JPH07175329A - Development device - Google Patents

Abstract

(57) [Abstract] [Purpose] To develop a dot distribution latent image with a magnetic brush of a two-component developer even in low density areas of an image without shading to obtain a high quality image without shading. It is in the developing device that made it possible. A carrier of a two-component developer used for magnetic brush development is a developing magnetic pole S of a magnet 29 in a developing sleeve 25.
1 has a magnetization intensity of 100 emu / cm ³ or less when a magnetic field having a peak value of the vertical magnetic field on the surface of the sleeve 25 is applied, and the developer has a toner diameter of r _t μm and a carrier diameter of r _c μm. , Toner bulk density ρ _t g / cm ³ , carrier bulk density ρ _c g / cm ³ , toner amount in developer nwt%
When the to and _{s = n · r c · ρ} c / (100-n) · r t · ρ t becomes s is to satisfy the 2.0 or more. [Effect] Carrier magnetization intensity is 100 emu / cm.
^{When it is 3} or less, rubbing can be eliminated, and when s is 2.0 or more, the concentration can be improved and the object can be achieved.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention uses a magnetic brush of a two-component developer having a toner and a magnetic carrier to form a dot distribution electrostatic latent image formed on an image carrier corresponding to a recorded image signal. The present invention relates to a developing device for developing.

[0002]

2. Description of the Related Art A photosensitive drum, which is an electrophotographic photosensitive member, is scanned and exposed by a laser beam modulated corresponding to a recorded image signal to form a dot distribution electrostatic latent image, that is, a dot-shaped latent image as an image. There is known an image forming method for forming electrostatic latent images correspondingly distributed. Among them, the so-called pulse modulation width (PWM) method of modulating the width of the laser drive pulse current (that is, the duration time) in accordance with the density of the recorded image is
High recording density (that is, high resolution) can be obtained, and high gradation can be obtained.

However, when a dot distribution electrostatic latent image is formed on the photosensitive drum by using the PWM method, a magnetic brush of a two-component developer is brought into contact with the photosensitive drum, and the electrostatic latent image is reversely developed. The image was rough in the halftone region where the reflection density was 0.3 or less. This rubbing did not occur so much in a text original or the like, and often occurred in a light density area of a photo original or the like.

Then, when the cause of the rubbing was investigated, the following was found. Normally, when forming a low-density latent image with a dot distribution latent image, the latent image on the photosensitive drum is not a broad latent image like an analog latent image as shown in FIG. , A two-dimensional distribution of local dot-shaped latent images. When attempting to reproduce even lower density, the dot-shaped latent image becomes blunt due to the influence of the photoconductor film thickness of the photosensitive drum, and as shown in FIG. 2, the maximum contrast Vo (potential difference between the non-exposed portion potential and the dot-shaped latent image The minimum potential difference of the absolute value) gradually becomes smaller. For example, when an image with a reflection density of about 0.2 is reproduced, the Vo of the dot-shaped latent image at that time is 150 to 2
It will be about 00V.

On the other hand, in the case of reversal development in which toner adheres to the light-exposed portion of the photosensitive drum, the DC voltage component of the vibration developing bias voltage is higher than the surface potential of the non-exposed portion (non-image portion) in order to prevent fogging. Also, since the absolute value is set to be 100 to 200 V lower, the maximum contrast Vo is 150 to
In the case of 200 V, the potential difference Vcont between the potential of the light exposure portion of the dot-shaped latent image and the DC voltage component of the developing bias is 0 to 50.
It will be about V.

The Vcont of 0 to 50 V is a very unstable contrast potential as to whether the toner reaches the photosensitive drum side or remains on the developing sleeve side. Therefore, when developing the above dot-shaped latent image with a two-component developer,
The contact state of the magnetic brush with the photosensitive drum has a great influence on the developing efficiency, and due to the lack of dots or the like corresponding to the unevenness of the ears of the magnetic brush, density unevenness that is unevenly distributed is likely to occur. This is shown in FIG.

In FIG. 3, the symbol P indicates each pixel. The dot-shaped latent images L1 to L1 corresponding to the low-density image are generated by the laser beam modulated to each pixel P by the PWM method.
L5 is formed. D1 to D4 are dot-shaped latent images L1
.About.L4 toner adhering regions, that is, developed regions.

The dot-shaped latent image L2 is completely developed. However, the dot-shaped latent images L1, L3, and L4 are only partially developed. The dot-shaped latent image L5 is not developed at all.

As described above, since the developed image having a defect in the development of the dot-shaped latent image is two-dimensionally distributed, the low-density area looks rough, and a color image is formed by superposing toner images of a plurality of colors. When formed, this rubbing is very noticeable, resulting in deterioration of image quality.

[0010]

SUMMARY OF THE INVENTION An object of the present invention is to provide a dot-distribution electrostatic latent image formed on an image bearing member with a magnetic brush of a two-component developer without causing shakiness even in a low density area of the image. It is an object of the present invention to provide a developing device capable of developing and obtaining a high-quality image without rubbing.

[0011]

The above object can be achieved by the developing device according to the present invention. In summary, the present invention is to carry a two-component developer having a toner and a magnetic carrier on a developer carrier containing a magnet and convey the developer to a developing unit facing the image carrier, where the magnet is used in the developing unit. A magnetic brush of the developer is formed by the developing magnetic field generated by the developing magnetic pole of, the magnetic brush is brought into contact with the image carrier, and the dot distribution electrostatic latent image formed corresponding to the recorded image signal is formed on the image carrier. In the developing device for developing, the magnetic carrier has a magnetization intensity of 100 emu / cm ³ or less when a magnetic field having a peak value of a vertical magnetic field on the surface of the developer carrier of the developing magnetic pole is applied, and The agent has a toner diameter r _t μm, a carrier diameter r _c μm, a toner bulk density ρ _t g / cm ³ , a carrier bulk density ρ _c g / cm ³ , and a toner amount nw in the developer.
when the t%, a developing apparatus is characterized in that _{s = n · r c · ρ} c / (100-n) · r t · ρ t becomes s is 2.0 or more.

Preferably, at the time of developing the dot distribution electrostatic latent image, a vibration bias voltage is applied to the developer carrying member, the image carrying member is an electrophotographic photosensitive member, and the dot distribution electrostatic latent image is , Is formed by exposing the electrophotographic photosensitive member with a light beam modulated by pulse width modulation corresponding to the density of the recorded image.

In the present invention characterized by the above, in order to prevent the occurrence of rubbing, the density of the magnetic brush, that is, the number of magnetic brushes per unit area of the developer carrying member is increased and the toner concentration is increased. As a result, a good halftone image can be obtained in the entire density region of the dot distribution latent image.

Regarding the dot-distributed electrostatic latent image targeted by the developing device of the present invention, one pixel indicates the minimum unit of gradation information, and in the multi-valued PWM system or the like, the minimum recording unit. Say that. That is, the pixel exposed by the light driven by the pulse of the time length corresponding to the minimum recording unit is the pixel of the highest density, and the exposed portion and the exposure exposed by the light driven by the pulse of the time length shorter than the above time length Pixels consisting of non-exposed portions that have not been exposed are pixels of halftone density, and pixels consisting of only non-exposed portions are pixels of the lowest density (white background).

On the other hand, in the dither method for outputting the pseudo gradation in the binary recording, for example, when outputting the pseudo gradation in the minimum recording unit of 2 × 2, one pixel is a set of four minimum recording units. There is.

[0016]

【Example】

Embodiment 1 FIG. 1 is an overall configuration diagram showing an example of an electrophotographic color printer to which a developing device of the present invention can be applied. This printer is provided with an electrophotographic photosensitive drum 3 which rotates in the direction of the arrow, and a primary charging device 4, a rotary developing device 1 having a plurality of developing devices around the photosensitive drum 3, a transfer charging device 10 and a cleaning means. 12 is provided, a laser scanner LS is provided above the photosensitive drum 3, and the image forming means is configured by these primary chargers 4 and the like.

The developing devices of the rotary developing device 1 are a magenta developing device 1M, a cyan developing device 1C, a yellow developing device 1Y and a black developing device 1K, which contain a two-component developer containing toner and carrier, Magenta toner, cyan toner, yellow toner, and black toner are used as toner.

The document to be copied is read by a document reading device (not shown). This reading device has a photoelectric conversion element such as a CCD that changes an original image into an electric signal, and outputs image signals corresponding to magenta image information, cyan image information, yellow image information, and monochrome image information of the original, respectively. To do. The semiconductor laser built in the scanner LS is controlled according to these image signals and emits the laser beam L. A laser beam can be emitted in response to an output signal from the computer.

The sequence of the entire color printer will be briefly described by taking the case of the full color mode as an example. First, the photosensitive drum 3 is uniformly charged by the primary charger 4. Next, scanning exposure is performed by the laser beam L modulated by the magenta image signal, and the photosensitive drum 3
A dot-distributed electrostatic latent image is formed on the latent image. The latent image is subjected to reversal development by the magenta developing unit 1M which is fixed at the developing position in advance, and is visualized as a magenta toner image.

On the other hand, the recording material such as paper which is taken out from the cassette C2 and supplied through the paper feed guide 5a, the paper feed roller 6 and the paper feed guide 5b is the gripper 7 of the transfer drum 9.
Is electrostatically wound around the transfer drum 9 and carried by the contact roller 8 and its opposite pole. The transfer drum 9 is rotating in the direction of the arrow shown in the figure in synchronization with the photosensitive drum 3, and the recording material carried on the transfer drum 9 is conveyed to a transfer portion facing the photosensitive drum 3, where the photosensitive drum 3 is moved. The upper magenta toner image is transferred by the transfer charger 10. The transfer drum 9 continues to rotate as it is, and again conveys the recording material toward the transfer portion in preparation for transfer of the cyan toner image to be formed next.

After the transfer of the magenta toner image is completed, the photosensitive drum 3 is destaticized by the destaticizing charger 11, is cleaned by the cleaning means 12 to remove the untransferred toner, and is then charged by the primary charger 4 again. The dot distribution electrostatic latent image is formed on the photosensitive drum 3 by the exposure of the laser beam L modulated by the cyan image signal. During this time, the developing device 1 rotates by a quarter and the cyan developing device 1 rotates.
C is fixed in advance at the developing position, and the electrostatic latent image formed on the photosensitive drum 3 is reverse-developed by the cyan developing device 1C to be visualized as a cyan toner image. The obtained cyan toner image is transferred onto the recording material on the transfer drum 3 from the magenta toner image in an overlapping manner at the transfer portion.

The above steps are carried out for yellow and black, and the dot distribution electrostatic latent image is formed on the photosensitive drum 3 by exposure of the laser beam L modulated by the respective image signals, and the yellow developing device. Magenta, cyan, yellow and black are formed on the recording material by forming a yellow toner image and a black toner image by development with 1Y and a black developing device 1K, and superposing and transferring the yellow toner image and the black toner image on the recording material. A color image is obtained by superimposing the four color toner images.

When the transfer of the four-color toner images is completed, the recording material is discharged by the separation charge-eliminating chargers 13 and 14, the gripper 7 releases the grip, and then the recording material is separated from the transfer drum 9 by the separation claw 15 and then conveyed. Belt 16 to fixer 17
Sent to. The recording material sent to the fixing device 17 is heated and pressed there to fix the toner image, and the toner image is mixed and fixed to the recording material to form a full-color permanent image. Thus, a series of full-color mode sequences is completed, and the full-color print image obtained by fixing is discharged outside the printer.

FIG. 4 shows a laser beam scanner installed in the printer of FIG. This scanner includes a semiconductor laser element 102, and this element 102 is a generator (laser driver circuit) 5 of an emission signal which is a drive signal for generating a laser beam.
00, and blinks according to the light emission signal generated by the light emission signal generator 500. The laser beam L emitted from the laser element 102 by blinking is made into substantially parallel light by the collimator lens system 103 and is incident on a rotating polygon mirror, that is, a polygon mirror 105.

By rotating the polygon mirror 105 in the direction of arrow B at a constant speed, the collimator lens system 103
The laser beam L of parallel light incident from is deflected and scanned in the direction of arrow C, which is the same as the axial direction of the photosensitive drum 3, which is the object to be scanned. The fθ lens group 100 provided in front of the polygon mirror 105 images the deflected laser beam L in a spot shape on the surface of the photosensitive drum 3, and the scanning speed thereof is constant on the surface of the photosensitive drum 3. And By scanning and exposing the photosensitive drum 3 with the laser beam L, a dot distribution electrostatic latent image is formed on the photosensitive drum 3.

Each of the developing devices 1M to 1K includes a primary charger 4
Since the reversal development is performed in which the toner charged to the same polarity as the charging polarity by the toner is attached to the bright portion potential portion of the latent image, the laser beam L exposes the area of the photosensitive drum 3 to which the toner should be attached.

In the present embodiment, the dot distribution electrostatic latent image is formed by multi-valued recording in which the minimum recording unit is one pixel using the PWM (pulse width modulation) method. This will be described. FIG. 5 is a circuit block diagram showing an example of the pulse width modulation circuit, and FIG. 6 is a timing chart showing the operation of the pulse width modulation circuit.

In FIG. 5, reference numeral 401 is a TTL latch circuit for latching an 8-bit digital image signal, and 402.
Is a level converter for converting a TTL logic level into a high-speed ECL logic level, and 403 is a D / A converter for converting an ECL logic level into an analog signal. 404 is PWM
An ECL comparator for generating a signal, a level converter 405 for converting an ECL logic level into a TTL logic level,
406 is a clock oscillator that oscillates the clock signal 2f,
Reference numeral 407 is a triangular wave signal generator that generates a substantially ideal triangular wave signal in synchronization with the clock signal 2f, and 408 is a frequency divider that divides the clock signal 2f by ½ to generate an image clock signal f. As a result, the clock signal 2f has a cycle twice that of the image clock signal f. In order to operate the circuit at high speed, ECL logic circuits are arranged everywhere.

The circuit operation of such a configuration will be described with reference to the timing chart of FIG. Signal a is clock signal 2
The signal f and the signal b indicate the image clock signal f and are associated with the image signal e as shown in the figure. Also in the triangular wave signal generator 407, in order to keep the duty ratio of the triangular wave signal at 50%, the clock signal 2f is once divided by 1/2 and then the triangular wave signal c is generated. Further, the triangular wave signal c is converted to the ECL level (0 to -1V) and becomes the triangular wave signal d.

On the other hand, the image signal is 00h (white) to FFh.
It changes up to (black), for example, in 256 gradation levels (the symbol "h" indicates hexadecimal display). The image signal e indicates an ECL voltage level obtained by D / A converting some image signal values. For example, the first pixel has the highest density pixel level of FFh, the second pixel has the halftone level of 80h, and the third pixel has the halftone level of 4h.
0h, the 4th pixel shows each voltage of 20h of the halftone level.

The comparator 404 compares the triangular wave signal d and the image signal e to obtain a PW having a pulse width (time length) T, t ₂ , t ₃ , t _4, etc. according to the pixel density to be formed.
Generate the M signal. This signal becomes narrower as the pulse width corresponding to the low-density pixel increases. Then, this PWM signal is converted into a TTL level of 0V or 5V to become a PWM signal f, which is input to the laser driver circuit 500. By changing the exposure time per pixel in accordance with the PWM signal value obtained in this way, 256 gradations can be obtained for one pixel.

The beam effect h in FIG. 6 shows the laser beam exposure area shape of the photosensitive drum corresponding to each drive pulse width. The area shape of each dot latent image also substantially corresponds to this exposure area shape. In FIG. 6, for the signal waveforms a to g, the horizontal axis indicates time, and for h, the horizontal axis indicates the distance in the beam scanning direction.

FIG. 7 is a sectional view showing an embodiment of the developing device of the present invention. The developing device of this embodiment includes a developing container 18 partitioned by a partition wall 19 into a developing chamber (first chamber) R1 and a stirring chamber (second chamber) R2, and a toner storage chamber R3 is provided above the stirring chamber R2. The toner storage chamber R3 contains a reinforcing toner (non-magnetic toner) 20. A replenishment port 21 is provided below the toner storage chamber R3, and the reinforcing toner 20 drops and is replenished into the stirring chamber R2 through the replenishment port 21 in an amount commensurate with the toner consumed in the development. In the developing chamber R1 and the agitating chamber R2, the two-component developer 22 in which the toner and the magnetic carrier are mixed is contained.

As the toner, a known toner obtained by adding a coloring material, a charge control agent and the like to a binder resin can be used, and the volume average particle diameter thereof is preferably in the range of 5 to 15 μm. The volume average particle diameter of the toner is measured, for example, by the following measuring method.

Call counter TA-II as a measuring device
A model (manufactured by Coulter) was used, and an interface (manufactured by Nikkaki) that outputs a number distribution and a volume distribution and a CX-i personal computer (manufactured by Canon) were connected. First-grade sodium chloride was used as the electrolyte and 1% NaC
1 aqueous solution was prepared.

A surfactant, preferably 0.1 to 5 ml of alkylbenzene sulfonate is added as a dispersant to 100 to 150 ml of the above-mentioned electrolytic solution, and 0.5 to 50 mg of a toner measurement sample is further added and suspended. Then, the electrolytic solution in which the sample is suspended is dispersed by an ultrasonic disperser for about 1 to 3 minutes, and then 100 μm is obtained by the above-mentioned call counter.
The particle size distribution of particles having a size of 2 to 40 μm is measured using the aperture of, and the volume distribution of the toner is obtained therefrom. This volume distribution gives the volume average particle size of the sample.

On the other hand, as the magnetic carrier, those having an extremely thin resin coating on the surface of magnetic particles can be preferably used, and the average particle diameter is preferably 5 to 70 μm.
The average particle size of this carrier is shown by the maximum chord length in the horizontal direction, and the measurement method was based on microscopy. The average particle size of the carrier can be obtained by randomly selecting 300 or more carriers, measuring the diameters and arithmetically averaging the diameters.

In the present developing device, a conveying screw 23 is installed in the developing chamber R1 of the developing container 18, and the rotation of the conveying screw 23 stirs the developer in the developing chamber R1 while the length of the developing sleeve 25 is increased. It is conveyed in the direction. Conveyor screw 24 also in stirring chamber R2
Is installed, and the developer in the agitating chamber R2 and the toner supplied thereto are agitated by the rotational driving of the conveying screw 24, and are conveyed in the longitudinal direction of the developing sleeve 25. The conveying direction by the conveying screw 24 is opposite to that by the screw 23.

The partition 19 of the developing container 18 has openings on the front side and the back side, and the developer conveyed by the screw 23 is transferred to the screw 24 through one of the openings and conveyed by the screw 24. The developed developer is transferred to the screw 23 through the other one opening, and is conveyed while being stirred as described above. Due to the friction with the magnetic carrier due to this stirring, the toner is charged to the polarity for developing the latent image.

An opening is provided in a portion of the developing container 18 near the photosensitive drum 3, and the developing sleeve 25 is installed in this opening. The developing sleeve 25 is
It is made of a non-magnetic material such as aluminum or non-magnetic stainless steel.

The developer 22 contained in the developing container 18
Is carried on the developing sleeve 25 by a magnet 29 installed in the developing sleeve 25, and the developing section 26 facing the photosensitive drum 3 by rotating the developing sleeve 25 in the direction of arrow b.
Be transported to. Then, a magnetic brush is formed in the developing unit 26 and comes into contact with the photosensitive drum 3 rotating in the arrow direction b,
The electrostatic latent image formed on the photosensitive drum 3 is developed.

At the time of development, a vibration bias voltage in which a DC voltage is superimposed on an AC voltage is applied to the developing sleeve 25 as a developing bias by the power source 27. The dark portion potential (non-exposed portion potential) and the bright portion potential (exposed portion potential) of the latent image are located between the maximum value and the minimum value of the vibration bias voltage. As a result, an alternating electric field whose direction changes alternately is formed in the developing section 26. In this alternating electric field, the toner and the carrier of the developer violently vibrate, the toner shakes off the electrostatic restraint to the developing sleeve 25 and the carrier and flies to the photosensitive drum 3, and adheres to the photosensitive drum 3 corresponding to the latent image. To do.

The difference between the maximum and minimum values of the vibration bias voltage (peak-to-peak voltage) is preferably 1 to 5 kV, and the frequency is 1.
-10 kHz is preferable. The waveform of the vibration bias voltage is
Square wave, sine wave, triangle wave, etc. can be used. The direct current voltage component of the vibration bias is a value between the dark part potential and the bright part potential as described above, but a value closer to the dark part potential than the minimum absolute bright part potential leads to the dark part potential region. It is preferable for preventing the adhesion of the fog toner.

The minimum gap between the developing sleeve 25 and the photosensitive drum 3 (minimum gap in the developing section 26) is preferably 0.2 to 1 mm.

A developer layer thickness regulating blade 28 is installed above the developing sleeve 25. The regulating blade 2
8 is a developer 22 which the developing sleeve 25 conveys to the developing section.
Layer thickness, that is, the amount of the developer 22 is regulated. The amount of the developer conveyed to the developing sleeve 25 depends on the developing magnetic pole S of the magnet 29.
The height of the developer formed in the developing unit 26 by 1 from the surface of the developing sleeve of the magnetic brush is 1.2 to the minimum gap of 0.2 to 1 mm when the photosensitive drum 3 is removed.
The amount is preferably 3 times.

The magnet 29 in the developing sleeve 25 is formed in a roller shape, and is arranged concentrically and non-rotatably in the developing sleeve 25. The developing magnetic pole S1 of the magnet 29 is located in the developing section 26, and the developing magnetic field formed in the developing section 26 by the developing magnetic pole S1 forms a magnetic brush on the developer on the developing sleeve 25. The magnetic brush contacts the photosensitive drum 3, and the toner in the magnetic brush is transferred to the dot distribution electrostatic latent image formed on the photosensitive drum 3 to develop the latent image. The toner in the magnetic brush adheres to the brush (brush) of the magnetic carrier or adheres to the surface of the developing sleeve 25, but any toner can be transferred to the exposed portion of the latent image.

The strength of the developing magnetic field by the developing magnetic pole S1 on the surface of the developing sleeve 25 (the magnetic flux density in the direction perpendicular to the surface of the developing sleeve) preferably has a peak value of 500 to 2000 gauss.

In this example, the magnet 29 has magnetic poles N1, N2, N3 and S2 in addition to the developing magnetic pole S1. As the developing sleeve 25 rotates, the developer 22 in the developing container 18 is drawn up by the magnetic pole 2 onto the developing sleeve 25, and the drawn up developer is conveyed from the magnetic pole S2 to N1, and the regulating member 28 in the middle thereof. Regulated by
A thin layer of developer 22 is formed. And the developing sleeve 2
The toner is conveyed to the developing unit 26 with the rotation of 5, and as described above, forms a magnetic brush in the magnetic field of the developing magnetic pole S1 and is used for the electrostatic latent image on the photosensitive drum 3.

After the development is completed, the developer is used as the developing sleeve 2.
When the magnetic poles N3 and N
The repulsive magnetic field between the two causes the stirring chamber R to rise from above the developing sleeve 25.
It falls within 1 and is collected. The collected developer is conveyed by the stirring screws 23 and 24 as described above.

As a result of a study using the developing device as described above, the density of the magnetic brush of the developer (developing sleeve 2
5) Increase the number of magnetic brushes per surface unit area,
Moreover, when the toner density is increased, a good halftone image can be obtained in the entire density area of the dot distribution latent image.

The density of the magnetic brush can be increased by decreasing the strength of magnetization of the magnetic carrier in the developing section.

The magnetic characteristics of the magnetic carrier are as follows: Riken Denshi Co., Ltd., DC magnetization BH characteristic automatic recording apparatus BHH-50
Can be measured using. At the time of this measurement, a cylindrical container (inner diameter 6.5 mm, height 10 mm) is filled with a carrier by applying a load of about 2 g, and the strength of the magnetization is measured while the carrier does not move in the container. It is important to do so.

In this embodiment, the developing magnetic pole S of the magnet 9 is used.
As 1, the peak value of the magnetic flux density in the normal direction on the surface of the developing sleeve was 1000 gauss. Therefore, the value of the magnetization of the carrier when the magnetic force was 1000 gauss and the magnetic brush in the developing section. The relationship of the density of was investigated. The result is shown in FIG.

As shown in FIG. 8, the carrier magnetization σ ₁₀₀₀ (e
mu / cm ³ ) and the density of magnetic brush ears α (books /
mm ² ) is in an inversely proportional relationship, and when the value on the curve of the inversely proportional curve is read from FIG. 8 and expressed by an equation, α × σ
₁₀₀₀ = 600. As can be seen from this, the smaller the carrier magnetization σ, the higher the magnetic brush ear density α.

In this example, a development experiment was conducted in which the toner particle size, the carrier particle size, the toner concentration and the strength of magnetization of the carrier at the developing portion were changed, and the obtained image was evaluated. The results are shown in Tables 1 to 5.

In this experiment, the recording density was set to 2 in the sub-scanning direction (moving direction of the photosensitive drum surface, that is, the photosensitive drum circumferential direction).
00 dpi, 400 dpi in the main scanning direction (beam scanning direction, that is, the axial direction of the photosensitive drum).

[0057]

[Table 1]

[0058]

[Table 2]

[0059]

[Table 3]

[0060]

[Table 4]

[0061]

[Table 5]

In Tables 1 to 5, the meanings of the symbols are as follows.

Glossiness term: A: Very smooth and smooth image quality B: Smooth and smooth image quality C: Smooth and smooth image quality D: Conspicuous smoothness E: Crusty Contrast is very noticeable Concentration term: A: Very high concentration B: High concentration C: Normal concentration D: Low concentration E: Very low concentration

In Tables 1 to 5, s is represented by the following formula and is a kind of surface area ratio as shown below, which is unitless.

[0065] _{s = n · r c · ρ} c / (100-n) · r t · ρ t However, r _t: toner diameter (μm), r _c: Carrier diameter (μm) ρ _t: toner bulk density ( g / cm ³ ), ρ _c : Carrier bulk density (g / cm ³ ) n: Amount of toner in developer (wt%)

As can be seen from Tables 1 to 5, the carrier magnetization intensity is 100 emu / cm ² or less and s is 2.
When it is 0 or more, it is possible to obtain a high-quality image in which the prevention of rubbing is good and the density is high.

This s corresponds to the ratio of the total surface area of the toner and the total surface area of the carrier, which is calculated by using the toner concentration for the surface area of the toner and the surface area of the carrier. Even if the toner concentration is the same, the developing efficiency naturally changes, for example, if the toner diameter or the carrier diameter changes. However, if the ratio s of the total surface area of the toner and the carrier is equal, the developing efficiency becomes substantially the same. That is, the ratio s corresponds to the toner density in which the toner diameter and the carrier diameter are taken into consideration.

Originally, regarding the shakyness of an image, the recording density (the distribution density of dot-shaped latent images, that is, the distribution density of pixels) is concerned. When the recording density is low, it is required that the ears of the magnetic brush have a high density. However, with respect to the density, when the density of the ears becomes high, the density is slightly decreased, and this does not change even when the recording density is low.

As can be seen from the results of Tables 1 to 5, the image density is improved and the roughness is eliminated by increasing s to secure the toner density and increasing the density of the magnetic brush ears. I was able to. Further, when various recording densities were examined, the same result was obtained regardless of the difference in the recording densities.

As described above, the developing efficiency is improved by increasing s with respect to the toner concentration, but the strength of magnetization is 150.
As can be seen from Table 1 at emu / cm ³ , the rubbing becomes worse. However, the strength of magnetization is 100 em
When u / cm ³ or less, the increase in s is favorable for rubbing. This is because if the ears of the magnetic brush are rough,
On the contrary, when the toner concentration is increased, the unevenness of the ears becomes conspicuous. On the contrary, when the ears are made dense, it is considered that the improvement of the developing property due to the increase of the toner concentration improves the dryness.
Due to this synergistic effect, the strength of carrier magnetization is 100e.
Under the condition of mu / cm ³ or less and s of 2.0 or more, it is possible to obtain an image with good prevention of rubbing and high density regardless of the recording density.

On the other hand, when the magnetization intensity of the carrier at the peak value of the developing magnetic field is less than ³⁰ emu / cm ³ , the developer transportability on the developing sleeve is poor and the image quality of the developed image deteriorates, The lower limit is preferably ³⁰ emu / cm ³ or more because the agent is easily scattered.

If the recording density is less than 60 pixels / mm ^{2, the} resolution is not good. Therefore, the present invention is preferably applied to the case of 60 pixels / mm ² or more. On the other hand, if the recording density is higher than 10,000 pixels / mm ² , it becomes difficult to reproduce the dot image with the dry toner. Therefore, in the present invention, the maximum density is preferably 10,000 pixels / mm ² or less.

Above, the peak value d of the developing magnetic field is 1000.
Although the condition was Gauss, similar results were obtained even when the peak value d was other than 1000 Gauss. Peak value d is 50
Magnetization strength σ _d (emu / cm ³ ) at 0, 800, 1500, 2000 gauss and magnetic brush density α
The relationship of (pieces / mm ² ) is shown in FIG. In either case,
It can be seen that the relationship of σ _d × α = 600 is satisfied.

Therefore, the density of the brush ears of the magnetic brush and the anti-rubbing property of the image in which the dot distribution latent image is developed do not depend on the peak value d of the developing magnetic field, but of the carrier in a magnetic field of strength d. It can be seen that it depends on the strength of magnetization σ _d .

Example 2 In the above examples, the magnetic carrier shown in FIG.
A carrier having a hysteresis characteristic shown in, that is, a soft ferromagnetic carrier (soft magnetic ferromagnetic carrier) was used,
In this example, a carrier having a hysteresis characteristic as shown in FIG. 11, that is, a hard ferromagnetic carrier (hard magnetic ferromagnetic carrier) was used.

Since the hard ferromagnetic carrier as shown in FIG. 11 has the coercive force Hc and the residual magnetization σ _r , the magnetization remains even when the external magnetic field is weakened (away from the developing section). Since the attractive force between the carriers becomes strong, it is more advantageous than the soft ferromagnetic carrier in preventing the carrier adhesion phenomenon in which the carrier adheres to the image portion and the image is disturbed.

In this embodiment, only the carrier of the developer is changed, and the latent image forming method (pulse width modulation method) and the apparatus structure are the same as those in the embodiment 1 using the soft ferromagnetic carrier.

The magnetic carrier has a coercive force Hc of about 2
000 oersteds, but several types of hard ferromagnetic carriers having different magnetization values σ ₁₀₀₀ and remanent magnetization σ _{r at} a magnetic force of 1000 gauss were tried. 12 shows the density of the ears when the ears of the magnetic brush are formed in the developing portion by using the developing magnetic pole S1 having the peak value d of 1000 gauss for the two-component developer using these carriers.

As shown in FIG. 12, also in the case of a hard ferromagnetic carrier, as in the case of the above-mentioned soft ferromagnetic carrier,
The formula α × σ ₁₀₀₀ = 600 holds.

Further, as shown in FIG. 13, even if the residual magnetization σ _r of the hard ferromagnetic carrier is different, α × σ _r = 600
The relationship is met. That is, it is understood that the density of the magnetic brush ears does not depend on the residual magnetization of the carriers but on the strength of the carrier magnetization in the peak magnetic field d.

With the above, the peak value d of the developing magnetic field is 100.
Although it was 0 gauss, the same result was obtained when the peak value d was other than 1000 gauss. That is, in any of the peak values d of 500, 800, 1500, and 2000 gauss, the relationship of σ _d × α = 600 is satisfied.

In the present invention, the magnetic carrier (soft ferromagnetic carrier, hard ferromagnetic carrier) contains ferrite as a main component, and the periodic table IA, IIA, IIIA, IVA, V
It contains at least one element selected from the group consisting of A, IB, IIB, IVB, VIB, VIIB, and VIII.
For example, Ni-Zn system, Li system, Li-Zn system, Mn-C
u-based ferrite can be used. The magnitude of the magnetization intensity σ _d of the carrier at the peak value d of the developing magnetic field can be adjusted by, for example, the carrier composition. Of course, the carrier material is not limited to the above.

The present invention can be applied to a device which expresses gradation by the dither method.

[0084]

As described above, according to the present invention,
It is possible to develop a dot-distributed electrostatic latent image into an image having a good gradation from a low density to a high density area without being rusted.

[Brief description of drawings]

FIG. 1 is an overall configuration diagram showing an example of an image forming apparatus to which a developing device of the present invention can be applied.

FIG. 2 is an explanatory diagram showing a potential of a dot-shaped latent image formed on a photosensitive drum of the image forming apparatus of FIG.

FIG. 3 is an explanatory diagram showing a developed image of a dot-shaped latent image.

FIG. 4 is an explanatory diagram showing a laser beam scanner installed in the image forming apparatus of FIG.

5 is a circuit block diagram showing an example of a pulse width modulation circuit used in the scanner of FIG.

6 is a timing chart showing an operation of the pulse width modulation circuit of FIG.

FIG. 7 is a sectional view showing an embodiment of the developing device of the present invention.

8 is a graph showing the relationship between the carrier magnetization of the two-component developer used in the developing device of FIG. 7 and the density of the magnetic brush ears.

FIG. 9 is a graph showing the relationship of carrier magnetization with respect to the strength of the peak value of the developing magnetic field.

FIG. 10 is a graph showing a hysteresis curve of a soft ferromagnetic carrier.

FIG. 11 is a graph showing a hysteresis curve of a hard ferromagnetic carrier.

FIG. 12 is a graph showing the relationship between the magnetization of soft and hard ferromagnetic carriers and the density of magnetic brush ears.

FIG. 13 is a graph showing the relationship between the magnetization of a hard ferromagnetic carrier and the density of magnetic brush ears.

[Explanation of symbols]

 3 Photosensitive Drum 18 Development Container 22 Two-Component Developer 25 Development Sleeve 27 Bias Power Supply 29 Magnet S1 Development Magnetic Pole

Claims (3)

[Claims] 1. A two-component developer having a toner and a magnetic carrier is carried on a developer carrying body containing a magnet and conveyed to a developing section facing the image carrying body, and the magnet is developed in the developing section. A magnetic brush of developer is formed by the developing magnetic field of the magnetic pole, the magnetic brush is brought into contact with the image carrier, and the dot distribution electrostatic latent image formed corresponding to the recorded image signal on the image carrier is developed. In the developing device, the magnetic carrier has a magnetization intensity of 100e when a magnetic field having a peak value of the vertical magnetic field on the surface of the developer carrier of the developing magnetic pole is applied.
mu / cm ³ or less, and the developer has a toner diameter of r _t μm, a carrier diameter of r _c μm, and a toner bulk density of ρ.
_t g / cm ^3, the carrier bulk density ρ _c g / cm ^3, when the amount of toner in the developer was nwt%, s = n · r c · ρ c / (100-n) · r t · A developing device characterized in that s of ρ _t is 2.0 or more. 2. The developing device according to claim 1, wherein an oscillating bias voltage is applied to the developer carrying member during the development of the dot distribution electrostatic latent image. 3. The image bearing member is an electrophotographic photosensitive member,
3. The developing device according to claim 1, wherein the dot-distributed electrostatic latent image is formed by exposing the electrophotographic photosensitive member with a light beam modulated by pulse width modulation corresponding to the density of the recorded image.