JPH0511512A - Two-component magnetic brush developing method - Google Patents

Abstract

PURPOSE:To allow the stable maintaining of a printing grade by determining developing density conditions, fogging density conditions and transfer conditions so as to satisfy specific relations. CONSTITUTION:The developer is constituted so as to satisfy all of equations I, II, III in the two-component magnetic brush developing method consisting of a toner and carrier. In the equations, epsilon0 is the dielectric constant of vacuum (8.854X10<-12> F/m), epsilonr1 is the specific dielectric constant of a toner layer; deltais the sp. gr. of the toner; P is the packing rate of the toner layer; Vts is a satd. toner voltage (effective latent image intensity: Vb-V0; Vb is a developing bias voltage; V0 is a latent image potential); (r)is a proportional constant (=0.5); M is a toner deposition; dm is the thickness of a photosensitive body; epsilonm is the specific dielectric constant of the photosensitive body; Vs is a uniform electrification potential; r1 is the radius of the toner; Fa12 is the mechanical adhesive force (4.2X10<-8>N) between the toner and the photosensitive body at about 127j; toner diameter; d1 is the thickness (=21) of the toner layer after development; (m) is the average mass of one piece of the toner; sigma co is an optimum transfer charge quantity (302 muc/m<2>); Tp is the sp. charge of the toner.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a two-component magnetic brush developing method used in an electrophotographic recording apparatus or the like, and relates to a parameter setting condition of a recording process. In particular, the toner charge amount is an important value that greatly affects the printing process such as development and transfer and affects the printing quality. In order to set this value, it is necessary to determine the toner charge amount in consideration of other process parameters.

[0002]

2. Description of the Related Art At present, an electrophotographic recording system is widely used which is capable of quickly and neatly printing on plain paper. FIG. 2 shows a configuration example of a printer using the electrophotographic recording method, and the recording principle will be described. For example, a photosensitive drum made of selenium is uniformly charged with positive charges by a corona charger. After that, exposure is performed according to the image pattern to form an invisible image of electric charge (latent image). Then, a toner, which is colored fine particles charged by a developing device, is attached to the latent image to form a visible image. A sheet of recording paper is placed on this toner image, and a corona charger is applied from the back to give a charge of the opposite polarity to the charged toner.
Toner is electrostatically transferred onto recording paper. The transferred toner image is melted by a heat roller or the like and fixed on the recording paper. On the other hand, the residual toner remaining on the photosensitive member without being transferred is removed by a fur brush cleaner or the like. This process is repeated to continuously print.

As a developing system for electrophotographic recording as shown in FIG. 2, a magnetic brush developing device is generally used. FIG. 3 shows an example of the configuration of the magnetic brush developing device. In the developing device, a rotatable non-magnetic metal sleeve is provided on a fixed magnet roller. As the developer, a carrier made of iron powder having a diameter of about 100 μm and a toner, which is a resin having a diameter of about 10 to 12 μm, are usually mixed at a certain ratio. When the toner and the carrier are agitated, they are charged by friction between the toner and the carrier and electrostatically adhere to the surface of the carrier.

FIG. 4 shows a state of a developing area in the magnetic brush developing device. FIG. 4A shows the state of the image portion in which the latent image portion is a visible image, and FIG. 4B shows the state of the portion other than the latent image portion, that is, the background portion. Bias voltage V _b applied to the developing device, generally uniform charging potential V _s and the latent image potential V
It is set between _o and. Then, in the image portion, an electric field is generated in the direction in which the charged toner moves to the surface of the photoconductor, and the electric field causes the charged toner to adhere to the surface of the photoconductor. On the other hand, in the background portion, an electric field is generated from the surface of the photoconductor toward the developing device, and a force acts in the direction of moving the charged toner to the developing device side. Therefore, normally, the charged toner mechanically attached to the surface of the photoconductor is electrostatically removed,
It was thought that it would be possible to print beautifully without stains on the background.

In the conventional two-component magnetic brush development system, the mechanism of fog generation has not been clarified, and the relationship between the amount of toner charge and the fog phenomenon has not been clarified. For this reason, in some cases, the background part is often printed in gray. Furthermore, the relationship between the toner charge amount and the image density, which is directly related to the print density, has not been clarified, and the toner charge amount is determined based on the feeling and experience of each developed electrophotographic device. It was the current situation. Therefore, a great deal of development man-hours are required.

In order to solve this problem, Japanese Patent Laid-Open No. 2-24
As shown in Japanese Patent No. 2267, a detailed study was conducted on the entire electrophotographic process. As a result, the toner charge amount is
It was clarified that it only affects the development and transfer processes. In the development process, the amount of toner adhering to the surface of the photoconductor, that is, the print density, and the stain (fog phenomenon) due to the toner adhering to the background portion other than the image portion, are greatly affected. It was clarified that the process affects the transfer efficiency of the toner image onto the recording paper.

The following is a theoretical / relationship between the toner adhesion amount during development, the fogging phenomenon, the electrostatic transfer efficiency of a toner image, and the toner charge amount, which affect the print quality.
A detailed description will be made experimentally, and the conditions of the toner charge amount necessary for obtaining good print quality will be obtained.

(A) Condition from toner adhesion amount at the time of development A theoretical study on the toner adhesion amount in the two-component magnetic brush developing method is described in the literature (Nakajima, Kimura, Horie, Matsuda: "Dynamics of Magnetic Brush Development. Analysis ”, IEICE, Journal (C),
J61-C, 9, pp. 601-606 (Showa 53-09).) Has been thoroughly studied.

The two-component magnetic brush development is a process of filling an electrostatic latent image with charged toner as shown in FIG. In the reversal development method as shown in the figure, toner of the same polarity is made to adhere to the latent image portion from which electric charges have been removed by the electric field between the magnetic brush and the photoconductor. As shown in FIG. 6, consider the case where the magnetic brush of the speed υ _m of the photosensitive drum moving at the speed υ _d is developing relative to each other. Here, the toner supply ability θ is defined as follows.

[Equation 4]

Here, n is the number of times of repeated development in consideration of multi-stage development and the like. Now, the developing process will be divided into the feeding and feeding of the toner to the developing area and the movement of the toner due to the electrostatic force of the toner on the surface of the photoconductor. The former is evaluated by the toner supply ability θ, and the latter is evaluated by the electric flux density in the developing area. The electric flux density can be represented by a product ε _o E (X) of a void electric field E (X) which is a variable of the toner layer thickness X and a dielectric constant ε _{o of} vacuum. It is considered that the toner adhesion amount M increases as the toner supply capability θ and the electric flux density ε _o E (X) increase. Here, the amount M of the adhered toner is evaluated by the area charge density Q of the adhered toner, and a slight increase dQ of Q is the electric flux density ε _o E
Assuming that (X) and θ are proportional to the minute increase dθ, the following equation is obtained.

DQ = γ · ε _o E (X) dθ (2) where γ is a proportional constant. The area charge density Q is represented by the thickness X of the adhered toner layer and the volume charge density ρ, and is given by the following equation. Q = ρX (3) Substituting equation (3) into equation (2) rearranges the following equation.

[Equation 5]

The differential equation is solved to find the relationship between the amount of adhered toner and the toner supply ability θ. First, the void electric field E
Calculate (X) from Fig. 7. In the figure, it is assumed that there is an adherent charged toner layer having a thickness X on a photoconductor having a thickness d _m , and a toner layer having a thickness d _{t is} on a magnetic brush across a gap g. The air gap electric field E (X) is given by the following equation.

[Equation 6]

Here, V _ts of the molecule is a theoretical saturated toner voltage, and the potential difference corresponding to the electrostatic attraction between the toner and the carrier is calculated from the potential difference between the developing bias and the photoconductor (effective latent image intensity). It is the deducted value. The term enclosed in parentheses is the voltage generated by the toner adhering to the photoconductor, that is, the toner voltage V _t . If the toner voltage reaches a saturation toner voltage V _ts, gap field development becomes zero ceases. Each term in the denominator is the effective distance of the toner layer on the toner layer, the gap, the photoconductor, and the developer on the developer. Among them, the void g is considered to be 1 μm or less, and thus can be omitted as compared with the others. In addition, the toner layer attached to the developer is considered to be attached to the photoconductor after development and incorporated into X, and is calculated, so that d _t can be omitted.

Substituting equation (4) into equation (5) and solving for X with θ = 0 as the initial value when θ = 0 gives the following equation.

[Equation 7] Assuming that the toner density is δ and the filling rate of the toner layer is p, the attached toner amount M is δpX. Further, when the volume charge density ρ is expressed using the toner specific charge T _p , the amount M of adhered toner is given by the following equation.

[Equation 8] However, the toner voltage V _t is given by the following equation.

[Equation 9]

Expression (7) shows the amount M of adhered toner when a toner voltage is applied, and expression (8) shows how the toner voltage V _t is determined in the developing process.
FIG. 8 shows experimentally obtained relations between the toner adhesion amount and the toner supply ability θ by using three kinds of developers having different toner specific charges. The solid line is a theoretical value when the proportionality constant γ = 0.5 and is an application example of the conductive carrier. On the other hand, the broken line is the theoretical value when γ = 0.3,
This is an application example in which the surface of the carrier is coated with a resin of about 1 μm. In addition, ○, ●, △, ×, □ indicates experimental data. It can be seen that the amount of adhered toner increases as the toner supply capability θ increases and as the toner specific charge decreases. Further, when the conductive carrier and the coating carrier are compared, the conductive carrier has a larger toner adhesion amount. Thus, the good agreement between the experimental value and the theoretical value shows the validity of theoretical formulas (7) and (8). The proportionality constant γ was further investigated, and depending on the surface condition of the carrier, it was 0.1 to 0.7.
It has become clear that it changes to some extent.

From the above examination, conversely, considering the toner adhesion amount M required, the toner specific charge, which is the charge amount of the toner required for development, is obtained by the following equation.

[Equation 10]

(B) Condition from Fogging Density The point of fog density has not been studied at all. Therefore, the results theoretically examined by the authors will be described in detail below. As shown in FIGS. 9 and 10,
Assuming that the reverse electric field for collecting the toner is applied between the photoconductor and the developing device in a state where the charged toner is attached to the photoconductor surface, the following equation is established.

[Equation 11]

From this, the electric field E _o acting on the surface of the photoreceptor is given by the following equation.

[Equation 12]

From the equation (11), the electrostatic force F _e acting on the charged toner is given by the following equation. F _e = q _t · E _o (12) where q _t is the amount of charge on the toner, and when the electrostatic force F _e acting on the charged toner becomes larger than the mechanical force F _a between the photoconductor and the toner. , Fogging does not occur. This condition is as follows. F _e > F _a (13) Substituting equation (12) and equation (11) into equation (13), the relationship between the reverse bias voltage (V _s −V _b ) that does not cause fog and the toner specific charge T _p Is calculated by the following equation.

[Equation 13]

Equation (14) indicates that the reverse bias voltage at which fogging does not occur takes a minimum value at a certain toner specific charge. Further, when the specific charge T _{p of the} toner in which fogging does not occur is obtained from the equation (14), the following equation is obtained.

[Numerical equation 14]

FIG. 11 shows the occurrence of fog obtained experimentally.
Areas with a circle, and areas with a fog with a cross.
is doing. Furthermore, the occurrence of fog obtained from theoretical formula (15)
The area without is shown with diagonal lines. Of the toner used in the calculation
Radius r_t, Toner density δ, relative dielectric constant of toner ε_r1, Photosensitive
Body thickness d_m, Relative permittivity ε of photoconductor_mAre each r _t
= 5 μm, δ = 1110 kg / m³, Ε_r1= 2.2, d_m= 50μ
m, ε_m= 6. Also, the mechanical adhesive force F_a= 4.2
× 10^-8Assume N. From Fig. 11, the validity of theoretical formula (15) is
It was revealed.

(C) Conditions from electrostatic transfer Regarding the transfer condition of the toner image, reference has already been made in the literature (Electrophotographic Society: “Basics and Applications of Electrophotographic Technology”, Section 3.5 “Toner Image Transfer Theory”, The Institute of Electrophotography, Corona Publishing, p. 171-1
91 (June 1988)). As shown in FIG. 12, the recording paper is conveyed so as to be in close contact with the toner layer on the photosensitive drum, and in the state where the toner image and the recording paper are in close contact, a charge having a polarity opposite to that of the charged toner is applied from the back surface of the recording paper. Then, the recording paper and the photoconductor are electrostatically adsorbed. In this state, the charged toner is attracted to the recording paper by the electric charge applied to the recording paper, and the toner image is transferred. In the electrostatic transfer, the electrostatic force F _e acting on the toner having the charge amount q is greater than the mechanical adhesion force F _a of the toner and the photosensitive member surface, the toner is transferred onto the recording sheet. The transfer mechanism is theoretically explained using the model of electrostatic transfer in FIG.

A charged toner layer d _t having a volume charge density ρ is provided on a photosensitive member having a thickness d _m , and a recording paper having a thickness d _p is provided thereon. The gap between the toner layer and the recording paper is g. Further, an electric charge ρ _c having a polarity opposite to that of the charged toner is applied on the recording paper. In this state, the force F _e (X) in the recording paper direction acting on the charged toner at the position X from the surface of the photoconductor is given by the following formula.

[Equation 15] Where ε _o : Dielectric constant in vacuum ε _r1 : Dielectric constant of toner layer

The volume charge density ρ of the toner layer is expressed by the following equation using the toner specific charge T _p . ρ = δpT _p (17) However, when δ: toner density p: toner layer filling factor (17) is substituted into equation (18), electrostatic force F _e (X) of the charged toner with respect to toner specific charge is obtained. It is obtained as a quadratic equation as in the following equation.

[Equation 16] The electrostatic force F _e (X) acting at a distance X from the surface of the photoconductor balances the mechanical adhesive force F _a, and the toner layer is divided at this point, and only the toner layer having a thickness of (d _t −x) Is transferred to the recording paper, the transfer efficiency η at this time is expressed by the following equation.

[Numerical formula 17]

Therefore, the toner specific charge for obtaining the highest transfer efficiency η = 1 is as follows.

[Equation 18]

FIG. 14 shows the experimental result of the relationship between the toner specific charge T _p and the transfer efficiency η. Here, before transfer, the toner was recharged by a corona charger to change the specific charge of the toner. Black circles indicate positively charged toner before recharging, and white circles indicate negatively charged toner. As is clear from the figure, the specific charge of the toner indicates that there is an optimum specific charge for transfer. The toner density δ which is the physical property value of the toner used in the experiment, the filling rate p of the toner layer, the thickness d _t of the toner layer and the mass m of one toner are δ = 1110 kg / m ³ , respectively.
p = 0.6, d _t = 20 μm, m = 5.8 × 10 ⁻¹³ kg, and
Each this time the toner and between the photoreceptor mechanical adhesion F _a and the transfer charge amount sigma _{c ^{a, F a = 4.2 × 10 -8}} N, σ c
= −320 μC / m ² (optimum transfer charge amount = σ _co ), and these values are shown by the solid line in FIG. Thus, the theory and experimental values were in good agreement, demonstrating the validity of the theory. When the transfer charge amount σ _co was investigated, a practical transfer efficiency of 70
As shown in Fig. 15, the transfer charge amount σ _{co at which the} above% is obtained is
It was revealed to be 100 to 500 μC / m ² .

(D) Overall toner specific charge conditions Therefore, in order to obtain good printing, the following conditions must be satisfied.・ Development density condition

[Formula 19]

Fogging conditions

[Equation 20]

-Transfer conditions

[Equation 21]

However, ε_oIs the dielectric constant of the vacuum (8.854 × 10^-12
F / m), ε_r1Is the relative dielectric constant of the toner layer, and δ is the toner ratio
Weight, p is the filling rate of the toner layer (= normally 0.2 to 0.8), V
_tsIs the saturated toner voltage (effective latent image intensity: V_b-V_o; V_b
Is the developing bias voltage, V_oIs the latent image potential), γ is a proportional constant
(= 0 to 1.0; usually 0 if conductive carrier is used.
4 to 0.7, but when using a coated carrier
Is 0.1 to 0.4. ), M is the toner adhesion amount (= sufficient
To obtain dark prints, toner on the surface of the photoconductor after development
Adhesion amount of toner that has one or more layers (kg / m²)
If the toner particle size is about 10 μm, 6 g / m²Degree
It becomes degree. ), D_mIs the thickness of the photoconductor, ε _mIs the ratio of the photoconductor
Dielectric constant, V_sIs a uniform charging potential, r_tIs the toner radius, F
_aIs the mechanical adhesive force between the toner and the photoconductor (= normal
In the minute development method, 10^-9~Ten^-6It is about N. ), D
_tIs the thickness of the toner layer after development, m is the average of one toner
Mass, σ_coIs the optimum transfer charge (= 100-500 μC / m
²).

[0031]

In the above theoretical examination, the mechanical force F _a is the van der Waals force, which is a fixed value (4.2 × 10 ⁻⁸ N) when the toner particle size is 10 to 13 μm. Is being considered. However, since F _a changes depending on the toner particle size, it was not possible to find a process condition that can cope with the change in the toner particle size. For this reason, when the toner particle size becomes small, if the process parameters are determined under the conditions of the above-mentioned theoretical study formula, the phenomenon that the toner adheres to the background portion of the image or the transfer efficiency decreases may occur. was there.

In the present invention, in the case of toner having a toner particle size other than around 12 μm, the influence of the printing process conditions such as the toner charge amount, the toner particle size, the developing bias, the surface potential, etc., over the entire electrophotographic process. About how
We will study from both theoretical and experimental points of view, set the process conditions based on the obtained results, and try to realize the two-component magnetic brush developing method that can keep the printing quality stable.

[0033]

According to the two-component magnetic brush developing method of the present invention, there is provided a two-component magnetic brush developing method comprising a toner and a carrier used in an electrophotographic apparatus.
It is characterized by satisfying all the conditions of the following formula. (A) Development density condition

[Equation 22] (B) Fogging density condition

[Equation 23] (C) Transfer condition

[Equation 24] And Ε _o is the dielectric constant of vacuum (8.854 × 10 ^-12 F /
m), ε _r1 is the relative dielectric constant of the toner layer, δ is the specific gravity of the toner,
p is the filling rate of the toner layer, V _ts is the saturated toner voltage (effective latent image intensity: V _b -V _o ; V _b is the developing bias voltage, V _o is the latent image potential), and γ is a proportional constant (= 0.5). ), M is the toner adhesion amount, d _m is the thickness of the photoconductor, ε _m is the relative permittivity of the photoconductor, V
_s is uniform charging potential, r _t is the toner radius, F _a12 mechanical adhesion force between the toner and the photoconductor the toner diameter in the vicinity of ^{12μm (4.2 × 10 -8 N)} , d t after development the thickness of the toner layer (≒ 2r _t), m is one toner average mass, sigma _co
Is the optimum transfer charge amount (320 μC / m ² ), and T _p is the toner specific charge. By adopting this configuration, it is possible to obtain the two-component magnetic brush developing method which can keep the printing quality stable.

[0034]

[Function] Focusing on the change of van der Waals force when the toner particle size is changed, F _a of the formulas (9), (15) and (20) is set as the following formula. F _a12 conventional van der Waals force during the toner diameter 12μm by the formula ^{(21) (4.2 × 10 -8} N), r t is the radius of the toner.

[Equation 25]

The present inventors have examined the van der Waals force F _a when the toner particle size changes. Van der Waals force F _a is described in the literature (Electrophotographic Society of Japan;
"Basics and Applications of Electrophotographic Technology", Corona (1988), p.465)
Than,

[Equation 26]

It becomes Therefore, van der Waals force F
_There is a proportional relationship between _a and the toner diameter (radius) r _t . Further, as described above, the measured value of F _a when the toner particle size is 12 μm is
It is also known to be 4.2 × 10 ⁻⁸ N. From these facts, F _a is given by equation (21).

[0037]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows a laser of a first embodiment to which the present invention is applied.
The structure of the printer is shown. As a photoconductor, thickness d_t= 19
μm, relative permittivity ε_r1= 3 (capacitance 1.4 μF / m ²) Yes
Rotate the photoconductor drum at a constant speed at a drum peripheral speed of 120 mm / s
Let Use a corona charger to evenly place this photosensitive drum at -60
It is uniformly charged to 0 V, and then the semiconductor laser light is used to image the image.
Exposure is performed according to the turn. Then, in the place where the light hit
The surface potential drops to about -100 V, and an electrostatic latent image is formed.
Is made. After that, bias voltage-
450 V was applied to make the electrostatic latent image visible. Transcription
In the section, the current value flowing through the corona wire is 200 μA
And the optimum transfer charge amount was given. By this
Then, the toner image was transferred onto the recording paper. Transfer efficiency at this time
About 85% was obtained. This toner image is
It was melted with a roller and fixed on recording paper. Meanwhile, photoconductor drum
The toner remaining on the top is felt by a fur brush with a diameter of 20 mm.
Removed from above the photoconductor drum.

In the developing section, the average particle size is 60 μm.
m magnetite carrier and a resin toner having an average particle size of 7 μm were mixed so that the weight mixing ratio of the toner was 4.0%. At this time, with respect to the specific charge of the toner, using the above theoretical formula, as shown in Table 1, (a) development density condition, (b) fog condition, and (c) transfer condition are adjusted according to the embodiment. First, the overall practical range was sought. Next, even if the toner specific charge drops under high temperature and high humidity, the toner specific charge at room temperature and humidity is close to the upper limit of 30 μC so that the lower limit of 13 μC / g, which is the overall practical range, is reached.
/ G. In order to adjust the toner specific charge to this set value, the carrier is controlled by controlling the thickness and the surface treatment temperature of the resin film coated on the surface of the magnetite core, and the toner is controlled by controlling the addition amount of the charge control agent. The required toner specific charge of 30 μC / g was achieved.

[0039]

[Table 1]

By using the developer having the toner specific charge thus determined, the low temperature and low humidity (10
Even at high temperature and high humidity (32 ℃, 85% RH), the print density was sufficiently dark and high-quality prints without background stains were obtained.

As a second embodiment, a printing experiment was conducted by examining the practical range in the case of using a toner having a small particle diameter of 5 μm. The conditions were the same as in the first embodiment. Table 2 shows the examination results. As a result of adjusting the toner specific charge by the same adjusting method as in the first embodiment, a sufficiently thick print density can be obtained even in a small particle size toner of 5 μm under low-temperature low-humidity to high-temperature high-humidity environmental conditions. High-quality printing was obtained without stains on the parts.

[0042]

[Table 2]

[0043]

According to the present invention, even in the electrophotographic process using the two-component magnetic brush developing method using a toner having a particle size other than about 12 μm, the process conditions required for obtaining good print quality Can be uniquely obtained.
Therefore, the number of development steps of the electrophotographic process can be significantly reduced, and good and stable image quality can be obtained.

[Brief description of drawings]

FIG. 1 is a diagram showing a configuration of a laser printer according to an embodiment of the present invention.

FIG. 2 is a diagram showing a configuration of a conventional printer.

FIG. 3 is a diagram showing a configuration of a magnetic brush developing device.

FIG. 4 is a diagram showing a state of a developing area, in which (a) is an image portion,
(B) is the potential thereof, and (c) is the background portion.

FIG. 5 is a diagram showing a reversal development model.

FIG. 6 is a diagram showing a state during development.

FIG. 7 is a diagram showing a magnetic brush development model.

FIG. 8 is a diagram showing the effect of toner specific charge.

FIG. 9 is a diagram showing a fog development model.

FIG. 10 is a diagram showing a development potential model.

FIG. 11 is a diagram showing a relationship between a toner specific charge and a reverse bias voltage so that fog density does not occur.

FIG. 12 is a diagram showing a basic transcription model.

FIG. 13 is a diagram showing an electrostatic transfer model.

FIG. 14 is a diagram showing a relationship between toner specific charge and transfer efficiency.

FIG. 15 is a diagram showing a relationship between a transfer charge amount and transfer efficiency.

Claims (3)

[Claims] 1. A two-component magnetic brush developing method comprising a toner and a carrier for use in an electrophotographic apparatus, wherein the following conditions are all satisfied. (A) Development density condition [Equation 1] (B) Fogging density condition [Equation 2] (C) Transfer condition [Equation 3] And Ε _o is the dielectric constant of vacuum (8.854 × 10 ^-12 F /
m), ε _r1 is the relative dielectric constant of the toner layer, δ is the specific gravity of the toner,
p is the filling rate of the toner layer, V _ts is the saturated toner voltage (effective latent image intensity: V _b -V _o ; V _b is the developing bias voltage, V _o is the latent image potential), and γ is a proportional constant (= 0.5). ), M is the toner adhesion amount, d _m is the thickness of the photoconductor, ε _m is the relative permittivity of the photoconductor, V
_s is uniform charging potential, r _t is the toner radius, F _a12 mechanical adhesion force between the toner and the photoconductor the toner diameter in the vicinity of ^{12μm (4.2 × 10 -8 N)} , d t after development the thickness of the toner layer (≒ 2r _t), m is one toner average mass, sigma _co
Is the optimum transfer charge amount (320 μC / m ² ), and T _p is the toner specific charge. 2. A developer for two-component magnetic brush development, which has a toner specific charge (T _p ) satisfying all the expressions of claim 1. 3. An electrophotographic apparatus using a two-component magnetic brush developing method satisfying the conditions of claim 1.