applied to actually form a color image. Color mixing occurred. When these developers were observed with an optical microscope, it was confirmed that yellow toner was present in the magenta and Sian developers. Therefore, it is clear that the color mixture of these developers was generated by scraping off the previously developed toner in the next developing step. When the charge amount of the toner image developed on the photosensitive drum 2 at this time was measured, it was 7 μC / g for yellow, 10 μC / g for magenta,
Sian was 7 μC / g. Therefore, the reason why the color mixing occurs here is that the amount of charge of the yellow toner previously developed on the photosensitive drum 2 is lower than that in the first embodiment, and the adhesion of the yellow toner to the photosensitive drum 2 is small. It is considered that the toner was easily scraped off during the repeated development due to its 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 this second embodiment, and FIG. It is a process diagram of a 2nd example.

In the color image forming apparatus shown in FIG.
The photoconductor belt 21 uniformly charged by the first scorotron 20 is exposed by the first exposure means 22 and
An electrostatic latent image of a color is formed. At this time, the exposure is performed on the image portion to remove the charge on 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 photosensitive belt 21. Subsequently, the photoconductor belt 21 is exposed by the second exposure means 24, an electrostatic latent image of the second color is formed, and development is performed by the blue developing device 25. The exposure at this time is performed on the image portion and the background portion, but the exposure is performed so that the image portion potential is lower than the background portion potential, and the reversal development is performed on the image portion. Further, the photoconductor belt 21 has a third
The image portion and the background portion are exposed by the exposure means 26 of the second exposure device in the same manner as the second exposure, a latent image of the third color is formed, and the reverse development is performed by the third green developing device 27. Then, the fourth exposure unit 2
Similarly, the image portion and the background portion are exposed at 8, a latent image of the fourth color is formed, and the reversal development is performed at the fourth red developing device 29.

Thus, black, blue,
Four color toner images of green and red are formed, and 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 photoconductor belt 21 is removed by the cleaner brush 32.
In the figure, reference numerals 33, 34 and 35 denote drive rolls for driving the photoconductor 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, the charging potential was -900V. Exposure means 22, 2
LEDs of 300 dpi were used as 4, 26, and 28. The diameter of the developing sleeve of each of the developing devices 23, 25, 27 and 29 used is 30 mm, the distance between the developing sleeve and the photoconductor 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
It is set to mm / s, and development is performed in a state where the developer does not contact the photoconductor belt 21. This is to prevent the toner image previously developed on the photoconductor belt 21 during the overlapping development from being disturbed in the subsequent development process.

The developing conditions will be described in detail below.

Here, a magnetized powder dispersion type polymer carrier having an average particle diameter of 40 μm and a saturation magnetization of 50 emu / g was used as in the first embodiment.

The carrier used for each developer is common, and 30 wt% of styrene-acrylic copolymer, 68 wt% of magnetite, and 2 wt% of nigrosine are kneaded and pulverized to form a carrier having a positive charging property. .

Regarding the toner, the average particle diameter of each toner was 10 μm. Black toner is 80% by weight of styrene-acrylic copolymer, 10% by weight of styrene-butadiene rubber,
Carbon black 10wt% 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 is 90 wt% polyester, 7 wt% copper phthalocyanine
%, And polytungstophosphoric acid 3 wt% were kneaded and pulverized to obtain colored particles having an average particle size of 10 μm, and then 7 wt% of hydrophobic silica was externally added to the colored particles to obtain a negatively chargeable toner. Green toner is polyester 90wt%, benzine derivative 6w
t% and 4 wt% of copper phthalocyanine were kneaded and pulverized to form colored particles having an average particle size of 10 μm, and 6 wt% of hydrophobic silica was externally added to the colored particles to obtain a negatively chargeable toner. Red toner is 90 wt% polyester, 4 wt% benzine derivative
%, And 6 wt% of polytungstophosphoric acid were kneaded and pulverized to give colored particles having an average particle diameter of 10 μm, and 5 wt% of hydrophobic silica was externally added to the colored particles to obtain a negatively chargeable toner. When the charge amount of the toner was measured with respect to these four kinds of developers, the black charge was -25 μC / g, the blue charge was −20 μC / g, the green charge was −15 μC / g, and the red charge 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. 6 (b)
As shown in, the latent image potential V _I1 of the first color is set to −7 by exposure.
The voltage is lowered to 00V and black toner is developed by a black developing device.
At this time, the black developing device has a DC component of −850 V, an AC component of frequency 3.5 kHZ, and V _PP = 1.5 k.
A developing bias having a DC component and an AC component of V is applied. After that, as shown in FIG. 6C, the image portion potential V _I2 of the second color is lowered to −500 V and the background portion potential V ₂ is lowered to −700 V by the exposure of the second color, and the blue toner is developed by the blue developing device. At this time, as a DC component in the blue developing device,
650V, as an AC component, frequency 3.0kHz, V
A developing bias having a DC component and an AC component of _PP = 1.5 kV is applied. Furthermore, after that, FIG.
As shown in, the exposure of the third color causes the potential V _I3 of the image portion of the third color.
Is reduced to -300 V and the background portion potential V ₂ is reduced to -500 V, and the green toner is developed by the green developing device. At this time, a developing bias having a direct current component of −450 V, an alternating current component of frequency 2.0 kHZ, and an alternating current component of V _PP = 1.5 kV is applied to the green developing device. After that, as shown in FIG. 6E, the image portion potential V _I4 of the fourth color is −100 V and the background portion potential V ₂ is −30 in the fourth color exposure.
It is dropped to 0V and red toner is developed by a red developing device.

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

In this way, when four colors of black, blue, green and red were printed, a stable image was obtained even after 100,000 sheets, and no color mixture was observed.

Comparative Example 2 In the second embodiment, only the AC component of the developing bias of the color developing device is changed, and the frequency is 1.0 kHz for blue.
Z, V _PP = 1.5kV, green is frequency 2.0kHZ, V
_PP = 1.5 kV, red has a frequency of 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 was generated by scraping off the previously developed toner in the next developing step.
At this time, the charge amount of the toner image developed on the photoconductor belt was measured. As a result, blue was -7 μC / g and green was -10 μC
C / g and red were -7 μC / g. Therefore, as in Comparative Example 1, the reason why the color mixture occurs here is that the charge amount of the blue toner developed on the photoconductor belt is lower than that in the second embodiment, and the blue toner is attached to the photoconductor belt. It is considered that the toner was easily scratched during the overdevelopment because the adhesion was small.

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

[0059]

According to the present invention, which has the above-described structure and operation, the charge amount of the developing toner is increased to increase the adhering force of the developing toner to the latent image carrier, thereby preventing the color mixture of the toner and the image deterioration. It is possible to suppress the occurrence, and it is possible to always obtain stable print quality.

[Brief description of drawings]

FIG. 1 is a configuration diagram showing 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 an image forming apparatus according to the present invention.

FIGS. 3A to 3E are graphs showing potentials in an image forming process in the same example.

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

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

6 (a) to 6 (e) are graphs showing the potentials of the image forming process in the same example.

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

[Explanation of symbols]

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 amount, F ₁ , F ₂ , F ₃ ,
F ₄ Frequency of AC component of developing bias.

Front page continuation (72) Inventor Taku Aoshima 2274 Hongo, Ebina City, Kanagawa Prefecture Fuji Xerox Co., Ltd.

Claims (1)

[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 the electrostatic latent image carrier and a developing process a plurality of times. In at least the second and subsequent developing means, a developing method is adopted in which a two-component developer composed of a toner and a carrier is held in a non-contact state with the electrostatic latent image carrier and the toner is ejected by an AC electric field. And the chargeability of the developer of the developing means is set to be lower in the latter stage, and the frequency of the AC component of the developing bias is set to be lower in the latter stage. Forming equipment.