JPH07121032A - Image forming device - Google Patents

Abstract

PURPOSE:To eliminate irregularities in a highlight, halftone density area, to prevent the occurrence of sticking of carrier and to obtain a high quality image, in an image forming device using two-component developer. CONSTITUTION:Inside a photo receptor drum 3, a magnet 31 whose polarity is the same as a developing magnetic pole 231 is arranged opposite to the developing magnetic pole 231 in a developing sleeve 22. At this time, the magnet 31 is arranged so that the intensity of a magnetic field is 0 gauss inside the photoreceptor drum 3 and on a straight line connecting the center of the developing sleeve 22 and the center of the photoreceptor drum 3.

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 having a developing device for developing an electrostatic latent image formed by an electrophotographic method or an electrostatic recording method.

[0002]

2. Description of the Related Art Conventionally, when developing a latent image formed on an image bearing member such as an electrophotographic photosensitive member, various two-component developers have been used. In particular, a two-component developer having a non-magnetic toner and a magnetic carrier enables compact and effective development by a magnetic brush development method or the like. For magnetic development, a one-component developer using a magnetic toner is used. However, since the one-component developer contains a black magnetic material, it is difficult to obtain a desired color, so that a color image is formed. The method using the above-mentioned two-component developer is superior to the above.

An example of a conventional two-component developing device will be described with reference to FIG.

The latent image formed on the surface of the photosensitive drum 103, which is an image carrier, faces the developing device 101. Developer 1
The developer container 136 of No. 01 contains the developer D, and the developer D is carried by the magnetic force of the fixed magnet 123 in the non-magnetic developing sleeve 122 on the developing sleeve 122 as the developer supporting means. It The developer D is coated on the surface of the developing sleeve 122 by the carrier return 126 and the regulating blade 124 by the rotation of the developing sleeve 122, and a portion facing the photosensitive drum 103 (developing portion).
Be transported to. The developer D is a non-magnetic toner of 8 μm and 50
The latent image on the photosensitive drum 103 is developed by toner by an AC bias applied to the developing sleeve 122 from a power source (not shown). The developer that has passed through the developing section is again taken into the developing container 136, and is stirred and conveyed by the screw 161.

In the magnetic brush developing method using the two-component developer as described above, the developer contributing to the development can be sufficiently developed, so that a high image density can be obtained. Since the ears are rough, the image often becomes dull especially in the highlight and halftone density regions.

[0006]

As a method for obtaining a final image of high density in the entire density region with less shading in the highlight and halftone density regions,
A two-component developing method in which the ears of the magnetic brush in the developing area are made dense, or the particle size of the magnetic carrier is reduced to
A two-component developing method for improving the (T + C) ratio (by reducing the particle size, the ears also tend to be dense) is considered. However, in order to make the ears dense, it is necessary to reduce the magnitude of magnetization (emu / cm ³ ) per unit volume of the magnetic carrier in the magnetic field at the developing pole position. In this case, the magnetic carrier is the developing sleeve. The magnetically restrained force is weakened, and carrier adhesion easily occurs. Also,
Even if the particle size of the carrier is made small, the amount of magnetization per carrier (emu / piece) becomes small, so that carrier adhesion easily occurs.

SUMMARY OF THE INVENTION Therefore, an object of the present invention is to provide an image forming apparatus equipped with a two-component developing device capable of eliminating the roughness in highlight and halftone density regions and preventing the occurrence of carrier adhesion.

[0008]

The above object can be achieved by an image forming apparatus according to the present invention. In summary, the present invention is
An image carrier that forms a latent image, a developer supporting unit that faces the image carrier and conveys a two-component developer containing toner and a magnetic carrier to a developing position, and provided inside the developer supporting unit. And a developing means having a plurality of magnetic field generating means, wherein the two-component developer is applied on the image bearing body by a magnetic brush developing method at a portion where the image bearing body and the developer supporting means face each other. In the image forming apparatus for developing the latent image of
In the image forming apparatus, a magnetic field generating unit having the same pole as the developing magnetic pole is disposed inside the image carrier at a position facing the developing magnetic pole of the magnetic field generating unit.

Preferably, the latent image formed on the image carrier is a dot latent image.

Preferably, the dot latent image is formed by modulating the light emission time of the light source per pixel in accordance with the image density value.

It is preferable to use a magnetic carrier having a magnitude of magnetization of 150 (emu / cm ³ ) or less at 1000 Gauss.

Further, when developing the latent image on the image carrier, it is preferable to apply a DC bias superposed with an AC bias.

[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 A first embodiment of the image forming apparatus according to the present invention will be described with reference to FIGS. 1 to 10.

FIG. 2 shows a typical electrophotographic color printer. This printer includes a photosensitive drum 3 as an image bearing member that rotates in the direction of the arrow, and a rotary developing device 1 including a charger 4 and developing devices 1M, 1C, 1Y, and 1BK around the photosensitive drum 3. An image forming unit including a transfer discharger 10, a cleaning unit 12, and a laser beam scanner disposed above the photosensitive drum 3 in the drawing is disposed.

The sequence of the entire color printer will be briefly described by taking the case of the full color mode as an example. First, the photosensitive drum 3 is uniformly charged by the charger 4. Next, a latent image is formed on the photosensitive drum 3 by the laser light E of the original (not shown) modulated by the magenta image signal, and then the latent image is formed by the magenta developing device 1M which is previously set at the developing position. Development is performed.

On the other hand, the paper feed guide 5a, the paper feed roller 6,
The transfer material that has advanced through the paper feed guide 5b is held by the gripper 7 in synchronization with a predetermined timing, and electrostatically wound around the transfer drum 9 by the contact roller 8 and its opposing pole. The transfer drum 9 rotates in the direction of the arrow in the figure in synchronization with the photosensitive drum 3, and the magenta developing device 1M
The visible image developed by the transfer charger 10 is transferred to the transfer charger.
Is transferred to the transfer material. The transfer drum 9 continues to rotate and is ready for the transfer of the next color (cyan in FIG. 2).

On the other hand, the photosensitive drum 3 is decharged by the charger 11, cleaned by the cleaning means 12, charged again by the charger 4, and exposed as described above by the next cyan image signal. During this time, the developing device 1 rotates and the cyan developing device 1C is fixed at a predetermined developing position to perform predetermined cyan development.

Subsequently, the above steps are carried out for yellow and black respectively, and when the transfer of four colors is completed, the four-color image on the transfer material is discharged by the chargers 13 and 14, and The gripper 7 is released and separated from the transfer drum 9 by the separation claw 15, so that the conveyor belt 1
The image is sent to a fixing device (hot-press roller fixing device) 17 by 6, and a series of full-color printing sequences are completed, and a predetermined full-color printing image is formed.

As shown in FIG. 3, the laser beam scanner forming the exposure means is composed of a semiconductor laser section 102, a polygon mirror 105 rotating at a high speed, and an f-θ lens 106, and the semiconductor laser section 102.
Receives a time-series digital image signal that is calculated and output by an electronic calculator or the like of the image reading device, oscillates a PWM-modulated laser beam corresponding to the signal, and exposes the surface of the photosensitive drum 3.

More detailed description will be given with reference to FIG.
Solid-state laser device 1 as a laser light source that is a light source unit
Reference numeral 02 is connected to a laser driver 500, which is a light emission signal generator that sends a light emission signal for generating a laser beam, and blinks according to the light emission signal of the laser driver. The laser light flux emitted from the solid-state laser element 102 is made into substantially parallel light by the collimator lens system 103. The collimator lens system 103 has an arrow A which is in the optical axis direction of the laser light by a focus adjusting means 104 described later.
It can be moved in a predetermined direction by a predetermined amount.

A polygon mirror, that is, a rotary polygon mirror 105.
Rotates in the direction of arrow B at a constant speed, reflects parallel light emitted from the collimator lens system 103, and scans in the direction of arrow C, which is a predetermined direction. Rotating polygon mirror 105
F-θ lens group 106 (106a, 10a, 10) provided in front of
6b and 106c) form an image of the laser light beam deflected by the polygon mirror 105 on a non-scanning surface, that is, a predetermined position on the photosensitive drum 3, and make the scanning speed constant on the surface to be scanned.

The laser beam L is guided through a reflecting mirror 107 onto a CCD (solid-state image pickup device) 108 as a detecting means and is scanned on a photosensitive drum 3 as a non-scanning surface. The CCD 108 is configured by arranging a large number of light detection distances in the direction of arrow C at positions that are substantially optically equivalent to the surface of the photosensitive drum 3 and the light source section. Further, the CCD 108 is connected to the control unit 100 which controls the laser driver 500 and the focus adjusting means 104.

An image processing section 111 is connected to the laser driver 500 and the control section 100.

In the above structure, when a desired image is formed, the image output signal P is first input from the image processing section 111 to the control section 100, and the laser driver 5 is used.
The image signal S is input to 00 to blink the solid-state laser element 102 at a predetermined timing.

Laser light emitted from the solid-state laser element 102 is converted into substantially parallel light by the collimator lens system 103, and is further rotated in the arrow B direction.
The image is scanned in the direction of arrow C by 05 and is imaged in a spot shape on the photosensitive drum 3 by the f-θ lens group 106. An exposure distribution for one scanning of an image is formed on the surface of the photosensitive drum 3 by the scanning of the laser light flux L, and the photosensitive drum 3 is rotated by a predetermined amount for each scanning, and an image signal is formed on the drum 3. A latent image having an exposure distribution corresponding to S is formed and recorded as a visible image on a transfer material by a well-known electrophotographic process.

The image output signal P is output from the image processing unit 111 prior to the image signal S, and the output is finished after the output of the image signal S is finished. The control unit 100 stops its operation while the image output signal P is input from the image processing unit 111. Therefore, the pixel size and the contrast can be kept constant during the image forming operation.

Next, the operation of the focus position adjusting means 104 for the laser beam L will be described.

First, an operation signal is input to the laser driver 500 from the control unit 100, and the laser driver 5
00 to generate a rectangular wave that is turned on and off at regular intervals as shown in FIG.
02 is blinked in response to this signal. The laser light from the solid-state laser element 102 is scanned as described above and is reflected by the reflecting mirror 107, so that the photosensitive drum 3
Is projected and scanned on the CCD 108 arranged at a position optically equivalent to

The control unit 100 resets the accumulated electric charge of each image of the CCD 108 before the laser beam L scans the CCD 108, and performs CCD scanning by spot scanning of one line.
After the charge is accumulated in each pixel of 08, the charge is read as an electric signal.

When the solid-state laser element 102 blinks the laser beam and scans once, the CCD 108 turns to the photosensitive drum 3.
Since it is located at a position that is optically equivalent to, the exposure distribution on the surface of the CCD 108 shows a strong and weak distribution shape according to the spot diameter of the laser beam L. Therefore, the output of each pixel of the CCD 108 has a distribution as shown in FIG. 4B, and the signal is sent to the control unit 100. In the control unit 100, the CCD
The maximum value of the output of 108 is θ _max and the minimum value is θ _min , and the contrast V is V = (θ _max −θ _min ) / (θ _max + θ _min ). taking measurement.

In this case, since the contrast V increases as the spot diameter in the scanning direction decreases, the preset value V ₀ is compared with V calculated by the equation (1) to obtain V.
Is not equal to the predetermined value V ₀ , a drive signal is sent from the control unit 100 to the focus adjusting means 104 to move the collimator lens system 103 by a predetermined amount in the arrow A direction. Then, each the contrast V at a position obtained by moving the collimator lens system 103 is measured, if fixing the collimator lens system 103 in this value and V ₀ are equal positions, to correct the defocus of the optical system V ₀ Therefore, the scanning spot system of the laser beam L can be minimized.

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,
Reference numeral 402 is a level converter for converting a TTL logic level into a high-speed ECL logic level, and 403 is a D / A converter for converting an ECL logic level into an analog signal. 404
Is an ECL comparator that generates 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 a clock signal 2f, 407 is a triangular wave generator that generates a substantially ideal triangular wave signal in synchronization with the clock signal 2f, and 408 is a clock It is a ½ frequency divider that divides the signal 2f by ½ to generate an image clock signal f. As a result, the clock signal 2f has a cycle twice that of the image clock signal f. In order to operate the circuit at high speed, E
CL logic circuit is arranged.

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 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 once set to 1
The triangular wave signal c is generated after dividing the frequency by 1/2. Further, the triangular wave signal c is converted to the ECL level (0 to -1V) and becomes the triangular wave signal d.

On the other hand, the image signal is from 00h (white) to FFh.
Up to (black), for example, 256 gradation levels change. The symbol "h" represents hexadecimal notation. And the image signal e
Shows the 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 generate a PWM signal having a pulse width (time length) T, t ₂ , t ₃ , t _4, etc. according to the pixel density to be formed. The pulse width corresponding to the low density pixel becomes narrower. Then, this PWM signal is converted into the TTL level of OV or 5V to become the PWM signal f, and the laser driver circuit 500.
Entered in.

In the circuit of FIG. 5, the latch circuit 401
A lookup table (not shown) is provided in the front stage of the. This lookup table can be
In the memory in which the data of the corrected result is stored, the memory is accessed by using the image signal of 1 pixel 8 bits as the address data, and the image signal of the desired γ-corrected data is output. Normally, one specific γ correction table in one pixel can be switched and used in one screen. That is, for each line scanning by the beam, for example, three types of tables are sequentially and repeatedly used, and the γ correction in the sub-scanning direction can be changed for each line to perform gradation correction.

As the look-up table, a so-called only γ table is set when the toner density is low so as not to be affected by the density peculiar to the toner of each color, for example, four colors of yellow, magenta, cyan and black. When the density is high, a γ table having the opposite characteristic is set and is provided for each formation. In order to correct the turbidity of the color of each color toner in the preceding stage of such a look-up table. Can be provided with a non-linear masking circuit, for example a secondary color masking circuit.

The above-mentioned PWM method is characterized in that dot area gradation is performed for each pixel and halftone can be expressed at the same time without lowering the density of pixels to be recorded.

The features of the present invention will be further described below with reference to examples. Incidentally, FIG. 7 shown below is an enlarged view of the vicinity of one developing device of the rotary developing device 1 used in the laser beam printer shown in FIG. 2, and the developing device is arranged at a developing position facing the photosensitive drum 1. Has been done.

The developing device (for example, 1M) is a developing sleeve 22 which is located close to the photosensitive drum 3 rotating in the direction of arrow a.
Is equipped with. The developing sleeve 22 is made of a non-magnetic material such as aluminum or SUS316. The developing sleeve 22 has a laterally elongated opening formed in the lower left wall of the developing container 36 in the longitudinal direction of the container so that a substantially right half peripheral surface projects into the container 36, and a left substantially half peripheral surface is exposed to the outside of the container 36 so as to be rotatably supported. And is laterally installed, and is rotationally driven in the direction of arrow b.

In the developing sleeve 22, a fixed permanent magnet (magnet) 23 as a fixed magnetic field generating means positioned in the illustrated position and orientation is arranged. The magnet 23 includes an N-pole magnetic pole 23a, an S-pole magnetic pole 23b, an N-pole magnetic pole 23c,
It has four magnetic poles, that is, an S-pole magnetic pole 23d. The magnet 23 may be an electromagnet instead of the permanent magnet.

The base is fixed to the side wall of the container on the upper edge side of the developer supply device in which the developing sleeve 22 is disposed, and the tip end side is projected to the inside of the container 36 from the upper edge position of the opening so that the upper edge of the opening becomes longer. Along the side, a non-magnetic blade 24 as a developer regulating member is arranged. The blade 24 is, for example, SUS3
16 is bent so that the cross section has a V shape. Further, a magnetic particle limiting member 26 is provided in which the upper surface is in contact with the lower surface side of the non-magnetic blade 24 and the front end surface is the developer guide surface 261. The portion formed by the non-magnetic blade 24, the magnetic particle limiting member 26, and the like is the restriction portion.

The developer is magnetic particles 27 and non-magnetic toner 37.
Contains. A seal member 40 is provided to seal the toner accumulated in the lower portion of the developing container 36. The seal member 40 has elasticity, is bent toward the rotation direction of the sleeve 22, and elastically presses the surface side of the sleeve 22. The seal member 40 has an end portion on the downstream side in the sleeve rotation direction in the contact area with the sleeve so as to allow the developer to enter the inside of the container.

In the developing container 36, the floating toner generated in the developing process is further applied with a voltage having the same polarity as that of the toner to adhere to the photosensitive drum side to prevent scattering, and a scattering prevention electrode plate 30 and toner concentration. Toner supply roller 6 that operates according to the output obtained by the detection sensor (not shown)
0 is placed. As the toner concentration detecting sensor, for example, a developer volume detecting method, a piezoelectric element, an inductance change detecting element, a piezoelectric element, an antenna method using an alternating bias, or a method for detecting optical density can be used. The non-magnetic toner 37 is replenished by stopping the rotation of the toner replenishing roller 60. The fresh developer supplied with the toner 37 is mixed and stirred while being conveyed by the screw 61. Therefore, tribo is applied to the replenished toner during this conveyance. Plate 63
Is cut out at both ends in the longitudinal direction of the developing device,
In this portion, the fresh developer conveyed by the screw 61 is delivered to the screw 62 rotating in the direction of arrow c.

The S pole 23d is a carrier pole. The collected developer after the development is collected in the container, and the developer in the container is conveyed to the regulation unit. In the vicinity of the S pole 23d, the fresh developer conveyed by the screw 62 provided close to the sleeve is exchanged with the collected developer after development.

The carrying screw 64 is used for the developing sleeve 22.
And a magnetic particle limiting member 26 for uniformizing the amount of the developer in the axial direction of the developing sleeve. The developer carried on the sleeve in accordance with the rotation of the sleeve is screw 64. Part of the developer layer that has been conveyed in the axial direction of the sleeve by the "convex" in the axial direction on the sleeve is in a direction opposite to the conveying direction of the developer on the sleeve through the space M in FIG. It is reversed and pushed back. The screw 64 conveys the developer in the opposite direction to the screw 62.

The construction of such a developing device is also effective when magnetic particles and non-magnetic or weakly magnetic toner are mixed in the developer container.

In the present invention, the toner is obtained by externally adding colored resin particles (including a binder resin, a colorant and, if necessary, other additives) and an external additive such as hydrophobic colloidal silica fine powder. Contains colored particles. In this embodiment, a toner having a volume average particle diameter of 8 μm is used which is a negatively chargeable polyester resin. As the magnetic particles, soft ferromagnetic (ferrite) carriers having a volume average particle diameter of 50 μm and exhibiting magnetic characteristics as shown in FIG. 8 are used.

By the way, as described above, it is known that it is effective to increase the magnetic brush density on the developing sleeve in order to eliminate the roughness in the highlight and halftone density regions. When developing a dot latent image, the latent image is sharper than an analog latent image, but since it becomes a local latent image, unevenness (chattering) corresponding to the ears of the magnetic brush is more likely to occur. Therefore, when developing the dot latent image, it is desired that the ears of the magnetic brush are denser. As a method of changing the magnetic brush density on the developing sleeve, there is a method of changing the magnetic characteristics of the magnetic carrier. Specifically, it is a method of reducing the magnitude of the magnetization of the magnetization carrier magnetized by the magnetic field of the developing magnetic pole. This corresponds to reducing the magnetic permeability when soft ferrite is used as the carrier. A DC magnetization BH characteristic automatic recording device BHH-50 manufactured by Riken Denshi Co., Ltd. was used for measuring the magnetization characteristics of the magnetic carriers below.
FIG. 9 shows the magnitude of magnetization per unit volume of the magnetic carrier in the magnetic field at the developing magnetic pole position (hereinafter, σd (emu / cm ³ ).
Called. d represents the magnetic brush density on the developing sleeve (the amount of leakage on the sleeve is constant) when the magnetic field of the developing pole is changed). d is 10 as a guide
Considering 00 Gauss, σ ₁₀₀₀ is evaluated as the magnitude of magnetization thereafter. As is clear from FIG. 9, as the value of σ ₁₀₀₀ is reduced, the magnetic brush becomes denser.

Next, the relationship between the magnetic brush density changed by this method, the image quality of the output image (a feeling of highlight / halftone roughness), and the degree of carrier adhesion is determined by using a general two-component developing method. Scanning direction 200d
The image sample of pi and 400 dpi in the sub-scanning direction was evaluated. As a result, the results shown in Table 1 below were obtained.

[0051]

[Table 1]

By making the ears of the magnetic brush denser by this method, the feeling of image roughness is reduced. That is, it can be seen from Table 1 that the smaller the magnitude of magnetization per unit volume and the value of σ ₁₀₀₀ , the better the image can be obtained. However, on the other hand, by reducing the value of σ _d, the force of the carrier being restrained on the sleeve at the developing pole position is weakened, and the carrier adheres mainly to the non-image area and the highlight area. According to Table 1, σ ₁₀₀₀ is 1400 (emu / cm ³ ) and there is some carrier adhesion, and when it is 150 (emu / cm ³ ) or less, carrier adhesion is large.

In the case of the soft ferrite carrier as described above, as apparent from FIG. 8, the external magnetic field is 10
The magnetization intensity is saturated near 00 Gauss. When one such magnetic carrier exists in a magnetic field of around 1000 Gauss or more, the tangential direction of the magnetic force line generated from the magnetic field generation source (magnetic charge, that is, magnet) is generated. It can be approximated that a force having the magnitude of the following equation (a) acts on the direction of the source.

[0054]

[Equation 1]

That is, the phenomenon that the carrier having a smaller value of σ ₁₀₀₀ tends to adhere to the carrier is because the value of m in the above equation is small in the developing area which is the closest portion between the photosensitive drum and the developing sleeve. It occurs because the magnetic force attracted to the sleeve side becomes smaller.

Therefore, according to the structure of the present invention, in the developing area which is the closest portion between the photosensitive drum and the developing sleeve, the magnetic force for attracting the magnetic carrier to the developing sleeve side, δ ·
The structure is strengthened by improving the vector H / δ / vector r (gradient of magnetic field). In this embodiment, as schematically shown in FIG. 1, the developing magnetic pole 231 and the photosensitive drum 3 inside the developing sleeve 22 are on a straight line connecting the center (axis) of the developing sleeve 22 and the center (axis) of the photosensitive drum 3. In addition, magnets 31 having the same polarity as the developing magnetic poles that are long in the axial direction of the photosensitive drum are fixedly arranged facing each other so that the magnetic field strength is 0 gauss at the position inside the photosensitive drum on this straight line. FIG. 10 shows the above arrangement.
A single magnet 23 of the developing magnetic pole on a straight line connecting the center (axis) of the developing sleeve and the center (axis) of the photosensitive drum.
1, the magnet alone 31 inside the photosensitive drum, and the magnetic field strengths of these two magnets are respectively represented by graphs. The strength of the magnetic field on the surface of the developing sleeve is 14 when the developing magnetic pole is a single magnet.
The intensity of the magnetic field on the surface of the photosensitive drum is 300 gauss when the magnet alone inside the photosensitive drum is used. 0 gauss is a position of 2.2 mm inside the photosensitive drum from the surface of the photosensitive drum. In order not to reduce the strength of magnetization of the carrier in the expression (a), the strength of the magnetic field in the vicinity of the photosensitive drum must be 1000 gauss, or at least 700 gauss in the developing area.

With the above configuration, the value of δ · vector H / δ ·
The vector r (in this case, r is the distance from the center of the magnet of the developing magnetic pole) can be improved, and the magnetic force attracted to the developing sleeve side can be increased even if a carrier having a small σ ₁₀₀₀ is used.

In the structure of this embodiment, the magnetic restraint can be increased while maintaining the density of the brushes of the magnetic brush in the developing magnetic pole, and the carrier adhesion can be reduced without impairing the image quality.

In this embodiment, σ ₁₀₀₀ = 60 (emu / cm
³ ), a carrier having a volume average particle diameter of 30 μm is used.
An AC bias is applied to the developing sleeve 22 from a power source (not shown). In this embodiment, Vpp2000V, Fr20
00 Hz is applied. Generally, in the two-component developing method, application of an AC bias increases the developing efficiency, resulting in a high quality image. Also in this example, by applying this AC bias, the unevenness of the ears became less noticeable. However, on the contrary, carrier adhesion is likely to occur.
However, with the structure as in this embodiment, a carrier having a σ ₁₀₀₀ of 150 (emu / cm ³ ) or less is used as the magnetic carrier, and the magnetic binding force is maintained while maintaining the magnetic brush density in the developing magnetic pole. Can be increased, and there is no highlight shading and image formation with reduced carrier adhesion can be performed.

Example 2 In the first example, a soft ferromagnetic carrier having the magnetic characteristics as shown in FIG. 8 was used as the carrier of the two-component developer, but in this example, the hysteresis characteristic as shown in FIG. 11 was used. Is used, and the hard ferromagnetic carrier has a coercive force Hc and a residual magnetization σr.

Since the hard ferromagnetic carrier has a remanent magnetization σr, it has a certain degree of magnetization even when the external magnetization is weakened (a state away from the developing sleeve).
The attractive force between the carriers or the force attracted to the developing magnetic pole of the carrier becomes stronger, which is more advantageous for carrier adhesion than the soft ferromagnetic carrier.

In this embodiment, the latent image forming method and apparatus structure are the same as in the first embodiment, and only the carrier of the developer is changed. As the carrier of the developer used, one having a coercive force Hc of about 2000 (Oe) was used.
Value of magnetization at magnetic force of 000 gauss σ ₁₀₀₀ (emu
/ Cm ³ ) and carriers having different residual magnetization σr were used.

Using such a hard ferromagnetic carrier, the relationship between the magnetization value and the magnetic brush density when the magnetic force was 1000 gauss was investigated. As can be seen from FIG. 12, the relationship between the value of σ ₁₀₀₀ and the density of ears becomes denser as the value of σ ₁₀₀₀ becomes smaller, as in the case of the soft ferromagnetic carrier. The relationship between the density of the magnetic brush changed by this method, the image quality of the output image (the highlight / halftone shading) and the degree of carrier adhesion is determined by the same two-component non-contact developing method as in the first embodiment.
Table 2 below shows the results of evaluation using image samples in the main operation direction of 200 dpi and the sub-scanning direction of 400 dpi.

[0064]

[Table 2]

The results are almost the same as those in the first embodiment, and a good image with less roughness is obtained as the ears are made denser.
Carriers adhere when the value of σ ₁₀₀₀ is 100 (emu / cm ³ ) or less.

With the above configuration, even when a hard ferromagnetic carrier is used as the magnetic carrier, the magnitude of magnetization per unit area is 100 (emu) in a magnetic field of 1000 gauss.
/ Cm ³ ) or less, it is possible to increase the image density while maintaining the magnetic brush density in the developing magnetic pole, without increasing the roughness of the highlight, and to form an image with reduced carrier adhesion.

Embodiment 3 In this embodiment, the latent image forming method, the apparatus structure and the developer are the same as those in the first embodiment, and the two-component developer and the photosensitive drum are arranged at the facing portion of the photosensitive drum and the developing sleeve. It is facing non-contact. A thin layer of the developer must be coated in order to make it non-contact, but as a general method, a method of reducing the leakage amount of the developer by a regulating blade is adopted. According to this method, the developing efficiency is lowered, and the density in the solid area of the image is lowered. The relationship between the size of σ ₁₀₀₀ and the height of the brush is that when the leak amount on the sleeve is constant,
By decreasing σ ₁₀₀₀ (the ears are denser), the height of the magnetic brush is lowered. That is, as described above, when the magnetic brush and the photosensitive drum are brought into non-contact with a thin layer coating as described above, by reducing the size of σ ₁₀₀₀ , even if the magnetic brush density is the same, the magnetic brush density is reduced. Becomes denser and the amount of developer can be increased. Such non-contact makes it possible to obtain an image with no graze especially in the highlight portion.

In such a structure, FIG. 7 of the first embodiment is shown.
In order to increase the density of the magnetic brush using the developing device shown in Fig. 1, the magnitude of magnetization per unit volume is 150 or less (emu) in a magnetic field of 1000 Gauss as a magnetic carrier.
/ Cm ³ ) was used. The distance d ₂ between the end of the magnetic blade 24 and the surface of the developing sleeve 22 is 300 μm,
A thin layer of developer is coated on the surface of the developing sleeve 22. The distance d ₁ between the photosensitive drum 3 and the surface of the developing sleeve 22 is 500.
It is set to μm, and the thin-layer coated developer and the photosensitive drum 3 are kept in non-contact with each other.

With the above structure, σ as a magnetic carrier is obtained.
By using a small carrier of ₁₀₀₀ , the magnetic restraining force can be increased while maintaining the density of the magnetic brush in the developing magnetic pole, and there is no highlight blurring and image formation with reduced carrier adhesion can be performed.

Example 4 In recent years, a multiple development process has been proposed and studied in which a toner image is directly superposed on a photosensitive member to form a color image. The present embodiment is characterized in that the present invention is applied to an image forming apparatus adopting such a multiple development process.

First, referring to FIG. 13, a conventional image forming apparatus adopting the above-mentioned multiple development process will be described. First, the charger 54 uniformly charges the photosensitive drum 53. After that, the exposure unit 5 is exposed from above the photosensitive drum 53.
The latent image is written by 5 and only the portion irradiated with the laser 56 is developed by reversal development. This process is repeated for three or four colors of magenta, cyan, yellow, and (black), and the toner images are superposed on the photosensitive drum 53 to form a color image. Then, this toner image is collectively transferred onto paper by the transfer means 57, and the residual charge on the photosensitive drum 3 is removed by the pre-exposure lamp 58. After that, the color image is obtained by passing through a fixing device (not shown) to obtain a color image.

In such a structure, the developing device 1 shown in FIG. 7 of the first embodiment is used, and the magnitude of magnetization per unit volume is 150 (in a magnetic field of 1000 gauss in order to increase the magnetic brush density. emu / cm ³ ) or less was used. Also in this embodiment, the latent image forming method, the apparatus configuration,
The developer is the same as that in the first embodiment.

With the above structure, as a magnetic carrier,
Using a carrier with a small σ ₁₀₀₀ , while maintaining the density of the magnetic brush in the developing magnetic pole, as a result, there is no highlight glare and the image density can be increased,
Moreover, it is possible to form an image with reduced carrier adhesion.

Fifth Embodiment Next, with reference to FIG. 14, a fifth embodiment of the image forming apparatus according to the present invention will be described.

The image forming apparatus in this embodiment is a full-color laser beam printer, but unlike the above-described embodiments, a dedicated image carrier for each color, that is, the photosensitive drums 73Y (yellow) and 73M in this embodiment. (Magenta), 7
It is equipped with 3C (cyan) and 73BK (black), and a dedicated laser beam scanner 70 around them.
Y, 70M, 70C, 70BK, developing devices 71Y, 71
M, 71C, 71BK, transfer dischargers 90Y, 90M,
90C, 90BK, cleaning devices 92Y, 92M, 9
2C and 92BK are arranged.

The transfer material is conveyed through the paper feed guide 75a, the paper feed roller 76 and the paper feed guide 75b in this order, receives corona discharge from the attraction charger 91, and is reliably attracted to the conveyor belt 90a.

After that, the image formed on each photosensitive drum is transferred by the transfer dischargers 90Y, 90M, 90C, and 90BK, the charge is removed from the conveyor belt 90a by the charge eliminator 93, and the image is fixed by the fixing device 97 to obtain a full-color image. An image is obtained.

In this embodiment, the latent image forming method, the apparatus structure and the developer are the same as those in the first embodiment, and 1000 magnetic carriers are used to increase the magnetic brush density.
In a Gaussian magnetic field, the magnitude of magnetization per unit volume is 15
Those having a value of 0 (emu / cm ³ ) or less were used.

With the above structure, as a magnetic carrier,
By using a carrier having a small σ ₁₀₀₀ , the density of the magnetic brush on the developing magnetic pole can be maintained and the density of the image can be increased without the shading of highlights, and the image formation with reduced carrier adhesion can be performed.

[0080]

As is apparent from the above description, the image forming apparatus according to the present invention has the same magnetic pole as the developing magnetic pole in a position facing the developing magnetic pole of the magnetic field generating means inside the image carrier. By arranging the magnetic field generating means,
Roughness in highlight and halftone density areas can be eliminated, carrier adhesion can be prevented, and a high-quality image can be obtained.

[Brief description of drawings]

FIG. 1 is an explanatory diagram showing a characteristic part of a first embodiment of an image forming apparatus according to the present invention.

FIG. 2 is an overall configuration diagram of an image forming apparatus according to a first embodiment.

FIG. 3 is a configuration diagram showing the scanning optical device of FIG. 2 in more detail.

FIG. 4 is a graph (a) showing an on / off state of a laser light source and an output distribution diagram (b) of each pixel of the CCD.

5 is a PWM circuit diagram used in the device of FIG. 2;

6 is a timing chart showing the operation of the circuit of FIG.

FIG. 7 is a block diagram showing the developing device of FIG.

FIG. 8 is a graph showing magnetic characteristics of soft ferromagnetic carriers.

FIG. 9 is a graph showing the relationship between the value of the magnetization and the density of the ears of the magnetic brush when the external magnetic field is 1000 gauss when a soft ferromagnetic carrier is used.

FIG. 10 is a graph showing the strength of the magnetic field from the center of the magnet of the developing magnetic pole in the first embodiment.

FIG. 11 is a graph showing the magnetic characteristics of the hard ferromagnetic carrier in the second example.

FIG. 12 is a graph showing the relationship between the value of magnetization and the density of the ears of a magnetic brush when an external magnetic field is 1000 gauss when a hard ferromagnetic carrier is used.

FIG. 13 is a schematic configuration diagram showing an image forming apparatus of a fourth embodiment.

FIG. 14 is a schematic configuration diagram showing an image forming apparatus of a fifth embodiment.

FIG. 15 is a schematic configuration diagram showing a conventional developing device.

[Explanation of symbols]

 1 Developing Device (Developing Means) 3 Photosensitive Drum (Image Bearing Body) 22 Developing Sleeve (Developer Supporting Means) 31 Magnet (Magnetic Field Generating Means) 231 Developing Magnetic Pole

Claims (5)

[Claims] 1. An image carrier for forming a latent image, a developer supporting means for conveying a two-component developer containing a toner and a magnetic carrier to a developing position, facing the image carrier, and the developer support. And a developing means provided with a plurality of magnetic field generating means provided inside the means, at a portion where the image carrier and the developer supporting means face each other, by the two-component developer by a magnetic brush developing method. In an image forming apparatus for developing a latent image on the image carrier, a magnetic field generating unit having the same pole as the developing magnetic pole is provided inside the image carrier at a position facing the developing magnetic pole of the magnetic field generating unit. An image forming apparatus characterized by being arranged. 2. The image forming apparatus according to claim 1, wherein the latent image formed on the image carrier is a dot latent image. 3. The image forming apparatus according to claim 1, wherein the dot latent image is formed by modulating the light emission time of the light source per pixel in accordance with the image density value. 4. The image forming apparatus according to claim 1, wherein the magnitude of magnetization of the magnetic carrier at 1000 Gauss is 150 (emu / cm ³ ) or less. 5. When developing a latent image on the image carrier,
5. The image forming apparatus according to claim 1, wherein a DC bias with an AC bias superimposed is applied.