JP3444942B2 - 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 with a developer composed of magnetic particles and toner particles. Regarding

[0002]

2. Description of the Related Art Recently, digitalization of a control unit has been advanced along with the full-color and systematization of image forming apparatuses such as copying machines and printers.

For example, a laser beam printer or the like which scans a laser beam and forms a dot latent image on a photosensitive member, which is a latent image carrier, by recording the desired image by turning the laser beam on and off is widely used. It is becoming known. Its typical application is binary recording of characters and figures. Since printing of such characters and figures does not require halftones, the printer structure can be made simple.

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

[0005]

However, when an image is output by a copying machine to which the above-described multi-value recording system is applied, there is a possibility that a rough image is generated in a halftone area having a reflection density of 0.3 or less. found. This rubbing did not occur so much in a text original or the like, but occurred in a low density area such as a photographic image.

[0006] Then, as a result of examining the cause of the rubbing, the following facts have become clear. In other words, when forming a latent image of a highlight part by a normal dot latent image, the microscopic latent image on the photoconductor is not a broad latent image like an analog latent image but a local latent image. There is. 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. 10. For example, when trying to reproduce an image with a reflection density of about 0.2,
The maximum contrast V ₀ of the latent image at that time is 150 to 20.
It will be about 0V.

Further, in the case of reversal development, the surface potential of the non-image portion is set to 100 to 200 V higher than the DC component of the developing bias in order to remove fog, so that when the maximum contrast V ₀ is 150 to 200 V. The potential difference V _{cont from} the DC component of the developing bias is about 0 to 50V.
The potential difference V _{cont of 0} to 50 V is a very unstable contrast whether the toner adheres to the photoconductor side or the sleeve side. Therefore, when a dot latent image is developed with a two-component developer, the contact state of the magnetic brush greatly contributes to the development efficiency, and there is roughness due to dot loss or the like corresponding to the unevenness of the brush of the magnetic brush. It will be easier.

Therefore, it is an object of the present invention to prevent the occurrence of roughness when developing a low-density image area, and
An object of the present invention is to provide an image forming apparatus including a magnetic brush developing device that develops a dot latent image with a two-component developer that can prevent the carrier from adhering to the latent image carrier.

[0009]

The above object can be achieved by an image forming apparatus according to the present invention. In summary, the present invention is
A latent image carrier on which a dot latent image is formed; a developer carrier that faces the latent image carrier and carries a two-component developer containing magnetic particles and toner particles and conveys the developer to a developing position; A magnetic brush developing device having a magnetic field generating means having a plurality of magnetic poles fixedly arranged inside a developer carrier,
An image forming apparatus for contact development dot latent image on the latent image carrier, wherein the magnetic particles are hard ferromagnetic carrier an external magnetic field has a magnetization at 0, the plurality of magnetic
A developing magnetic pole of one is the magnetic flux density peak Ru is fixedly disposed proximal region of the latent image bearing member of poles, the developer
Upstream and below the developing magnetic pole in the rotation direction of the carrier.
The magnetic pole arranged on the flow side is different from the developing magnetic pole.
The angle formed by the position of the magnetic flux density peak of the developing magnetic pole and the position of the magnetic flux density peak of the magnetic pole arranged on the upstream side in the rotation direction of the developer carrying member is set within 40 °,
Further, in the image forming apparatus, the density of the magnetic brush formed in the magnetic flux density peak portion of the developing magnetic pole is 6 / mm ² or more.

[0010]

Preferably, the electric field at the time of developing the toner particles by the developing device is an alternating electric field.

Preferably, the latent image carrier is an electrophotographic photosensitive member, and the electrophotographic photosensitive member is exposed by a light flux modulated by a pulse width modulated signal corresponding to the density of a recorded image. A dot distribution electrostatic latent image is formed.

[0013]

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 configuration 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 document G is to be copied is set on the document table 10 on the lower side.
Then, copying is started by pressing the copy button.
A unit 9 in which an original irradiation lamp, a short focus lens array, and a CCD sensor are integrally formed scans the original while irradiating the original, and the light reflected from the original surface of the irradiation scanning light is imaged by the short focus lens array. Is incident on the CCD sensor. The CCD sensor has a light receiving part, a transfer part,
It is composed of an output section. The optical signal is converted into an electric signal in the CCD light receiving part, and is sequentially transferred to the output part in synchronization with the clock pulse in the transfer part, and the charge signal is converted into a voltage signal in the output part, amplified and reduced in impedance to be output. It The analog signal thus obtained is subjected to known image processing, converted into a digital signal, and then sent to the printer section.

FIG. 4 shows a schematic structure of a laser scanning section 100 for scanning a laser beam in the above apparatus. When scanning the laser light by the laser scanning unit 100, first, the laser light emitted from the solid-state laser element 102 by the light emission signal generator 101 based on the input image signal is substantially parallel by the collimator lens system 103. Is converted into a light beam and further scanned in the direction of the arrow C ₀ by the rotating polygon mirror 104 rotating in the direction of the arrow b, and an image is formed in a spot shape on the surface to be scanned 106 such as the photosensitive drum by the fθ lens groups 105a, 105b and 105c. To be done. The surface to be scanned 106 is scanned by such laser light.
An exposure distribution for one scan of an image is formed on the upper side, and if the surface to be scanned 106 is scrolled by a predetermined amount in a direction perpendicular to the scanning direction for each scan, the surface to be scanned 106 is responsive to the image signal. The exposure distribution is obtained.

Referring again to FIG. 1, the printer section receives the above image signal and forms an electrostatic latent image as follows using the laser scanning section. The photosensitive drum 1, which is a latent image carrier, is rotationally driven in the direction a at a predetermined peripheral speed around a central support shaft, and in the course of its rotation, a uniform charging process of positive or negative polarity is performed by the charger 3. The surface of the photosensitive drum 1 corresponds to the original image by scanning the uniformly charged surface of the solid-state laser element 103, which is turned on and off according to an image signal, by a rotating polygon mirror 104 that rotates with a light beam. An electrostatic latent image is sequentially formed.

FIG. 2 is a schematic structural view of the developing device 2 for developing the electrostatic latent image formed on the photosensitive drum 1 as seen from the back side of FIG. As shown in FIG. 1, the developing device 2 includes a developing sleeve 11 that is disposed so as to face the photosensitive drum 1 and that conveys the developer to a developing position, a roller magnet 12 that is fixedly disposed in the developing sleeve 11, and a developing device. Agitating screws 13 and 14 for agitating the developer, a regulating blade 15 arranged vertically to the developing sleeve 11 for forming 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 accommodating 14 is provided. The magnet 1
2 has magnetic poles S1, N2, N3, S4 and N1.

In such a developing device, first, the developer drawn up by the N3 pole with the rotation of the developing sleeve 11 is conveyed from the S4 pole to the N1 pole in the process.
The layer thickness is regulated by the regulation blade 15, and a thin layer is formed on the developing sleeve 11. Here, when the thin-layered developer is conveyed to the main developing pole S1, the spikes are formed by its magnetic force. The electrostatic latent image is developed by the developer formed in the spike shape. After that, 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.

In this way, the toner image formed on the photosensitive drum 1 is electrostatically transferred onto the transfer material by the transfer charger 7, as shown in FIG. After that, the transfer material is electrostatically separated by the separation charger 8 and conveyed to the fixing device 6, where it is thermally fixed and an image is output.

On the other hand, the surface of the photosensitive drum 1 after transfer of the toner image is repeatedly used for image formation after the cleaner 5 removes adhered contaminants such as transfer residual toner.

Next, in the present embodiment, the PWM method (pulse width modulation method) is used to perform multi-valued recording in which the minimum recording unit is one pixel. Therefore, 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, 401 is a TTL latch circuit for latching an 8-bit digital image signal, and 402 is T.
A level converter for converting the TL logic level into a high-speed ECL logic level, and a D / A converter 403 for converting the ECL logic level into an analog signal. Reference numeral 404 is an ECL comparator for generating a PWM signal, 405 is a level converter for converting an ECL logic level into a TTL logic level,
406 is a clock oscillator that oscillates the clock signal 2f,
Reference numeral 407 denotes a triangular wave generator that generates a substantially ideal triangular wave signal in synchronization with the clock signal 2f, and 408 denotes the clock signal 2f.
1 is divided by 2 to generate the image clock signal f 1
/ It is a 2-minute cycle. As a result, the clock signal 2f has a cycle twice that of the image clock signal f.
In order to operate the circuit at high speed, ECL logic circuits are arranged everywhere.

The circuit operation of the above structure 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 are associated with the image signal as shown in the drawing. Also, in order to keep the duty ratio of the triangular wave signal at 50% inside the triangular wave generator 407, the clock signal 2f
Is divided by 1/2 and the triangular wave signal c is generated. Further, the triangular wave signal c is at the 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.
Up to (black), for example, 256 gradation levels change. The symbol "h" indicates hexadecimal notation. The image signal e indicates an ECL voltage level obtained by D / A converting some image signal values. For example, the first pixel indicates the highest density pixel level FFh, the second pixel indicates the intermediate tone level 80h, the third pixel indicates the intermediate tone level 40h, and the fourth pixel indicates the intermediate tone level 20h.

The comparator 404 compares the triangular wave signal d and the image signal e to obtain P of pulse width (time length) T, t ₂ , t ₃ , t ₄ etc. according to the pixel density to be formed.
Generate a WM signal. The pulse width corresponding to the low density pixel becomes narrower. And this PWM signal is OV or T of 5V.
The PWM signal f is converted into the TL level and is input to the laser driver circuit 500.

By changing the exposure time per pixel in accordance with the PWM signal value thus obtained, it is possible to obtain 256 gradations with one pixel.

Incidentally, FIG. 6h shows the laser beam exposure area shape of the photosensitive member corresponding to each drive pulse width. The area shape of each dot latent image also substantially corresponds to this exposure area shape.

In FIG. 6, the horizontal axis relating to the signal waveforms a to g is time, and the horizontal axis relating to h is the distance in the beam scanning direction.

As the toner, a known toner obtained by adding a colorant, a charge control agent and the like to a binder resin can be used, and 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, one measured by the following measuring method is used.

As a measuring device, a Coulter counter T
A-II type (manufactured by Coulter, Inc.)
Interface that outputs the volume average distribution (made by Nikkaki)
And CX-i personal computer (Canon) are connected, and the electrolyte is 1% Na using primary sodium chloride.
Prepare a cl aqueous solution.

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

On the other hand, as the magnetic carrier, those having an extremely thin resin coating on the surface of magnetic particles can be used, and in the present embodiment, those having an average particle diameter of 50 μm were used. The average particle diameter of the magnetic carrier is indicated by the maximum chord length in the horizontal direction, and the measurement method is a microscope method in which 300 or more magnetic carriers are randomly selected, the diameter is measured, and the arithmetic mean is taken to obtain the carrier particles of this embodiment. The diameter.

Now, with reference to FIG. 2 again, the developing process by the two-component magnetic brush method of the developing device 2 will be described in more detail. A conveying screw 13 is housed in the developing chamber R1. By the rotational driving of the carrying screw 13, the developer in the developing chamber R1 is carried in the longitudinal direction of the developing sleeve 11.

A transfer screw 14 is housed in the storage chamber R2. The conveying screw 14 conveys the toner along the longitudinal direction of the developing sleeve 11 by the rotation thereof. The developer transport direction by the screw 14 is opposite to that of the screw 13. Conveyor screw 1
An opening is provided on the front side and the back side of the partition wall 19 that separates the screws 3 and 14 from each other in the longitudinal direction.
The developer conveyed by is delivered to the screw 14 from the other one of the openings. During this time, the toner is charged with 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 near 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 convey the developer containing the toner and the magnetic carrier to the developing section. The magnetic brush of the developer carried on the developing sleeve 11 is the photosensitive drum 1 which rotates in the direction of arrow a in the developing section.
And the electrostatic latent image is developed with toner in this developing section.

An oscillating bias in which a direct current voltage is superimposed on an alternating current voltage is applied to the developing sleeve 11 by a power source. 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 portion. In this alternating electric field, the toner and the magnetic carrier violently vibrate, 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 between the maximum value and the minimum value of the vibration bias voltage (peak-to-peak voltage) was 2 kV, and the frequency was 2 kHz. A rectangular wave, a sine wave, a triangular wave, or the like can be used as the waveform of the vibration bias voltage, but the rectangular wave is used in this embodiment. The DC voltage component has a value between the dark portion potential and the light portion potential of the latent image, but the dark portion potential is closer to the dark portion potential than the minimum absolute light portion potential. It is preferable for preventing the adhesion of the fog toner.

Further, the minimum gap between the developing sleeve 11 and the photosensitive drum 1 (this minimum gap position is within the developing section) is 0.
It is preferably 2-1 mm, and in this embodiment, it is 0.5 mm. A developer layer thickness regulating blade 15 is provided so that the lower end portion is close to the surface of the developing sleeve 11.
As a result, the developing sleeve 11 carries and conveys it to the developing section.
The layer 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 the developing magnetic field formed by the developing magnetic pole S1 at the developing portion, and the magnetic brush comes into contact with the photosensitive drum 3 to develop the dot-distributed electrostatic latent image. At that time, both the toner attached to the brush (brush) of the magnetic carrier and the toner attached to the sleeve surface instead of the brush 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 such a configuration, the developer drawn up by the N3 pole by the rotation of the developing sleeve 11 is transported from the S4 pole to the N1 pole, and the layer thickness is regulated by the regulation member 15 in the middle thereof, and a thin developer layer is formed. It Then, the developer that has risen in the magnetic field of the developing magnetic pole S1 develops the electrostatic latent image on the photosensitive drum 1. After that, 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 screw 13,
It is stirred and conveyed by 14.

In FIG. 3 showing a modified example of the developing device, the magnet 121 fixedly arranged inside the developing sleeve 11 includes N2, S2, S3, N in addition to the developing magnetic pole S1.
4, S4 and N1 poles. With such a configuration,
The developer drawn up by the S3 pole by the rotation of the developing sleeve 11 is conveyed in the order of N4 pole → S4 pole → N1 pole, and the layer thickness thereof is regulated by the regulation member 15 in the middle thereof, and a thin developer layer is formed. . Then, the developer that is erected 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 repulsive magnetic field between the S2 and S3 poles causes the developer on the developing sleeve 11 to drop into the stirring chamber R1. The developer dropped into the stirring chamber R1 is stirred and conveyed by the screws 13 and 14.

When the above-mentioned problems were examined by using such a developing device, in order to eliminate the above-mentioned roughness, the density (the number per unit area) of the magnetic brush formed by the developer is set. It has been found that it is necessary to raise the height in the developing section.

As one method for realizing the high-density magnetic brush, there is a method of lowering the magnetization intensity of the magnetic carrier used for the developer. However, if the low magnetization of the magnetic carrier is lowered, the 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 with a polarity opposite to that of the toner, so that the magnetic carrier adheres to the photosensitive drum due to the fogging potential on the non-image portion.
It is hard to generate because the magnetic force attracts the developing sleeve to a great extent, but if the magnetization intensity of the magnetic carrier is lowered, the electric force becomes larger than the magnetic force, and the magnetic carrier easily adheres to the photosensitive drum.

As a method for increasing the density of the magnetic brush without causing the carrier adhesion phenomenon as described above, a magnetic flux density pattern in the normal direction of the developing sleeve is obtained by using a carrier having a high magnetic carrier magnetization. There is a method of making the density of ears denser by changing the density.

In the present embodiment, magnetic carriers having an average particle size of 50 μm and soft magnetic properties shown in the graph of FIG. 7 and having a magnetization intensity of 64, 120, 208 emu / cm ³ in an external magnetic field of 1000 Gauss are used. Using,
Developing sleeves (a to
The density and the image quality of the magnetic brush ears of the developing magnetic pole S1 when f) was used were examined.

[0046]

[Table 1]

The developing sleeves (a to f) shown in FIG.
The characteristics shown in (a) to (f) of FIG. In FIG. 8, the horizontal axis indicates the position in the circumferential direction of the developing sleeve in the rotational direction with reference to the developing magnetic pole S1, and the vertical axis indicates the distribution form of the magnetic flux density Br near the developing magnetic pole S1. As a result of using such a developing sleeve, as shown in Table 2 below, the developing magnetic pole S
It has been found that the density of the ears of the developing magnetic pole S1 is increased by bringing the different pole near the upstream portion of No. 1.

The symbols A to D in Table 2 indicate the following. A: It is extremely smooth and has no roughness. B: Roughness is not noticeable and smooth. C: There is some roughness. D: Conspicuous rubbing.

[0049]

[Table 2]

Regarding carrier adhesion, as can be seen from Table 2 above, the lower the value of magnetization, the more disadvantageous.
In this way, when the density of the magnetic brush was increased and an image was output and 6 lines / mm ² or more per unit area, it was possible to obtain a good image with no graze even in the low density region. Thus, 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 N1 having the magnetic flux density peak is provided at a position within 40 ° upstream of the developer sleeve rotation direction. By increasing the density of the ears of the magnetic pole brush to 6 brushes / mm ² or more, it is possible to obtain a good image with no graze.

Example 2 In the first example, a soft ferromagnetic carrier having the characteristics shown in the graph of FIG. 7 was used, but in this example, a hard ferromagnetic carrier having the characteristics shown in the graph of FIG. 9 was used. A ferromagnetic carrier was used. As magnetic carrier, average particle size 50μ
m, the strength of magnetization in an external magnetic field of 1000 Gauss is 6
8,120,197 emu / cm ³ , remanent magnetization in an external magnetic field of 0 gauss is 64,114,191 emu / cm
The one used was ³ . Other configurations and the like were the same as those in the first embodiment. The developing sleeve is also the first
Various developing sleeves having a magnetic force as shown in Table 1 were used as in the example. The results of this example are shown in Table 3 below.

The symbols A to D in Table 3 indicate the following. A: It is extremely smooth and has no roughness. B: Roughness is not noticeable and smooth. C: There is some roughness. D: Conspicuous rubbing.

[0053]

[Table 3]

Also in this embodiment, it was found that the density of the ears of the developing magnetic pole S1 is increased by bringing the different pole N1 near the upstream of the developing magnetic pole S1 as in the first embodiment.
Regarding the carrier adhesion, as can be seen from Table 3, the lower the value of the magnetization is, the more disadvantageous it is, but it is slightly more advantageous than the first embodiment, and the intensity of the magnetization in the external magnetic field of 1000 gauss is 120, 197 emu / In the case of cm ^3, no carrier adhesion phenomenon was observed. In this way, when the density of the magnetic brush was increased and an image was output and 6 lines / mm ² or more per unit area, it was possible to obtain a good image with no graze even in the low density region. Thus, 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 N1 having the magnetic flux density peak is provided at a position within 40 ° upstream of the developer sleeve rotation direction. By increasing the density of the ears of the magnetic brush to 6 brushes / mm ² or more, it is possible to obtain a good image free of roughness.

Example 3 As a method for improving the image quality, there is a method of reducing the particle size of the toner in addition to the method of increasing the density of the ears. However, when the particle size of the toner is made small, it is necessary to make the particle size of the magnetic carrier small in order not to change the toner mixing ratio due to the specific surface area.

In the first and second embodiments, the volume average particle diameter of the toner is 8 μm and the average particle diameter of the magnetic carrier is 50 μm.
A developer having a mixture ratio of 5 μm and a weight mixing ratio of 5:95 was used as the developer. In the present embodiment, a toner having a volume average particle size of 6 μm and a magnetic carrier having an average particle size of 38 μm was used. A developer was obtained by mixing at a mixing ratio of 5:95.

Also in this embodiment, similarly to the second embodiment, a hard ferromagnetic carrier having the characteristics shown in the graph of FIG. 9 is used, and more specifically, the strength of magnetization at an external magnetic field of 1000 gauss is 68, 120, 197 emu / cm ³ ,
Remnant magnetization in an external magnetic field of 0 Gauss is 64, 114, 1
91 emu / cm ³ was used. Other configurations and the like were performed in the same manner as the first and second. As for the developing sleeve, various developing sleeves exhibiting magnetic forces as shown in Table 1 were used as in the first and second embodiments. The results of this example are shown in Table 4 below.

(Here, the symbols A to D in Table 2 indicate the following.) A: Very smooth with no roughness. B: Roughness is not noticeable and smooth. C: There is some roughness. D: Conspicuous rubbing.

[0059]

[Table 4]

Also in this embodiment, it has been found that the density of the ears of the developing magnetic pole S1 is increased by bringing the different pole N1 near the upstream portion of the developing magnetic pole S1 as in the first and second embodiments. Regarding the carrier adhesion, as compared with the second embodiment, it became easier to adhere by reducing the particle size of the magnetic carrier. Further, as can be seen from Table 4, the lower the value of the magnetization is, the more disadvantageous it is to the magnetization intensity of the magnetic carrier. When the magnetization intensity in the external magnetic field of 1000 Gauss is 68 or 120 emu / cm ³ , Carrier adhesion phenomenon was observed.

In this way, when the toner particle size is reduced, the density of the magnetic brush is increased, and an image is output, and when the number of particles per unit area is 6 / mm ² or more, the effect of the toner particle size is also added and the effect is reduced. The first that there is no roughness even in the concentration range
Further, it was possible to obtain a better image than that of the second embodiment.

In this way, 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 N1 having the magnetic flux density peak is provided at the position within 40 ° upstream of the developing sleeve rotational direction. As a result, the density of the brush ears of the magnetic brush is increased, and by setting the density to 6 / mm ² or more, it is possible to obtain a good image free of roughness.

[0063]

As is apparent from the above description, the present invention
Is a latent image carrier on which a dot latent image is formed, and a latent image carrier
Component development including magnetic particles and toner particles facing the
A developer carrier that carries the developer and conveys it to the developing position, and
A plurality of magnetic poles fixedly arranged inside the image carrier
A magnetic brush developing device having a magnetic field generating means,
Image forming apparatus for contact-developing a dot latent image on a latent image carrier
In, a magnetic particle is a hard ferromagnetic carrier having a magnetization when the external magnetic field is 0, and is one of a plurality of magnetic poles.
There is a developing magnetic pole of the magnetic flux density peak Ru is fixedly disposed proximal region of the latent image bearing member, you in the rotation direction of the developer carrying member
The magnetic poles located upstream and downstream of the developing magnetic pole.
A different poles with respect to the image pole, magnetic disposed upstream in the rotational direction of the position and the developer carrying member of the magnetic flux density peak of the developing magnetic pole
The angle formed with the position of the magnetic flux density peak of the pole is set within 40 °, and the density of the magnetic brush formed at the magnetic flux density peak portion of the developing magnetic pole is 6 / mm ² or more, Even in the low-density image area, it is possible to prevent the occurrence of shakiness, obtain a good image, and prevent the carrier from adhering to the latent image carrier.

[Brief description of 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 showing a first embodiment of a developing device of the image forming apparatus of FIG.

FIG. 3 is a modification of the developing device in FIG.

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

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

FIG. 6 is a diagram illustrating a signal waveform of a PWM circuit.

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

FIG. 8 is a radial magnetic flux density distribution in the vicinity of developing magnetic poles of a plurality of developing sleeves used in an example of the present invention.

FIG. 9 is a graph showing a hysteresis curve of a hard ferromagnetic carrier.

FIG. 10 is an explanatory diagram of a potential of a dot latent image.

[Explanation of symbols]

1 Photosensitive drum (latent image carrier) 2 Development device 11 Development sleeve (Development carrier) 12 Magnet (magnetic field generating means) 121 magnet (magnetic field generating means)

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Claims (3)

(57) [Claims] 1. A latent image carrier on which a dot latent image is formed,
A developer carrier that faces the latent image carrier and carries a two-component developer containing magnetic particles and toner particles and conveys the developer to a developing position, and a plurality of developer carriers fixedly arranged inside the developer carrier. A magnetic brush developing device having magnetic field generating means having magnetic poles , wherein the magnetic particles are magnetized when an external magnetic field is 0. hard ferromagnetic is the carrier, a developing magnetic pole der its magnetic flux density peak one is among the plurality of magnetic poles Ru is fixedly disposed proximal region of the latent image bearing member are
The developing magnetic pole in the rotation direction of the developer carrier.
Magnetic poles arranged on the upstream side and the downstream side of the developing magnetic pole
Before being arranged on the upstream side in the rotation direction of the developer carrying member and the position of the magnetic flux density peak of the developing magnetic pole.
The angle formed with the position of the magnetic flux density peak of the magnetic pole is set within 40 °, and the density of the magnetic brush formed at the magnetic flux density peak portion of the developing magnetic pole is 6 / mm ² or more. Image forming apparatus. 2. The electric field at the time of developing the toner particles by the developing device is an alternating electric field.
1. The image forming apparatus of 1 . 3. The latent image carrier is an electrophotographic photosensitive member, and the electrophotographic photosensitive member is exposed with a light beam modulated by a pulse-width modulated signal corresponding to the density of a recorded image to form dots. The image forming apparatus according to claim 1, which forms a distributed electrostatic latent image.