JP3226740B2 - Image forming method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming method for developing an electrostatic latent image formed by an electrophotographic method or an electrostatic recording method to obtain an image.

[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 in accordance with an image signal to be recorded to form a dot distribution electrostatic latent image (that is, a dot-like latent image into an image). There is known a latent image forming method for forming an electrostatic latent image distributed corresponding to the image. Among them, a so-called pulse modulation width (P) that modulates the width of the drive pulse current of the laser (that is, the duration) in accordance with the density of the recorded image.
The WM) method can obtain high recording density (that is, high resolution) and high gradation.

However, when a dot latent image was formed on the photosensitive drum by using the PWM method, and a magnetic brush of a two-component developer was brought into contact with the photosensitive drum, the electrostatic latent image was reversely developed.
The obtained image was rough in a halftone region having a reflection density of 0.3 or less. 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.

[0004] Then, when the cause of the occurrence of roughness was investigated and examined, the following was found. Normally, when a latent image of a low density portion is formed by a dot latent image, when viewed microscopically, the latent image on the photosensitive drum is not a broad latent image like an analog latent image, but as shown in FIG. It has a two-dimensional distribution of local dot latent images. If an attempt is made to reproduce a lower density, the dot latent image becomes dull due to the effect of the photoconductor thickness of the photosensitive drum, and as shown in FIG. (The difference in the potentials of the two) gradually decreases. For example, when trying to reproduce an image having a reflection density of about 0.2, Vo of the dot latent image at that time is about 150 to 200V.

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

[0006] The Vcont of 0 to 50 V is a very unstable contrast potential whether the toner reaches the photosensitive drum side or remains on the developing sleeve side. 2 for that
The contact state of the magnetic brush of the component developer with the photosensitive drum
This greatly affects the development efficiency of the dot latent image and is developed in accordance with the unevenness of the magnetic brush ears (that is, the area where the magnetic brush is in contact with the photosensitive drum is not developed, and the area that is not in contact is developed. ). For this reason, roughness due to missing dots etc. (density is distributed as fine unevenness)
Is more likely to occur.

FIG. 3 shows this. In FIG.
Indicates one pixel. Dot latent images L1 to L5 corresponding to low-density images are formed on each pixel P by a laser beam modulated by the PWM method. D1 to D4 indicate toner-attached areas of the dot latent images L1 to L5, that is, developed areas. The dot latent image L2 has been completely developed. However, the dot latent images L1, L3, L4 are only partially developed. The dot latent image L5 has not been developed at all.

[0008] As described above, the two-dimensional distribution of the developed image having a defect in the development of the dot latent image makes it possible to see the low-density region in a sudden manner. When this is formed, the roughness is very conspicuous, and the image quality is degraded.

[0009]

By the way, as a method of reducing the roughness in the highlight and halftone density regions, it is conceivable to form the magnetic brush of the developing section densely. As a method for obtaining a high-density final image in the entire density region, the particle size of the magnetic carrier is reduced and the mixing ratio of the toner to the developer, that is
It is conceivable to improve the // (T + C) ratio (note that
By reducing the particle size of the carrier, the density of the magnetic brush also tends to increase).

The most effective method for making the brushes of the magnetic brush denser is that in the magnetic field at the position of the developing magnetic pole of the magnet in the developing sleeve, the magnitude of the magnetization per unit volume of the magnetic carrier [emu / cm ^3] ] Is reduced. However, when the magnitude of the magnetization is reduced, the force with which the magnetic carrier is restrained on the developing sleeve is weakened, and the carrier pulling phenomenon that the carrier adheres to the photosensitive drum side is likely to occur. , The magnitude of magnetization [emu / piece] per carrier is reduced, and carrier pulling is likely to occur).

Further, in order to make the ears of the magnetic brush dense and reproduce the low-concentration region smoothly and prevent the carrier from adhering, it is necessary to reduce the size of the carrier per unit volume while maintaining a certain level of residual magnetization. Is effective.

An object of the present invention is to provide a dot distribution electrostatic latent image with two dots.
Image formation that can be developed with a magnetic brush of component developer to obtain high-quality images in all density regions without low-density parts such as halftones, and to eliminate carrier adhesion together Is to provide a way.

[0013]

The above object is achieved by the image forming method according to the present invention. In summary, the present invention provides a method of forming a dot latent image on an image carrier and forming a latent image on a developer carrier having a magnetic field generating means in the developing section facing the image carrier. In the image forming method of carrying a component developer and carrying it, forming a magnetic brush of the developer by the magnetic field of the magnetic field generating means in the developing section, and developing the latent image on the image carrier with the magnetic brush to obtain an image, The magnetic carrier is a hard ferromagnetic carrier, and the magnitude of magnetization per unit volume of the carrier is 1 in the magnetic field of the developing magnetic pole located at the developing section of the magnetic field generating means.
00 [emu / cm ³ ] or less, and the average charge amount Q / M per unit weight of the carrier is c [μC /
g], and the mixing ratio of the toner to the developer is T / (T + C)
C and T / (T + C)
When 2 ≦ T / (T + C) <6, 1 / c ≧ −1 / 10 · T / (T + C) +0.85 When 6 ≦ T / (T + C), the relationship c ≦ 5 is satisfied. This is an image forming method.
Preferably, the dot distribution electrostatic latent image is formed by modulating a light emission time of a light source per pixel in accordance with an image density value.

[0014]

【Example】

Embodiment 1 FIG. 4 is an overall configuration diagram showing an image forming apparatus to which the present invention can be applied. The image forming apparatus is an electrophotographic color printer. This printer includes an electrophotographic photosensitive drum 3 rotating in the direction of an arrow, and a rotary developing device 1 including a primary charger 4 and developing devices 1M, 1C, 1Y, and 1K around the photosensitive drum 3, a transfer charger. A laser beam scanner LS is provided above the photosensitive drum 3, and an image forming unit is constituted by the primary charger 4 and the like.

The developing devices of the rotary developing device 1, that is, a magenta developing device 1M, a cyan developing device 1C, and a yellow developing device 1Y
The black developing device 1K contains a two-component developer containing a toner and a carrier, and uses a magenta toner, a cyan toner, a yellow toner, and a black toner as the toner, respectively.

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 converts 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 built in the laser beam scanner LS is controlled according to these image signals,
A laser beam L is emitted. Note that the laser beam L 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 a 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 a dot latent image is formed on the photosensitive drum 3. This latent image is reversal-developed by a magenta developing device 1M fixed at a developing position in advance,
Visualized as a magenta toner image.

On the other hand, the transfer material such as paper taken out of the cassette 2 and supplied through the paper feed guide 5a, the paper feed roller 6, and the paper feed guide 5b is transferred to the gripper 7 of the transfer drum 9.
, And is electrostatically wound and carried on the transfer drum 9 by the contact roller 8 and its opposite pole. The transfer drum 9 rotates in the direction of the arrow shown in FIG. 3 in synchronization with the photosensitive drum 3, and the transfer material carried on the transfer drum 9 is transported to a transfer unit facing the photosensitive drum 3, where the photosensitive drum 3 3 is transferred by the transfer charger 10. The transfer drum 9 continues to rotate as it is,
In order to prepare for the transfer of the toner image of the color to be formed next (the cyan toner image in FIG. 4), the transfer material is transported again toward the transfer portion.

After the transfer of the magenta toner image is completed, the photosensitive drum 3 is neutralized by the neutralizing charger 11 and is cleaned by the cleaning means 12 to remove the untransferred toner. Then, the photosensitive drum 3 is charged again by the primary charger 4. Exposure of the laser beam L modulated by the cyan image signal of
A dot latent image is formed on the photosensitive drum 3. During this time, the developing device 1 rotates and the cyan developing device 1C is fixed at the developing position, and the latent image on the photosensitive drum 3 is reversely developed by the cyan developing device 1C to be visualized as a cyan toner image. The obtained cyan toner image is transferred onto the transfer material on the transfer drum 3 in the transfer section from above the magenta toner image.

The above-described steps are performed for yellow and black, forming a dot latent image on the photosensitive drum 3 by exposure to a laser beam L modulated with respective image signals, a yellow developing unit 1Y, and black The formation of a yellow toner image and a black toner image by development in the developing device 1K, and the superimposed transfer of the yellow toner image and the black toner image onto the transfer material, the transfer of the four colors of magenta, cyan, yellow and black onto the transfer material. Is obtained.

When the transfer of the toner images of four colors is completed, the transfer material is discharged by the separation / discharge chargers 13 and 14, the gripping by the gripper 7 is released, separated from the transfer drum 9 by the separation claw 15, and then conveyed. Fixing device 17 with belt 16
Sent to The transfer material sent to the fixing device 17 is heated and pressurized there to be fixed, whereby the toner images of each color are mixed and fixed to the transfer material to form a full-color print image. Thus, a series of the full-color mode sequence is completed, and the full-color print image obtained by fixing is discharged out of the printer.

FIG. 5 shows a laser beam scanner LS installed in the printer of FIG. The scanner includes a semiconductor laser element 102. This element 102 is connected to a light emission signal generator (laser driver circuit) 500 which is a drive signal for generating a laser beam. Flickers accordingly. The light beam of the laser beam L emitted from the laser element 102 by blinking is converted into substantially parallel light by a collimator lens system 103, and is turned into a rotating polygon mirror.
That is, the light enters the polygon mirror 104.

The polygon mirror 104 is rotated at a constant speed in the direction of arrow B so that the collimator lens system 103 is rotated.
Deflects the parallel laser beam L incident thereon, and scans in the direction of the arrow C in the same direction 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 104 forms the deflected laser beam L into a spot image on the surface of the photosensitive drum 3 and sets the scanning speed at a constant speed on the surface of the photosensitive drum 3. And By the scanning exposure of the photosensitive drum 3 with the laser beam L, a dot distribution electrostatic latent image (for short, a dot latent image or a dot latent image) is formed on the photosensitive drum 3.

Each of the developing devices 1M to 1K includes a primary charger 4
In this case, the laser beam L exposes an area of the photosensitive drum 3 to which the toner is to be attached because the toner charged to the same polarity as the toner is applied to the bright portion potential portion of the latent image.

In this embodiment, a dot latent image is formed by multi-value recording using a minimum recording unit of one pixel using a PWM (pulse width modulation) method. This will be 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;
Is a level converter for converting a TTL logic level to a high-speed ECL logic level, and 403 is a D / A converter for converting an ECL logic level to an analog signal. 404 is PWM
An ECL comparator for generating a signal; a level converter for converting an ECL logic level to a TTL logic level;
406 is a clock oscillator for oscillating the clock signal 2f,
Reference numeral 407 denotes a triangular wave signal generator that generates a substantially ideal triangular wave signal in synchronization with the clock signal 2f, and 408 denotes a frequency divider that generates the image clock signal f by dividing the clock signal 2f by １ ／. Thus, the clock signal 2f has a cycle twice as long as the image clock signal f. In addition,
ECL logic circuits are provided everywhere to operate the circuit at high speed.

The operation of the circuit 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 e as shown. Also, in the triangular wave signal generator 407, the triangular wave signal c is generated after once dividing the clock signal 2f by 1/2 in order to keep the duty ratio of the triangular wave signal at 50%. Further, this triangular wave signal c has an ECL level (0 to -1 V).
Into a triangular wave signal d.

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

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 or the like corresponding to the pixel density to be formed.
Generate a WM signal. This PWM signal becomes narrower as the pulse width corresponds to the lower density pixel. And the PWM signal is 0
PWM signal f converted to TTL level of V or 5V
And input to the laser driver circuit 500. By changing the exposure time per pixel corresponding to the PWM signal value obtained in this way, 256 pixels per pixel can be obtained.
Gradation can be obtained.

The beam effect h in FIG. 7 indicates the shape of the laser beam exposure area of the photosensitive drum corresponding to each drive pulse width. The area shape of each dot latent image substantially corresponds to this exposure area shape. In FIG. 7, the horizontal axis represents time for the signal waveforms a to g, and the horizontal axis represents distance in the beam scanning direction for h.

FIG. 8 is a sectional view of a developing device used as each of the developing devices 1M to 1K of the rotary developing device 1 of FIG.
The developing device includes a developing container 18 containing a two-component developer 22 in which a toner and a magnetic carrier are mixed. According to the present invention, the two-component developer 22 is a mixture of a non-magnetic toner and a hard ferromagnetic carrier (these will be described later). An opening is formed in the developing container 18 at a position close to the photosensitive drum 3, and a developing sleeve 25 is rotatably disposed in the opening. A blade 28 is provided above the developing sleeve 25 at a predetermined interval from the developing sleeve 25.

The developing sleeve 25 is formed of a cylinder made of a non-magnetic material such as aluminum or non-magnetic stainless steel, and rotates in a direction indicated by an arrow (forward direction) during a developing operation. Inside the developing sleeve 25, a cylindrical magnet 29 serving as a magnetic field generating means is fixedly arranged in a non-rotating manner. The magnet 29 has five magnetic poles N1, N2, N3, S1, and S2 around the surface thereof, and the magnetic pole N3 disposed near the developing position facing the photosensitive drum 3 has a magnetic pole of the developer at the developing position. A developing magnetic pole forming a brush, and other magnetic poles S1,
S2, N1, and N2 are magnetic poles for transporting the developer.

Reference numeral 32 denotes a developing roller, and the developing sleeve 25 and the magnet 29 inside the developing sleeve 25 may be collectively referred to as a developing roller.

It is necessary that the blade 28 does not change the magnetic field distribution of the magnet 29 at the position where the blade 28 is provided.
16 and the like. Blade 28
The gap between the leading end of the developer and the developing sleeve 25 is the amount of the two-component developer 22 carried on the developing sleeve 25 and conveyed to the developing unit, specifically, the layer thickness of the developer on the developing sleeve 25. Regulate. The developer 22 on the developing sleeve 25 is
By passing between the tip of the blade 28 and the surface of the developing sleeve 25, the thickness is regulated so as to be in contact with the photosensitive drum 3 at the developing position, and then reaches the developing position.

In the developing container 18, a developer stripping means 30 of a rotating blade member is installed at a position close to a lower portion of the developing sleeve 25, and a stirring screw 23 is installed in a lower portion of the container 18. 23, a developer supply means 3 of a rotating belt member having a receiving plate 31a protruding therefrom.
1 is provided, and the upper end of the supply member 31 is close to the side of the developing sleeve 25. A replenishing chamber 20 containing replenishing toner is provided at an upper position on the back side in the container 18.

The developer 22 contained in the developing container 18
Is magnetically attracted by the magnetic pole N2 of the magnet 29, is held on the developing sleeve 25, and is conveyed to the developing unit via the magnetic pole S1 with the rotation of the developing sleeve (on the way,
The layer thickness is regulated by the blade 28). In the developing section, the developer is formed on the magnetic brush by the magnetic pole N3 and is used for development. Then, the developer that has completed the development passes through the transport pole S2 while being held on the developing sleeve 25, and is stored in the container 1.
8, when transported to the magnetic pole N1, it is retained at the position of the magnetic pole N1 by the repelling magnetic field formed by the magnetic poles N1 and N2 constituting the repelling magnetic pole.

The retained developer is scraped off from the developing sleeve 25 by the rotation of the peeling member 30, stirred with the developer 22 in the container 28 by the stirring screw 23, and dropped from the supply chamber 20 as necessary. Mixed with the toner. As a result, the concentration of the toner consumed in the development of the developer is restored. Thereafter, the developer is scraped up by the receiving plate 31a of the supply member 31, is conveyed to the developing sleeve 31, and repeats the above-described process of magnetic attraction by the magnetic pole N2.

In the present invention, as the toner of the two-component developer, a known toner obtained by adding a colorant, a charge control agent, and the like to a binder resin can be used.
A range of from 15 μm to 15 μm is preferred. The volume average particle size of the toner was measured by, for example, the following measurement method.

As a measuring device, a call counter TA-II is used.
Using a mold (manufactured by Coulter), an interface (manufactured by Nikkaki) for outputting the number distribution and volume distribution and a CX-i personal computer (manufactured by Canon) were connected. As the electrolytic solution, using reagent grade 1 sodium chloride, 1% Na
A Cl aqueous solution was prepared.

In 100 to 150 ml of the above electrolyte solution, 0.1 to 5 ml of a surfactant, preferably an alkylbenzene sulfonate, is added as a dispersant, and 0.5 to 50 mg of a toner measurement sample is further added and suspended. Then, the electrolytic solution in which the sample was suspended was subjected to dispersion treatment for about 1 to 3 minutes using an ultrasonic
Using an aperture of m, the particle size distribution of the particles of 2 to 40 μm is measured, and then the volume distribution of the toner is determined. This volume distribution gives the volume average particle size of the sample.

In the present invention, a hard ferromagnetic carrier, that is, hard ferromagnetic particles is used as the magnetic carrier. The hard ferromagnetic particles include, for example, metals such as iron, nickel, cobalt, manganese, chromium,
The method for producing these hard ferromagnetic particles is not particularly limited.

In this embodiment, as the hard ferromagnetic carrier,
Weight average particle size 20 to 100 μm, preferably 30 to 70
Ferrite particles containing neodymium, samarium, barium, etc., having a coercive force of at least 1,000 Oersts, preferably at least 200 Oersts, were used after being coated with a resin.

The average particle size of the carrier is indicated by the maximum chord length in the horizontal direction, and the measurement was performed by a microscope. An average particle size of the carrier can be obtained by randomly selecting 300 or more carriers, actually measuring the diameter, and arithmetically averaging the diameters.

The resistance of the magnetic carrier is 10 ⁷ Ωcm or more,
Preferably, a material having a resistivity of 10 ⁸ Ωcm or more was used. The carrier resistance is such that the measurement electrode area is 4 cm and the distance between the electrodes is 0.
The measurement was performed using a 4 cm sandwich type cell.
A load of 1 kg is applied to one of the electrodes sandwiching the carrier,
A voltage E [V / cm] is applied between the two electrodes under the load, the current value flowing to the measuring circuit at that time is measured, and then the resistance value of the carrier is measured.

The average charge amount per unit weight (abbreviated average charge amount) Q / M of the magnetic carrier due to frictional charging is:
Average charge amount per unit weight of toner due to frictional charge (abbreviated average charge amount) Q / M _(T) (To distinguish from the average charge amount Q / M of the carrier, the average charge amount of the toner Q / M _(T)) . The method of measuring the average charge Q / M _(T) of the toner is as follows.

FIG. 9 shows an apparatus for measuring the average charge amount of the toner. As shown in FIG. 9, the measuring device includes a metal measuring container 42 having a 500-mesh conductive screen 43 at the bottom. Weight ratio of toner and magnetic carrier whose charge amount is to be measured: toner / carrier =
0.5 / 9.5 in a 50-100 ml polyethylene bottle, shake by hand for about 10-40 seconds, then mix about 0.5-1. 5
g is put in the measuring container 42, and a lid 44 is placed thereon. At this time, the entire measurement container 42 was weighed, and this was
(G). Next, the measuring container 42 is connected to the suction device 4 of the apparatus.
1 (at least the part in contact with the measuring container 42 is an insulator), suction is performed through the suction port 47, and the air flow control valve 46 is adjusted to adjust the pressure of the vacuum gauge 45 to 250 mmA.
q. In this state, suction is sufficiently performed, preferably for 2 minutes, to remove the toner resin by suction. At this time, the potential of an electrometer 49 connected in parallel with a capacitor (capacitance C (μF)) 48 between the measurement container 42 and the ground is read, and the reading is set to V (volt). Further, the entire measurement container 42 after suction is weighed, and this is defined as W2 (g). The average charge amount of the toner at this time is calculated as in the following equation. The measurement conditions are 23 ° C. and 60% RH.

Average charge amount of toner [μC / g] = C · V
/ (W1-W2)

In this embodiment, as the magnetic carrier, a carrier made of fluorine resin-styrene resin coated ferrite particles and having 70 to 90 wt% of particles of 250 mesh pass (pass) and 350 mesh on (non-pass) is used. did. Specifically, a vinylidene fluoride-tetrafluoroethylene copolymer and styrene-acrylic acid 2
Ferrite particles coated with a 0.2 to 0.7 wt% resin mixture of -ethylhexyl-methyl methacrylate at a ratio of 5: 5 (weight ratio) were used. Toner and carrier
After being left for at least 12 hours in an environment of 23 ° C. and 60% RH, it is used for measuring the amount of charged electric charge. The charge amount was measured at 23 ° C, 6
Perform under 0% RH environment.

The average charge amount of the toner is obtained as described above. The total charge amount due to friction between the toner and the carrier in the developer is opposite to each other and has an absolute value of the toner and the carrier. Almost equal. Accordingly, the average charge amount per unit weight of the carrier Q / M is the average charge amount per unit weight of the toner Q / M _(T) and the ratio of the toner to the developer, that is, the C / (T + C) ratio (accordingly, /
(T + C) ratio, it can be easily obtained by the back calculation. For example, the T / (T + C) ratio is 5 wt%, and the Q / M _{(T ) of the} toner is 3
Assuming 0 μC / g, the C / (T + C) ratio is 95 w
and the average charge Q / M of the carrier is about 1.58.
μC / g.

At the time of development, an oscillating bias voltage obtained by superimposing a DC voltage on an AC voltage is applied to the developing sleeve 25 by a power supply 27 as a developing bias. 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 carrier of the developer vibrate violently, and the toner shakes off the electrostatic restraint on the developing sleeve 25 and the carrier, flies to the photosensitive drum 3, and adheres to the photosensitive drum 3 corresponding to the latent image. I do.

The difference between the maximum value and the minimum value of the vibration bias voltage (peak-to-peak voltage) is preferably 1 to 5 kV, and the frequency is 1 kV.
-10 kHz is preferred. The waveform of the oscillation bias voltage is
Square waves, sine waves, triangular waves, etc. can be used.

As described above, the DC voltage component of the vibration bias is a value between the dark portion potential and the bright portion potential, but the value closer to the dark portion potential than the smallest bright portion potential in absolute value is the dark portion potential. It is preferable to prevent the fog toner from adhering to the potential region.

The developer 2 carried on the developing sleeve 25
2, the amount of the developer transported to the developing unit 26 by the blade 28 is regulated.
The height of the developer from the surface of the developing sleeve of the magnetic brush may be 1.2 to 3 times the minimum gap between the developing sleeve 25 and the blade 28 when the photosensitive drum 3 is removed. preferable.

The magnet 29 in the developing sleeve 25 has a developing magnetic pole N3 in the developing section 26 facing the photosensitive drum 3, and the developing magnetic pole N3 forms a developing magnetic field on the developing sleeve 25 by a developing magnetic field formed in the developing section 26. A magnetic brush is formed on the agent. The magnetic brush contacts the photosensitive drum 3, and the toner in the magnetic brush transfers to a 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 ears (brush) of the magnetic carrier,
Although attached to the surface of the developing sleeve 25, any toner can be transferred to the exposed portion of the latent image.

It is preferable that the peak value of the strength (magnetic flux density in a direction perpendicular to the surface of the developing sleeve) of the developing magnetic field by the developing magnetic pole N3 on the surface of the developing sleeve 25 is 500 to 2000 Gauss.

Investigations were conducted using the above-described developing device. As a result, the density of the magnetic brush of the developer on the developing sleeve 25 in the developing section (the unit area of the surface of the developing sleeve) was confirmed in order to eliminate the image roughness. It has been found that it is necessary to increase the number of magnetic brushes per unit), and to increase the density of the magnetic brushes, it is necessary to reduce the intensity of magnetization of the magnetic carrier in the developing unit.

Next, the relationship between the value of the magnetization of the magnetic carrier and the density of the magnetic brush in the developing section will be described. In this embodiment, since the peak value of the magnetic flux density on the surface of the developing sleeve is 1000 Gauss, the developing magnetic pole N3 is used.
First, soft ferromagnetic carrier (soft ferrite carrier)
Was examined for a magnetic force of 1000 Gauss. The soft ferromagnetic carrier is a carrier having magnetic properties as shown in FIG. As a result, the result was as shown in FIG.

As can be seen from FIG. 10, the value of the magnetization of the carrier at the peak magnetic flux density of the developing magnetic field and the density of the ears of the magnetic brush are in inverse proportion. That is, the density of the ears of the magnetic brush is α [lines / mm ² ], and the strength of magnetization of the carrier at a magnetic force of 1000 Gauss is σ ₁₀₀₀ [em
u / cm ³ ], α × σ ₁₀₀₀ = 600.
That is, the smaller the σ ₁₀₀₀ is, the denser the brush ears are. However, conversely, when the binding force decreases, carrier adhesion tends to occur.

Further, when considering the relationship between the density of the magnetic brush ears of the developer and the roughness, the recording density (the distribution density of dot latent images, that is, the distribution density of pixels) is involved. When the recording density is low, it is difficult for the spike to appear even if the ear density is somewhat coarse, but when the recording density is high, the spike density also needs to be high.

Therefore, the recording density is set to 200 dpi, 400 dpi in the sub-scanning direction (moving direction of the photosensitive drum).
And in the main scanning direction (beam scanning direction),
An image was formed under the conditions of 200 dpi and 400 dpi, and the image quality and the state of carrier adhesion were examined. The two-component developer used for development has a T / (T + C) ratio of 5 wt.
%, And the average charge Q / M _{(T) of} the toner was −40 μC / g. This soft ferromagnetic carrier capable of imparting an average charge amount of −40 μC / g to the toner is referred to as “carrier A”.
And Table 1 shows the results.

[0061]

[Table 1]

In Table 1, the meanings of the symbols are as follows.

A: Very smooth image quality without "graininess" B: Smooth image quality without "greyness" C: Smooth image quality without "greyness" noticeable D: "greyness" Notable E: "Gastiness" is very noticeable. ○: Carrier does not adhere at Vb (fogging potential) 300 V. △: Carrier does not adhere at Vb 150 V. ○: Carrier does not adhere at Vb 150 V.

As shown in Table 1, the value of σ ₁₀₀₀ is 75
When the density of spikes of the magnetic brush was 8 or more per 1 mm ² using a carrier of emu / cm ³ or less, the roughness was hardly noticeable even when the recording density was high. This depends on the visual limit of the human eye.

FIG. 11 shows the spatial frequency ν [line / m
m] and the number L of levels that can be recognized and the density difference ΔD. L = 10 ³ exp (−0.72ν) ·
{1-exp (-0.52v)} + 1.

In general, the fluctuation range of the density in the low density portion (density 0.2 to 0.3) where the roughness is conspicuous is about 0.
02, and from FIG. 11, the spatial frequency ν is about 2.7.
If it is higher than line / mm, a density fluctuation of about 0.02 cannot be recognized by human eyes. That is, the density of the magnetic brush is 7.3 brushes / mm ² (2.7 × 2.7 = 7.3).
In the case described above, the high frequency becomes rough for the above-mentioned reason, making it difficult to recognize. Therefore, the ear density is 1 mm ²
When there were eight or more pieces per day, the roughness was hardly noticeable.

On the other hand, the carrier intensity σ ₁₀₀₀ at the peak value of the developing magnetic field (the magnetic field generated by the developing magnetic pole) is 30 emu.
If it is less than / cm ³ , the transportability of the developer on the developing sleeve is poor, and the image quality of the developed image is deteriorated and the developer is easily scattered. Therefore, the magnetization intensity of the carrier is 3
It is preferably 0 emu / cm ³ or more.

As described above, the peak value d of the developing magnetic field is 1000
Although the condition was Gaussian, the same result was obtained when the peak value d was other than 1000 Gauss. Peak value d is 50
Magnetization intensity σ _d [emu / cm ³ ] at 0, 800, 1500 and 2000 Gauss and density α of ears of magnetic brush
FIG. 12 shows the relationship of [books / mm ² ]. In any case, it can be seen that the relationship of σ _d × α = 600 is satisfied.

The peak value d is 500, 800, 150
In both cases of 0 and 2000 gauss, the density of the ears of the magnetic brush becomes 8 or more / mm ² when σ _d is 75 em.
u / cm ³ or less, and the density of the magnetic brush ears is 10 / mm ² or more when σ _d is 60 emu / c.
m ³ or less.

Therefore, the density of the ears of the magnetic brush and the prevention of roughness of the image formed by developing the dot distribution latent image do not depend on the peak intensity d of the developing magnetic field, but the carrier of the carrier in the magnetic field of the intensity d. It can be seen that it depends on the magnetization strength σ _d .

However, as shown in Table 1, when a soft ferromagnetic carrier having a magnetization intensity of 100 emu / cm ³ or less is used, the T / (T + C) of the developer is 5 wt.
% And toner Q / M _(T) of −40 μC / g, carrier adhesion occurred when Vb was 150 V.

Generally, the adhesion of a magnetic carrier is a phenomenon that occurs when a magnetic force acting on a carrier is smaller than an electrostatic force and is attracted to a photosensitive drum side. Even when the T / (T + C) ratio of the region of the magnetic brush in contact with the photosensitive drum is extremely low, the carrier adheres even when the toner in the developing region is attracted to the developing sleeve by the fog removing voltage (Vb). Is easy to occur.

As countermeasures against this, there are a method of increasing the force acting on the magnetic carrier and a method of weakening the electric charge held by the magnetic carrier and reducing the electrostatic force in the direction in which the magnetic carrier is attracted to the photosensitive drum side. The electrostatic force Fe acting on one particle of the magnetic carrier in the direction of attracting the photosensitive drum side attracts the electric charge q of one carrier particle and the electric field that attracts the carrier to the photosensitive drum side (that is, attracts the carrier of Vpp to the photosensitive drum side). (The peak voltage in the direction + fog removal voltage). Or T
There is a method in which the // (T + C) ratio is increased to make it difficult for the T / (T + C) ratio of the region in contact with the photosensitive drum in the magnetic brush to decrease.

A hard ferromagnetic carrier as shown in FIG. 14 is characterized by having a coercive force Hc and a residual magnetization σr. Since the hard ferromagnetic carrier has a residual magnetization σr, the magnetization remains even in a state where the external magnetic field is weakened (in a state away from the developing unit). This is advantageous in preventing carrier adhesion (a phenomenon in which a carrier adheres to an image portion and disturbs an image).

Therefore, an image was formed in the same manner as in the case of using the soft ferromagnetic carrier shown in Table 1 except that the carrier of the developer was changed to a hard ferromagnetic carrier. The hard ferromagnetic carriers used each have a coercive force Hc of about 2
000 Oersted (Oe), but the magnetization strength σ ₁₀₀₀ and the residual magnetization σ r at the magnetic pole of 1000 Gauss are different. The density of the spikes of the magnetic brush of the hard ferromagnetic carrier formed in the developing portion by the developing magnetic pole N3 having a peak value d of 1000 Gauss is shown by white circles in FIG. 15 (black circles in FIG. I wrote the same one as 10). The two-component developer used was T / (T + C)
The ratio is 5 wt% and the average charge Q / M _{(T) of the} toner is −40.
μC / g. This hard ferromagnetic carrier capable of giving the toner an average charge of −40 μC / g is referred to as “carrier A ′”. Table 2 shows the evaluation results (graininess, carrier adhesion) of the images obtained by developing the dot distribution electrostatic latent images. In Table 2, the meanings of the symbols are the same as in Table 1.

[0076]

[Table 2]

As can be seen from FIG. 15, also in the case of the hard ferromagnetic carrier, α × σ ₁₀₀₀ = 600 as in the case of the soft ferromagnetic carrier described above. Further, as can be seen from Table 2, as the magnetic brush density is increased, a good image without roughness is obtained, and the value of σ ₁₀₀₀ is 75 emu /
When the density of spikelets is 8 / mm ² or more at cm ³ or less,
Although the recording density was high, the roughness was not noticeable. As can be seen from comparison with Table 1, when Vb is 150 V, σ ₁₀₀₀ is 60 emu / cm ³ or more, and no carrier is attached.

On the other hand, as shown in FIG. 16, even if the residual magnetization σr [emu / cm ³ ] of the hard ferromagnetic carrier is different,
The equation α × σ _d = 600 holds. That is, the density of the ears of the magnetic brush does not depend on the residual magnetization of the carrier, but on the intensity of the magnetization of the carrier at the peak magnetic field d [Gauss].

In the above description, the peak value d of the developing magnetic field is 1000
Although the condition was Gaussian, the same result was obtained when the peak value d was other than 1000 Gauss. That is, in any case where the peak value d is 500, 800, 1500, or 2000 Gauss, the relationship of σ _d × α = 600 is satisfied.

The peak value d is 500, 800, 150
In both cases of 0 and 2000 gauss, the density of the ears of the magnetic brush becomes 8 or more / mm ² when σ _d is 75 em.
u / cm ³ or less, and the density of the magnetic brush ears is 10 / mm ² or more when σ _d is 60 emu / c.
m ³ or less.

As described above, the average charge amount Q of the carrier
/ M is determined by the T / (T + C) ratio and the frictional charging between the toner and the carrier. For example, when the toner amount is increased while the carrier amount is kept the same, the number of times of contact of the toner per carrier increases, and the charge per carrier (or the average charge amount Q / M of the carrier) increases. I do.
Conversely, the number of times of contact with the carrier per toner decreases, and the charge per toner (or the average charge amount Q / M _{(T) of the} toner) decreases.

That is, FIG. 17 is a diagram plotting the reciprocal of the average charge amount Q / M of the carrier when the T / (T + C) ratio is changed, and FIG. 18 is a diagram where the T / (T + C) ratio is changed. FIG. 7 is a diagram in which the average charge amount Q / M _(T) of the toner at the time is plotted. A, B, and C in FIGS. 17 and 18 indicate different developers in which the types of the carrier and the toner (the coating agent of the carrier and the binder of the toner) are changed. FIG.
18 that the T / (T + C) ratio increases, the toner Q / M _(T) increases, and the carrier Q / M increases.

The surface properties of the carrier coating agent are changed to change the triboelectric charging characteristics with the toner, and the Q / M of the carrier is made larger than that of the carrier A (this hard ferromagnetic carrier is referred to as “carrier B”). The T / (T + C) ratio is 5 wt%
And T / (T + C) using this carrier B
When the ratio was changed to 6%, the image quality (highlight, halftone roughness) of the output image and the degree of carrier adhesion were examined.

The image sample is an image formed by a two-component developing method, and is 200 dpi, 400 dpi in the main scanning direction.
i, 200 dpi, 400 dpi in the sub-scanning direction
It was made. The results are shown in Tables 3 and 4. At this time, the density of the magnetic brush did not change,
This is because the density of the magnetic brush is dominated by the magnetic properties of the carrier.

[0085]

[Table 3]

[0086]

[Table 4]

When Table 3 is compared with Table 2, the average charge amount Q / of the carrier is larger than that of Carrier A ', as in Carrier B.
It can be seen that the use of a material having a high M facilitates carrier adhesion. Also, when Table 4 is compared with Table 3, T /
By increasing the (T + C) ratio, the average charge Q of the toner
It can be seen that / M _(T) decreases and the average charge Q / M of the carrier increases, but the carrier is less likely to adhere.

When the T / (T + C) ratio is 2% or less,
In the non-image area, while the fogging voltage is being applied,
There is almost no toner on the photosensitive drum side, and even though the Q / M of the carrier is small, the carrier is almost saturated in Q / M. When the saturated Q / M value is 5 μC / g or less, the carrier does not adhere.

According to the results of the above experiment, when the area shaded in FIG. 1 is satisfied, that is, when the average charge amount Q / M per unit weight of the carrier is c [μC / g], 2
The weight mixing ratio with respect to the component developer is T / (T + C) [wt
%], When c and T / (T + C) are 2 ≦ T / (T + C) <6, 1 / c ≧ −1 / 10 · T / (T + C) +0.856 6 ≦ T / (T + C) When satisfying the relationship of c ≦ 5, a good image could be obtained even at Vb-300V without carrier adhesion.

When an image was formed using the above-described image forming apparatus, it was possible to suppress the roughness of the low-density portion without causing carrier adhesion and to obtain a high-quality image.

Embodiment 2 Recently, a multi-developing process system in which a toner image is superimposed directly on an image carrier to form a color image has been proposed and studied.

This process will be described with reference to FIG. 19. The photosensitive drum 3 is uniformly charged by the charger 4 and the photosensitive drum 3 is exposed by the laser beam L from the exposure means to form a first color, for example, a magenta component color. Is written on the photosensitive drum 3 by reversal developing only the portion irradiated with the laser by the magenta developing unit 1M.
A magenta toner image of a color is formed. The same process as above is repeated for the second and subsequent colors, for example, cyan (C) and yellow (Y), or these and black (K), so that the three colors of magenta, cyan and yellow, or Are formed by superimposing four color toner images obtained by adding black to the toner images. The three or four color toner images are collectively transferred onto the transfer paper supplied to the photosensitive drum 3 by the transfer charger 10 to obtain a color image on a transfer material. Thereafter, the transfer material is fixed by a fixing device, so that color mixing of the toner images of each color and fixing to the transfer material are performed, thereby forming a full-color print image. The photosensitive drum 3 on which the transfer of the toner image has been completed
After cleaning by the cleaner 20, the residual charge is removed by the pre-exposure lamp 21 to prepare for the next image formation.

In the image forming method of such a multiple development process, the magnitude of magnetization per unit volume in a magnetic field of 1000 Gauss is 100 emu, as in the first embodiment.
/ Cm ³ or less, and the average charge Q / M per unit weight of the carrier is c [μC /
g], and the mixing ratio of the toner to the developer is T / (T + C)
Assuming that [wt%], when c and T / (T + C) are 2 ≦ T / (T + C) <6, 1 / c ≧ −1 / 10 · T / (T + C) +0.856 6 ≦ T / ( T + C) By satisfying the relationship of c ≦ 5, it was possible to obtain a good halftone image in the entire density region without carrier adhesion and little roughness.

Embodiment 3 FIG. 20 is a schematic structural view showing still another embodiment of the image forming apparatus which can be used for carrying out the present invention. Although the image forming apparatus is a full-color laser beam printer, unlike the second embodiment, it has dedicated photosensitive drums 3Y, 3M, 3C, and 3K for each color. Around these photosensitive drums 3Y, 3M, 3C, 3K, respective laser beam scanners 80Y, 80M, 80
C, 80K, developing units 1Y, 1M, 1C, 1K, transfer chargers 10Y, 10M, 10C, 10K, cleaner 12Y,
12M, 12C and 12K are arranged. In the same manner as described above, a yellow toner image, a magenta toner image, a cyan toner image, and a black toner image are formed on the photosensitive drums 3Y, 3M, 3C, and 3K, respectively.

The transfer material is supplied to the conveyor belt 9a via the paper feed guide 5a, the paper feed roller 6, and the paper feed guide 5b.
Upon receiving the corona discharge from the attraction charger 81, it is reliably held on the transport belt 9a by electrostatic attraction. The transfer material held on the transport belt 9a is sequentially transported by the transport belt 9a to transfer sections facing the respective photosensitive drums 3Y to 3K, and each photosensitive drum is transferred by a transfer charger 10 to 10K arranged in each transfer section. The toner images of the respective colors on 3Y to 3K are sequentially superimposed and transferred onto the transfer material. The transfer material to which the four color toner images have been transferred is neutralized by the static eliminator 82 and separated from the conveyor belt 9a, and then fixed by the fixing unit 17 to form a full-color print image.

Also in this embodiment, the magnitude of magnetization per unit volume in a magnetic field of 1000 Gauss is 100 emu.
/ Cm ³ or less, and the average charge Q / M per unit weight of the carrier is c [μC /
g], and the mixing ratio of the toner to the developer is T / (T + C)
When [wt%], c and T / (T + C) are 2 ≦ T / (T + C) <6. 1 / c ≧ −1 / 10 · T / (T + C) +0.856 6 ≦ T / (T + C) In the case of (1), by satisfying c ≦ 5, a favorable halftone image was obtained in all density regions without carrier adhesion and with little roughness.

[0097]

As described above, according to the image forming method of the present invention, a dot distribution electrostatic latent image is developed by a magnetic brush of a two-component developer without causing carrier adhesion, and a halftone or the like is obtained. It is possible to obtain a high-quality image with good image quality in the entire density region without any roughness in the low density portion.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram showing a preferable range of an average charge amount Q / M and a T / (T + C) ratio of a hard ferromagnetic carrier of a two-component developer used in the present invention.

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

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

FIG. 4 is an overall configuration diagram showing a color electrophotographic apparatus to which the present invention can be applied.

FIG. 5 is an explanatory diagram showing 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 showing a signal waveform of the PWM method.

FIG. 8 is a sectional view showing a developing device in a rotary developing device that can be used in the present invention.

FIG. 9 is a perspective view showing an apparatus for measuring the average charge amount of the toner of the two-component developer.

FIG. 10 is a diagram showing a correlation between magnetization of soft ferromagnetic carriers and ear density.

FIG. 11 is an explanatory diagram of a human eyesight limit.

FIG. 12 is an explanatory diagram showing the relationship between the intensity of the peak value of the developing magnetic field and the magnetization of the soft ferromagnetic carrier.

FIG. 13 is an explanatory diagram showing magnetic characteristics of a soft ferromagnetic carrier.

FIG. 14 is an explanatory diagram showing magnetic characteristics of a hard ferromagnetic carrier.

FIG. 15 is a diagram showing the correlation between the magnetization of the hard ferromagnetic carrier and the density of the ears, together with the correlation for the soft ferromagnetic carrier.

FIG. 16 is a diagram showing a correlation between magnetization of hard ferromagnetic carriers and ear density.

FIG. 17 is an explanatory diagram showing the relationship between T / (T + C) of a two-component developer and the reciprocal of the average charge amount of a carrier.

FIG. 18 is an explanatory diagram illustrating a relationship between T / (T + C) of a two-component developer and an average charge amount of a toner.

FIG. 19 is an overall configuration diagram illustrating another example of an image forming apparatus to which the present invention can be applied.

FIG. 20 is an overall configuration diagram showing still another example of an image forming apparatus to which the present invention can be applied.

[Explanation of symbols]

 Reference Signs List 1 developing device 3 photosensitive drum 9 transfer drum 22 two-component developer 25 developing sleeve 28 blade 29 magnet

Claims (2)

(57) [Claims] A latent image is formed on an image carrier, and a developing section facing the image carrier is provided with a toner carrier and a magnetic carrier on a developer carrier containing a magnetic field generating means.
In an image forming method of carrying a component developer and transporting, forming a magnetic brush of the developer by a magnetic field of a magnetic field generating means in a developing unit, and developing an latent image on the image carrier by the magnetic brush to obtain an image, The magnetic carrier is a hard ferromagnetic carrier, and the magnitude of magnetization per unit volume of the carrier is 100 [emu / cm ³ ] or less in a magnetic field of a developing magnetic pole located at a developing section of the magnetic field generating means; And the average charge amount Q / M per unit weight of the carrier is c
[ΜC / g], and the mixing ratio of the toner to the developer is T /
When (T + C) [wt%], c and T /
When (T + C) is 2 ≦ T / (T + C) <6, 1 / c ≧ −1 / 10 · T / (T + C) +0.85 When 6 ≦ T / (T + C), it satisfies the relationship of c ≦ 5. Characteristic image forming method. 2. The image forming method according to claim 1, wherein the dot latent image is formed by modulating a light emission time of a light source per pixel in accordance with an image density value.