JP3093512B2 - Developing device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a developing method for developing an electrostatic latent image using a developer having a toner and a magnetic carrier, that is, a so-called two-component developer.

[0002]

2. Description of the Related Art An electrophotographic photosensitive member is scanned and exposed by a laser beam modulated in accordance with an image signal to be recorded, and a dot distribution electrostatic latent image, that is, a dot-like latent image is distributed corresponding to the image. An image forming method for forming an electrostatic latent image is known.

Among them, the so-called pulse width modulation (PWM) method, which modulates the width of a driving pulse current of a laser (that is, the duration time) in accordance with the density of a recorded image, has a high recording density (that is, a high recording density). Resolution) and a high gradation can be obtained.

However, an electrostatic latent image having a dot distribution is formed on a photoreceptor by using, for example, the above-mentioned PWM method.
When a magnetic brush of a component developer is brought into contact with the photoreceptor and the electrostatic latent image is reversely developed, roughness is generated in a halftone area of 0.3 or less in reflection density of a formed developed image. Oops. This roughness did not occur so much in a text document or the like, but often occurred in a low density area such as a photo document.

[0005] Then, when the cause of the occurrence of roughness was examined, the following was found.

When a latent image in a low density portion is formed by a normal dot distribution latent image, the latent image on the photosensitive member is
It is not a broad latent image like an analog latent image but has a two-dimensional distribution of local dot-like latent images as shown in FIG. When trying to reproduce a low density, the dot-shaped latent image becomes dull and the maximum contrast V
₀ (the difference between the potential of the non-exposed portion and the absolute value of the absolute value in the dot-shaped latent image) gradually decreases. For example, an attempt to reproduce the image of the degree of reflection density 0.2, V ₀ of the dot-like latent image at that time, becomes approximately 150～200V.

On the other hand, in the case of reversal development in which toner is adhered to a light-exposed portion of a photoreceptor, in order to prevent fogging, the DC voltage component of the vibration developing bias voltage is higher than the surface potential of the non-exposed portion (non-image portion). Is set to be 100 to 200 V lower in absolute value, the potential difference Vcont between the potential of the light exposure portion of the dot-like latent image and the DC voltage component of the developing bias when V ₀ is 150 to 200 is about ₀ to 50 V Become. When Vcont is 0 to 50 V, the contrast is very unstable whether the toner adheres to the photoconductor side or remains on the developer carrier side. Therefore, when the dot-like latent image is developed by the two-component developer, the state of contact of the magnetic brush with the photoreceptor greatly contributes to the development efficiency. Roughness (distribution of fine unevenness in density) is likely to occur.

FIG. 3 shows this. In FIG. 3, P indicates one pixel. A dot-like latent image L1 corresponding to a low-density image is applied to each pixel P by a laser beam modulated by the PWM method.
To L5. D1 to D4 are dot latent images L
1 to L4 indicate toner-attached areas, that is, developed areas.

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

As described above, the two-dimensional distribution of the defective developed image of the dot-shaped latent image makes it possible to see the low density area in a noticeable manner. In particular, a color image is formed by superimposing a plurality of color toners. In this case, the roughness is particularly conspicuous, and deteriorates the image quality.

It has been proposed in Japanese Patent Application No. 173253/1992 to use magnetic carrier particles having a small magnetization in a developing magnetic field in order to prevent the above roughness. By using the magnetic carrier particles having a small magnetization in the developing magnetic field, the density of the magnetic brush is increased, the occurrence of the above-mentioned defective developed image can be prevented, and a low-density image without roughness can be formed.

However, when the magnetic field having the maximum value of the vertical magnetic field on the surface of the developer carrier of the developing magnetic pole is applied, the magnetization becomes 10%.
A magnetic carrier of 0 emu / cm ³ or less, and
When a magnetic carrier whose soft magnetic material is a soft magnetic material (a magnetic material having a coercive force of 125 Oe or less) is used, the magnetic carrier adheres to the image carrier and is removed from the developer carrier. The phenomenon of adhesion is likely to occur.

This is because, in the above carriers, the magnetic coupling force between the carriers and the magnetic restraining force on the developer carrier are weak, and the electrostatic adhesion force on the image carrier is reduced by the magnetic force. This is because as a result of overcoming, the amount of carrier particles taken out of the developing unit while being attached to the image carrier increases.

In any case, when such carrier adhesion occurs, the transfer of the developed image to the transfer material is inhibited in the transfer section, thereby deteriorating the image quality.
Inconveniences such as rubbing and damage of the image carrier occur.

[0015]

SUMMARY OF THE INVENTION The object of the present invention is to
When the magnetic field having the maximum strength of the perpendicular magnetic field of the developing magnetic pole is applied on the surface of the developer carrying member, the strength of magnetization is 10
An object of the present invention is to provide a developing method capable of preventing a carrier from adhering to an image carrier when a developer having a magnetic carrier of 0 emu / cm ³ or less and a toner is used.

[0016]

The main object of the present invention is to provide a soft magnetic material having a magnetization intensity of 100 em when a maximum magnetic field of a vertical magnetic field of a developing magnetic pole is applied on the surface of a developer carrier.
u / cm ³ or less of a developer having a magnetic carrier and a toner is carried by the developer carrying member, and the layer thickness is regulated by a layer thickness regulating member. In a developing method of developing an electrostatic latent image formed on the image carrier by bringing a magnetic brush of the developer into contact with an image carrier in a development magnetic field formed by the developing magnetic pole in the developing section, the developer The layer thickness is set such that the free standing of the magnetic brush at the closest portion between the carrier and the image carrier is 2.0 times or less the closest distance between the developer carrier and the image carrier. A developing method characterized in that the electrostatic latent image is developed by regulating the layer thickness of the developer by a regulating member.

Here, the free standing of the magnetic brush is as follows.
The height of the magnetic brush closest to the developer carrier and the image carrier when the image carrier is removed. In other words, in the case where there is no image carrier, the length of the magnetic brush of the developer formed on the developer carrier at the position corresponding to the closest portion is referred to as a magnetic brush in this specification. Let's say it is free standing. Therefore, in the process of developing an electrostatic latent image, the thickness of the magnetic brush (thickness of the developer layer) at the closest part is naturally reduced to the distance between the image carrier and the developer carrier. It is compressed, not free standing.

[0018]

FIG. 4 shows an electrophotographic color printer to which the present invention can be applied. This printer includes an electrophotographic photosensitive drum 3 that rotates in the direction of an arrow. Around the photosensitive drum 3, a charger 4 and developing devices 1M, 1C, 1Y, and 1Bk
The image forming means includes a rotary developing device 1, a transfer discharger 10, a cleaning means 12, and a laser beam scanner LS disposed above the photosensitive drum 3 in the drawing. Each developing device supplies a two-component developer containing toner particles and carrier particles to the drum 3. The developer of the developing device 1M contains magenta toner, the developer of the developing device 1C contains cyan toner, the developer of the developing device 1Y contains yellow toner, and the developer of the developing device 1Bk contains black 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, for converting a document 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 document, respectively. I do.
The semiconductor laser incorporated in the scanner LS is controlled according to these image signals and emits a laser beam L. The output signal from the computer can be printed out.

The sequence of the entire color printer will be briefly described by taking a full color mode as an example. First, the photosensitive drum 3 is uniformly charged by the charger 4. Next, scanning exposure is performed by the laser light L modulated by the magenta image signal, and a dot distribution electrostatic latent image is formed on the photosensitive drum 3, and this latent image is formed by a magenta developing device fixed at a developing position in advance. Reversal development by 1M.

On the other hand, a transfer material such as paper taken out of the cassette C and advanced through the paper feed guide 5a, the paper feed roller 6, and the paper feed guide 5b is held by the gripper 7 of the transfer drum 9 and is brought into contact with the transfer material. Is electrostatically wound around the transfer drum 9 by the use roller 8 and its opposite pole. The transfer drum 9
It rotates in the direction shown by the arrow in synchronization with the photosensitive drum 3, and
The magenta visual image developed by the magenta developing device 1M is transferred to a transfer material by a transfer charger 10 in a transfer section. The transfer drum 9 continues to rotate as it is to prepare for transfer of an image of the next color (cyan in FIG. 1).

On the other hand, the photosensitive drum 3 is neutralized by the charger 11 and is cleaned by the cleaning means 12. The photosensitive drum 3 is charged again by the charger 4 and is irradiated with the laser beam L modulated by the next cyan image signal. Upon exposure, an electrostatic latent image is formed. During this time, the developing device 1 rotates, and the cyan developing device 1C is fixed at a predetermined developing position, and performs reversal development of the dot distribution electrostatic latent image corresponding to cyan to form a cyan visible image.

Subsequently, the above-described steps are performed on the yellow image signal and the black image signal, respectively, and when the transfer of the four-color visualized image (toner image) is completed, the transfer material is charged into each of the chargers 13 and 14. , The gripper 7 is released, the toner is separated from the transfer drum 9 by the separation claw 15, and the fixing device (heat-pressure roller fixing device) 1 is conveyed by the transport belt 16.
7 The fixing device 17 fixes the visible images of four colors overlapping on the transfer material. Thus, a series of full-color print sequences is completed, and a required full-color print image is formed.

In FIG. 5, the semiconductor laser element 102 is connected to a laser driver 500 which is a light emission signal generator for sending a light emission signal (drive signal) for generating a laser beam, and responds to the light emission signal of the laser driver. Flickers. Laser beam L emitted from laser element 102
Are made substantially collimated light by the collimator lens system 103.

A polygon mirror, ie, a rotating polygon mirror 105
Scans parallel light emitted from the collimator lens system 103 in the direction of arrow C by rotating at a constant speed in the direction of arrow B. F-θ provided in front of the rotating polygon mirror 105
The lens group 100 forms a laser beam deflected by the polygon mirror 105 into a spot on the surface to be scanned, that is, the photosensitive drum 3, and makes the scanning speed constant on the surface to be scanned.

By performing the scanning exposure of the photoconductor with the laser beam L, an electrostatic latent image having a dot distribution is formed on the photoconductor 3.

Each of the developing units performs reversal development in which toner charged to the same polarity as the charging polarity of the charger 4 is applied to the bright portion potential portion of the latent image. Expose the area to be exposed.

In the present embodiment, the PWM method (pulse width modulation) is used for multi-value recording in which the minimum recording unit is one pixel, so that the PWM method will be briefly described.

FIG. 6 is a circuit block diagram showing an example of the pulse width modulation circuit, and FIG. 7 is a timing chart showing the operation of the pulse width modulation circuit.

In FIG. 6, reference numeral 401 denotes a TTL latch circuit for latching an 8-bit digital image signal, and 402 denotes a TTL latch circuit.
A level converter 403 for converting a logic level to a high-speed ECL logic level, and a D / A converter 403 for converting an ECL logic level to an analog signal. 404, an ECL comparator for generating a PWM signal; 405, a level converter for converting an ECL logic level to a TTL logic level;
407 is a clock oscillator for starting the clock signal 2f.
Is a triangular wave generator that generates a substantially ideal triangular wave signal in synchronization with the clock signal 2f.
This is a half-period in which the image clock signal f is generated by dividing the frequency by two. As a result, the clock signal 2f has a period twice as long as the image clock signal f. Note that ECL logic circuits are provided everywhere in order to operate the circuit at high speed.

The circuit operation having such a configuration will be described with reference to the timing chart of FIG. The signal a indicates the clock signal 2f and the signal b indicates the image clock signal f, which is related to the image signal as shown in the figure. Also, inside the triangular wave generator 407, the clock signal 2f is once frequency-divided by か ら to generate the triangular wave signal c in order to keep the duty ratio of the triangular wave signal at 50%. Further, this triangular wave signal c is converted into an ECL level (0 to -1 V) to become a triangular wave signal d.

On the other hand, the image signal is from 00h (white) to FFh
(Black), for example, at 256 gradation levels. The symbol 'h' indicates hexadecimal notation. The image signal e indicates the ECL voltage level obtained by D / A converting some image signal values. For example, the first pixel indicates a voltage of FFh of the highest density pixel level, the second pixel indicates a voltage of 80h of the halftone level, the third pixel indicates a voltage of 40h of the halftone level, and the fourth pixel indicates a voltage of 20h of the halftone level.

The comparator 404 compares the triangular wave signal d with the image signal e to obtain a pulse width (time length) T, t ₂ , t ₃ , t _4, etc. corresponding to the pixel density to be formed.
Generate a WM signal. The pulse width corresponding to the low density pixel becomes narrower. And this PWM signal is 0V or 5V T
The signal is converted to a TL level and becomes a PWM signal f, which is input to the laser driver circuit 500. By changing the exposure time per pixel corresponding to the PWM signal value obtained in this way, it is possible to obtain 256 gradations with one pixel.

7h shows the laser beam exposure area shape of the photosensitive member corresponding to each drive pulse width. The area shape of each dot latent image substantially corresponds to this exposure area shape.

In FIG. 7, for the signal waveforms a to g, the horizontal axis represents time, and for h, the horizontal axis represents the distance in the beam scanning direction.

Each of the developing devices 1M to 1Bk for visualizing the dot distribution electrostatic latent image formed on the photosensitive drum 3 has a developer container 18 as shown in FIG.

The inside of the developer container 18 is partitioned into a developing chamber (first chamber) R1 and a stirring chamber (second chamber) R2 by a partition wall 19, and a toner storage chamber R3 is formed above the stirring chamber R2. A replenishment toner (non-magnetic toner) 20 is stored in the toner storage chamber R3. The toner storage room R3
Is provided with a supply port 21, and an amount of supply toner 20 corresponding to the toner consumed in the development is dropped and supplied into the stirring chamber R2 through the supply port 21.

On the other hand, a developer 22 in which the toner particles and the magnetic carrier particles are mixed is contained in the developing chamber R1 and the stirring chamber R2.

As the toner, a known toner obtained by adding a coloring material, a charge control material and the like to a binder resin can be used, and a toner having a volume average particle of 5 to 15 μm can be suitably used. Here, as the volume average particle diameter of the toner, for example, the one measured by the following measurement method is used.

As a measuring device, Coulter Counter T
A-II type (manufactured by Coulter), number average distribution,
Interface to output volume average distribution (made by Nikkaki)
And a CX-i personal computer (manufactured by Canon Inc.), and the electrolyte was 1% NaCl using primary grade sodium chloride.
Prepare a Cl aqueous solution.

The measuring method is as follows.
In 150 ml, 0.1 to 5 ml of a surfactant (preferably an alkylbenzene sulfonate) is added as a dispersant,
Further, 0.5 to 50 mg of a measurement sample was added.

The electrolytic solution in which the sample was suspended was subjected to dispersion treatment for about 1 to 3 minutes by an ultrasonic disperser, and the particle size of 2 to 40 μm was measured using the Coulter Counter TA-II using a 100 μm aperture as an aperture. Measure the distribution and determine the volume distribution.

From the obtained volume distribution, the volume average particle size of the sample is obtained.

On the other hand, as the magnetic carrier, those obtained by applying a very thin resin coating on the surface of magnetic particles are preferably used, and the average particle diameter is preferably 5 to 70 μm. The average particle size of the carrier is indicated by the maximum chord length in the horizontal direction. The measuring method is to randomly select 300 or more carriers by microscopy, measure the diameter, and take the arithmetic average to obtain the average particle size of the present invention. did.

In the developing chamber R1, the conveying screw 2
3 are accommodated. The developer 22 in the developing chamber R1 is transferred to the developing sleeve 25 by the rotation of the transport screw 23.
Is conveyed in the longitudinal direction.

A transport screw 24 is accommodated in the storage room R2. The transport screw 24 transports the toner along the longitudinal direction of the developing sleeve 25 by its rotation. The developer conveying direction by the screw 24 is opposite to that by the screw 23.

The partition wall 19 is provided with openings on the near side and the back side, and the developer conveyed by the screw 23 is delivered to the screw 24 from one of the openings, and the developer conveyed by the screw 24. Is delivered to the screw 23 from the other one of the openings.

The toner is charged to a polarity for developing a latent image by friction with the magnetic carrier particles.

An opening is provided in a portion of the developer container 18 close to the photosensitive drum 3, and a non-magnetic developing sleeve 25 made of aluminum, non-magnetic stainless steel or the like is provided in the opening.

The developing sleeve 25 rotates in the direction of arrow b to carry and transport the developer in which the toner and the carrier are mixed to the developing section 26. The magnetic brush of the developer carried on the sleeve 25 contacts the photoreceptor 3 rotating in the direction of arrow a in the developing section 26, and the electrostatic latent image is developed in the developing section 26.

The power supply 27 supplies a power to the sleeve 25.
An oscillation bias voltage obtained by superimposing a DC voltage on an AC voltage is applied. The dark portion potential (non-exposed portion potential) and the light 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 unit 26. In this alternating electric field, the toner and the carrier vibrate violently, and the toner shakes off the electrostatic restraining force on the sleeve and the carrier and adheres to the drum 3 corresponding to the latent image.

The difference (peak-to-peak voltage) between the maximum value and the minimum value of the vibration bias voltage is preferably 1 to 5 kV, and the frequency is preferably 1 to 10 KHz. As the waveform of the oscillation bias voltage, a rectangular wave, a sine wave, a triangular wave, or the like can be used.

The DC voltage component has a value between the dark portion potential and the bright portion potential of the latent image, and may have a value closer to the dark portion potential than the minimum bright portion potential in absolute value. This is preferable for preventing fog toner from adhering to the dark portion potential region.

The minimum gap between the sleeve 25 and the photosensitive drum 3 (the minimum gap position is in the developing section 26) is 0.1 mm.
Preferably it is 2 to 1 mm.

Reference numeral 28 denotes a developer layer thickness regulating blade, which regulates the layer thickness of the two-component developer carried and transported by the sleeve 25 to the developing section 26. The developing unit 26 is regulated by the blade 28.
Amount of developer is conveyed to the magnetic brush of the developer is formed by the magnetic field in the developing portion by the developing magnetic pole S ₁ described later,
It is preferable that the free height on the sleeve surface at the minimum gap between the sleeve and the drum is 1.2 to 2 times the minimum gap value between the sleeve and the photosensitive drum.

The roller-shaped magnet 2 is provided in the developing sleeve 25.
9 is fixedly arranged. The magnet 29 has a developing magnetic pole S ₁ facing the developing unit 26. Developing magnetic pole S ₁ is a magnetic brush before development is formed by the developing magnetic field formed in the developing unit 26, the magnetic brush developing the dot distribution electrostatic latent image in contact with the photosensitive drum 3. At this time, the toner adhering to the ears (brushes) of the magnetic carrier and the toner adhering to the sleeve surface instead of the ears are transferred to the exposed portion of the latent image and developed.

Sleeve 2 for developing magnetic field by developing magnetic pole S ₁
It is preferable that the peak value (maximum value) of the strength on the surface (magnetic flux density in a direction perpendicular to the sleeve surface) is 500 to 2000 Gauss.

[0058] In the magnet in this example, in addition to the developing magnetic pole S _1, and a _{N 1, N 2, N 3} , S 2 pole.

With this configuration, as in the related art, the developer pumped at the N ₂ pole by the rotation of the developing sleeve 25 is transported from the S ₂ pole to the N ₁ pole, and is regulated by the regulating member 28 on the way. To form a thin layer of developer. In the pole, the developer raised in the magnetic field of the developing magnetic pole S ₁ develops the electrostatic latent image on the image carrier 3. Then N ₃ pole, the developer on the developing sleeve 25 by a repulsive magnetic field of the N ₂ machining gap falls to a stirred chamber R _1. The developer that has fallen into the stirring chamber R ₁ is stirred and transported by the screws 23 and 24.

As described in Japanese Patent Application No. 173253/1992, in order to eliminate such roughness, the density (the number of magnetic brushes per unit area) of the magnetic brush formed by the developer is reduced. It was found that it was necessary to increase the height in the developing section.

Further, it has been found that a method of increasing the density of the magnetic brush can be achieved by reducing the intensity of magnetization of the magnetic carrier in the developing section.

The magnetic characteristics of the magnetic carrier were measured by a DC magnetization BH characteristic automatic recording device BHH of Riken Denshi Co., Ltd.
-50 can be used. At this time, the diameter (inner diameter)
A cylindrical container having a height of 6.5 mm and a height of 10 mm is filled with a carrier under a load of about 2 kg, and the magnetization intensity is measured while the carrier does not move in the container.

[0063] Now, firstly, as a developing magnetic pole S _1, on the sleeve surface, the peak value of the normal direction of the magnetic flux density with respect to the sleeve surface was used a 1000 gauss, magnetic force (magnetic flux density) of 1000 FIG. 9 shows the relationship between the value of the magnetization of the carrier in the case of Gauss and the density of the magnetic brush in the developing unit.

As can be seen from FIG. 9, the relationship between the value of the carrier magnetization and the density of the ears of the magnetic brush at the peak magnetic flux density of the developing magnetic field is inversely proportional. Assuming that the ear density is α (pieces / mm ² ) and the carrier magnetization value at a magnetic force of 1000 Gauss is σ ₁₀₀₀ (emu / cm ³ ), α × σ ₁₀₀₀ = 600. That is,
the smaller the σ ₁₀₀₀ it sees the density of the ear becomes dense.

When considering the relationship between the density of the developer ears and the roughness, the recording density (the distribution density of the dot-like latent images, that is, the distribution density of the pixels) is involved. When the recording density is low, it is difficult for the ears to be rough even if the ear density is somewhat coarse. However, when the recording density is high, the ears also need to have a high density. Therefore, in the present embodiment, the recording density is set to 200 dpi, 300 dpi, 400 d in the sub-scanning direction (photoconductor moving direction).
and 600 dpi, and 200 dpi, 400 dpi, and 60 dpi in the main scanning direction (beam scanning direction).
An experiment was performed for the case where the resolution was changed to 0 dpi. Table 1 shows the relationship between the density of the magnetic brush and the roughness at each recording density.

In Table 1, the meanings of the symbols are as follows: A: Very smooth image quality without "graininess". B: Smooth image quality without "graininess". C: Smooth image quality with no noticeable “gutter”. D: "Gasatsu" is conspicuous. E: "Gasatsu" is very conspicuous.

[0067]

[Table 1]

As can be seen from the results shown in Table 1, even when the recording density was low, when the number of pixels per magnetic brush was 25 or less, the roughness was almost inconspicuous.

Further, when the ear density was 8 or more per 1 mm ² , the recording density was high, and when the number of pixels per magnetic brush was 25 or more, the roughness was hardly noticeable. This depends on the visual limit of the human eye.

FIG. 10 shows the spatial frequency ν (1 line / m
m) and the number L (density difference) of recognizable levels are shown.
(L = 10 ³ e ^-0.72ν (1-e ^-0.52ν ) +1)

In general, the fluctuation range of the density in the low density portion of the image density of 0.2 to 0.3 where the roughness is conspicuous is about 0.02, and from FIG.
When it is higher than 7 (1 line / mm), the above-described density fluctuation cannot be recognized by human eyes. In other words, the density of the magnetic brush is 7.3 (lines / mm ² )
In the case of (2.7 × 2.7 = 7.3) or more, the high frequency becomes rough for the above-mentioned reason, making it difficult to recognize. Therefore, when the ear density was 8 or more per 1 mm ² , the recording density was high, and when the number of pixels per magnetic brush was 25 or more, the roughness was hardly noticeable.

Here, assuming that the number of pixels per 1 mm ² is X, the above-mentioned relationship α × σ ₁₀₀₀ = 600
Assuming that one or more magnetic brushes are required for each pixel, the following equation is obtained: σ ₁₀₀₀ ≦ 600 × 25 / X = 1500 / X.

Further, as shown in Table 1, in order for the density of the magnetic brush to be 8 brushes / mm ² or more, the condition of σ ₁₀₀₀ ≦ 75 may be satisfied.

On the other hand, if the magnetization intensity of the carrier at the peak value of the developing magnetic field is smaller than 30 (emu / cm ³ ), the transportability of the developer on the sleeve is poor, and the image quality of the developed image is deteriorated. Since the scattering of the developer is likely to occur, 30
(Emu / cm ³ ) or more.

When the recording density X is smaller than 60 pixels / mm ² , the resolution is not good, and therefore, the recording density X is 60 pixels / mm ^2.
It is preferable to apply the present invention to the case of mm ² or more.
On the other hand, if the recording density X is greater than 10,000 pixels / mm ² , it is difficult to reproduce a dot image by dry toner particles. Therefore, the present invention is preferably applied to a case where the recording density X is 10,000 pixels / mm ² or less.

As described above, in the relationship between the number of pixels X per 1 mm ² and σ ₁₀₀₀ , when the hatched area in FIG. That is, it is possible to suppress the occurrence of a defective developed image such as D1, D3, and D4 in FIG. 2 and the remaining of a non-developed dot-like latent image such as L5.

X at the point where the above two equations intersect is 2
00 (pixels / mm ² ), the following can be said.

That is, when the peak value (d Gauss) of the magnetic flux density in the normal direction of the sleeve surface on the sleeve surface of the developing magnetic field is applied, the magnetization strength of the magnetic carrier is represented by σd
In the case of (emu / cm ³ ), the use of a magnetic carrier satisfying the following conditions made it difficult to cause roughness, and a good halftone image could be obtained in all density regions. Σd ≦ 15000 / X if X <200 σd ≦ 75 if X ≧ 200

From Table 1, it was found that most of the magnetic brushes could not be visually recognized by setting the number of magnetic brushes to 1 or more per 15 pixels or 10 or more / mm ² . For that purpose, it is necessary to use a magnetic carrier satisfying the following conditions. X <For 150 .sigma.d <9000 / X (15 or one per pixel) If the X ≧ 150 .sigma.d <60 (10 present / mm ² or more) (the X points two equations intersect 150 (pixel / mm ² )

In the above description, the developing magnetic field peak value d is 1000
Although the case was Gaussian, the result was the same when the peak value d was other than 1000 Gauss.

That is, d (Gauss) is 500, 800, 1
The magnetization intensity σd (em
u / cm ³ ) and the density α of the magnetic brush ears (books / mm ² )
Is shown in FIG. In each case, σd × α = 6
00 is satisfied.

D (Gauss) is 500, 800, 15
In each of the cases of 00 and 2000, the density of the magnetic brush ears is 8 / mm ² or more when σd is 75 emu /
a case of cm ³ or less, the density of the ears becomes ten / mm ² or more σd was the case for 60 emu / cm ³ or less.

Therefore, the density of the ears of the magnetic brush and the anti-rattleness of the image obtained by developing the dot distribution latent image do not depend on the peak intensity d (Gauss) of the developing magnetic field, but in the d-Gauss magnetic field. Of carrier magnetization σd (emu /
cm ³ ).

The number of pixels per 1 mm ² is as follows.
Indicates the minimum unit of gradation information, and indicates PW which is multi-value recording.
In the M system and the like, it refers to the minimum recording unit. That is,
Pixels exposed by light driven by a pulse having a time length corresponding to the minimum recording unit are pixels having the highest density, and the pixels exposed by light driven by a pulse having a time length shorter than the above-described time length are not exposed. The pixel consisting of the exposed part is a pixel of halftone density, and the pixel consisting of only the non-exposed part is the pixel of the lowest density (white background).

On the other hand, in a dither method or the like for outputting a pseudo gradation in binary recording, for example, when outputting a pseudo gradation in a 2 × 2 minimum recording unit, a set of 4 minimum recording units is set to 1
Pixels.

The magnetic material that is a component of the magnetic carrier particles includes periodic tables IA, IIA, IIIA, and IV.
A, VA, IB, IIB, IVB, VB, VIB, VI
It contains at least one element selected from the group IB and VIIIB. For example, Ni-Zn based, Li
System, Li-Zn system, Mn-Cu system ferrite can be used.

The magnitude of σd and the coercive force can be adjusted by appropriately adjusting the composition of the magnetic material.

Of course, the material of the carrier is not limited to the above.

By the way, during the course of the above examination, the following has been found.

That is, regarding the roughness, there are cases where carrier adhesion is remarkable and cases where carrier adhesion does not occur despite the same evaluation result as shown in Table 1.

The following was found when this was examined.

That is, the magnetic material constituting the carrier is the same as that shown in FIG.
In the case of a magnetic material having a hysteresis characteristic as shown in FIG. 2, that is, a magnetic material having no or small coercive force, that is, a so-called soft magnetic material, carrier adhesion may become remarkable ( The soft magnetic material has a coercive force of 125.
Oested (0e) or less magnetic material. ).

The magnetization σd is 100 emu / c
When magnetic carrier particles having a particle size of m ³ or less are used, carrier adhesion may be remarkable.

The magnetic carrier particles having the above-mentioned magnetic properties may or may not cause carrier adhesion. This is due to the size of the gap between the developing sleeve 25 and the developer layer thickness regulating blade 28. It turned out that it corresponds. In other words, even if the evaluation of the roughness is the same, if the gap becomes large, carrier adhesion occurs,
When the gap became small, no carrier adhesion occurred.

The sleeve 25 and the blade 28
The size of the gap corresponds to the degree of pressure of the developer layer in the developing section. The degree of compression can be expressed by the ratio between the free standing of the magnetic brush of the developer at the closest portion between the sleeve 25 and the drum 3 and the closest distance (referred to as SD gap) between the sleeve 25 and the drum 3. Therefore, this ratio was changed to evaluate the degree of carrier adhesion.

Table 2 shows an example of the evaluation results.

In Table 2, A indicates no carrier adhesion, B indicates slight carrier adhesion but does not hinder practical use, C indicates considerable carrier adhesion and deteriorates the transferred image, and D Indicates that significant carrier adhesion has occurred. In the evaluations A and B, the carrier adhesion was substantially prevented.

[0098]

[Table 2]

As can be seen from the above results, if the latent image is contact-developed with a developer layer in which the free standing of the developer magnetic brush at the nearest portion of the sleeve and drum is 2.0 times or less of the SD gap, The magnetic material is a soft magnetic material, and σd is 100e
When using magnetic carrier particles of mu / cm ³ or less, carrier adhesion can be prevented, and a good transfer image can be obtained. This is presumably because the degree of pressure of the developer in the developing section is in an optimum range for preventing the carrier from adhering.

The amount of developer conveyed to the developing unit is regulated by the gap between the sleeve 25 and the blade 28. Before the magnetic brush of the developer reaches the developing unit,
Once fallen on the surface of the sleeve, when entering the magnetic field region of the developing magnetic pole S _1, go stand up on the back surface of the sleeve.
At this time, the length of the magnetic brush slightly varies.

Therefore, by preventing this irregularity, the effect of preventing carrier adhesion can be further ensured.

An example will be described below.

In FIG. 13, the magnet 29 has magnetic poles S ₁ , S ₂ , S
_3, addition is made of N _1, N ₂ is a developing device similar to the configuration of FIG. 8.

In FIG. 13, a broken line Br indicates a magnetic flux density distribution on the sleeve surface of a magnetic field component perpendicular to the sleeve surface. A magnetic field component perpendicular to the sleeve surface causes the magnetic brush to stand on the sleeve surface.

[0105] Br ₁ shows a magnetic flux density of the vertical magnetic field component of the developing magnetic pole S _1, Br ₂ is adjacent to the magnetic pole S _1, and relates to the sleeve rotation direction, the magnetic poles are positioned upstream of the magnetic pole S ₁ shows a magnetic flux density of the vertical magnetic field components of the N _1.

After the height of the magnetic brush on the sleeve 25 is regulated by the regulating blade 28, the magnetic brush falls on the sleeve as the magnetic flux density Br ₂ decreases, and the boundary between Br ₂ and Br ₁ , that is, the vertical direction, At or near the position R where the magnetic field component becomes zero, the height on the sleeve becomes minimum. Then, the magnetic brush starts to rise again on the sleeve by the magnetic field of Br ₁ from a position slightly beyond the position R.

Reference numeral 30 denotes a developer leveling plate made of non-magnetic stainless steel or the like, which is in contact with the surface of the developer layer downstream of the position R with respect to the rotational direction of the sleeve and upstream of the developing section.

The gap between the leading end of the leveling plate 30 and the sleeve 25 is preferably wider than the SD gap and smaller than the free height of the magnetic brush at the closest position of the sleeve and the drum.

In any case, the leveling plate 30 comes in contact with the upper portion of the magnetic brush of the developer rising on the sleeve 25 immediately before the developing section to level the magnetic brush, so that the length of the magnetic brush is evenly adjusted. Ensuring the anti-adhesion effect more reliably.

Note that the tip of the leveling plate 30 is defined by the magnetic flux density distribution B
effect leveling and allowed to position to the position of 30% or more of the magnetic flux density of the peak value of r ₁ is high.

In FIG. 14, a thin (for example, 100 μm thick) such as a synthetic resin (for example, polyethylene terephthalate) is used.
Lightly (for example, a linear pressure of 0.1 g / c).
This is an example in which the developer magnetic brush is leveled by contacting the sleeve 25 with m).

The position where the leveling sheet 31 contacts the sleeve 25 is the same as the position where the leading end of the leveling plate 30 is positioned in FIG.

When the transfer of the developer is started by rotation of the sleeve 25, the sheet 31 is lifted by the developer, and
It is separated from the sleeve 25. Then, the upper portion of the developer magnetic brush is stroked to make the brush uniform in height.

Regardless of the elastic sheet or the rigid plate shown in FIG. 13, when the toner comes into contact with the developer, the toner is also rubbed by the leveling member. Is selected so as to be frictionally charged to a polarity for developing, which contributes to an improvement in image density.

[0115]

As described above, according to the present invention, when the magnetic field having the maximum strength of the perpendicular magnetic field of the developing magnetic pole on the surface of the developer carrier having the soft magnetic material is applied, the intensity of the magnetization is reduced. 100
When a developer containing a magnetic carrier and toner having an emu / cm ³ or less is used, a developing method capable of preventing the carrier from adhering to the image carrier can be realized.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram of a preferable range in which the relationship between the magnetization intensity of magnetic carrier particles and the recording density is obtained.

FIG. 2 is an explanatory diagram of a potential of a dot-like latent image.

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

FIG. 4 is a diagram illustrating an example of a color electrophotographic apparatus to which the present invention can be applied.

FIG. 5 is an explanatory diagram of a laser beam scanner that can be used in the present invention.

FIG. 6 is an explanatory diagram of a PWM circuit.

FIG. 7 is an explanatory diagram of a signal waveform of a PWM method.

FIG. 8 is an explanatory diagram of a developing device that can be used in the present invention.

FIG. 9 is a correlation diagram between carrier magnetization and ear density.

FIG. 10 is an explanatory diagram of a visual acuity limit.

FIG. 11 is an explanatory diagram of the relationship between the magnetization of a carrier and the intensity of a development peak magnetic field.

FIG. 12 is an explanatory diagram of a hysteresis curve of a soft ferromagnetic carrier.

FIG. 13 is an explanatory diagram of a developing device having a leveling member.

FIG. 14 is an explanatory diagram of another example of a developing device having a leveling member.

[Explanation of symbols]

3 electrophotographic photosensitive drum 22 the two-component developer 25 developing sleeve 27 bias power source 29 magnet 102 semiconductor laser S ₁ developing polar

──────────────────────────────────────────────────続 き Continuation of the front page (72) Inventor Yukio Nagase 3-30-2 Shimomaruko, Ota-ku, Tokyo Inside Canon Inc. (56) References JP-A-2-132459 (JP, A) JP-A-60 -263955 (JP, A) JP-A-63-41864 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) G03G 15/08-15/095 G03G 9/08 G03G 15/06 101 B41J 2/435-2/47

Claims (3)

(57) [Claims] 1. A magnetic carrier having a soft magnetic material and having a magnetization intensity of 100 emu / cm ³ or less when a maximum magnetic field of a vertical magnetic field of a developing magnetic pole is applied on a surface of a developer carrier. A developer having a toner is carried by the developer carrying member, and after the layer thickness is regulated by a layer thickness regulating member, the layer is conveyed to a developing section, and the developing magnetic pole in the developer carrying body is formed in the developing section. A developing method for developing an electrostatic latent image formed on the image carrier by bringing a magnetic brush of a developer into contact with the image carrier in a developing magnetic field, wherein the developer carrier and the image carrier are closest to each other. The layer thickness regulating member regulates the layer thickness of the developer such that the free standing of the magnetic brush at the portion is not more than 2.0 times the closest distance between the developer carrier and the image carrier. And developing the electrostatic latent image. 2. The image bearing member is an electrophotographic photosensitive member,
2. The developing method according to claim 1, wherein said electrophotographic photoreceptor is exposed to a light beam modulated by a pulse width modulated signal corresponding to the density of a recorded image to form a dot distribution electrostatic latent image. 3. The developing method according to claim 1, wherein an oscillating bias voltage is applied to said developer carrier.