JPH04278974A - Method and device for developing prevented carrier from being attracted - Google Patents

Abstract

PURPOSE:To miniaturize the whole of a developing device by setting magnetism generated on the surface of a developing sleeve within a specified range specially the magnetic force of a tangent line direction at a point that the rubbing of a developer layer is finished. CONSTITUTION:Only toner is transferred on an electrostatic latent image formed on the photosensitive layer 24 of a faced photosensitive body 22 in a developing area, that means, a rubbing part A between a magnetic brush and a latent image part is developed. Besides, the point P on the sleeve surface 51 corresponds to the position where the rubbing of developer is finished and it is the intersection point of a straight line connecting the finish position of the rubbing part on the layer 24 and the center of the sleeve and the sleeve surface. At the point P, the prescribed magnetic force acts from an inside magnet roll 52 and it is vectorized and resolved in a vertical direction to the sleeve surface and in the tangent line direction. Then, the scalar amount thereof is measured and the value thereof is set within the fixed range.

Description

Detailed Description of the Invention (Field of Application of the Invention) The present invention relates to a developing device such as a copying machine or a printer, and a developing method used in such a developing device. The present invention relates to a developing device and a developing method that do not cause transfer to a photoreceptor and can be downsized.

(Prior Art) In the field of electrophotography, two-component magnetic developers are widely used as a means for developing electrostatic latent images. Two-component developers usually consist of toner particles containing a colorant and magnetic carrier particles, and are stirred during development. The toner and carrier are triboelectrically charged by stirring, and the toner is attracted to the surface of the carrier due to the electrostatic charge. The developer in this state is supplied onto a developing sleeve having a magnet inside, and is formed into a magnetic brush by attraction of the internal magnet. The developer is conveyed in this state by the sleeve and sent to the photoreceptor having the electrostatic latent image.

The developer is rubbed against the surface of the photoreceptor as a magnetic brush, and the charged toner is transferred to the electrostatic latent image surface by Coulomb force based on the potential difference with the electrostatic latent image surface to form a toner image. On the other hand, the magnetic carrier is attracted by the magnet in the sleeve and remains on the sleeve. The toner image on the electrostatic latent image surface is transferred and fixed onto a subsequent transfer paper or the like to form an image.

FIG. 5 is an explanatory diagram of a general developing device. As shown in FIG. 5, the developing device 2 is provided with a developer supply mechanism 4, and a developer 6 is supplied from above. The developer 6 is supplied into the lower developing device 10 through a supply port 8 with a feeder, and is agitated by agitators 12, 12 within the developing device 10. In the case of a two-component developer, the carrier and toner are charged by friction and bond to each other.

Inside the developing device 10 is a developing sleeve 16 having a large number of magnetic poles.
The developer 14 that has been triboelectrically charged is supplied to the developing sleeve 16, and a magnetic brush is formed by the developer on the sleeve surface. The length of the magnetic brush is adjusted by a doctor blade 20, and the magnetic brush is conveyed to the nip position of the photosensitive layer 24 of the electrophotographic photosensitive drum 22.

The distance between the photosensitive drum 22 and the developing sleeve 16 is DD−.
The developing sleeve 16 and the photosensitive drum 22 are rotatably supported by a machine frame (not shown) so that the moving directions (arrows) at the nip position are the same (or opposite). driven by

A corona charger 26 connected to a variable high voltage power supply 25 and an exposure optical system 28 are arranged around the photoreceptor drum 22.
is disposed upstream of the developing device 10 to form an electrostatic latent image with a predetermined surface potential. A bias power supply 33 equipped with a voltage adjustment mechanism 30 is connected between the photosensitive drum 22 and the developing sleeve 12, and a bias power source 33 having a voltage adjustment mechanism 30 is connected to the photosensitive drum 22 and the developing sleeve 12. A value (bias potential) can be applied. Further, a transfer mechanism 34 for transferring the toner image 27 onto copy paper is provided around the photosensitive layer 24 and downstream of the developing section.

In such a configuration, the developer 14 is transferred to the developing sleeve 1.
8 to form a magnetic brush 18 and a doctor blade 20
After being regulated by , the toner image 27 is reacted with the electrostatic latent image on the photosensitive layer 24 at the nip position to form a toner image 27 on the photosensitive layer 24 .

(Problem to be Solved by the Invention) Normally, in order to increase the image density during development, it is considered to increase the electric field strength between the developing sleeve and the photoreceptor, and the voltage applied between the developing sleeve and the photoreceptor is increased. It is possible to increase the size or to shorten the distance between the developing sleeve and the photoreceptor. However, such excessive voltage application may cause carrier particles to migrate to the surface of the photoreceptor, making it easy to cause so-called carrier pull. Further, when the magnetic binding force of the magnetic brush is strengthened by using carriers with low saturation magnetization in order to improve image quality, carrier attraction becomes more likely to occur.

Conventionally, in order to prevent this kind of carrier pull, the magnetic force on the surface of the developing sleeve was made stronger, and the positional relationship between the alternating N and S poles in the developing sleeve and the developing area was changed. , for example, where the magnetic flux center position of the main magnet is tilted 2 to 15 degrees upstream from the position facing the photoreceptor (Japanese Patent Laid-Open No. 62
JP-A No. 17775), and one in which the magnetic force when entering the developing area and the magnetic force when exiting the developing area are changed (Japanese Patent Laid-Open No. 59-228681) has been proposed.

However, although these methods prevent carrier pull to some extent, the developer magnetic brush in the developing area becomes hard due to strong magnetism, and the carrier particles adversely affect the latent image area of the photoreceptor, making it impossible to obtain good image quality. .

In addition, digital copying machines, which have recently become mainstream, reproduce halftones using a laser dot array, but the quality of this image depends on how to prevent toner scattering around the dots (scattering and adhesion around the dot image). has become an issue.

SUMMARY OF THE INVENTION An object of the present invention is to provide a developing device and a developing method that can produce high-quality images without causing developer scattering such as so-called carrier pull during development.

Another object of the present invention is to provide a high-density development method that allows toner to be transferred only to the latent image area of finely formed dot areas, thereby forming a high-quality surface in intermediate tones such as dot areas. It's about doing.

In addition, in recent years, with the development of full-color developing systems that require multiple developing devices, it has been desired to make the developing devices more compact. There is an urgent need to develop a developing device that does not cause drag.

Therefore, another object of the present invention is to provide a developing device and a developing method in which the sleeve is miniaturized.

(Means for Solving the Problems) According to the present invention, a magnetically formed developer layer on the surface of the developing sleeve is rubbed against the surface of the photoreceptor drum to develop the electrostatic latent image on the surface of the photoreceptor drum. A developing method in which the magnetic force X in the tangential direction of the developing sleeve surface is 430 Gauss or more at the position where the developer layer finishes rubbing against the photoreceptor drum surface, or the magnetic force X in the tangential direction is 430 Gauss or more. Provided is a developing method that prevents carrier attraction, characterized in that the magnetic force Y in the normal direction is less than Gauss, Y≧-X+800 (Gauss), and an alternating bias potential is set between a developing sleeve and a photoreceptor drum. be done.

According to the present invention, in a developing device that performs development by rubbing a developing layer formed on the surface of a developing sleeve against the surface of a photoreceptor by magnetic force, the developer layer finishes sliding against the surface of the photoreceptor drum. When the magnetic force X in the tangential direction and the magnetic force Y in the normal direction of the sleeve surface at the position have X≧430 (Gauss), Y≧0, and when the magnetic force X in the tangential direction is less than X<430 (Gauss). The magnetic force Y in the normal direction is Y≧-X+800 (
There is provided a developing device characterized in that a developing sleeve having a Gaussian relationship and a photosensitive drum are disposed in close proximity to each other and an alternating bias potential is applied thereto.

The present invention can also be characterized in that the amplitude of the bias potential when the alternating bias potential is set is set to be below the photoreceptor surface potential and above the residual potential.

The present invention further provides a maximum magnetic force on the sleeve surface of 1500.
It can be characterized by making it less than Gaussian.

The present invention may be characterized in that the developing sleeve is used with a diameter of 20 mm or less.

Note that the normal direction is the radial direction of the sleeve, and the magnetic force in the normal direction refers to the value in the normal direction when magnetic force is vectorized. Furthermore, the tangential direction refers to the direction of the tangential line when drawn on the circumferential surface of the sleeve at the position where the developer finishes sliding, and the tangential magnetic force is the tangential direction when the magnetic force is vectorized. refers to value.

(Function) The present invention sufficiently prevents carrier pull during development by setting the magnetic force and the direction of the magnetic force within a specific range at the position on the developing sleeve surface where the developer finishes sliding against the photoreceptor. In particular, by applying an alternating bias voltage between the developing sleeve and the photoreceptor surface while the developer is in the developing area, This is based on the knowledge that dot-type electrostatic images can be developed with good image quality.

In the present invention, the magnetic state that appears on the developing sleeve surface when the developing sleeve surface and the photoreceptor surface are opposed and the developing layer is conveyed between the opposing surfaces and rubbed against the photoreceptor surface is important. It is important that the magnetic force X in the tangential direction at the position where the agent finishes rubbing against the photoreceptor surface is 430 Gauss or more. As mentioned above, the tangential direction means that the direction of the magnetic lines of force is parallel or oblique to the sleeve surface. This can be determined by the overall relationship between the strength and distance of N and S magnetic poles alternately provided in the developing sleeve, and the sleeve diameter. When the tangential magnetic force X satisfies the above range, the brush of the developer layer is sufficiently bent with respect to the photoreceptor surface at a position away from the photoreceptor. It is understood that such a brush condition has a large effect on the sliding part, and even if the diameter of the sleeve is small and the strength of the main pole at the sliding part cannot be ensured, it is possible to prevent carrier pull or A surface image without chipping at the rear end of the surface image can be obtained. In addition, in such a state, it is possible to lower the strength of the main pole and use a carrier with low saturation magnetization.
Forms a magnetic brush of soft developer to enable clear development. Furthermore, the magnetic brush of the developer is bound so as to stick to the sleeve surface in the sliding area, preventing carrier scattering in this area, and unlike conventional methods, carrier scattering in the sliding area away from the main pole is prevented. Prevented.

Furthermore, when the tangential magnetic force
It is important to

The normal direction is a direction extending perpendicularly to the sleeve surface, and means a state in which the magnetic brush of the developer stands up.

As is clear from Table 1 of Examples to be described later, in a device that satisfies the above relationship, carrier pull does not occur and X≧
As in the case of 430 Gauss, the developer is stuck to the sleeve surface of the sliding portion, and the above effect is fully exerted.

It is also important for the present invention that the maximum magnetic force appearing on the main pole or on the alternating poles in the sleeve, ie on the sleeve surface, is less than 1500 Gauss, especially less than 1200 Gauss. If the magnetic force of the main pole is too large, a soft magnetic brush cannot be formed, depending on the diameter of the sleeve, and a clear image cannot be obtained.

In the present invention, it is also important to set an alternating bias potential between the developing sleeve and the photoreceptor drum. In normal development or reverse development, a constant DC bias electric field is formed between the developing sleeve and the photoreceptor drum to facilitate the transfer of toner to the latent image area on the photoreceptor surface. in this case,
The polarity of the charge on the toner is opposite to that of the electrostatic portion, and is set so that the polarity is the same as that of the photoreceptor surface other than the electrostatic portion, or the potential thereof is approximately the same as that of the photoreceptor surface. In the present invention, it is important to superimpose an alternating bias electric field in addition to the DC bias electric field, and by superimposing the alternating bias electric field, it is desirable to vary the bias potential between the photoreceptor drum surface potential and the residual potential. Further, the cycle of the alternating bias potential at this time depends on the circumferential speed of the sleeve, but in consideration of high-speed development, it is desirable to be in a range of .

Normally, the above-mentioned toner scattering appears around the dot areas that are electrostatic images, and this toner is considered to be excessive migration and is not electrically attracted to the latent image area. It is thought that this occurs as a result of losing a place to go, and is thought to be physically or slightly electrically attached around the latent image area. However, when alternating bias potentials are superimposed as in the present invention, the toner physically attached to the periphery of the dots is adsorbed and collected again by the carrier, which is the opposite polarity, when the bias potential is in a periodic low potential state. As a result, unnecessary adhesion due to toner scattering around the dots is prevented, and the image becomes clearer.

In this case, as mentioned above, the carrier has a magnetic field set to a specific value, so it has a degree of freedom on the surface of the developing sleeve and is likely to move slightly due to electrical amplitude, and this state of the carrier prevents toner from scattering. This is thought to make collection more effective.

Therefore, by applying an alternating bias potential between the photoreceptor and the developing sleeve, the image of the electrostatic portion formed with dots is made clearer when the carrier is in good condition.

(Embodiments of the Invention) Preferred embodiments of the present invention will be described in detail below with reference to the accompanying drawings. Further, FIGS. 1 and 2 are explanatory diagrams of main parts of the developing device according to the present invention, and other parts are shown in FIG.
Since it has almost the same structure as the device shown in its entirety,
A detailed explanation thereof will be omitted.

As shown in FIG. 1, in the entire developing sleeve 16, a magnet roll 52 having a large number of magnetic poles N and S is housed in a sleeve 51 made of a non-magnetic material such as aluminum, and this magnet roll 52 is fixed and the sleeve 51 is provided to be driven and rotated in the direction of the arrow, that is, in the counterclockwise direction. A magnetic brush, which is the two-component developer layer 18, is formed on the outer peripheral surface of the sleeve 51, and the magnetic brush 18 is
As the sleeve 51 rotates, it moves in the same direction as the sleeve rotation direction.

Only toner is transferred to the electrostatic latent image formed on the photoreceptor layer 24 of the opposing photoreceptor 22 in the development area, that is, the rubbing area with the magnetic brush (portion A in FIG. 1). The image area is developed. Also, the point P shown in FIG. 1 on the sleeve surface 51
corresponds to the position where the sliding portion of the developer ends, and is the intersection of the sleeve surface and the straight line connecting the sliding portion end position on the photoreceptor layer 24 and the center of the sleeve.

In the present invention, as shown in FIG. 2, a predetermined magnetic force is applied from an internal magnet roll 52 at a point P, and the magnetic force is directed into a vector in a normal direction Y (perpendicular to the sleeve surface) and a tangential direction X. Decompose it and measure its scalar quantity,
The value is set within a certain range. FIG. 3(A) shows a chart of the magnetic force acting in the normal direction on the sleeve surface. Further, FIG. 3(B) shows a chart of the magnetic force generated on the sleeve surface and acting in the tangential direction. The equidistant reference circles of the sleeve 51 indicate a scalar amount (Gauss) of magnetic force, and indicate the strength of the magnetic field in the normal and tangential directions on the sleeve surface.

In the present invention, the sleeve surface point P where the sliding ends
The magnetic force in the tangential direction X is X≧430 Gauss,
The formation of such a magnetic force is performed by adjusting the positional relationship and distance between the N and S poles of the magnet roll 52 within the sleeve 51 according to the radius of the sleeve. Further, when the magnetic force in the tangential direction X is less than 430 Gauss, the value in the normal direction Y is limited and set so that Y≧−X+800 Gauss.

Further, in the present invention, it is desirable that the maximum magnetic force of the N and S poles be 1500 Gauss or less, particularly 1200 Gauss or less. On a sleeve set within such a range, the ears of the magnetic brush tend to remain soft.

Further, the diameter of the developing sleeve 16 in the present invention can be used within a range of 15 to 50 mm, and the area occupied by the developing sleeve 16 in the developing mechanism is reduced. The cutting length of the magnetic brush is 0.5 to 2.0 mm, especially 0.7 mm.
1.0mm to 1.0mm, and the photoreceptor-developing sleeve distance (D
-S distance) is preferably in the range of 0.5 to 2.0 mm, particularly 0.5 to 1.0 mm.

Another condition is that the ratio (vS/vD) between the circumferential speed of the photoreceptor vD and the circumferential speed vS of the developing sleeve is generally 1 to 5, particularly 1.
It is preferably in the range of 5 to 3.

Examples of the photoreceptor include photoreceptors conventionally used in electrophotography, such as selenium photoreceptors, amorphous silicon photoreceptors, zinc oxide photoreceptors, cadmium selenide photoreceptors, cadmium sulfide photoreceptors, and various organic photoreceptors. etc. are all used.

As a developing condition, the DC bias voltage applied between the developing sleeve and the photoreceptor conductive substrate has an average electric field strength of 10
0 to 1000V/mm, especially 125 to 700V/m
It is preferable that the alternating bias voltage is in the range of m, and the alternating bias voltage to be superimposed on it is 100 to 800 V, especially one with an amplitude of 300 to 700 V. It is desirable that it be in between. Also, the period (frequency) of the alternating bias potential is 0.
A range of 2 to 4 kHz, particularly 0.5 to 3 kHz is preferred.

Conventionally, as developer ferrite, for example, iron zinc oxide (ZnFe2
O4), iron yttrium oxide (Y3Fe5O2), iron cadmium oxide (CdFe2O4), iron gadolinium oxide (Ga3Fe6O12), iron copper oxide (CuFe2O4)
, iron lead oxide (PbFe12O19), iron nickel oxide (
NiFe2O4), neodymium iron oxide (NdFeO3)
, barium iron oxide (BaFe12O19), magnesium iron oxide (MgFe2O4), iron manganese oxide (MnF
Sintered ferrite particles having a composition consisting of one or more types of iron oxide (LaFeO3), lanthanum iron oxide (LaFeO3), etc. are used, and in particular, a metal component selected from the group consisting of Cu, Zn, Mg, Mn and Ni. Soft ferrite containing at least one, preferably two or more of the following:
Zinc-magnesium ferrite is used, and among these ferrites, one that satisfies the above conditions is used.

The saturation magnetization of the carrier is 40 to 65 emu/g, especially 4
It is desirable that it be in the range of 5 to 56 emu/g. The magnetic carrier is a ferrite carrier that satisfies the above conditions,
Particularly preferred are spherical ferrite carriers, the particle size of which is from 20 to 140 μm, especially from 50 to 100 μm.
It is desirable that it be within the range of .

The electrical resistance of a ferrite carrier varies not only depending on its chemical composition, but also on its particle structure, manufacturing method, and coating type and thickness. Generally, its volume resistivity is 5×108 to 5×1
0...Ω・cm, especially 1×109 to 1×10・・Ω・
It is better to be within the cm range.

The toner used in the present invention is one in which a colorant and a charge control agent, or further toner ingredients known per se, are blended in a fixing resin medium. The toner used in the present invention also includes 1
×10-... to 5×10-8S/cm, especially 5×10
It preferably has a conductivity of -10 to 1 x 10-9 S/cm, and its dielectric constant is 2.5 to 4.5, especially 2.
．． It is desirable that it be in the range of 5 to 4.2.

The fixing resin medium, colorant, charge control agent, and other toner compounding agents for the toner are preferably selected and combined so as to obtain the above characteristics. First, as the fixing resin medium, styrene resin, acrylic resin, styrene-acrylic resin, polyester, nipoxy resin, rosin-modified maleic acid resin, silicone resin, xylene resin, polyvinyl butyral resin, etc. are used. In addition, the resin used is
It is generally preferred to have an acid number of 0 to 25. Also,
From the viewpoint of fixing properties, the glass transition temperature (T
g).

As the colorant contained in the resin, any known inorganic or organic pigments, dyes, etc. may be used alone or in combination of two or more. For example, carbon black such as furnace black and channel black; iron black such as triiron tetroxide; titanium dioxide such as rutile type or anatase type; phthalocyanine blue: phthalocyanine green; cadmium yellow; molybrene orange; pyrazolone red: fast violet. Examples include B.

As the charge control agent, any charge control agent known per se may be used, such as oil-soluble dyes such as nigrosine base (CI50415), oil black (CI20150), and spirone black, and 1:1 type or 2:1 type metals. complex dye,
Metal (complex) salts of (alkyl)salicylic acid and naphthonic acid are used.

The particle size of the toner particles is 8 to 14 μm on a volume basis, especially 10 μm as measured by a Coulter counter.
The particle size is preferably in the range of 12 μm to 12 μm, and the particle shape may be amorphous particles produced by a molten acid kneading/pulverization method, or spherical particles produced by a dispersion or suspension polymerization method.

In addition, the electrical resistance of the developer as a whole is 1 x 10 8 to 1 x 10 Ω・cm, especially 5 x 10 9 to 5 x 10
A range of Ω·cm is preferred for the purposes of the present invention.

Experimental examples showing examples and comparative examples are shown below.

(Experiment Example 1) Developing conditions A modified machine of Laser Printer LPX-1 (trade name) manufactured by Sanda Kogyo Co., Ltd. Developing sleeve diameter: 20 mm, Developing sleeve circumferential speed: 210 mm/sec, Photoconductor drum diameter: 6 mm, Photoconductor drum Peripheral speed: 70 mm/sec, photoreceptor drum: negatively charged organic photoreceptor, photoreceptor surface potential: -70
0V, development bias potential: -500V, development magnetic pole: The position of the maximum magnetic force in the normal direction is on the line connecting the center of the magnet roller and the photoreceptor drum, various magnet rolls are used, toner: polyester and carbon black. The volume-based median diameter of the dispersed particles is 11 μm, and the true density is 1.1.
1 g/cm3 toner, carrier: Carrier made of resin coated ferrite core with saturation magnetization of 55 emu/g, electrical resistance of 5 x 109 Ωcm, and true density of 5 g/cm3.

Image formation was performed under the above development conditions. Table 1 shows the results.
And when the magnetic force in the tangential direction shown in Figure 4 is X and the magnetic force in the normal direction is Y, the magnetic force in the tangential direction is 430 Gauss or more, or if the magnetic force in the tangential direction is less than 430 Gauss, the fourth Straight line Y=-X+800 shown in the figure (Y: magnetic force in the normal direction, X:
It can be seen that in the region represented by Y≧−X+800 with the boundary (magnetic force in the tangential direction) as the boundary, carrier attraction is effectively prevented from occurring.

(Experimental Example 2) Regarding the case in which carrier attraction did not occur in Experimental Example 1, an AC voltage (frequency:
1kHz) [■-150~-650V, ■-150~-
800V, ■-50 to -650V, ■-50 to -750
When an image was formed by applying V], toner scattering around the dot image was reduced. In particular, the effect was remarkable when the voltage was applied in the range from the peak of (1) to the peak (2), and a good surface image was clear and sharp. On the other hand, under the condition (2), the degree of fogging tended to be worse than that when only DC voltage was applied. In case (2), carrier adhesion tended to be worse than in case only DC voltage was applied. The case (2) had a tendency to be worse in terms of toner scattering than the case where only a DC voltage was applied.

As a result, it was confirmed that the scattering of toner around the dot image was suppressed by the application of AC voltage, and that high-quality images were produced, especially when the peak to peak was between the surface potential of the photoreceptor and the residual potential. Ta.

(Effects of the Invention) As described above, according to the present invention, the magnetism generated on the surface of the developing sleeve, which is a developer conveying member, particularly the magnetic force in the tangential direction at the point where the developer layer finishes sliding, can be controlled in a specific manner. By setting the magnetic force within a specific range or by setting the magnetic force in the tangential direction and the normal direction within a specific range, it is possible to reduce the carrier pull during development, and also to reduce the diameter of the developing sleeve to reduce the overall size of the developing device. Miniaturization can be achieved.

Furthermore, since the alternating bias potentials were applied in a state in which the carriers had a degree of freedom in the development area due to the characteristics of the magnetic force,
A clear image can be formed by preventing toner scattering that occurs in an electrostatic image area formed by dots.

[Brief explanation of the drawing]

FIGS. 1, 2, and 3 are explanatory views of the main parts of the developing method and apparatus according to the present invention, and FIGS. 4(A) and (B) are
FIG. 5, a magnetic force distribution chart at points S and P, is a diagram showing the relationship between the magnitudes of magnetic force in the normal direction and the tangential direction. Patent applicant: Sanda Kogyo Co., Ltd. Agent: Patent attorney: Ikuo Suzuki

Claims (5)

[Claims] 1. A developing device that performs development by rubbing a developing layer formed on the surface of a developing sleeve by magnetic force against the surface of a photoreceptor, at a position where the developer layer finishes sliding against the surface of the photoreceptor drum. When the magnetic force X in the tangential direction and the magnetic force Y in the normal direction of the sleeve surface have X≧430 (Gauss), Y≧0, and when the magnetic force X in the tangential direction is less than X<430 (Gauss), the normal A developing device characterized in that a developing sleeve in which a magnetic force Y in a direction has a relationship of Y≧−X+800 (Gauss) and a photosensitive drum are provided in close proximity to each other and an alternating bias potential is applied thereto. 2. A developing method in which an electrostatic latent image on the photosensitive drum surface is developed by rubbing a magnetically formed developer layer on the surface of the developing sleeve with the photosensitive drum surface, the developer layer being rubbed against the photosensitive drum surface. At the position where the layer finishes rubbing against the surface of the photoreceptor drum, the magnetic force X in the tangential direction of the developing sleeve surface is set to 430 Gauss or more, or the magnetic force Y in the normal direction is set when the magnetic force X in the tangential direction is less than 430 Gauss. A developing method for preventing carrier pull, characterized in that Y≧-X+800 (Gauss) and setting an alternating bias potential between a developing sleeve and a photoreceptor drum. 3. The developing method according to claim 2, wherein the amplitude of vibration of the bias potential when the alternating bias potential is set is within a range of below the photosensitive drum surface potential and above the residual potential. 4. The developing method according to claim 2, wherein the maximum magnetic force on the sleeve surface is 1500 Gauss or less. 5. The developing method according to claim 2, wherein the diameter of the developing sleeve is 20 mm or less.