JP3308681B2 - Image forming device - 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 apparatus for forming an image by developing an electrostatic latent image formed by an electrophotographic method or an electrostatic recording method using a developer comprising magnetic particles and toner particles. About.

[0002]

2. Description of the Related Art In recent years, digitalization of a control unit has been promoted with the development of full-color and systemized image forming apparatuses such as copying machines and printers.

For example, devices such as a laser beam printer which scans a laser beam and forms a dot latent image on a photosensitive member as a latent image carrier by turning on and off the laser beam to record a desired image are widely known. Have been. A typical use is for binary recording of characters, figures, and the like. Since the recording of such characters and figures does not require halftones, the printer can have a simple structure.

On the other hand, there is a printer capable of forming a halftone even with the above-mentioned binary recording system. As such a printer, a printer employing a dither method, a density pattern method, or the like is well known. However, as is well known, there is a disadvantage that a high resolution image cannot be obtained with a printer employing a dither method, a density pattern method, or the like. Therefore, in recent years, a multi-value recording method for forming a halftone in a minimum recording unit without lowering the high recording density has been proposed. In this method, halftone formation is performed by pulse width modulation (PWM) of a laser beam with an image signal. According to this method, a high-resolution and high-gradation image can be formed.

[0005]

However, when an image is output by, for example, a copying machine to which the above-described multi-value recording method is applied, roughening occurs in a halftone area having a reflection density of 0.3 or less. There was found. This roughness was not so much generated in a character document or the like, but was often generated in a low density area such as a photographic image.

[0006] Then, as a result of examining the cause of the occurrence of rough, the following became clear. That is, when a latent image of a highlight portion is formed by a normal dot latent image, when viewed microscopically, the latent image on the photoconductor is not a broad latent image like an analog latent image but a local latent image. I have. Furthermore, an attempt to reproduce the lower density image, accent latent image from the effects of the film thickness of the photosensitive member gradually decreases the maximum contrast V ₀ as shown in FIG. 17. For example, when trying to reproduce an image with a reflection density of about 0.2,
Maximum contrast V ₀ which the latent image at that time is from 150 to 20
It becomes about 0V.

Further, in the case of reverse development, the surface potential of the non-image portion to take the head, since it is 100~200V set higher than the DC component of the developing bias, when the maximum contrast V ₀ is 150~200V The potential difference V _{cont from} the DC component of the developing bias is about 0 to 50 V.
The fact that the potential difference V _cont is 0 to 50 V is a very unstable contrast whether the toner is applied to the photoconductor side or the sleeve side. Therefore, when developing a dot latent image with a two-component developer, the contact state of the magnetic brush greatly contributes to the development efficiency, resulting in the occurrence of roughness due to missing dots corresponding to unevenness of the magnetic brush ears. It becomes easier.

In order to eliminate the roughness caused by the unevenness of the ears, it is effective to increase the density of the magnetic brush. As one method for realizing a high-density magnetic brush, there is a method of reducing the intensity of magnetization of a magnetic carrier used for a developer. However, when the intensity of magnetization of the magnetic carrier is reduced, a phenomenon that the magnetic carrier adheres to the non-image portion may occur. This phenomenon is a phenomenon in which the magnetic carrier is charged to the opposite polarity to the toner and adheres to the photoreceptor due to the fogging potential for the non-image area. It is difficult to generate because the force attracted to the sleeve is large. However, when the magnetization strength of the magnetic carrier is reduced, the electrostatic force becomes larger than the magnetic force, so that the magnetic force easily adheres to the photoconductor.

SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide a magnetic brush for developing a dot latent image with a two-component developer capable of preventing a magnetic carrier from adhering to a non-image area and obtaining a good image without roughness. An object of the present invention is to provide an image forming apparatus including a developing device.

[0010]

The above object is achieved by an image forming apparatus according to the present invention. In summary, the present invention provides:
A latent image carrier on which a dot latent image is formed, and a developer carrier that carries a two-component developer containing magnetic carrier particles and toner particles and conveys the developer to a development position, facing the latent image carrier;
And a magnetic brush developing device having a plurality of magnetic field generating means fixedly arranged inside the developer carrier, wherein the image forming apparatus contacts and develops a dot latent image on the latent image carrier. 1 of magnetic carrier particles used for component developer
Intensity of magnetization per cubic centimeter is a developing area, a .sigma.d in an external magnetic field of the developer carrying member surface, when the average particle diameter of the magnetic carrier particles is ^{A, σd (emu / cm 3} ) × A (μm ) ≦ 6000, the maximum magnetic flux density peak in the magnetic field generating means is 800 gauss or more on the surface of the developer carrier, and the magnetic flux density peak of the magnetic field generating means is The position of the magnetic flux density peak of the magnetic field generating means fixedly arranged in the vicinity area of the latent image carrier and the position of the magnetic flux density peak of the magnetic field generating means arranged downstream of the developer carrier in the rotation direction are formed. Angle is 40 °
Image forming apparatus.

Preferably, the magnetic pole of the magnetic field generating means disposed on the downstream side in the rotation direction of the developer carrier with respect to the magnetic pole of the magnetic field generating means fixedly arranged in the vicinity area of the latent image carrier. It is different.

[0012]

Further, the angle between the position of the magnetic flux density peak of the magnetic field generating means and the position of the magnetic flux density peak of the magnetic field generating means arranged on the upstream side in the rotation direction of the developer carrier is within 40 °. Is preferred.

Preferably, the electric field when developing the toner particles by the developing device is an alternating electric field.

The latent image carrier is an electrophotographic photosensitive member,
It is preferable that the electrophotographic photosensitive member is exposed to a light beam having a pulse width modulated corresponding to the density of a recorded image to form a dot distribution electrostatic latent image.

[0016]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the image forming apparatus according to the present invention will be described below in more detail with reference to the drawings.

Embodiment 1 FIG. 1 is a schematic diagram showing an example of an electrophotographic copying machine to which the present invention can be applied. In the figure, first, the surface on which the original G is to be copied is set on the original platen 10 at the lower side.
Next, copying is started by pressing the copy button.
A unit 9 having a document irradiating lamp, a short focus lens array, and a CCD sensor integrally scans the document while irradiating the document, so that the reflected light of the irradiation scan light on the document surface is imaged by the short focus lens array. Incident on the CCD sensor. The CCD sensor is a light receiving unit, a transfer unit,
It consists of an output unit. The light signal is converted to an electric signal in the CCD light receiving unit, and is sequentially transferred to the output unit in synchronization with the clock pulse in the transfer unit. The charge signal is converted to a voltage signal in the output unit, and is amplified and reduced in impedance and output. You. The analog signal thus obtained is subjected to well-known image processing, converted into a digital signal, and sent to a printer unit.

FIG. 4 shows a schematic configuration of a laser scanning section 100 for scanning a laser beam in the above-described apparatus. When the laser beam is scanned by the laser scanning unit 100, first, the laser beam emitted from the solid-state laser element 102 is emitted from the solid-state laser element 102 by the light emission signal generator 101 based on the input image signal, so that the laser beam is emitted by the collimator lens system 103. such is converted into the light flux, the imaging further rotary polygon mirror 104 by the fθ lens unit 105a while being scanned in the arrow C ₀ direction of rotation in the direction of arrow b, 105b, in a spot shape on the scanned surface 106, such as a photosensitive drum by 105c Is done. By the scanning of the laser light, the surface to be scanned 106 is
An exposure distribution for one scan of the image is formed on the upper surface, and the scanned surface 106 is scrolled by a predetermined amount perpendicularly to the above-mentioned scanning direction for each scan. An exposure distribution is obtained.

Referring again to FIG. 1, the printer section receives the image signal and forms an electrostatic latent image using the laser scanning section as follows. The photosensitive drum 1 serving as a latent image carrier is driven to rotate in a direction around a center support shaft at a predetermined peripheral speed, and receives a uniform charging process of positive or negative polarity by a charger 3 during the rotation process. The light of the solid-state laser element 103, which is turned on and off in response to an image signal, is scanned on the uniformly charged surface by a rotating polygon mirror 104 that rotates with a light beam. Latent images are sequentially formed.

FIG. 2 is a schematic structural view of the developing device 2 for developing an electrostatic latent image formed on the photosensitive drum 1 as viewed from the back side in FIG. As shown in FIG. 1, a developing device 2 is provided to face the photosensitive drum 1 and transports a developer to a developing position, a roller-shaped magnet 12 fixedly arranged in the developing sleeve 11, and a developing device. Agitating screws 13 and 14 for agitating the developer, a regulating blade 15 disposed perpendicularly to the developing sleeve 11 to form a thin layer of the developer on the surface of the developing sleeve 11, the developer and the agitating screw 13,
A developing container 16 that houses the developing container 14 is provided. In addition, magnet 1
2 has magnetic poles S1, N2, N3, S4 and N1.

In such a developing device, first, the developer pumped up by the N3 pole as the developing sleeve 11 rotates is transported from the S4 pole to the N1 pole,
The thickness of the layer is regulated by the regulating blade 15, and a thin layer is formed on the developing sleeve 11. Here, when the developer formed as a thin layer is conveyed to the developing main pole S1, the magnetic force forms ears. The electrostatic latent image is developed by the spike-shaped developer. Thereafter, the developer on the developing sleeve 11 is returned into the developing container 16 by the repulsive magnetic field of the N2 pole and the N3 pole.

The toner image thus formed on the photosensitive drum 1 is electrostatically transferred onto a transfer material by a transfer charger 7 as shown in FIG. Thereafter, the transfer material is electrostatically separated by the separation charger 8 and conveyed to the fixing device 6, where it is heat-fixed to output an image.

On the other hand, the surface of the photosensitive drum 1 after the transfer of the toner image is subjected to removal of contaminants such as untransferred toner by the cleaner 5 and is repeatedly used for image formation.

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

FIG. 5 is a circuit block diagram showing an example of the pulse width modulation circuit, and FIG. 6 is a timing chart showing the operation of the pulse width modulation circuit.

In FIG. 5, reference numeral 401 denotes a TTL latch circuit for latching an 8-bit digital image signal;
A level converter 403 converts the TL logic level into a high-speed ECL logic level, and a D / A converter 403 converts the ECL logic level into 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;
406 is a clock oscillator for oscillating the clock signal 2f,
Reference numeral 407 denotes a triangular wave generator that generates a substantially ideal triangular wave signal in synchronization with the clock signal 2f, and 408 denotes a clock signal 2f.
Is divided by 1/2 to generate the image clock signal f.
/ 2 min cycle. Thus, the clock signal 2f has a cycle 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.

Next, the operation of the circuit having the above 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. The clock signal 2f is also provided inside the triangular wave generator 407 to maintain the duty ratio of the triangular wave signal at 50%.
Is once divided by か ら before generating a triangular wave signal c. Further, the triangular wave signal c has an ECL level (0 to -1).
V) and becomes 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 or the like corresponding to the pixel density to be formed.
Generate a WM signal. The pulse width corresponding to the low density pixel becomes narrower. The PWM signal is OV 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 in accordance with the PWM signal value obtained in this way, it is possible to obtain 256 gradations per pixel.

FIG. 6h shows the shape of the laser beam exposure area 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. 6, the horizontal axis for the signal waveforms a to g is time, and the horizontal axis for h is the distance in the beam scanning direction.

As the toner, a known toner in which a colorant, a charge controlling agent and the like are added to a binder resin can be used. In this embodiment, a toner having a volume average particle diameter of 8 μm was 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 is 1% Na using primary sodium chloride.
Prepare the cl aqueous solution.

The electrolytic solution in which the sample is suspended is subjected to a dispersion treatment for about 1 to 3 minutes by an ultrasonic disperser, and the particle size distribution of particles of 2 to 40 μm is measured by the above-mentioned Coulter Counter TA-II using an aperture of 100 μm. Is measured to determine the volume distribution. From these determined volume distributions,
The volume average particle size of the sample is obtained.

On the other hand, as the magnetic carrier, magnetic particles coated with an extremely thin resin on the surface thereof can be used. In this embodiment, those having an average particle diameter of 50 μm are used. The average particle size of the magnetic carrier is indicated by the maximum chord length in the horizontal direction. The measuring method is as follows. Diameter.

Referring to FIG. 2 again, the developing process of the developing device 2 by the two-component magnetic brush method will be described in more detail. A transport screw 13 is accommodated in the developing chamber R1. The developer in the developing chamber R <b> 1 is transported in the longitudinal direction of the developing sleeve 11 by the rotation of the transport screw 13.

A transport screw 14 is accommodated in the storage room R2. The transport screw 14 transports the toner along the longitudinal direction of the developing sleeve 11 by its rotation. The direction in which the developer is transported by the screw 14 is opposite to that of the screw 13. Transport screw 1
Openings are provided in the partition wall 19 that separates 3 and 14 in the longitudinal direction from each other in the near side and the back side.
Is transferred to the screw 14 from the other one of the openings. During this time, the toner is charged to a polarity for developing the latent image by friction with the magnetic carrier.

An opening is provided in a portion of the developing container 16 close to the photosensitive drum 1, and a non-magnetic developing sleeve 11 made of aluminum, non-magnetic stainless steel, or the like is provided in the opening. The developing sleeve 11 rotates in the direction of arrow b to carry and transport the developer in which the toner and the magnetic carrier are mixed to the developing unit. The magnetic brush of the developer carried on the developing sleeve 11 is a photosensitive drum 1 that rotates in the direction of arrow a in the developing section.
, And the electrostatic latent image is developed by toner in this developing section.

The developing sleeve 11 is supplied with a vibration bias in which a DC voltage is superimposed on an AC voltage by a power supply. 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 potential. As a result, an alternating electric field whose direction changes alternately is formed in the developing section. In this alternating electric field, the toner and the magnetic carrier vibrate violently, and the toner shakes off the electrostatic restraint on the developing sleeve and the magnetic carrier, and adheres to the photosensitive drum 1 corresponding to the latent image. The difference (peak-to-peak voltage) between the maximum value and the minimum value of the oscillation bias voltage was 2 kV, and the frequency was 2 kHz. As the waveform of the oscillation bias voltage, a rectangular wave, a sine wave, a triangular wave, or the like can be used. In the present embodiment, a rectangular wave is used. The DC voltage component is a value between the dark portion potential and the bright portion potential of the latent image, but it is determined that the absolute value is closer to the dark portion potential than the minimum bright portion potential. Is preferable in preventing fog toner from adhering.

The minimum gap between the developing sleeve 11 and the photosensitive drum 1 (this minimum gap position is in the developing section) is 0.1 mm.
The thickness is preferably from 2 to 1 mm, and in this example, it was 0.5 mm. A developer layer thickness regulating blade 15 is provided with the lower end portion close to the surface of the developing sleeve 11,
As a result, the developing sleeve 11 is carried and conveyed to the developing section.
The thickness of the component developer is regulated.

A roller-shaped magnet 12 is fixedly arranged in the developing sleeve 11. The magnet 12 has a developing magnetic pole S1 facing the developing section. A magnetic brush of developer is formed by a developing magnetic field formed by the developing magnetic pole S1 in the developing section, and the magnetic brush contacts the photosensitive drum 3 to develop a dot distribution electrostatic latent image. At this time, the toner adhering to the ears (brushes) of the magnetic carrier and the toner adhering to the sleeve surface other than the ears are transferred to the exposed portion of the latent image and developed.

In this embodiment, as described above, the magnet 12 is
In addition to the developing magnetic pole S1, it has N2, N3, S4 and N1 poles. With this configuration, the developer pumped up at the N3 pole by the rotation of the developing sleeve 11 is transported from the S4 pole to the N1 pole, and the thickness of the developer is regulated by the regulating member 15 on the way to form a thin developer layer. You. Then, the developer spiked in the magnetic field of the developing magnetic pole S1 develops the electrostatic latent image on the photosensitive drum 1. Thereafter, the developer on the developing sleeve 11 falls into the stirring chamber R1 due to the repulsive magnetic field between the N2 pole and the N3 pole. The developer dropped into the stirring chamber R1 is a screw 13,
It is stirred and transported by 14.

In FIG. 3 showing a modification of the developing device, the magnet 121 fixed inside the developing sleeve 11 has N2, S2, S3, N in addition to the developing magnetic pole S1.
4, S4 and N1 poles. With such a configuration,
The developer pumped by the S3 pole by the rotation of the developing sleeve 11 is transported in the order of N4 pole → S4 pole → N1 pole, and the regulating member 15 regulates the layer thickness along the way to form a thin developer layer. . Then, the developer rising in the magnetic field of the developing magnetic pole S1 develops the electrostatic latent image on the image carrier 3. afterwards,
After passing through the N2 pole, the developer on the developing sleeve 11 drops into the stirring chamber R1 due to the repulsive magnetic field between the S2 and S3 poles. The developer that has fallen into the stirring chamber R1 is stirred and transported by the screws 13 and 14.

When the above-mentioned problem was examined using such a developing apparatus, the density (the number of magnetic brushes per unit area) of the magnetic brush formed by the developer was reduced in order to eliminate the aforementioned roughness. It was found that it was necessary to increase the height in the developing section.

It has also been found that one method for realizing a high-density magnetic brush is to reduce 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 carrier having a height of 6.5 mm and a height of 10 mm is filled with a magnetic carrier under a load of about 2 kg, and the strength of the magnetization is measured while the magnetic carrier does not move in the container.

In this embodiment, a soft ferromagnetic carrier having BH characteristics as shown in the graph of FIG. 7 was used.
The relationship between the intensity of this magnetization and the density of the ears of the magnetic brush will be described. Using the developing sleeve 1 in which the peak value of the magnetic flux density in the normal direction of the developing magnetic pole S1 (see FIGS. 2 and 3) is 1000 Gauss, the magnetization of the magnetic carrier when the magnetic force (magnetic flux density) is 1000 Gauss. When the relationship between the value and the density of the magnetic brush in the developing section was examined, it was found that the relationship was as shown in the graph of FIG. As shown in the graph, the lower the magnetization value of the magnetic carrier, the higher the density of the magnetic brush ears. However, at this time, the phenomenon of magnetic carrier adhesion to the non-image area is likely to occur.

Here, the carrier adhesion phenomenon will be described. Carrier adhesion is a phenomenon that occurs when the magnetic force acting on the magnetic carrier is smaller than the electrostatic force and the magnetic carrier is attracted to the photosensitive drum side. A magnetic force is applied to the magnetic carrier in the direction of attracting the developing sleeve toward the developing sleeve due to the magnetic field generated by the magnet roller, and an electrostatic force [(peak voltage Vpp, a direction in which the magnetic carrier is attracted toward the photosensitive drum) is attracted toward the photosensitive drum by the developing bias. + (Fogging voltage)]. The magnetization of the magnetic carrier is represented by σ (emu /
cm ^3), the magnetic carrier one volume V (cm ^3), when the magnetic field of the magnet roller and B (gauss), magnetic force F _r of the developing sleeve radially acting on one magnetic carrier
(Dyn / number) can be expressed as: F _r = ∇ _r (σV · B)... CGS unit system. Even with magnetic carriers having the same charge amount, when the magnetization σ is small in a magnetic field of the same magnitude, the magnetic force is reduced and the carrier is easily attached.

In the case of the magnetic carrier as described above, FIG.
As can be seen from the graph of FIG.
The magnetization intensity is saturated near Gauss. Therefore, when one such magnetic carrier exists in a magnetic field near 1000 Gauss or more, σV can be obtained before the gradient as in the following equation.

F _r (dyn / piece) = ∇ _r (σV · B) = σV · ∇ _r (B)... CGS unit system, and the magnitude of this force is represented by the absolute value of the strength of the magnetic field, It is proportional to the gradient in the direction perpendicular to the sleeve surface, and the direction of the force is the direction toward the center of the developing roller. Therefore, the electrostatic force acting on one carrier in a direction to attract the development bias to the photosensitive drum side is F _e, when using the same charge amount of the carrier, the developing sleeve, the contact at the time of the photosensitive drum actuating NIP (carrier photosensitive drum (In the area in contact with the substrate), carrier adhesion starts when F _e > F _r .

[0052] Here, F _e, for F _r is in the following relation ^{_{respectively, F e = 4πα 2 δE (}} α: the radius of the carrier, [delta]: charge density of the carrier surface,
E: electric field intensity) F _r = σ · (4/3) πα ³ · ∇ _r (B) (σ: intensity of magnetization per unit volume of carrier) From the following equation, carrier adhesion occurs. Easier to do.

Σ · α · ∇ _r (B) <3δE From the above equation, when 3δE is constant, when σ · α is small and ∇ _r (B) is not high, carrier adhesion occurs. Here, α is the radius of one carrier, not the average particle size.

However, as the developing magnetic pole S1, a magnetic flux having a peak value of 1,000 gauss in the direction normal to the developing sleeve surface on the developing sleeve surface is used, and the average particle diameter A of the magnetic carrier and the developing area are determined. The product σd · of the average magnetization value σd of the carrier per unit area at
As a result of examining the relationship between A and the start voltage of carrier adhesion (DC component of development bias... Fog removal bias),
As shown in Table 1 below and FIG. 9, it was found that the carrier adhesion start voltage has a correlation with σd · A, and that the larger σd · A, the more difficult the carrier adhesion.

[0055]

[Table 1]

In order to maintain a good image, the DC component in the carrier attachment direction bias for removing fog is as follows.
It is desired that carrier adhesion does not occur up to about 200V. From Table 1 and FIG. 9, in the case of σd · A <6000, the carrier attachment direction bias DC component is 20%.
At 0 V or less, carrier adhesion occurs.

In this embodiment, as described above, σd · A <60
This is particularly effective in the case of 00, and the occurrence of carrier adhesion can be prevented even when a carrier having a low σd or a carrier having a small A is used.

In this embodiment, this phenomenon is prevented by the pattern of the magnet fixed inside the developing sleeve. Result of examination variously changing the pattern of the magnet in the developing sleeve, it is that in order to prevent the carrier adhesion phenomenon of magnetic attraction force F _r in the radial direction in the downstream portion of the developing region, it is more advantageous to increase It turns out. In order to increase the magnetic attraction force _Fr , it is necessary to increase the gradient of the magnetic flux density change in the radial direction by the magnet in the downstream portion of the developing area.

[0059] In this embodiment, in order to increase the magnetic attractive force F _r in the radial direction in the development area downstream section,
An N2 pole having a different polarity is disposed in the vicinity and downstream of the developing magnetic pole S1.
A peak pole position of the magnetic flux density of the magnet used in the present embodiment, the magnetic flux density B _r in the radial direction are shown in Table 2 below. The magnetic pole pattern of the magnet was centered on the developing magnetic pole S1, and the rotation direction of the developing sleeve was at a positive angle.

[0060]

[Table 2]

[0061] Here, the magnetic attraction force F _r in the radial direction can be expressed by the following equation as described above.

[0062] F _r (dyn / _{number) = ∇ r (σV · B} ) = σV · ∇ r (B) ··· CGS unit system ∇ _r (B) magnetic binding force F _r of the relative by the magnet by determining the it is possible to know the magnitude, it is possible to know the distribution of the magnetic binding force F _r, the peak position of the magnetic binding force F _r. In the present embodiment, by measuring the magnetic flux density at a position away 1mm from the magnetic flux density and the surface of the developing sleeve in the developing sleeve surface, it calculates the value of the magnetic binding force F _r in the developing area for various developing sleeve in Table 2 Relative evaluation was performed.

Each of FIGS. 10 and 11 (a) to (f)
In each figure, the horizontal axis shows the position in the circumferential direction of the developing sleeve by an angle, and the vertical axis shows the developing magnetic pole S1 in FIG.
Distribution form of magnetic flux density B _r in the neighborhood indicates a relative value of the magnetic attraction force F _r in FIG. 11.

As can be seen from FIGS. 10 and 11, when the proximity pole N2 is brought closer to the development magnetic pole S1, the magnetic attraction force _Fr increases. In addition, when the downstream pole is brought closer, _Fr
There was up, it can be seen that the closer to the upstream pole F _r of the upstream portion is up. When the development of carrier adhesion was examined using such various developing sleeves, FIG.
Were obtained. In the graph of FIG. 12, a magnetic carrier having an average particle diameter of 50 μm and a magnetization intensity of 64 emu / cm ³ at an external magnetic field of 1000 gauss is used. The alternating electric field of the developing bias has an amplitude of 2 kv and a frequency of 2 k.
Hz, the fog removal bias DC component
It shows the number of carriers attached to a non-image area per unit area.

As can be seen from the above results, as shown in the magnetic pole patterns (c), (d) and (e), the developing magnetic pole S
By providing the different pole N2 within 40 ° downstream of 1, the carrier adhesion to the non-image area is reduced. The developing magnetic pole S
No significant effect was obtained above 40 ° downstream of 1. Further, when the different pole N2 is provided within 40 ° both upstream and downstream of the developing magnetic pole S1, the magnetic field gradient can be further increased, and the carrier adhesion can be further reduced. From FIGS. 10 and 11, it can be seen that the carrier adhesion phenomenon is affected by _{Fr at} the downstream portion of the developing region. The above-described development area is not a nip caused by the developer but an area where the toner can fly.

As described above, the magnetic flux density peak of the developing magnetic pole S1 is arranged in the area close to the photosensitive drum and the developing sleeve, and the different pole N2 having the magnetic flux density peak is provided at a position within 40 ° downstream of the developing sleeve in the rotation direction. Thereby, carrier adhesion can be prevented, and a good image without roughness can be obtained.

Embodiment 2 In the first embodiment, the magnetic flux density of the developing magnetic pole S1 is set to 10
00 gauss, but in this embodiment, the developing magnetic pole S1
Has been increased to 1500 gauss. The configuration other than the above is the same as in the first embodiment. When Gauss is increased, the magnetic attraction force _Fr also increases due to the magnetic field gradient. In the magnet used in this embodiment, the magnetic pole pattern was set to the same pole position as in the first embodiment as shown in Table 2, and was simply raised by Gauss of the developing magnetic pole S1. This allows
The carrier adhesion phenomenon was caused by an average particle diameter of the carrier of 50 μm and an magnetization intensity of 64 emu / at an external magnetic field of 1000 Gauss.
In the case of cm ³ , the alternating electric field of the developing bias has an amplitude of 2 kv and a frequency of 2 kHz, it decreases as shown in the graph of FIG. Also in this case, as shown in (c), (d) and (e) of the magnetic pole pattern, by providing the different pole N2 within 40 ° downstream of the developing magnetic pole S1, carrier adhesion to the non-image area can be prevented. Greatly reduced. In addition, 40 ° downstream of the developing magnetic pole S1.
As described above, no significant effect was obtained when different polarities were provided. Further, both upstream and downstream of the developing magnetic pole S1 are 40
When the different poles were provided within °, the magnetic field gradient could be further increased and the carrier adhesion could be further reduced.

In addition, the case where the magnetic force is weakened is examined, and when the magnetic force is weakened to 800 Gauss or less, the magnetic attraction force F is reduced.
_r weakened, and the effect of the present invention could not be sufficiently obtained.

As described above, the magnetic flux density peak of the developing magnetic pole S1 is arranged in the vicinity of the photosensitive drum and the developing sleeve, and the different pole N2 having the magnetic flux density peak is provided at a position within 40 ° downstream of the developing sleeve rotation direction. As a result, it is possible to prevent carrier adhesion and obtain a good image without roughness.

Embodiment 3 In the first and second embodiments, a soft ferromagnetic carrier having the characteristics shown in the graph of FIG. 7 was used. In the present embodiment, the characteristics shown in the graph of FIG. Was used. As the magnetic carrier, a hard ferromagnetic carrier having an average particle size of 50 μm and a magnetization intensity of 68 emu / cm ³ at an external magnetic field of 1000 Gauss was used. Other configurations were the same as those of the first embodiment. As for the developing sleeve, similarly to the first embodiment,
A developing sleeve having a magnetic pole pattern as shown in Table 2 was used. As a result, in this example, the result as shown in the graph of FIG. 15 was obtained. In the graph of FIG.
Alternating electric field of developing bias has amplitude of 2 kv and frequency of 2 kHz
5 shows the number of carriers attached to the non-image area per unit area with respect to the DC component of the fog removal bias. The result is slightly better than the result of the first embodiment. As in the first embodiment, the dependence on the magnet is different from that of the different pole N2 within 40 ° downstream of the developing magnetic pole S1, as shown in (c), (d) and (e) of the magnetic pole pattern.
, The carrier adhesion to the non-image area is reduced. Note that no significant effect was obtained at 40 ° or more downstream of the developing magnetic pole S1. Further, when the different pole N2 is provided within 140 ° of the developing magnetic pole S1 both upstream and downstream of the developing magnetic pole S1, the magnetic field gradient can be further increased and the carrier adhesion can be further reduced. 11 and the carrier adhesion phenomena Figure 15, the downstream portion of the developing region, the magnetic force F _r acting on the developing sleeve radially acting on one magnetic carrier
It can be seen that the influence is exerted. The above-mentioned development area is an area where the toner can fly, not a nip caused by the developer.

As described above, the magnetic flux density peak of the developing magnetic pole S1 is arranged in the area close to the photosensitive drum and the developing sleeve, and the different pole N2 having the magnetic flux density peak is located at a position within 40 ° downstream of the developing sleeve rotation direction. By providing the carrier, it is possible to prevent carrier adhesion and obtain a good image without roughness.

Embodiment 4 As a method for improving the image quality, there is a method for reducing the particle diameter of the toner in addition to the method for increasing the density of ears.
However, when the particle size of the toner is reduced, it is necessary to reduce the particle size of the magnetic carrier so as not to change the toner mixing ratio due to the specific surface area.

In the first, second and third embodiments, the developer was obtained by mixing a toner having a volume average particle diameter of 8 μm and a magnetic carrier having an average particle diameter of 50 μm at a weight mixing ratio of 5:95. In this embodiment, the volume average particle size is 6 μm
The toner was mixed with a magnetic carrier having an average particle size of 35 μm at a weight mixing ratio of 5:95 to obtain a developer. Also,
In this embodiment, as in the third embodiment, a hard ferromagnetic carrier having the characteristics shown in the graph of FIG. 14 is used.
8 emu / cm ^3, the residual magnetization in the external magnetic field 0 Gauss was used for 64emu / cm ^3. Other configurations were the same as those of the second embodiment. As for the developing sleeves, the developing magnetic poles S1 of various developing sleeves shown in Table 2 were increased to 1500 gauss as in the second embodiment. In the present embodiment, the results shown in the graph of FIG. 16 were obtained. The graph of FIG. 16 shows that the alternating electric field of the developing bias has an amplitude of 2 kv and a frequency of 2 kHz.
The figure shows the number of carriers attached to a non-image portion per unit area with respect to the fog removal bias DC component. As with the first embodiment, the dependence on the magnet is reduced by providing the different pole N2 within 40 ° of the downstream portion of the developing magnetic pole S1 as shown in (c), (d) and (e) of the magnetic pole pattern. Carrier adhesion to the image area is reduced. It should be noted that no significant effect was obtained when the different pole was provided at a position 40 ° or more downstream of the developing magnetic pole S1. Further, when the different pole N2 is provided within 40 ° both upstream and downstream of the developing magnetic pole S1, the magnetic field gradient can be further increased, and the carrier adhesion can be further reduced. The carrier adhesion development from FIGS. 11 and 16, it can be seen that the downstream portion of the developing region, the magnetic force F _r acting on the developing sleeve radially acting on one magnetic carrier are affected. The above-mentioned development area is an area where the toner can fly, not a nip caused by the developer. By reducing the toner particle size as in this embodiment,
When the density of the magnetic brush was increased and an image was output, the effect of the toner particle diameter was added, and there was no roughness in the low density area, and a better image than the first, second and third embodiments could be obtained. .

As described above, the magnetic flux density peak of the developing magnetic pole S1 is arranged in the vicinity of the photosensitive drum and the developing sleeve, and the different pole N2 having the magnetic flux density peak is located within 40 ° downstream of the developing sleeve in the rotating direction. By providing the carrier, it is possible to prevent carrier adhesion and obtain a good image without roughness.

[0075]

As is apparent from the above description, in the image forming apparatus according to the present invention, the intensity of magnetization per cubic centimeter of the magnetic carrier particles used in the two-component developer is as follows.
When the magnetic field is Ad in the external magnetic field of the developing region and the surface of the developer carrier, and the average particle size of the magnetic carrier particles is A, the magnetic property that satisfies the relationship of σd (emu / cm ³ ) × A (μm) ≦ 6000 is satisfied. By using the carrier particles, the density of the ears of the magnetic brush can be increased,
Roughness of an image can be prevented. When the density of the brush of the magnetic brush is increased according to the formula, the magnetic carrier tends to adhere to the latent image carrier. However, the maximum magnetic flux density peak in the magnetic field generating means is 800 at the surface of the developer carrier. A magnetic field that is equal to or more than Gauss, and the magnetic flux density peak of the magnetic field generating means is fixedly arranged in the vicinity of the latent image carrier, and the magnetic flux density peak of the magnetic field generating means and the magnetic field arranged downstream of the developer carrier in the rotation direction With a configuration in which the angle between the position of the magnetic flux density peak of the generating means and the position of the magnetic flux density peak is within 40 °, carrier adhesion can also be prevented.
As described above, according to the present invention, it is possible to achieve both prevention of image roughness and prevention of carrier adhesion, and to obtain a good image.

[Brief description of the drawings]

FIG. 1 is a schematic configuration diagram showing an embodiment of an image forming apparatus according to the present invention.

FIG. 2 is a configuration diagram illustrating a first embodiment of a developing device of the image forming apparatus of FIG. 1;

FIG. 3 is a modified example of the developing device of FIG. 2;

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

FIG. 5 is a schematic circuit diagram of a PWM circuit.

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

FIG. 7 is a graph showing a hysteresis curve of a soft ferromagnetic carrier.

FIG. 8 is an explanatory diagram showing a relationship between the intensity of magnetization of a magnetic carrier and the density of a magnetic brush.

FIG. 9 is a graph showing a relationship between a fog removal bias and a carrier attached to a non-image area when the magnetization strength of the magnetic carrier is different.

FIG. 10 is a graph showing a magnetic flux density distribution in a radial direction near a developing magnetic pole of a plurality of developing sleeves used in an embodiment of the present invention.

FIG. 11 is a graph showing relative values of magnetic attraction force distribution in the radial direction near the developing magnetic poles of a plurality of developing sleeves used in the example of the present invention.

FIG. 12 is a graph showing a relationship between a fog removing bias and a carrier attached to a non-image portion in the first embodiment.

FIG. 13 is a graph showing a relationship between a fog removing bias and a carrier attached to a non-image portion in the second embodiment.

FIG. 14 is a graph showing a hysteresis curve of a hard ferromagnetic carrier.

FIG. 15 is a graph showing a relationship between a fog removing bias and a carrier attached to a non-image portion in the third embodiment.

FIG. 16 is a graph showing a relationship between a fogging bias and a carrier attached to a non-image portion in the fourth embodiment.

FIG. 17 is an explanatory diagram of a potential of a dot latent image.

[Explanation of symbols]

 Reference Signs List 1 photosensitive drum (latent image carrier) 2 developing device 11 developing sleeve (developer carrier) 12 magnet (magnetic field generating means) 121 magnet (magnetic field generating means)

──────────────────────────────────────────────────続 き Continuation of the front page (72) Inventor Masaru Hibino 3-30-2 Shimomaruko, Ota-ku, Tokyo Inside Canon Inc. (56) References JP-A-5-46028 (JP, A) JP-A-3 -4268 (JP, A) JP-A-1-102588 (JP, A) JP-A-62-267783 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) G03G 15/09

Claims (5)

(57) [Claims] A latent image carrier on which a dot latent image is formed;
A developer carrier that carries a two-component developer including magnetic carrier particles and toner particles and conveys the developer to a developing position, facing the latent image carrier; and a plurality of developer carriers fixedly disposed inside the developer carrier. And a magnetic brush developing device having a magnetic field generating means for contact-developing a dot latent image on the latent image carrier, wherein the magnetization of magnetic carrier particles used for a two-component developer per cubic centimeter is Σd (emu / cm ³ ) × A (μm) ≦ 6000 when the strength of the magnetic field is Ad in the external magnetic field of the developing region and the surface of the developer carrier, and the average particle size of the magnetic carrier particles is A. Using magnetic carrier particles that establish the relationship, the maximum magnetic flux density peak in the magnetic field generating means is 800 gauss or more on the surface of the developer carrier, and the magnetic flux density peak of the magnetic field generating means is The angle formed between the position of the magnetic flux density peak of the magnetic field generating means and the position of the magnetic flux density peak of the magnetic field generating means arranged on the downstream side in the rotation direction of the developer carrier is fixedly arranged in the proximity area of the image carrier. An image forming apparatus characterized by being within 40 °. 2. A magnetic pole of said magnetic field generating means, which is arranged on the downstream side in the rotation direction of said developer carrier, with respect to a magnetic pole of said magnetic field generating means, which is fixedly arranged in an area adjacent to said latent image carrier. The image forming apparatus according to claim 1, wherein the image forming apparatus is a pole. 3. The angle between the position of the magnetic flux density peak of the magnetic field generating means and the position of the magnetic flux density peak of the magnetic field generating means arranged on the upstream side in the rotation direction of the developer carrier is within 40 °. 3. The method according to claim 1, wherein
Image forming apparatus. 4. The image forming apparatus according to claim 1, wherein an electric field when developing the toner particles by the developing device is an alternating electric field. 5. The latent image carrier is an electrophotographic photosensitive member, and the electrophotographic photosensitive member is exposed to a light beam having a pulse width modulated corresponding to the density of an image to be recorded to form a dot distribution electrostatic latent image. The image forming apparatus according to claim 1, wherein the image forming apparatus is formed.