JPH1048926A - Image-forming device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image-forming device having less influence due to environmental change or aged change and better half tone reproducibility. SOLUTION: The slope of the curve of developing toner amount against a developing bias electric potential and a developing contrast electric potential determined by the electric potential at the total exposure part of an image support is established to satisfy: |Vs|<|Vb|-|Vd|, 15<=Cv<=55(%), 0.8<=Ps/Pd<=2.1, 120<=(TcXRXPs/Pd)<=960 Where, Vs: Developing contrast electric potential at point located at one-fifth slope applied when developing contrast electric potential is around 0; Vd: Electric potential at total exposure part of image support; Vb: Developing bias electric potential; Cv(%): Toner covering rate applied to carrier surface; Pd: Image support moving speed; Ps: developer support moving speed; TC: Toner concentration (%); and R: Carrier particle diameter (μm).

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 used for an electrophotographic digital printer, a digital copier and the like.

[0002]

2. Description of the Related Art Hitherto, digital image forming apparatuses for reproducing multiple gradations by an electrophotographic method have been widely used. In particular, in recent years, full-color digital image forming apparatuses have been remarkably spread. In a full-color digital image forming apparatus, for example, an input signal is converted into digital image signals of four colors of yellow, magenta, sian, and black, and an image carrier is exposed by light corresponding to each digital image signal. Form an electrostatic latent image on the carrier, yellow,
These electrostatic latent images are successively developed by magenta, cyan, and black developing devices to form a visible image on the image carrier, and the visible image on the image carrier is sequentially transferred onto recording paper by a transfer unit. To obtain a full-color image.

As a developing method of a full-color digital image forming apparatus, a two-component developer comprising a carrier having magnetism and a toner containing a coloring material is used, and the two-component developer is used as a developer carrier having a magnetic pole on its surface. A two-component magnetic brush development in which a magnetic brush is formed on the developer carrier by supplying the developer onto the developer carrier, and the toner is transferred to an electrostatic latent image on the image carrier via the magnetic brush. The scheme is well known. This two-component magnetic brush developing method is widely used because it is more advantageous than other methods in terms of stable development density and charging performance.

[0004]

In the two-component magnetic brush developing system, the developing bias applied to the developer carrier, the developing contrast potential determined by the exposed portion of the image carrier, and the developing contrast potential are determined. In general, there is a substantially linear relationship between the amount of toner to be developed and that shown in FIG.

FIG. 17 is a graph of development characteristics when the development contrast potential and the amount of developed toner have a linear relationship and do not have a saturation region. As shown in FIG. 17, the development characteristic graph in this case shows a substantially straight line, and its inclination mainly depends on the toner charge amount. For example, when the toner charge amount is low, the slope becomes steep, and the developing contrast potential is set within this slope region. Therefore, the developing toner amount at the same developing contrast potential becomes larger than when the toner charge amount is high. . Therefore, this developing method has a drawback that the amount of toner charge changes and the amount of developed toner changes due to environmental changes such as temperature and humidity, changes over time of the developer, or changes in toner concentration. . Further, the amount of developing toner also changes due to a change in the contrast potential itself due to a change in the sensitivity of the photoconductor, and the image density changes.

As a method for preventing such a change in image density, a developing method utilizing the saturation characteristics of the amount of developed toner and the image density has been widely adopted. For example, a phenomenon is used in which the image density on the recording paper is substantially saturated at a certain contrast potential or higher even if the change in the amount of developed toner with respect to the contrast potential is linear as shown in FIG. This saturation phenomenon is remarkable in a black developer, and if the developing contrast potential is set so that one or more toner layers are developed on the recording paper, even if the amount of the developed toner fluctuates slightly, it appears on the recording paper. A stable image density can be obtained. However, this method has a problem that halftone reproduction is sacrificed in an analog image forming apparatus that irradiates an original with light and irradiates the image carrier with exposure light corresponding to the intensity of the reflected light. is there. In the case of a digital system in which halftone reproduction is performed by the area gradation method, the above method can be adopted as long as an image forming apparatus for a black and white image is used. The toner may be damaged, or the thickness of the toner may be too large. Further, in an image forming apparatus for a color image, saturation of image density becomes unclear because an image is formed by mixing a plurality of color toners, an excess amount of development lowers saturation, or changes glossiness. Or Therefore, in a color image, it is difficult to stabilize the image density by the above method.

As another method for preventing the fluctuation of the image density, Japanese Patent Application Laid-Open No. 61-272672 discloses a method of controlling a developing bias based on a potential detection result at an exposure amount corresponding to a black density. Also, JP-A-60-2097
No. 53 discloses a method for controlling the exposure amount by detecting the temperature and humidity of an image carrier. These methods based on so-called process control cannot cope with a case where a change in sensitivity occurs due to a temporal change in a part of the image carrier, or a sensitivity unevenness due to a change in the thickness of the image carrier during manufacturing.

[0008] In the case of the feedback-based process control, there is a problem that the fluctuation cannot be immediately followed due to a time delay from detection to control. By the way, in order to stabilize the developing toner amount with respect to the developing contrast potential fluctuation and the toner charge amount fluctuation, FIG.
The linearity of the amount of the developing toner with respect to the developing contrast potential as shown in FIG. 18 is not required. Rather, as shown in FIG. .

FIG. 18 is a graph showing a developing characteristic in which a developing toner amount has a saturation region with respect to a developing contrast potential. As shown in FIG. 18, in this development characteristic graph, the development toner amount shows a tendency to increase to the right as the development contrast potential increases, but when the development contrast potential reaches Vs, the development amount becomes saturated, and Even if the development contrast potential is increased, the amount of developed toner changes at a substantially constant value. The region above this Vs point is the saturation region.

In such a saturated region, the amount of developed toner does not depend on the amount of charge of the toner, so that the amount of development is constant irrespective of environmental changes or changes over time, and the gradation is maintained. Further, even when the development contrast potential changes due to, for example, a change in the sensitivity of the photosensitive member, a stable image can be obtained without changing the development toner amount. As a typical development method using the development characteristics as shown in FIG. 18, a one-component development method is known.
The description is given in Vol. 0, No. 1, pp. 8-17 (1981). In this one-component developing method, the toner saturation amount is equal to the toner supply amount determined by the product of the amount of the layer-forming toner on the developer carrier and the speed ratio between the image carrier and the developer carrier. However, in a device having a gap between the image carrier and the developer carrier as used as an experimental device in the above-mentioned paper, a sufficient saturation region cannot be obtained. Further, for example, in a halftone image, not only the toner flies from the position facing the image carrier, but also the surrounding toner flies by being attracted by the electrostatic latent image, so that a stable developing toner amount can be obtained. I can't. Therefore, the one-component developing method cannot be used in an image forming apparatus that requires halftone reproduction.

The Journal of the Electrophotographic Society, Vol. 30, No. 3, pp. 293-301 (1991) describes a contact-type one-component developing system. In this method, it is possible to maintain a stable saturated density against potential fluctuations even in halftone reproduction, but the amount of toner adhered to the developer carrier, which determines the amount of developed toner, varies with environmental changes and aging. Greatly fluctuates. This is because the adhesion of the toner to the developer carrier is determined by the mirror image force and the mechanical frictional force due to the toner charge, and the toner charge greatly changes due to environmental changes and aging. Therefore, this method
There is a drawback that the image density fluctuates due to a change in the toner charge amount due to environmental changes or aging.

In order to avoid these drawbacks, it is advantageous to use a two-component developing system in which the magnetic attraction is used to peel off the developer from the developer carrier and supply the peeled developer to the image carrier. is there. As a development method by the two-component development method,
For example, Japanese Unexamined Patent Publication (Kokai) No. 61-130959 discloses that a ferrite sintered particle having a volume resistivity of 6.0 × 10 ^{4 to} 2.5 × 10 ⁶ Ω · cm is used as a carrier and combined with an insulating toner. A method has been disclosed in which the amount of the developing toner with respect to the contrast potential shows a saturation characteristic when used as a component developer. However, in the example described here, the electrostatic latent image is formed by directly irradiating the reflected light of the original to the image carrier, and as shown in FIG. Unless only a linear region is used, sufficient gradation cannot be obtained. Therefore, it can be said that the concept using the saturation region is basically different from the concept described above. Although the carrier of the developer disclosed in this publication is composed of only ferrite sintered particles, generally, such a carrier cannot sufficiently control the charge of the toner on the carrier side. Side must perform charging control.
At this time, it is difficult to impart charging characteristics after imparting the fixing characteristics and cleaning properties required for the toner to the toner. Further, there is a problem that the carrier is contaminated by fine powder particles or toner components used as an external additive of the toner, the chargeability is reduced, and the life of the developer is shortened. In order to avoid these problems, generally, the carrier is constituted by a core material made of sintered ferrite particles and a coating material made of a resin or the like that coats the core material, thereby controlling the charge by the carrier. And the reduction of toner adhesion on the carrier surface by selecting a coating material. For example, JP-A-1-120
As disclosed in Japanese Patent Publication No. 566, a carrier having good chargeability and good surface contamination can be obtained by selecting a specific coating material. However, in this method, since the carrier surface is coated with the resin, the carrier becomes a high-resistance carrier, so that the developing electric field is reduced, and the developing characteristic reaches a saturation point in a potential region where the image carrier is used. The development is performed in a substantially linear development characteristic region. As described above, it is not impossible to change the inclination of the development characteristic curve by lowering the charge amount of the toner to make the potential region used by the image carrier a saturated region, but since a portion of reverse polarity distribution is generated. Fog may occur.

As a two-component developing system utilizing the saturation characteristics (see FIG. 18), for example, L. B. Shein
Author, "Electrography and Dave"
"Physical Physics" (1992), p. 152, describes the results of an experiment of the two-component developing system performed for analyzing the developing mechanism. The region where the developing toner amount is saturated with respect to the developing contrast potential is described. There is no suggestion of a technical idea of effectively using.

As described above, at present, a developing technique utilizing a saturated developing characteristic which has high stability over a long period of time and is suitable for halftone reproduction has not been completed. Therefore,
As a method of solving these problems, a method of obtaining a stable halftone reproduction by applying a two-component developing method having a saturation characteristic to a binarized electrostatic latent image is considered. A specific method for using the development area has not been established. This method also has problems such as insufficient saturation characteristics and the possibility of image quality defects. For example, the saturation development amount depends on the toner concentration, but if the toner concentration is too low, the toner coverage on the carrier surface decreases, and a low-resistance carrier is used. Since the dielectric breakdown occurs and the latent image is destroyed, a brush-like white spot occurs. Further, when the toner coverage decreases, the carrier into which the electric charge has been injected due to the low resistance carrier is attracted to the image carrier side, and the carrier adheres. On the other hand, if the toner concentration is too high, the proportion of the toner, which is an insulator, increases, and the resistance of the entire developer becomes high, and the developer is out of the region where saturation characteristics can be obtained. The saturated development amount also depends on the speed ratio between the developer carrier and the image carrier, but if the speed ratio is too low, the image quality may be rough due to insufficient contact between the developer and the image carrier even when the toner concentration is high. Conversely, if the speed ratio is too high, a phenomenon occurs in which the density at the rear end of the image is increased, and in a color image, the hue changes particularly, which is one of the causes of deteriorating the image quality.

SUMMARY OF THE INVENTION In view of the above circumstances, an object of the present invention is to provide an image forming apparatus which is hardly affected by environmental changes and changes over time and has good halftone reproducibility.

[0016]

According to a first aspect of the present invention, there is provided an image forming apparatus which carries an electrostatic latent image and moves in a predetermined direction. An electrostatic latent image forming means for forming an electrostatic latent image on the image carrier by irradiating the charged image carrier with exposure light carrying image information on the image carrier; A developer carrying member, which is provided opposite to and carries a two-component developer composed of a toner and a carrier, and moves at the developing position in the same direction as the moving direction of the image carrier; A developing bias is applied between the image carrier and the image carrier, and the toner in the developer carried by the developer carrier is supplied to the electrostatic latent image on the image carrier, thereby forming Developing means for developing an electrostatic latent image of the toner and forming a visible image with toner In the image forming apparatus provided with the above, the slope of the curve of the amount of the developing toner with respect to the developing contrast potential determined by the developing bias potential and the potential of the entire exposed portion of the image carrier becomes 1/5 of the slope when the developing contrast potential is near 0. The development contrast potential at a certain point is Vs, the potential of the entire exposed portion of the image carrier is Vd, and the development bias potential is Vs.
b, the toner coverage on the carrier surface is Cv (%),
The moving speed of the image carrier is Pd, and the moving speed of the developer carrier is P
s, TC is the toner concentration (%), and R is the carrier particle size (μm). | Vs | <| Vb | − | Vd | 15 ≦ Cv ≦ 55 (%) 0.8 ≦ Ps / Pd ≦ 2 .1 120 ≦ (TC × R × Ps / Pd) ≦ 960.

The second object of the present invention to achieve the above object is as follows.
The image forming apparatus carries an electrostatic latent image and moves in a predetermined direction.After charging the image carrier, the image carrier is irradiated with exposure light carrying image information on the image carrier. An electrostatic latent image forming means for forming an electrostatic latent image on the image carrier, and a two-component developer comprising toners and carriers of each color, which is disposed at a predetermined developing position so as to face the image carrier. And a plurality of developer carriers that move in the same direction as the moving direction of the image carrier at the developing position, and apply a developing bias between the developer carrier and the image carrier. Supplying the toner in the developer carried by the developer carrier to the electrostatic latent image on the image carrier to develop the electrostatic latent image on the image carrier to form a visible image by the toner An image forming apparatus provided with a developing unit.
The developing contrast potential at the point where the slope of the curve of the amount of developing toner relative to the developing contrast potential determined by the developing bias potential and the entire exposed portion potential of the image carrier is 1/5 of the slope when the developing contrast potential is near 0 is Vs. The potential of the entire exposed portion of the image carrier is Vd, the developing bias potential is Vb, and the toner coverage on the carrier surface is Cv.
(%), The image carrier moving speed is Pd, the developer carrier moving speed is Ps, the toner concentration (%) is TC, and the carrier particle size (μ).
m) is R, | Vs | <| Vb | − | Vd | 15 ≦ Cv ≦ 40 (%) 1.0 ≦ Ps / Pd ≦ 1.8 120 ≦ (TC × R × Ps / Pd) ≦ 560 is set so as to satisfy the following relationship.

Here, the first and second image forming apparatuses of the present invention have a toner density detecting means for detecting a toner density on the developer carrying member, and a result detected by the toner density detecting means. May be provided with toner supply amount control means for controlling the supply amount of toner to the developing means based on the image forming apparatus. Further, the first and second image forming apparatuses of the present invention may be provided with the developer A speed ratio for controlling a ratio of the moving speed of the developer carrier to the moving speed of the image carrier based on a result detected by the toner density detecting means; It may have a control means.

The electrostatic latent image forming means forms a latent image contrast at the time of exposure of about 50% area ratio on the image carrier by a charging potential of the image carrier and a surface potential at full exposure. It may form an electrostatic latent image that matches the latent image contrast to be performed.

[0020]

Embodiments of the present invention will be described below. FIG. 1 is a schematic configuration diagram of a color copier incorporating an embodiment of the image forming apparatus of the present invention. Among the components of the color copying machine shown in FIG.
Corresponds to the image carrier of the present invention, and the electrostatic latent image forming charger 2, the document reading unit 10, the light beam scanning unit 20, the light beam pulse width modulation circuit (PWA) 30, and the like are referred to in the present invention. The rotary developing device 3 corresponds to a developing unit according to the present invention.

The operation of the color copying machine will be described with reference to FIG. A document is set downward on the document reading unit 10, and an image recorded on the document is photoelectrically read by the document reading unit 10 to generate an image signal. The light beam scanning unit 20 generates an on / off modulated light beam based on the image signal, and places the light beam 25 on the photosensitive drum 1 rotating in the direction of the arrow in the rotation direction of the photosensitive drum 1. Scanning is repeatedly performed in the main scanning direction perpendicular to the (sub-scanning direction), that is, in the direction perpendicular to the plane of FIG.

After the photosensitive drum 1 is uniformly charged by the electrostatic latent image forming charger 2, it is repeatedly scanned in the main scanning direction by the light beam 25 from the light beam scanning unit 20.
The light beam 25 is turned on and off in accordance with an image signal by a light beam pulse width modulation circuit 30. The light beam 25 exposes the photosensitive drum 1 to form an electrostatic latent image on the photosensitive drum 1. An image is formed. The electrostatic latent image formed on the photosensitive drum 1 moves to a developing position facing the rotary developing device 3. The rotary developing device 3 includes yellow, cyan,
It is composed of four developing devices each having magenta and black toner. Each developing device employs a reversal developing method using two-component magnetic brush development. The rotary developing device 3 rotates every time an electrostatic latent image corresponding to each color is developed,
The electrostatic latent image is developed with the toner corresponding to the color.
At this time, a bias voltage is applied to the developer carrier 31 used for development, and toner adhesion to the background portion of the electrostatic latent image is suppressed.

The toner image obtained by the development reaches a transfer position facing the transfer drum 4 by the rotation of the photosensitive drum 1. Recording paper (not shown) conveyed from the paper tray 11 via a predetermined paper conveyance path 12 is attracted to the transfer drum 4 on the outer periphery of the transfer drum 4 by the operation of the paper suction charger 4a. The toner image formed on the photosensitive drum 1 is transferred to the transfer position facing the photosensitive drum 1 with the rotation of the transfer drum 4 in the direction of the arrow. Is electrostatically transferred onto the recording paper adsorbed on the outer periphery of the transfer drum 4.

After the transfer, the photosensitive drum 1 is cleaned by a cleaner 5 to remove the remaining toner, is irradiated with light by a pre-exposure device 6 and is neutralized, and is again charged by a charger 2 for forming an electrostatic latent image. Charging for forming the next electrostatic latent image is performed. On the other hand, while yellow, cyan, magenta, and black toner images are sequentially formed on the photosensitive drum 1, one recording sheet is attracted to the transfer drum 4 and rotates with the rotation of the transfer drum 4. The recording paper attracted to the transfer drum 4 is also conveyed to the transfer position in synchronization with the arrival of the toner image of each color at the transfer position, and the toner images of each color are sequentially superimposed on the one recording paper. Is transferred to

When the transfer of the toner images of the four colors of yellow, cyan, magenta, and black onto the recording paper adsorbed on the transfer drum 4 is completed, the recording paper adsorbed on the transfer drum 4 is transferred to the peeling charger 4c. As a result, the electrostatic attraction with the transfer drum 4 is removed, the toner is separated from the transfer drum 4 by the separation claw 4e, fixed by the fixing device 9, and further carried out of the apparatus. On the other hand, the transfer drum 4 from which the recording paper has been peeled is neutralized by the neutralizing charger 4d, and when the image is formed again, the next recording paper is sucked in the same manner as described above.

FIG. 2 is a partially enlarged view of the rotary developing device used in the color copying machine of the present embodiment. As mentioned above,
The rotary developing device 3 is provided with four developing devices having yellow, cyan, magenta, and black toners respectively. FIG. 2 shows one of the developing devices in an enlarged manner. I have. As shown in FIG.
A plurality of fixed magnets 34 and fixed magnets 34 arranged in
, And a screw auger 35 and the like.

The developer carrier 31 is supplied with a DC bias AC voltage from a bias power source 37. The two-component developer in which the toner and the carrier are mixed is supplied to the developer carrier 31 by the screw auger 35, and the two-component developer is surrounded by the developer carrier 3 around the developer carrier 31.
A magnetic brush is formed by the magnetic force of the fixed magnet 34 in the unit 1 and is conveyed in the direction of the arrow A along with the sleeve that rotates in the direction of the arrow A. After the magnetic brush is regulated to a certain thickness by the layer thickness regulating member 36, when the developer carrier 31 and the image carrier 1 are transported to the developing positions facing each other,
The electrostatic latent image formed on the image carrier 1 is rubbed and developed. After the development is completed, the developer is conveyed to the downstream side in the rotation direction A, and reaches the developer peeling area where the magnetic attraction force is reduced by the fixed magnets 34a and 34b having the same polarity.
Peeled from 1. The peeled developer is stirred by the screw auger 35. The developer agitated by the screw auger 35 is supplied with new toner, and the agitated and re-adjusted developer is again supplied to the developer carrier 3.
1 is supplied.

The developer carrier 31 is set at a predetermined moving speed by a controller (not shown), and the moving speed is determined by a condition described later (see FIG. 8). The moving speed of the carrier 31 may be made variable, or the moving speed of the image carrier 1 may be made variable. As described above, in the present embodiment, a two-component magnetic brush developing system is employed as a developing unit for stably realizing the saturation characteristics. In the developing method using the saturation characteristic according to the present invention, the gradient of the curve of the amount of developing toner with respect to the developing contrast potential determined by the developing bias potential and the potential of the entire exposed portion of the image carrier is close to zero. The development contrast potential at a point which is 1/5 of the slope at the time is Vs, and the slope of the curve of the amount of developing toner with respect to the development contrast potential determined by the development bias potential and the potential of the entire exposed portion of the image carrier is such that the development contrast potential is close to zero. The development contrast potential at a point at 1/5 of the inclination at a certain time is Vs, the potential of the entire exposed portion of the image carrier is Vd, and the development bias potential is Vb.
By setting such that | Vs | <| Vb | − | Vd | is satisfied, stable image formation is achieved by using a saturated region in which the developing toner amount hardly fluctuates even when the developing contrast potential fluctuates. Means to perform

By the way, unlike the conventional developing method in which the amount of developing toner depends on the developing contrast potential and the amount of toner charge, in the present embodiment, even when the amount of toner charge changes, the amount of toner to reach the saturation region is not changed. Only by changing the slope of the curve, the developing toner amount itself does not change by setting the developing contrast potential in the saturation region A as shown in FIG. The saturation development amount in the saturation region A is determined according to the target development amount of the final image. The target development amount of the final image depends on the density reproducibility and gradation of the final image,
It is determined based on image quality such as graininess and glossiness. In the case of a color image, in addition to these items, items such as the width of a reproduction color gamut, image quality defects, and transferability at the time of transfer after development are taken into consideration. In the developing method of the present embodiment, the saturated developing amount is determined by the amount of toner supplied to the developing area of the developing device. The amount of toner supplied is determined by the toner density and the moving speed of the developer carrier and the image carrier. Depends on speed ratio.
If the toner concentration is too low, the toner coverage on the carrier surface will decrease, and a brush-like white spot due to dielectric breakdown under a high electric field and carrier adhesion to the image carrier will occur. Conversely, if the toner concentration is too high, the resistance as a developer increases and saturation characteristics cannot be obtained. Therefore, in the first image forming apparatus of the present invention, the toner coverage on the carrier surface is in the range of 15 to 55%. Set to.

However, in particular, in the case of a color image, the target development amount is set lower than that of black from the viewpoint of multiple transferability and the like, so that the toner coverage is set lower than that of the first image forming apparatus of the present invention. I do. That is, the second aspect of the present invention
In the image forming apparatus, the toner coverage on the carrier surface is set in the range of 15 to 40%. next,
A method for calculating the toner coverage will be described. First, it is assumed that the carrier and the toner are uniform spheres having a radius R and a radius r, respectively. Assuming that a toner layer having a radius r is formed around a carrier having a radius R, the toner concentration TC and the toner coverage Cv can be expressed by the following equations.

K = r / R TC = ((6K + 12K ² + 8K ³ ) ρ _t · Cv · p)
/ (Ρ _c + (6K + 12K ² + 8K ³ ) ρ _t · Cv ·
p) × 100 (%) Cv = TC · ρ _c / (1−TC) (6K + 12K ² +8)
K ³ ) ρ _t · p where ρ _t : specific gravity of toner ρ _c : specific gravity of carrier p = π / 3√3: dense packing density of toner (planar approximation) In order to prevent adhesion to the body and achieve both sufficient image density and image quality,
It is desirable to set it within the range of 80 μm.

The carrier used in such a developing means preferably has a volume resistivity of 10 ⁸ Ω · cm or less. Because high resistance carrier (≧ 10 ⁹ Ω · cm)
When the developer is used, the development electric field is weak, so that the rise of the development curve becomes gradual. Because of the electric field existing in the developer layer, not only the toner on the surface of the developer layer but also the toner inside the developer layer is an image carrier. This is because the development contrast potential becomes extremely high and saturation characteristics cannot be obtained until the development electric field is completely neutralized. Is sharp, the inside of the developer layer becomes close to conductivity, and there is no electric field. Therefore, only the toner very close to the surface of the developer layer contributes to the development, and the amount of the developed toner is saturated.

The volume resistivity of the carrier coating material is desirably from 10 ¹ Ω · cm to 10 ⁸ Ω · cm, and the volume resistivity of the carrier core material is 10 ¹ Ω · cm.
-It is desirable that the diameter be not more than cm. This is because the resistance of the entire carrier is determined by the balance between the resistance of the coating material and the resistance of the core material, and the appearance of the above-described saturation region is affected by the resistance of the entire carrier. When the resistance is smaller than the resistance of the material, the charge tends to collect on the coating material side, and the charge is induced on the surface of the carrier closer to the image carrier than the inside of the carrier. As a result, the carrier is strongly attracted to the image carrier side and the carrier adheres. This is because there is a risk of causing white spots during transfer.
Also, if a low-resistance carrier is used in a state where the toner concentration is low, a brush-like white spot may occur due to dielectric breakdown occurring between the carrier and the image carrier under a high electric field and destroying the latent image. There is. Since this phenomenon is caused by the low surface resistance of the carrier, the volume resistivity of the coating material is 10 ¹ Ω · cm or more and 10 ⁸ Ω · cm or more.
It is desirable to make the following.

Further, it is most effective to increase the magnetic force of the carrier to avoid the adhesion of the carrier to the image carrier.
The saturation magnetization of the entire carrier is 60 emu / g
As described above, it is preferable that the density be 70 emu / g or more. As a developing bias applied to the developing means, a bias obtained by superimposing an alternating electric field on a DC electric field is used.
It is desirable that pp be 100 V or more and 700 V or less, and that the frequency be 4 kHz or more and 20 kHz or less. This is because the developing bias does not directly affect the saturation characteristics, but by applying an alternating electric field, the non-electrostatic adhesion, magnetic force, and mirror image between the toner and the carrier or between the toner and the developer carrier This is because the effect of reducing the influence of the force and stabilizing the appearance of the saturated state can be obtained. Further, the occurrence of fogging in the non-image portion can be suppressed by vibrations caused by the alternating electric field. However, if Vpp is too large, leakage occurs due to low carrier resistance. Therefore, use in the above range is desirable. Further, if the frequency of the developing bias is too low, fogging of a non-image portion occurs. If the frequency is too high, the follow-up property of the toner is deteriorated and the above effect is lost. Therefore, it is desirable to use the toner within the above range.

Next, in this embodiment, the electrostatic latent image forming means determines whether the latent image contrast at the time of exposure of the image carrier having an area ratio of about 50% is the charging potential of the electrostatic image carrier and the surface potential at the time of full exposure. Almost matches the latent image contrast formed with the potential,
A case where a binarized electrostatic latent image is formed will be described. Conventionally, since the light beam has a distribution at the time of, for example, exposure at an intermediate image area ratio at a high screen ruling, the contrast of the exposure energy profile at the latent image level is reduced, and the developing electric field is weakened. May worsen. This reduction in the contrast of the exposure energy profile can be solved by increasing the distance between pixels or reducing the beam diameter. However, under the recent demand for higher image quality, it is necessary to simply increase the distance between pixels. In other words, it is necessary to avoid reducing the number of lines. Therefore, when a conventional image carrier is used, it is necessary to reduce the beam diameter. However, as shown in FIG. 3, in the case of an image carrier having a so-called S-shaped light potential attenuation characteristic, by appropriately adjusting the exposure energy, the contrast of the exposure energy profile is maintained even if the beam diameter is somewhat large. be able to.

FIG. 3 is a graph showing an S-shaped photo-potential decay characteristic. As shown in FIG. 3, the light potential attenuation characteristic of the image carrier does not fluctuate up to a certain region, but attenuates to almost zero potential through a sharp transient region. By using such an image carrier, it is possible to form a binarized electrostatic latent image. By setting the developing bias so that development is performed within the range of the saturation region, the amount of developed toner can be reduced even when the toner charge decreases due to environmental changes or changes over time or the sensitivity of the image carrier becomes uneven. Stable and good image reproduction becomes possible. This method is particularly suitable for a color image forming apparatus or the like that requires multiple gradations.

Next, examples performed using the apparatus of this embodiment will be described together with comparative examples. Here, the carriers used in the present embodiment and the comparative example will be described first.
The carrier used in this example is coated with a resin in which carbon black is dispersed. The carbon black is not particularly limited, and an appropriate one is selected from the relationship with the coating agent and the electric resistivity. In addition,
The conductive powder is not necessarily limited to carbon black as long as it satisfies the above requirements.
Further, a specific type may be used alone, or a plurality of types may be mixed and used.

As the core material of the carrier, magnetite exhibiting low resistance characteristics is used, but is not limited to magnetite alone. Hereinafter, characteristics of the carriers A, B, and C used in the present embodiment will be described. (Carrier A: 10 ^-1 Ω · cm) Particle size: 50 μm Carbon black: (VXC72, manufactured by Cabot) 15% by volume Core material: magnetite (MX030A, manufactured by Fuji Electric Chemical) 100 parts by weight Toluene: 13.5 weight Part Styrene-methacrylate copolymer (copolymerization ratio 20:80) 1.8 parts by weight Core material resistance: 10 ⁻⁵ Ω · cm Coating material resistance: 10 ¹ Ω · cm Saturation magnetization: 70 emu / g (Carrier B: 10) ² Omega · cm) particle size: 50 [mu] m carbon black: (VXC72, manufactured by Cabot Corporation) 12 vol% core material: magnetite (MX030A, Fujidenkikagaku Co.) 100 parts by weight toluene: 13.5 parts by weight styrene - methacrylate copolymerization polymer (copolymerization ratio 20:80) to 1.8 parts by weight of the core material resistance: 10 ^-5 Ω · cm coating material resistivity: 10 ³ Ω · cm Sum magnetization: 70 emu / g (Carrier C: 10 ⁸ Ω · cm) particle size: 50 [mu] m Carbon Black: (VXC72, manufactured by Cabot Corporation) 8 vol% core material: magnetite (MX030A, manufactured Fujidenkikagaku Ltd.) 100 parts by weight of toluene : 13.5 parts by weight Styrene-methacrylate copolymer (copolymerization ratio: 20:80) 1.8 parts by weight Core material resistance: 10 ⁻⁵ Ω · cm Coating material resistance: 10 ⁵ Ω · cm Saturation magnetization: 70 emu / g Next, characteristics of the carriers D to G of the comparative example will be described. (Carrier D: 10 ⁶ Ω · cm) Particle size: 50 μm Carbon black: (VXC72, manufactured by Cabot) 14% by volume Core material: ferrite (F-300, manufactured by Powder Tech) 100 parts by weight Toluene: 12.3% by weight part styrene - methacrylate copolymer (copolymerization ratio 20:80) to 1.7 parts by weight of the core material resistance: 10 ⁸ Ω · cm coating material resistance: 10 ¹ Ω · cm saturation magnetization: 60 emu / g (carrier E: Xerox Company Ltd. carrier: 10 ⁹ Ω · cm) particle size: 50 [mu] m core material: ferrite (F-300, manufactured by powder Tech Co.) core material resistance: 10 ⁸ Ω · cm saturation magnetization: 60 emu / g (carrier F: 10 ¹¹ Omega・ Cm) Particle size: 50 μm Core material: Ferrite 100 parts by weight Toluene: 14 parts by weight Styrene-methacrylate copolymer (copolymerization ratio 20: 8) ): 1.0 part by weight of the saturation magnetization: 50 emu / g (Carrier G: 10 ⁶ Ω · cm) particle size: 50 [mu] m Carbon Black: (VXC72, manufactured by Cabot Corporation) 10 vol% core material: ferrite (C28-FB, Fuji 100 parts by weight Toluene: 13.5 parts by weight Styrene-methacrylate copolymer (copolymerization ratio 20:80): 1.8 parts by weight Core material resistance: 10 ⁴ Ω · cm Coating material: 10 ⁴ Ω Cm 2 Saturation magnetization: 50 emu / g Further, a commercially available color toner is used as the toner, mixed with the above-mentioned carrier, and then mixed with a turbula shaker mixer (manufactured by Inversina) for 3 minutes. The average particle size of the toner is 7 μm.

Next, a method of measuring the resistivity of the carrier used in the embodiment of the present invention will be described. FIG. 4 is a schematic diagram of an apparatus for measuring the resistivity of a carrier. As shown in FIG. 4, the carrier resistivity measuring device 50 includes a rotatable cylinder 52 having a fixed magnet 51 provided therein, a cell 53 disposed opposite to the cylinder 52, and a cell 53 on both sides of the cell 53. A guard electrode 54 is provided, a DC power supply 55 for applying a bias to the cell 53, and a voltmeter 56 and an ammeter 57 for measuring a bias voltage and a bias current. The cylinder 52 simulates a developer carrier of a developing device, and the cell 53 simulates an image carrier of an image forming apparatus.

Measurement of carrier resistivity configured as described above
A developer is supplied to the surface of the cylinder 52 of the device 50 to form a developer layer.
A developer layer 59 having a predetermined thickness is formed by the thickness regulating plate 58,
10 ⁶ When a bias voltage of V / m is applied, the ammeter 57
Measure the value of the current flowing through. Then, it faces the cell 53.
Carrier resistivity from the volume occupied by the developer layer
Put out. The size of the cell 53 is 60 in the cylinder axis direction.
mm and 5 mm in the cylinder width direction. Also, cell 53
The gap between the cylinder and the cylinder 52 was 2.2 mm. developing
The thickness of the developer layer is determined by bringing the developer layer 59 into
Developer is clogged with the cell 53 when the 52 is rotated.
Adjust the thickness so that it does not grow. Note that low-resistance carriers
If the bias voltage is 10⁶ Shake before raising to V / m
In some cases, an electric field
From the relationship between 1/2 power and current density, 10⁶Resistance value at V / m
Was extrapolated and used to determine the carrier resistivity.

Next, examples and comparative examples will be described using the above-mentioned carriers A to G. (Embodiment 1) FIG. 5 shows carriers A, B, C, D, E, F
5 is a graph showing a development amount curve by a developer prepared by using FIG. In FIG. 5, carriers A, B, C, D, E, and F are mixed with a negatively charged toner, and the mixed toner is put into the rotary developing device 3 of the image forming apparatus shown in FIG. The development amount curve obtained by performing the test is shown. The experimental conditions at this time are as shown in Table 1.

[0042]

[Table 1]

As shown in Table 1, the image carrier and the developer carrier were rotated in a different mode, that is, in a whisper mode in which they moved in the same tangential direction at the contact point. The speed ratio between the image carrier and the developer carrier is 1. As shown in FIG. 5, carriers A having a volume resistivity of 10 ⁸ Ω · cm or less,
In B, C, and D, the developing curves show saturation characteristics, and it can be seen that the amount of developing toner is kept substantially constant even when the developing contrast potential rises to 500 V or more. In contrast, carriers E and F having a volume resistivity of 10 ⁹ Ω · cm or more are used.
In this case, the development curve does not show the saturation characteristic, and the amount of the developed toner continues to increase as the contrast potential increases. The rising slope of the development curve is steeper as the carrier resistance is lower. This gradient also depends on the toner charge amount.For example, the gradient increases as the toner charge amount decreases, but it is still dependent on the carrier resistance even after normalizing the gradient in each developer with the toner charge amount. It became clear. The reason for this is that, in the case of a carrier having a low resistance, the development curve rises sharply due to the strong development electric field, but the inside of the developer layer is close to conductivity and there is no electric field, so only the toner near the surface of the developer layer is exposed. Contributes to the development, and the toner near the surface of the developer layer is completely developed, and the development amount reaches saturation. On the other hand, in the case of a carrier having a high resistance, the rise of the development curve is slow due to a weak development electric field, but since the electric field exists in the developer layer, only the toner on the surface of the developer layer contributes to development. However, it is considered that the toner inside also contributes to the development and the saturation hardly occurs. (Example 2) FIG. 6 is a graph showing the results of development tests under various environmental conditions.

FIG. 6 shows a development amount curve obtained by performing the same development test as in Example 1 under various temperature and humidity conditions using the carriers B and E. The experimental conditions are the same as those shown in Table 1. The development contrast potential was 500 V. As shown in FIG. 6, the volume resistivity of 10 ⁸
When the carrier B of Ω · cm or less (the present embodiment) is used, the developing amount in the saturated region does not change even if the toner charge amount is changed by changing the temperature and humidity under the same toner concentration,
Carrier E with volume resistivity of 10 ⁹ Ω · cm or more (Comparative Example)
In the case where is used, the development amount fluctuates due to the influence of a change in toner charge amount accompanying a change in temperature and humidity. As described above, this phenomenon is a problem of the conventional developing method.

From the experimental results shown in FIG. 6, it is clear that the saturation characteristics do not depend on the toner charge amount. (Embodiment 3) FIG. 7 is a graph showing the relationship between the developer weight on the developer carrying member and the developing toner amount ratio.

FIG. 7 shows how the amount of developing toner changes when the weight of the developer per unit area on the developer carrier is changed using the carriers B and E. The vertical axis in FIG. 7 indicates the weight of the developer on the developer carrier, with an initial value of 45 m.
g / cm ² is normalized as 1 and displayed as a ratio to the initial value. The development contrast potential was 500 V.

As shown in FIG. 7, the volume resistivity is 10 ⁹ Ω ·
cm of the carrier E (comparative example), the developer toner amount ratio increases as the developer weight per unit area on the developer carrier increases.
^When the carrier B (the present embodiment) of ⁸ Ω · cm is used,
Even if the developer weight per unit area increases, the developing toner amount ratio hardly changes. This is because, as described above, when a carrier having a high resistance is used, the developing electric field is weak and an electric field exists in the developer layer, so that the toner on the surface of the developer layer contributes to the development and Since the toner inside also contributes to development, as the developer weight per unit area on the developer carrier increases, the amount of developed toner also increases. On the other hand, when a low-resistance carrier is used, the virtual developing electrode approaches the vicinity of the surface of the developer layer, but as the weight of the developer per unit area increases, the number of conductive paths in the developer layer further increases. The developing electrode position further approaches the image carrier side, and only the toner near the surface of the developer layer contributes to the development. Therefore, the developer weight per unit area increases, but the toner affected by the developing electric field is limited to the toner existing near the surface of the developer layer. As a result, it is considered that the amount of the developed toner does not change. Therefore, when a low-resistance carrier is used, the saturation development amount does not change even if the developer weight per unit area on the developer carrier changes due to, for example, a change in the operation state of the layer thickness regulating member. it is obvious. (Embodiment 4) FIG. 8 is a graph showing the relationship between the toner density with respect to the amount of developed toner and the speed ratio between the developer carrier and the image carrier.

FIG. 8 shows the change in the amount of developed toner when the toner concentration and the ratio of the moving speed of the developer carrier and the image carrier (hereinafter referred to as speed ratio) are changed using the carrier C. The results are shown. The development contrast potential at this time was 500 V, magenta toner was used as the toner, and the carrier particle size was 50 μm. FIG.
As shown in (2), the amount of developed toner linearly increases with an increase in the speed ratio, and the slope of the straight line is determined by the toner density. As the toner density increases from 3% to 4% and further to 5%. The slope of the straight line is steep. FIG.
Since the developing toner amount is saturated at a contrast potential of 500 V within the range of the toner concentration in the above, it can be seen that the saturated developing toner amount depends on the toner concentration and the speed ratio.
From the results of FIG. 8, the relationship between the toner concentration and the speed ratio for obtaining the target amount of developed toner can be obtained by calculation.

By the way, if the speed ratio between the developer carrier and the image carrier is too low, even if the toner concentration is set to be high, the image quality becomes rough due to insufficient contact with the image carrier.
If the temperature is too high, an image defect such as an increase in the density of the rear end of the image occurs. In particular, in the case of a color image, the requirements regarding image quality are severe, and a change in density leads to a change in hue. Therefore, it is desirable that the speed ratio between the developer carrier and the image carrier be 1.0 or more and 1.8 or less. .

FIG. 9 is a graph showing the relationship between the toner density and the speed ratio for obtaining the target developing toner amount. The target developing toner amount is 0.45 mg / unit area of the solid portion.
cm ² , 0.55 mg / cm ² , 0.65 mg / cm ²
3 levels. The target amount of the developed toner is determined based on image quality such as density reproducibility, gradation, granularity, and glossiness of the final image. In the case of a color image, the reproduction color gamut is wide and the image quality is defective. It is determined from such as. If the amount of the developing toner per unit area is too low, there is a problem that the density reproducibility, gradation, granularity, glossiness, etc. are reduced, or the width of the reproduction color gamut is reduced, and conversely, If the amount of the developing toner per unit area is too high, the gradation is deteriorated, especially the transferability is deteriorated. Therefore, in view of these problems, in the case of a color image, 0.65 mg /
cm ² is set. By the way, when a toner having a particle diameter of 7 μm is used and the layer is developed in a finely packed structure with one layer, the amount of the developed toner per unit area is calculated to be 0.51 mg.
/ Cm ² . Therefore, it is possible to realize a lower developing toner amount while solving the above problems by increasing the weight of the coloring material of the toner or improving the transferability. Set to.

As shown in FIG. 9, when the target developing toner amount decreases, both the toner concentration and the speed ratio need to be set accordingly. Next, the effects of the toner coverage and the speed ratio on image quality defects and saturation stability will be described. Table 2 is a table showing the evaluation results of the effects of the toner coverage on image quality defects and saturation stability.

[0052]

[Table 2]

This evaluation test was performed using the carrier C. The target developing toner amount is 0.8 for a black image.
~ 1.0 mg / cm ² , 0.45 for color images
The test was performed by changing the speed ratio within a certain range in relation to the target development amount at 0.65 mg / cm ² . From this evaluation result, when the toner coverage Cv is 15% or less, even if the speed ratio is increased, the toner supply amount is insufficient, and the image roughness is conspicuous, and there is a chance that the low-resistance carrier surface comes into contact with the image carrier. Because of the increase, dielectric breakdown occurred, and a brush-like white spot that destroyed the latent image was remarkably generated. Further, there is a phenomenon that the density of the rear end portion of the image becomes higher as the speed ratio is increased in order to obtain the target developing toner amount. A toner coverage Cv in a range of 15% or more and 40% or less;
In the range of not less than% and not more than 55%, some image quality defects are observed, but as a whole it is almost within the allowable range. But,
When the toner coverage Cv is in the range of 40% or more and 55% or less,
With the increase in developer resistance, the development amount tends to increase, and the saturation stability starts to slightly decrease. This may cause a problem in the case of a color image whose required characteristics are strict in terms of hue and gradation. When the toner coverage Cv becomes 55% or more, the saturation stability is further lost and reaches an unacceptable level. Therefore, it is necessary to use the toner in a range of 15% or more and 55% or less, and it is desirable to use the toner in a range of 15% or more and 40% or less for a color image with strict image quality requirements.

Table 3 shows the evaluation results of the effect of the speed ratio on image quality defects and saturation stability.

[0055]

[Table 3]

In this evaluation test, the carrier C was used as in the case of Table 2, and the target developing toner amount was also measured under the same conditions as in the case of Table 2. The test was performed by changing the toner coverage Cv within a certain range in relation to the target development amount. From this evaluation result, when the speed ratio Ps / Pd is 0.8 or less, the image quality is rough even if the toner coverage Cv is increased to the upper limit, and the brush-like white spots exceed the allowable range. Is 0.8 or more and 1 or less, the color image reaches an unacceptable level due to deterioration of the graininess of the halftone image. When the speed ratio Ps / Pd is not less than 1 and not more than 2.1, all the image defects are almost within an allowable range.
As the speed ratio Ps / Pd increases, the density of the trailing edge of the image begins to become conspicuous, causing hue fluctuation and gradation fluctuation in a color image. When the speed ratio Ps / Pd is 2.1 or more, the toner coverage Cv is used in accordance with the target toner density, and as a result, white spots become remarkable as in Table 2. Further, the increase in the density at the rear end of the image also becomes more remarkable. From these viewpoints, the speed ratio Ps / Pd is 0.8
It is necessary to use within the range of not less than 2.1 and not more than 2.1, and it is desirable to use the speed ratio Ps / Pd in the range of not less than 1.0 and not more than 1.8 in a color image.

Further, the toner concentration TC obtained from the toner coverage Cv, the carrier particle size R, and the speed ratio P
Due to the relationship with the product of s / Pd (TC × R × Ps / Pd), when the carrier particle size R is very small, the saturation magnetization of the carrier itself is weakened, and as a result, carrier scattering or carrier transport to the image carrier is caused. Adhesion occurs. The carrier particle size R
Is larger than a predetermined value, the gap between the carriers is increased, and as a result, it is difficult to obtain the saturation characteristic, and the image quality level is deteriorated. Therefore, the product (TC × R ×
Ps / Pd) needs to be in the range of 120 or more and 960 or less, and in a color image, the product (TC × R ×
Ps / Pd) is preferably in the range of 120 or more and 560 or less.

The image defects shown in Tables 2 and 3 are a serious problem particularly in the case of a color developer. This is because a color image requires particularly high image quality, and in the case of a color image, a slight change in density leads to a change in gradation or hue. In the case of a black image, the target developing toner amount is 0.80 to 1.00 mg / cm ² because the background of the recording paper is easily seen and the fine line reproducibility of the line image is problematic.
Therefore, even when the toner density is as high as 8% and the toner coverage is 55%, the speed ratio is 1.2, which is higher than that of the color image. Conversely, even when the toner coverage is reduced by lowering the toner concentration, the increase in the density at the rear end of the image due to the increase in the speed ratio is not conspicuous, and as a result, both the toner coverage and the speed ratio are within the allowable range from the image quality side. Becomes wider.

From the above results, in a color image, the toner coverage is desirably from 15% to 40%, and the speed ratio is desirably from 1.0 to 1.8. In a black image, the upper limit of the toner coverage can be increased to 55%, and the speed ratio can be used from 0.8 to 2.1. In the present embodiment, no image quality defect was observed within the range of the toner coverage and the speed ratio for achieving the target toner development amount, and good saturation characteristics could be obtained. (Embodiment 5) Table 4 is a table showing the resistance values of the core material and the coating material in the carriers A, B, C, and D and the state of carrier adhesion to the image carrier.

[0060]

[Table 4]

As shown in Table 4, in the case of the carriers A, B and C according to the present embodiment, there is no problem since the carrier is not attached to the image bearing member. In the test, carrier adhesion to the image carrier was remarkable. This is because carrier D has a core resistance of 10 ⁸ Ω · cm and a coat resistance of 10 ¹ Ω · cm, and the resistance of the coat material is lower than the resistance of the core material. This is presumably because charge is induced on the surface of the carrier closer to the image carrier than inside, and as a result, the carrier is more likely to be attracted to the image carrier side, causing carrier adhesion. In the carriers A and B whose coat resistance is higher than the core resistance, the carrier adhesion occurs only partially, and in the carrier C, no carrier adhesion is observed. As already described with reference to FIG.
Carriers A, B, C, having a volume resistivity of 10 ⁸ Ω · cm or less
Although saturation characteristics can be obtained by using D,
Of these, only the carrier D causes carrier adhesion to the image carrier, so that the core resistance of the carrier is preferably lower than the coat resistance. (Embodiment 6) FIG. 10 is a graph showing the relationship between the carrier magnetization and the carrier adhesion amount to the image carrier when the carriers A, B, D and E are used.

As shown in FIG. 10, when the carriers A, B, and E of this embodiment are used, the amount of the carrier adhered to the image carrier is small, but when the carrier D of the comparative example is used, the amount of the carrier adhered. There are many. In the carrier D, since the magnetization of the core material is low, the magnetization of the entire carrier is as low as 50 emu / g, which is considered to be the main factor of carrier adhesion. Therefore, from the viewpoint of preventing carrier adhesion, the carrier magnetization is 60 mu /
g or more. (Embodiment 7) FIG. 11 is a graph of a development curve when the development bias frequency and the peak voltage of the alternating component are changed.

In this example, an experiment was performed using the carrier C under the same conditions as those shown in Table 1 except for the developing bias. As shown in FIG. 11, the developing bias frequency and the peak voltage of the alternating component (hereinafter referred to as Vp-p
), There is no significant change in the development curve, and it is apparent that the development bias does not affect the saturated development amount. However, the peak voltage of the alternating component is 800 V
As described above, a leak occurred between the developer carrier and the image carrier, and image defects such as white spots and density reduction occurred. When the developing bias frequency was 4 kHz or less, non-image area fog occurred. FIG. 12 shows the result.

FIG. 12 is a graph showing the relationship between the developing bias frequency and the non-image area fog. The vertical axis of FIG. 12 represents the lightness L ^* , and the lower the value, the more the fog. From the results shown in FIGS. 11 and 12, it is desirable to use a developing bias in which an alternating electric field having a voltage of 100 V or more and 700 V or less and a frequency of 4 Hz or more and 20 kHz or less is superimposed on a DC electric field. (Embodiment 8) FIG. 13 is a graph showing the result of an experiment for maintaining the image density using the image carrier having the optical attenuation characteristics shown in FIG. 3 and the carrier C. FIG.
4 is a graph showing the results of an experiment on maintaining the skin color reproduction color difference using the image carrier having the light attenuation characteristics shown in FIG. 3 and the carrier C.

Table 5 is a table showing the experimental conditions in FIG. 13 and FIG.

[0066]

[Table 5]

As shown in FIGS. 13 and 14, despite the increase in the number of output sheets, the reproducibility of the solid image density is stable, and the reproducibility of halftones such as flesh color is also stable. . This is because the electrostatic latent image has a sufficient contrast potential even in the halftone range, and a stable intermediate potential is obtained with respect to the binarized electrostatic latent image in combination with the use of the saturated region in the present embodiment. It was confirmed that tone reproducibility was obtained. (Embodiment 9) FIG. 15 is a graph showing the influence on the developing characteristics due to the aging of the carrier.

The used carriers are carrier B (this embodiment) and carrier E (comparative example). In order to prepare a simulated developer which has been aged over time, a new developer was used for about 6 hours (10,000) using the developing device shown in FIG.
Continuous stirring). Table 6 shows the toner charge amount before and after the stirring process.

[0069]

[Table 6]

The charge amount after stirring of both carriers B and E is reduced to about ／ of the initial charge amount. Here, the amount of the developed toner indicates the weight of the developed toner on the image carrier. FIG.
As shown in FIG. 5, in the case of the carrier B, there is almost no effect on the development characteristics due to the aging, but in the case of the carrier E, the amount of the developing toner after the aging changes significantly more than in the initial state. The shape of the curve has changed significantly. From this, it is understood that the amount of the developed toner does not depend on the toner charge amount, but is determined by the toner supply amount.

Next, another embodiment of the image forming apparatus of the present invention will be described. As described above, the saturation development amount depends on the toner density and the speed ratio, and the target development density is determined by this. Therefore, the development density is not affected by the potential fluctuation and the toner charge amount fluctuation, but is affected by the toner density fluctuation and the speed ratio fluctuation. Therefore, it is desirable to provide a toner supply amount control means for detecting a change in the toner density and controlling the toner supply amount, or a speed ratio control means for detecting a change in the speed ratio and controlling the speed ratio. Conventionally, means for detecting the toner concentration on the developer carrying member is known, but the charge amount also fluctuates due to the change in the toner concentration, and the fluctuation in the charge amount directly changes the development amount. Also, since the charge amount changes, the toner supply amount cannot be directly determined from the toner density detection result, and complicated correction using a data table or the like is required. However, in the present embodiment, since there is no variation in the development amount due to the variation in the charge amount, the toner density control means for directly determining the toner supply amount from the toner density detection result can be operated. Further, since the toner supply amount can be controlled by changing the speed ratio, it is also possible to control the speed ratio based on the toner density detection result. Either the toner supply amount control means or the speed ratio control means may be operated, or both may be operated.

FIG. 16 is a schematic diagram showing a second embodiment of the image forming apparatus of the present invention. As shown in FIG. 16, this image forming apparatus is similar to the image forming apparatus shown in FIG. 1, and the difference between the two is that the image forming apparatus has a toner density detecting means 42, a comparing / arithmetic circuit 43 , Control circuit 4
Fourth, the speed ratio changing means 45 is added.

The added portions will be described below. The toner concentration detecting means 42 disposed opposite to a predetermined rotation position of the developer carrier 41 of the four developing devices is
The density of the toner carried by the developer carrier 41 is detected, and the detection result is transferred to the comparison / arithmetic circuit 43. The comparison / calculation circuit 43 transfers the value of the speed ratio calculated from the detected toner density and the target toner development amount to the control circuit 44. The control circuit 44 controls the moving speed of the developing carrier 41 or the image carrier 1 or both by the speed ratio varying means 45. Further, the control circuit 44 controls the toner supply device 4
6 is controlled to supply a predetermined amount of toner to the rotary developing device 3 based on the relationship with the speed ratio.
Regarding the relationship between the speed ratio control and the toner supply amount control, first, when the detected toner density is slightly lower than the predetermined target toner density, it is preferable to control the speed ratio from the viewpoint of followability. That is, in the case of the toner supply amount control, a time delay from the detection of the toner concentration to the supply of the toner from the toner dispenser (not shown) to the rotary developing device 3 is unavoidable. It is difficult to do the operation. On the other hand, in the case of the speed ratio control, a time delay from when the toner density is detected to when the comparison and calculation are performed and the speed control is started is small, and control with high tracking performance can be performed. However, if the toner concentration changes significantly due to long-term use, it cannot be handled only by the speed ratio control, and toner replenishment is required. Therefore,
In the first stage, it is desirable to perform speed ratio control, and to appropriately perform the toner supply amount control in the second stage based on the toner density detection result. Further, by providing a developing amount detecting means (not shown) in the vicinity of the image carrier as needed, more accurate control can be performed. In the present invention, the image density can be determined from the toner density and the speed ratio without being affected by the toner charge amount. There is no need for complicated data tables that take into account volume fluctuations.

[0074]

As described above, according to the image forming apparatus of the present invention, the two-component developing system utilizing the saturation region is adopted, and the toner concentration, the carrier particle size, the developer carrier and the image carrier are used. By keeping the product of the speed ratios within a predetermined range, the toner is stable against environmental changes, changes in toner charge due to aging, and fluctuations in sensitivity of the image carrier, and has no image defects and good halftone reproducibility. An image forming apparatus capable of forming an image can be obtained.

[Brief description of the drawings]

FIG. 1 is a schematic configuration diagram of a color copier incorporating an embodiment of an image forming apparatus of the present invention.

FIG. 2 is a partially enlarged view of a rotary developing device used in the color copying machine of the present embodiment.

FIG. 3 is a graph of an S-shaped light potential attenuation characteristic.

FIG. 4 is a schematic diagram of an apparatus for measuring the resistivity of a carrier.

FIG. 5 is a graph showing a development amount curve by a developer prepared using carriers A, B, C, D, E, and F;

FIG. 6 is a graph showing the results of development tests under various environmental conditions.

FIG. 7 is a graph showing a relationship between a developer weight on a developer carrying member and a developing toner amount ratio.

FIG. 8 is a graph showing a relationship between a toner concentration with respect to a developing toner amount and a speed ratio between a developer carrier and an image carrier.

FIG. 9 is a graph showing a relationship between a toner concentration and a speed ratio for obtaining a target developing toner amount.

FIG. 10 is a graph showing the relationship between carrier magnetization and carrier adhesion amount to an image carrier when carriers A, B, D, and E are used.

FIG. 11 is a graph of a development curve when a development bias frequency and a peak voltage of an alternating component are changed.

FIG. 12 is a graph showing a relationship between a developing bias frequency and a non-image portion fog.

FIG. 13 is a graph showing a result of an image density maintenance test performed using the image carrier having the light attenuation characteristic and the carrier C shown in FIG.

FIG. 14 is a graph showing the results of an experiment of maintaining the skin color reproduction color difference using the image carrier having the light attenuation characteristic shown in FIG. 3 and the carrier C;

FIG. 15 is a graph showing an influence on a developing characteristic due to a change with time of a carrier.

FIG. 16 is a schematic diagram illustrating a second embodiment of the image forming apparatus of the present invention.

FIG. 17 is a graph of development characteristics when the development contrast potential and the amount of developed toner have a linear relationship and do not have a saturation region.

FIG. 18 is a graph of a developing characteristic in which a developing toner amount has a saturation region with respect to a developing contrast potential.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Image carrier (photosensitive drum) 2 Charger for forming an electrostatic latent image 3 Rotary developing device 4 Transfer drum 4a Charger for adsorbing paper 4b Transfer charger 4c Charger for peeling 4d Charger for static elimination 5 Cleaner 6 Pre-exposure Device 9 Fixing device 10 Document reading section 11 Paper tray 12 Paper transport path 20 Light beam scanning section 25 Light beam 30 Light beam pulse width modulation circuit 31, 41 Developer carrier 42 Toner density detection means 43 Comparison / arithmetic circuit 44 Control circuit 45 speed ratio changing means 46 toner supply device

 ──────────────────────────────────────────────────の Continued on the front page (72) Inventor Kazuhiko Yanagita 1600 Takematsu, Minamiashigara-shi, Kanagawa Prefecture Inside Fuji Xerox Co., Ltd. (72) Inventor Kanishi 1600 Takematsu, Minamiashigara-shi, Kanagawa Prefecture Inside Fuji Xerox Co., Ltd.

Claims (5)

[Claims] 1. An image carrier that carries an electrostatic latent image and moves in a predetermined direction, and by irradiating the image carrier with an exposure light carrying image information on the image carrier after charging the image carrier. An electrostatic latent image forming means for forming an electrostatic latent image on the image carrier; and a two-component developer comprising a toner and a carrier, which is provided at a predetermined developing position so as to face the image carrier. A developer carrier that is carried and moves in the same direction as the moving direction of the image carrier at the developing position, and a developing bias is applied between the developer carrier and the image carrier; Developing means for developing the electrostatic latent image on the image carrier by forming a latent image on the image carrier by supplying the toner in the developer carried by the developer carrier to the electrostatic latent image on the image carrier The developing bias potential and the total exposure of the image carrier. The development contrast potential at the point where the slope of the curve of the amount of developing toner with respect to the development contrast potential determined by the development potential is 1/5 of the slope when the development contrast potential is near 0 is Vs, and the potential of the entire exposed portion of the image carrier is Vs. Vd, the developing bias potential is Vb, and the toner coverage on the carrier surface is Cv.
(%), The image carrier moving speed is Pd, the developer carrier moving speed is Ps, the toner concentration (%) is TC, and the carrier particle size (μ).
m) is R, | Vs | <| Vb | − | Vd | 15 ≦ Cv ≦ 55 (%) 0.8 ≦ Ps / Pd ≦ 2.1 120 ≦ (TC × R × Ps / Pd) ≦ 960 is set so as to satisfy the following relationship: 2. An image bearing member carrying an electrostatic latent image and moving in a predetermined direction, and irradiating the image bearing member with exposure light carrying image information after charging the image carrier. An electrostatic latent image forming means for forming an electrostatic latent image on the image carrier, and a two-component system comprising toner and carrier of each color, which is provided at a predetermined developing position so as to face the image carrier. A plurality of developer carriers that carry the developer and move in the same direction as the moving direction of the image carrier at the developing position, and apply a developing bias between the developer carrier and the image carrier. At the same time, the toner in the developer carried by the developer carrier is supplied to the electrostatic latent image on the image carrier, thereby developing the electrostatic latent image on the image carrier to form a visible image by the toner. A developing bias potential and an image bearing device. The developing contrast potential at the point where the slope of the curve of the amount of developing toner with respect to the developing contrast potential determined by the potential of all the exposed portions of the holding body is 1/5 of the slope when the developing contrast potential is near 0 is Vs, The potential of the entire exposed portion is Vd, the developing bias potential is Vb, and the toner coverage on the carrier surface is Cv.
(%), The image carrier moving speed is Pd, the developer carrier moving speed is Ps, the toner concentration (%) is TC, and the carrier particle size (μ).
m) is R, | Vs | <| Vb | − | Vd | 15 ≦ Cv ≦ 40 (%) 1.0 ≦ Ps / Pd ≦ 1.8 120 ≦ (TC × R × Ps / Pd) ≦ 560 is set so as to satisfy the following relationship: 3. A toner density detecting means for detecting a toner density on the developer carrying member, wherein a toner supply amount to the developing means is controlled based on a result detected by the toner density detecting means. 3. The image forming apparatus according to claim 1, further comprising a toner supply amount control unit. 4. A toner concentration detecting means for detecting a toner concentration on the developer carrying member, the developer carrying member corresponding to the image carrier moving speed based on a result detected by the toner concentration detecting means. 3. The image forming apparatus according to claim 1, further comprising a speed ratio control unit that controls a ratio of a moving speed. 5. The electrostatic latent image forming means forms a latent image contrast on the image carrier at the time of exposure of about 50% in area ratio by a charging potential of the image carrier and a surface potential at the time of full exposure. The image forming apparatus according to claim 1, wherein the image forming apparatus forms an electrostatic latent image that matches a latent image contrast to be formed.