JP2003323050A - Image forming apparatus and method for forming image - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To stably obtain a high-quality image by suppressing carrier adhesion in an image forming apparatus using a small-diameter image carrier and a small-diameter developer carrier for miniaturization, and employing a two- component developing device using the small-particle diameter carrier for high image quality, and a method for forming an image. <P>SOLUTION: In the image forming apparatus in which a developing sleeve 7 has a diameter of 30 mm or smaller and a photoreceptor body 8 has a diameter of 60 mm or smaller, the weight mean particle diameter of the magnetic carrier is 20 μm or larger and 40 μm or smaller and the volume resistance of the carrier is 10<SP>15</SP>(ohm×cm) or less: the magnetic flux density in the normal direction of the developing pole P1 of a magnet roller 7a included in the developing sleeve is 115 mT or more: the magnetic flux density of a magnetic pole P2 of the magnet roller 7a at the downstream side of the developing pole in the rotating direction of the developing sleeve is 85 mT or more: a developing gap is 0.4 mm or less. Developing bias employs DC bias. <P>COPYRIGHT: (C)2004,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming apparatus such as a copying machine, a facsimile and a printer, and an image forming method. More specifically, the present invention relates to image forming using a two-component developer including a magnetic carrier and a toner. The present invention relates to an apparatus and image formation.

[0002]

2. Description of the Related Art Conventionally, an image forming apparatus such as an electrophotographic copying machine has a magnetic carrier and a toner.
A two-component developing device for developing using a component developer,
A one-component developing device that performs development using only toner is known. The two-component developing device usually includes a magnet roller formed of a magnet body having a plurality of magnetic poles inside and a developing sleeve that is a rotatably supported cylindrical developer carrier. While carrying a magnetic carrier having toner adhered on the surface of the developing sleeve, the developing sleeve is conveyed to a developing area, which is a portion facing the image carrier, and development is performed by a magnetic brush composed of a two-component developer. In the two-component developing device, charging is performed by stirring and mixing the magnetic carrier and the toner, so that the chargeability of the toner is stable and a relatively stable and good image can be obtained. However, there is a drawback in that the carrier ratio is deteriorated and the toner in the developer is consumed to change the toner concentration, so that the mixing ratio of the toner and the magnetic carrier is changed. In order to suppress the fluctuation of the mixing ratio of the toner and the magnetic carrier, a toner concentration control device is provided, and the toner is replenished as necessary to suppress the fluctuation.

On the other hand, the one-component developing device conveys the toner carried on the surface of the developer carrying member to the developing area for developing. It does not have the drawback that the carrier deterioration of the two-component developing device and the toner concentration control device must be provided, but it has the drawback that the chargeability is difficult to stabilize.

The magnetic carrier used in the above-described two-component developing device is provided with a film for preventing toner filming on the carrier surface.
It is desired to form a uniform surface of the carrier, prevent surface oxidation, prevent deterioration of moisture sensitivity, protect the photoreceptor from scratches or abrasion of the carrier. It is also necessary to extend the life of the developer, control the charging polarity, or adjust the charge amount. For this purpose, a hard and high-strength coating layer is usually provided by coating with a suitable resin material. For example, one coated with a specific resin material (Japanese Patent Laid-Open No. 58-108548) is disclosed. Further, those in which various additives are added to the coating layer (Japanese Patent Laid-Open Nos. 54-155048, 57-40267 and 58-10854).
No. 9, JP-A-59-166968, JP-B-1
-19584, JP-B-3-628, and JP-A-6-202381). further,
There is disclosed a carrier using an additive adhered to the surface of the carrier (JP-A-5-273789).
Further, a coating film containing conductive particles larger than the coating film thickness is used (JP-A-9-16030).
No. 4) is disclosed. In addition, Japanese Patent Laid-Open No. 8-630
JP-A No. 7-76 describes using a benzoguanamine-n-butyl alcohol-formaldehyde copolymer as a main component in a carrier coating material. Also, the second patent
Japanese Patent No. 683624 discloses that a crosslinked product of a melamine resin and an acrylic resin is used as a carrier coating material.

In order to further improve durability,
The applicant of the present invention discloses in Japanese Patent Laid-Open No. 2001-188388 that "in a carrier having a coat film containing at least a binder resin and particles, the particle diameter D and the binder resin film thickness h
Of 1 <[D / h] <5. Since particles of this carrier are more convex than those of the coating film, contact with a strong impact on the binder resin due to friction with the toner or friction between carriers due to stirring for frictionally charging the developer. Can be relaxed. As a result, it is possible to prevent toner spent on the carrier, and also to prevent film scraping of the binder resin, which is the place where electrification occurs. Significantly improved.

[0006]

By the way, in the above-mentioned two-component developing device, there is a tendency that the particle size of the toner and the magnetic carrier is made smaller in accordance with the demand for higher image quality. However, the smaller the particle size of the magnetic carrier, the smaller the magnetization and the more likely the carrier is attached to the image carrier. In addition, with the recent demand for downsizing of machines, there is a tendency to reduce the diameter of a photosensitive drum as an image bearing member and the sleeve diameter of a developing sleeve as a developer bearing member. However, as the diameter of the drum and the diameter of the sleeve become smaller, the magnetic binding force of the magnetic brush tip to the carrier at the downstream of the developing area (outlet side) becomes smaller, and the carrier adhesion deteriorates. Due to such deterioration of carrier adhesion, deterioration of the photoconductor drum, the cleaning blade of the photoconductor drum, the intermediate transfer member, and the like is promoted, and white spots due to carrier adhesion occur on the image.

As a method for improving carrier adhesion, among the magnetic poles of the magnet roller inside the developing sleeve, a method of increasing the magnetic force of the developing pole arranged to face the photosensitive drum and the magnetic pole on the downstream side of the developing pole can be considered. . This method improves the magnetic restraint force when the magnetic brush moves away from the developing pole to the downstream magnetic pole, and suppresses separation of the carrier at the tip of the magnetic brush.

Further, a method of reducing the resistance of the magnetic carrier can be considered. This is to lower the resistance of the carrier to easily dissipate the counter charge remaining in the carrier after developing the solid portion, and reduce the carrier adhesion to the edge portion due to the counter charge. However, as the resistance of the carrier is lowered, the electric charge is more likely to leak, and in the case where the AC bias is applied as the developing bias, an abnormal image may occur due to the leak.

Further, a method of narrowing the gap between the photosensitive drum and the developing sleeve (hereinafter referred to as the developing gap PG) can be considered. This narrows the development gap PG to increase the electric field strength and reduce carrier adhesion. However, the narrowing of the developing gap easily causes problems such as developer overflow in the developing area, locking of the developing roller, and sticking of the developing sleeve. This is the deterioration of the developer over time,
The initial pumping amount is set to a high value in consideration of a decrease in the pumping amount due to deterioration of the developing sleeve surface and the like, but when the pumping amount is set to a high value, the fluctuation causes a large packing in the developing area. This is because the packing in the developing area causes problems such as the above-mentioned agent overflow, developing roller lock, and developing sleeve sticking.

However, only by adopting one of such improvement methods, in the apparatus in which the carrier adhesion is deteriorated due to the reduction of the particle diameter of the magnetic carrier or the diameter of the photosensitive drum or the developing sleeve, the carrier adhesion is deteriorated. It has been found that it is difficult to control the tolerable level. Further, if only one of such improvement methods is promoted, side effects may occur as described above.

The present invention has been made in view of the above background, and an object thereof is to use an image bearing member and a developer bearing member having a small diameter for downsizing, and a small particle diameter for improving image quality. To provide an image forming apparatus and an image forming method which employ a two-component developing device using a carrier, which can suppress carrier adhesion and stably obtain a high-quality image. Is.

[0012]

In order to achieve the above-mentioned object, the invention of claim 1 has a magnetic field generating means fixed inside, and a two-component developer comprising a magnetic carrier and a toner on the surface. A toner image of an electrostatic latent image on an image carrier is formed by using a developing device equipped with a developer carrier composed of a non-magnetic developing sleeve that carries and rotates. Diameter 30 mm or less, diameter of the image carrier is 60 mm
In the following image forming apparatus, the magnetic carrier has a weight average particle diameter of 20 μm or more and 40 μm or less and a volume resistance of 10 ¹⁵ (Ω · cm) or less, and is a magnetic field generating means inside the developer carrier. , The magnetic flux density in the normal direction of the developing pole arranged to face the image carrier is 115 mT.
As described above, the magnetic flux density of the magnetic pole on the downstream side of the developing pole with respect to the rotating direction of the developer carrying body is 85 mT or more, and the gap between the developer carrying body and the image carrying body is 0.4 mm or less, The developing bias applied to the developer carrying member is a DC bias. In the image forming apparatus of claim 1, in order to improve carrier adhesion, the weight average particle diameter of the magnetic carrier, the magnetic flux density of the developing pole and the magnetic pole on the downstream side of the developing pole, and the gap between the developer carrier and the image carrier. The developing bias is set to the above condition. This significantly improves carrier adhesion while suppressing the side effects of individual methods,
It is a well-balanced condition. As a result, even in an apparatus in which the carrier adhesion is deteriorated due to the reduction in the particle diameter of the magnetic carrier or the reduction in the diameter of the image carrier or the developer carrier, the carrier adhesion is suppressed within the allowable level. The details will be described below.
First, in order to reduce the size of the apparatus, the diameter of the developer carrying member should be 30
The image carrier has a diameter of 60 mm or less. Further, in order to improve the image quality by preventing the generation of traces and roughness of the magnetic carrier, a small particle size carrier having a weight average particle size of 40 μm or less is used as the magnetic carrier. On the other hand, if the weight average particle diameter of the magnetic carrier is smaller than 20 μm, the carrier adhesion becomes too bad, and it is difficult to suppress it to an acceptable level under other conditions. Therefore, the weight average particle diameter of the magnetic carrier is set to 20 μm or more and 40 μm or less. Next, in order to reduce carrier adhesion, the magnetic flux density in the direction normal to the developing pole of the magnetic field generating means inside the developer carrier is set to 115 mT or more, and the magnetic flux density of the downstream magnetic pole of the developing pole is set to 85 mT or more. Further, the volume resistivity of the magnetic carrier and ^{10 1 5 (Ω} · cm) or less.
Furthermore, the gap between the developer bearing member and the image bearing member is 0.4 mm.
The developing bias applied to the developer bearing member is D
C bias. By combining with such conditions, it was found by experiments described later that carrier adhesion can be suppressed to a level at which there is no problem in the image. As a comparative example, the magnetic flux density in the normal direction of the developing electrode is 110 mT, the magnetic flux density of the downstream magnetic pole of the developing electrode is 75 mT, the volume resistance of the magnetic carrier is 10 ¹⁶ (Ω · cm), the developer carrier and the image carrier. The gap between and was 0.5 mm. In this case, it was not possible to suppress the carrier adhesion to a level at which it does not pose a problem on the image. In addition, when only the developing bias is set to the AC bias in order to improve the image quality, the side wall of the carrier is low in resistance and the charge leaks to cause an abnormal image. According to a second aspect of the present invention, in the image forming apparatus according to the first aspect, the magnetic carrier has a coating film containing at least a binder resin and particles, and the particle diameter D and the binder resin film thickness h are 1 <[ D / h] <10. According to a third aspect of the present invention, there is provided a developer carrier comprising a non-magnetic developing sleeve which has magnetic field generating means fixed inside and which carries and rotates a two-component developer comprising a magnetic carrier and toner on the surface. A toner image is formed from the electrostatic latent image on the image carrier using the developing device provided,
In the image forming method using the developer carrier having a diameter of 30 mm or less and the image carrier having a diameter of 60 mm or less, the magnetic carrier has a weight average particle diameter of 20 μm or more and 40 μm or less and a volume resistance of 10 ¹⁵ (Ω · cm) or less, the magnetic field generating means inside the developer carrier, and the magnetic flux density in the normal direction of the developing pole arranged so as to face the image carrier is 115 mT or more, The magnetic flux density of the magnetic pole on the downstream side of the developing pole is 85 m with respect to the rotation direction of the agent carrier.
It is T or more, the gap between the developer carrying member and the image carrying member is 0.4 mm or less, and the developing bias applied to the developer carrying member is a DC bias. According to a fourth aspect of the present invention, in the image forming method according to the third aspect, the magnetic carrier has a coat film containing at least a binder resin and particles, and the particle diameter D and the binder resin film thickness h are 1 <[ D / h] <10.

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION An embodiment in which the present invention is applied to an image forming apparatus such as a copying machine, a facsimile and a printer will be described below. In the image forming apparatus according to the present exemplary embodiment, a charging device, an exposure device, a developing device, a transfer device, and a cleaning device are sequentially arranged around a drum-shaped photoconductor as an image carrier. Further, it is provided with a paper feeding / conveying device for feeding the transfer paper from the paper feeding tray, and a fixing device for fixing the toner onto the transfer paper after the transfer paper on which the toner image is transferred is separated from the photoconductor. In the image forming apparatus configured as described above, the surface of the rotating photoconductor is uniformly charged by the charging device, and then a laser beam or the like of the exposure device is irradiated based on the image information to form a latent image on the photoconductor. Form.
A toner image is formed by attaching toner charged by a developing device to the electrostatic latent image formed on the photoconductor. On the other hand, the transfer paper is fed from the paper feed tray by the paper feed / conveyance device, and then conveyed to the transfer section where the photoconductor and the transfer device face each other. Then, the transfer device applies a charge having a polarity opposite to that of the toner image on the photoconductor to the transfer paper, thereby transferring the toner image formed on the photoconductor to the transfer paper. Next, the transfer paper is separated from the photoconductor and is sent to a fixing device, and the toner is fixed on the transfer paper to obtain an image.

FIG. 1 is a schematic configuration diagram of a developing device of an image forming apparatus according to this embodiment. The developing device used in the image forming apparatus will be described in detail with reference to FIG. The developing device 1 is disposed on the side of the photoconductor 8 and is a non-magnetic developer carrier that supports a two-component developer (hereinafter referred to as "developer") containing toner and a magnetic carrier on its surface. A developing sleeve 7 is provided. This developing sleeve 7
Is attached so as to be partially exposed from an opening formed on the side of the photoconductor 1 of the developing casing, and is rotated in a direction of an arrow b in the drawing by a driving device (not shown). Further, inside the developing sleeve 7, a magnet roller 7a composed of a fixed magnet group as a magnetic field generating means is fixedly arranged. The developing device 1 also includes a doctor 9 as a developer regulating member that is a rigid body that regulates the amount of the developer carried on the developing sleeve 7. A developer accommodating portion 4 for accommodating a developer is formed on the upstream side of the doctor 9 in the rotation direction of the developing sleeve 7, and the first and second agitating means for agitating and mixing the developer in the developer accommodating portion 4 are formed. Screws 5 and 6 are provided. Further, a toner replenishing port 23 arranged above the developer accommodating portion 4, a toner hopper 2 filled with toner to be replenished in the developer accommodating portion 4, and a toner feeding device for connecting the toner replenishing port 23 and the toner hopper 2 to each other. 3 and 3 are provided.

In the developing device 1 having the above structure, the first and second stirring screws 5 and 6 are rotated to stir the developer in the developer accommodating portion 4 so that the toner and the magnetic carrier are opposite to each other. It is triboelectrically charged. This developer is supplied to the peripheral surface of the developing sleeve 7 that is rotationally driven in the direction of arrow b, and the supplied developer is carried on the peripheral surface of the developing sleeve 7, and the rotation direction of the developing sleeve 7 (the arrow It is conveyed in the b direction). Next, the amount of the transported developer is regulated by the doctor 9, and the regulated developer is transported to the developing area where the photoconductor 8 and the developing sleeve 7 face each other. In this developing area, the toner in the developer electrostatically transfers to the electrostatic latent image on the surface of the photoconductor 8 and the electrostatic latent image is visualized as a toner image.

Further, in the image forming apparatus of this embodiment,
For downsizing, a photosensitive member 8 having a diameter of 60 mm or less and a developing sleeve 7 having a diameter of 30 mm or less were used. As the developing sleeve 7, a non-magnetic material such as stainless steel, aluminum, or ceramics, or a material obtained by coating a non-magnetic material of these with sandblasting was used. The doctor 9 has rigidity and magnetism. Examples of the magnetic material include metal materials such as iron and stainless steel, and resin materials containing magnetic particles such as ferrite and magnetite. Alternatively, another member such as a metal plate made of such a magnetic material may be fixed to the doctor 9 directly or indirectly.

As the magnetic carrier of the developer, one having a weight average particle diameter d of 20 μm or more and 40 μm or less was used. If the weight average particle diameter d is larger than 40 μm, the magnetic carrier may cause a trace of ear and a rough feeling.
On the other hand, if the weight average particle diameter of the magnetic carrier is smaller than 20 μm, the carrier adhesion will be excessively deteriorated, and it will be difficult to suppress it to an acceptable level under other conditions described later.

Further, as a magnetic carrier, it has a coat film containing at least a binder resin and particles, and the particle diameter D and the binder resin film thickness h are in the range of 1 <[D / h] <10, preferably 1 A range of <[D / h] <5 was used. As a specific material of the carrier core material, one known as a magnetic carrier for a two-component developer, for example, ferrite, magnetite, iron, nickel or the like may be appropriately selected and used according to the use and purpose of use of the carrier. Further, specific materials for the particles of the coat film include alumina and silica. As the alumina, particles having a particle size of 10 μm or less are preferable, and any of those that have not been surface-treated and those that have been surface-treated such as hydrophobic treatment can be used. As the silica, those used for toner and those other than silica can be used, and all those not surface-treated and those surface-treated such as hydrophobic treatment can be used. In addition, carbon black or an acid catalyst may be used alone or in combination as the charge and resistance modifier. As the carbon black, any of those generally used for carriers or toners can be used. As the acidic catalyst, one having a catalytic action can be used. For example, those having a reactive group such as a fully alkylated type, a methylol group type, an imino group type, and a methylol / imino group type, are not limited to these.
Examples of the binder resin for the coat film include those known as coat film binder resins for magnetic carriers for two-component developers, such as acrylic resin.

Since the particles of the magnetic carrier having such a coating film are more convex than those of the coating film, the agitation for triboelectrifying the developer may cause friction with the toner or friction between the carriers. It is possible to reduce the contact with the binder resin which is accompanied by a strong impact. This makes it possible to prevent toner spent on the carrier. At the same time, it is possible to prevent the abrasion of the binder resin film, which is the place where the charge is generated. Therefore, the carrier surface shape change with time is small, and the durability can be significantly improved. When D / h is 1 or less, the particles are buried in the binder resin, which is not preferable because the effect is significantly reduced. When D / h is 10 or more, the contact area between the particles and the binder resin is small, so that sufficient restraining force cannot be obtained and the particles are easily detached, which is not preferable.

A technique similar to the above magnetic carrier, in which conductive particles larger than the coating resin film thickness are contained in the coating film (JP-A-9-160304).
Is mentioned. The difference from this is the resistance of the particles contained in the coat film. In this technique, the particles are used as a conductive path so as not to increase the resistance of the carrier. However, in this embodiment, the particles are not used as conductive paths. That is, in the present invention, the particles are not used as a resistance adjusting material as in the prior art, but are used as a protective material for the coating film resin and a surface shape adjusting material.

The present invention will be specifically described below by experiments. First, the magnetic carrier used in the experiment will be described. Magnetic carrier 1 (magnetic carrier of the present embodiment) Acrylic resin solution (solid content 50% by weight) 56.0 parts Guanamin solution (solid content 77% by weight) 15.6 parts Alumina particles [0.3 μm, specific resistance 10 ¹⁴ ( Ω · cm)] 160.0 parts Toluene 900 parts Butyl cellosolve 900 parts These were dispersed with a homomixer for 10 minutes to prepare a coating film forming solution. Fired ferrite powder [F-30 as core material
0: average particle size; 35 μm (manufactured by Powder Tech Co., Ltd.)], and this coating film forming solution was applied by a Spira coater (manufactured by Okada Seiko Co., Ltd.) to a film thickness of 0.15 μm and dried. The obtained carrier was baked in an electric furnace at 150 ° C. for 1 hour. After cooling, the bulk of the ferrite powder was crushed using a sieve having openings of 100 μm to obtain a carrier. Here, the ratio D / h of the particle diameter D = 0.3 μm of the alumina particles contained in the carrier coat film and the binder resin film thickness h = 0.15 μm is 2.0. The volume resistance of this carrier is 10 ¹⁵ (Ω · cm). The binder resin film thickness can be measured by observing the cross section of the carrier with a transmission electron microscope to observe the coating film covering the carrier surface. Therefore, the average value of the film thicknesses is taken as the film thickness. . Further, the volume resistance of the carrier is obtained by putting the carrier between parallel electrodes for resistance measurement (gap 2 mm), applying DC500V, and converting the resistance value after 30 seconds measured by a high resist meter into the volume resistivity. . Magnetic carrier 2 (conventionally used small particle size magnetic carrier) Acrylic resin solution (solid content 50% by weight) 56.0 parts Guanamine solution (solid content 77% by weight) 15.6 parts Toluene 900 parts Butylcellosolve 900 parts The coating film forming solution was prepared by dispersing with a homomixer for 10 minutes. The above-mentioned coating film is formed with a spira coater (Okada Seiko Co., Ltd.) so as to have a film thickness of 0.15 μm, using fired ferrite powder [F-300: average particle size; 35 μm (Powdertec Co.)] as a core material. The solution was applied and dried.
The obtained carrier was baked in an electric furnace at 150 ° C. for 1 hour. After cooling, open the ferrite powder bulk 10
It was crushed using a 0 μm sieve and used as a carrier.

(Experiment 1) The magnetic carriers 1 and 2 were set in the developing device shown in FIG. 1, and A4, 300,000 sheets of black monochromatic color were run in the image forming device. The mechanical conditions of the image forming apparatus are as follows. Linear velocity 300 (mm / sec) Photoconductor diameter 30 (mm) Development sleeve / photoconductor linear velocity ratio 2 Development gap PG 0.4 (mm) Doctor gap DG 0.65 (mm) Pumping amount (initial) 60 (mg / Cm ² ) developing sleeve diameter 25 (mm) main pole angle 0 ° main pole magnetic flux density (P1) 120 (mT) main pole downstream side magnetic pole density (P2) 85 (mT) charging potential VD-700 (V) exposure After-potential VL-60 (V) Development bias Vb-500 (V) The magnetic flux density is measured by a magnetic force distribution measuring device (3D magnetic measuring device manufactured by Excel System Products Co., Ltd.), Gauss Meter Co., Ltd. (Manufactured by Mitsui Chemical Co., Ltd.) was used and the measurement was carried out by the sleeve butting method. FIG. 2 shows changes in the pumped-up amount when running under these conditions, and shows the reduction rate (%) with respect to the initial pumped-up amount. Figure 2
Therefore, the magnetic carrier 1 which is the magnetic carrier of the present embodiment
Has less variation in the pumping amount than the magnetic carrier 2, and the reduction rate has not reached 20% even at 300,000 sheets of A4, which is the standard of developer life. The pumping amount is
If the reduction rate exceeds 20%, the solid becomes thin,
It has been confirmed by experiments that the image quality is deteriorated, such as the appearance of a trace image. Therefore, if the rate of decrease is within 20%, it can be said that the image is not affected. on the other hand,
The magnetic carrier 2 which is a conventional small particle size carrier is A4,
At 300,000 sheets, the reduction rate reached 40%, which had a bad effect on the image, such as thinning of the solid image and the appearance of an ear mark image. As a result, when the magnetic carrier of the present embodiment is used, the decrease in the pumping amount is small, and it is not necessary to set the initial pumping amount higher. Therefore, the development gap P
Even when G is narrowed, it is possible to set a condition that prevents the developer from overflowing, developing roller lock, and developing sleeve sticking.

Next, regarding the volume resistance of the magnetic carrier,
This will be described based on Experiment 2. (Experiment 2) The magnetic carrier 1 and other conditions are the same and the volume is the same.
Resistance 10¹⁴10,¹⁵10, ¹⁶(Ω ・ cm) and strange
Made into At this time, the volume resistance of 14, 15, 16 (L
Image of 2-dot vertical line image for ogR (Ω · cm))
The number of white spots caused by carrier adhesion
As shown in FIG. The evaluation method is the same as in Experiment 1.
, Turn off the transfer bias and shake the background potential
By doing so, accelerated evaluation was performed. From Figure 3, the volume of the carrier
Resistance is 10¹⁶With carrier at (Ω · cm) level
The clothes are remarkable. The volume resistance of the carrier is 10¹⁵(Ω ・ c
m), 10¹⁴Carry by lowering (Ω · cm)
It can be seen that the adhesion is greatly improved.

Next, the magnetic flux densities in the normal direction of the developing pole P1 of the magnet roller 7a inside the developing sleeve 7 and the downstream magnetic pole P2 of the developing pole will be described based on Experiment 3. (Experiment 3) Using the above magnetic carrier 1, in the same image forming apparatus as in Experiment 1, the magnetic flux densities in the normal direction of the developing pole P1 of the magnet roller 7a and the downstream magnetic pole P2 of the developing pole are changed as shown in Table 1. Let

[Table 1] FIG. 4 shows the number of white spots caused by carrier adhesion at the edge portion of the 2-dot vertical line image at this time. As for the evaluation method, the transfer bias was turned off and the background potential was shaken in the same manner as in Experiment 2 for accelerated evaluation. From FIG. 4, magnetic flux densities in the normal direction of the developing pole P1 and the magnetic pole P2 on the downstream side of the developing pole are 115 mT and 85 m, as in the example mag.
It can be seen that the carrier adhesion is significantly improved by increasing T.

Next, the developing gap GP between the developing sleeve 7 and the photosensitive drum 8 will be described based on Experiment 4. (Experiment 4) Using the above magnetic carrier 1, in the same image forming apparatus as in Experiment 1, only the developing gap Gp is set to 0.4, 0.5.
It was changed to mm. FIG. 5 shows the number of white spots caused by carrier adhesion at the edge portion of the 2-dot vertical line image at this time. As for the evaluation method, the transfer bias was turned off and the background potential was shaken in the same manner as in Experiment 2 for accelerated evaluation. From FIG. 5, it can be seen that carrier adhesion is significantly improved by narrowing the development gap GP to 0.4 mm.

Next, the developing bias is based on Experiment 5.
Explain. (Experiment 5) The above magnetic carrier 1 and other conditions
Is the same and the volume resistance is 10¹⁴10,¹⁵10, ¹⁶(Ω
・ Cm) was changed. Using these magnetic carriers,
In the same image forming apparatus as in Experiment 1, only the developing bias is DC,
An abnormal image due to charge leakage when changed to AC
The state of occurrence is shown in Table 2. DC bias is -500
(V), AC has a frequency of 5 kHz, Vpp 1.0 k
V and Duty 50% are superimposed.

[Table 2] According to Table 2, when the carrier resistance is 10 ¹⁵ (Ω · cm) or less, an abnormal image due to charge leakage occurs when an AC bias is superposed. However, with the DC bias, an abnormal image due to charge leakage does not occur even if the carrier resistance is reduced to 10 ¹⁵ and 10 ¹⁴ (Ω · cm). Therefore, when the volume resistance of the carrier is reduced to reduce carrier adhesion from Experiment 2 described above, AC cannot be superimposed as the developing bias. On the other hand, when the DC bias is used as the developing bias, the carrier resistance becomes 10
It can be lowered to ¹⁴ (Ω · cm) and carrier adhesion can be significantly improved.

From the examination results of Experiments 1 to 5 described above, the conditions shown in Table 3 are adopted in the image forming apparatus of the present embodiment as conditions for improving carrier adhesion without causing side effects.

[Table 3] FIG. 6 shows the number of white spots due to carrier adhesion at the edge portion of the 2-dot vertical line image under this condition. As for the evaluation method, the transfer bias is turned off as in Experiment 2.
Then, acceleration evaluation was performed by shaking the background potential. From FIG. 6, it can be seen that the above-mentioned conditions have a great effect of improving carrier adhesion and can be reduced to a level at which there is no problem on the image. On the other hand, when the same evaluation was performed under the conditions shown in Table 3 (conventional general conditions) as a comparative example, carrier adhesion was extremely poor.

As described above, according to the image forming apparatus of the present embodiment, side effects such as charge leakage occur even in a device in which the particle diameter of the magnetic carrier is reduced and the photosensitive drum and the developing sleeve are also reduced in diameter. Without suppressing carrier adhesion. Therefore, it is possible to stably obtain a high-quality image while achieving miniaturization. Further, a magnetic carrier having a coat film containing a binder resin and particles and having a particle diameter D and a binder resin film thickness h of 1 <[D / h] <10 is used.
This magnetic carrier is strong against stress and has improved durability,
Deterioration of the developer is suppressed. Therefore, it is possible to reduce the decrease in the pumping amount due to the change in the surface shape of the magnetic carrier over time, and it is not necessary to set the initial pumping amount higher. Therefore, in order to reduce carrier adhesion, the development gap P
Even when G is narrowed, side effects such as developer overflow, developing roller lock, and development sleeve sticking do not occur.

[0029]

According to the inventions of claims 1 to 4, a two-component developing device using an image carrier and a developer carrier having a small diameter for downsizing, and a carrier having a small particle size for improving image quality. In the image forming apparatus and the image forming method adopting, there is an excellent effect that carrier adhesion can be suppressed and a high quality image can be stably obtained.

[Brief description of drawings]

FIG. 1 is a schematic configuration diagram of a developing device employed in an image forming apparatus according to this embodiment.

FIG. 2 is a graph showing a pumping amount reduction rate when magnetic carriers 1 and 2 are used.

FIG. 3 is a graph showing the number of white spots due to carrier adhesion when the volume resistance of the magnetic carrier is changed.

FIG. 4 is a graph showing the number of white spots caused by carrier adhesion when the conditions of the magnet roller are changed.

FIG. 5 is a graph showing the number of white spots due to carrier adhesion when the development gap PG is changed.

FIG. 6 is a graph showing the number of white spots due to carrier adhesion in the image forming apparatus of the present embodiment and the image forming apparatus of the comparative example.

[Explanation of symbols]

1 Development device 2 Toner hopper 3 Toner flow device 4 developer storage 5 1st stirring screw 6 Second stirring screw 7 Development sleeve 7a Magnet roller 8 photoconductor 9 Doctor 23 Toner supply port

   ─────────────────────────────────────────────────── ─── Continued front page    F-term (reference) 2H005 BA06 BA07 CA17 CB07 EA01                       EA05 EA10                 2H031 AA01 AC09 AC20 AD05 AD16                       BA05 BA09 CA07 DA05                 2H035 CA07 CB03

Claims (4)

[Claims] 1. A developer carrier comprising a non-magnetic developing sleeve, which has a magnetic field generating means fixed inside and carries a two-component developer comprising a magnetic carrier and a toner on its surface and rotates. An image forming method for converting an electrostatic latent image on an image bearing member into a toner image by using a developing device, in which the diameter of the developer bearing member is 30 mm or less and the diameter of the image bearing member is 60 mm or less. In the device, the weight average particle diameter of the magnetic carrier is 20 μm or more and 40 μm or more.
m or less and a volume resistance of 10 ¹⁵ (Ω · cm) or less, which is a magnetic field generating means inside the developer carrier, and is in a direction normal to a developing pole arranged so as to face the image carrier. The magnetic flux density is 115 mT or more, the magnetic flux density of the magnetic pole on the downstream side of the developing pole is 85 mT or more with respect to the rotation direction of the developer carrier, and the gap between the developer carrier and the image carrier is 0.4 mm or less. The image forming apparatus is characterized in that the developing bias applied to the developer carrying member is a DC bias. 2. The image forming apparatus according to claim 1, wherein the magnetic carrier has at least a coat film containing a binder resin and particles, and the particle diameter D and the binder resin film thickness h are 1 <[D.
/ H] <10. 3. A developer carrying member comprising a non-magnetic developing sleeve, which has a magnetic field generating means fixed inside and carries a two-component developer consisting of a magnetic carrier and a toner on its surface and rotates. A toner image of an electrostatic latent image on an image bearing member is formed by using a developing device, wherein the developer bearing member has a diameter of 30 mm or less and the image bearing member has a diameter of 60 mm or less. In the image forming method used, the weight average particle diameter of the magnetic carrier is 20 μm or more and 40 μm or more.
m or less and a volume resistance of 10 ¹⁵ (Ω · cm) or less, which is a magnetic field generating means inside the developer carrier, and is in a direction normal to a developing pole arranged so as to face the image carrier. The magnetic flux density is 115 mT or more, the magnetic flux density of the magnetic pole on the downstream side of the developing pole is 85 mT or more with respect to the rotation direction of the developer carrier, and the gap between the developer carrier and the image carrier is 0.4 mm or less. And a developing bias applied to the developer carrying member is a DC bias. 4. The image forming method according to claim 3, wherein the magnetic carrier has at least a coating film containing a binder resin and particles, and the particle diameter D and the binder resin film thickness h are 1 <[D.
/ H] <10.