JPH06250505A - Image forming device - Google Patents

Abstract

PURPOSE:To provide an image forming device by which fog-free printing can be realized by setting developing bias voltage in the optimal value in the image forming device using an electrophotographic technology. CONSTITUTION:In an image forming device having an image carrier 101 and a developing roller 107, latent image strength being (developing bias voltage VB-exposure part electric potential VL) and the developing bias voltage VB to determine inverse bias voltage being (nonexposure part electric potential VS-developing bias voltage VB) are set so that a value by which fog due to implantation of electric charge having reversed polarity in a developing part accompanying an increase in inverse bias is not generated, becomes the lower limit and a value, by which the fog due to decrease in resiliency to an image carrier 101 or electrified toner accompanying a decrease in the inverse bias is not generated, becomes the upper limit.

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 using electrophotography.

With the rapid development of the information industry in recent years, various information systems have been developed, the demand for output machines (printers) suitable for them has increased, and various printers have been commercialized.

[0003]

2. Description of the Related Art Generally, recording methods are classified into impact methods and non-impact methods. The impact method has the advantages of small size, low price, maintenance-free, etc.
There is a problem in terms of noise. On the other hand, the non-impact method includes a serial method and a page method. In the serial method,
Although it has advantages of small size and low price, the thermal transfer method has a problem that the recording speed is slow, and the inkjet has a problem such as clogging.

As a page system, an electrophotographic system is typified, and although it has advantages such as high-speed printing and high printing quality, there are problems in cost reduction and device miniaturization due to the complicated process. Was considered. However, with respect to recent electrophotographic printers, downsizing and price reduction have advanced, and the present situation is that they are entering the area of impact printers. Next, an outline of this electrophotographic process will be described.

In this process, a uniform charge layer is formed by rotating a photosensitive member and printing a high voltage on a discharge wire on the surface thereof. Next, an electrostatic latent image corresponding to the image signal is formed on the surface by an exposure process. And
The electrostatic latent image is visualized with a developer composed of charged colored particles called toner (toner image formation). The toner image is
Usually, after transfer to a transfer material (recording paper), it is fixed by heat and pressure.

This electrostatic latent image developing system is roughly classified into the following:
One component development and two component development. In the case of two-component development, the developer used is composed of a toner and a carrier, and mechanical stirring causes frictional charging between the toner and the carrier to impart an electric charge to the toner. However, in the case of the two-component developing method, it is necessary to always keep the toner concentration constant, and there is a problem that stable control thereof is difficult.

On the other hand, the one-component developing method is a method in which the toner is developed by itself without mixing with the carrier, and magnetic powder is contained in the toner for carrying the toner on the developing roller and supplying the toner to the developing section. Some of them contain, and some use non-magnetic toner. Recently, in consideration of colorization, attention has been paid to the development of a color printer using a non-magnetic toner.

[0008]

By the way, in the conventional developing system, development is performed in a state where the toner supply amount is saturated in the developing section. The outline of this type of electrophotographic process is shown in FIG.
Shown in. In this case, the toner adhesion amount M on the photosensitive drum (image bearing member) 1, a latent image intensity formed on the photosensitive member 1 (the developing bias voltage V _B - irradiation part potential V _L), the toner specific charge T _P It depends on the size of. That is, the developing bias voltage is a factor that determines the toner adhesion amount (print density).
In FIG. 6, 2 is a developing roller, 3 is a carrier,
4 is the toner, V _S is the potential of the non-exposed portion, and V _s -V _B is the reverse bias voltage.

However, since the latent image intensity and the toner specific charge change due to the environmental change, the toner adhesion amount also changes, and there is a problem that the density and resolution of the printed image deteriorate. Therefore, we have proposed a developing method in which toner adhesion is always constant against environmental changes (Japanese Patent Application No. 3-156644, filed June 27, 1991). This method will be described below with reference to FIG.
Reference numeral 5 is a blade for controlling the toner layer thickness.

In this method, since the toner adhesion amount can form a very thin and uniform toner layer on the developing roller 2,
It can be uniquely determined by the amount of toner conveyed to the developing unit. The toner adhesion amount M is expressed by the following equation. M = 2r _t δρn (v _d / v _p ) r _t : toner radius δ: toner filling rate ρ: toner density n: number of toner layers on the developing roller v _p : photosensitive drum peripheral speed v _d : developing roller peripheral speed

Therefore, it is possible to obtain a stable toner adhesion amount because it does not depend on the latent image intensity and the toner specific charge, which easily change with respect to environmental changes. Further, the developing bias voltage in this developing method is set to a sufficiently large value in order to provide a margin for fluctuations in the surface potential of the photoconductor and fluctuations in the toner charge amount due to environmental changes.

However, depending on whether the developing bias is large or small, it may cause fogging. Explaining this point in detail, the influence of the reverse bias voltage described above with reference to FIG. When the reverse bias voltage is large, electric charges having the opposite polarity are injected into the toner, and the toner charged with the opposite polarity adheres to the background portion of the print and causes fog.

If the reverse bias voltage is small, the repulsive force of the normally charged toner with the photoconductor at the background of printing is small, and the toner adheres to the background of printing and fog occurs.

It is an object of the present invention to provide an image forming apparatus capable of realizing fog-free printing by setting a developing bias voltage to an optimum value.

[0015]

In order to achieve the above object, according to the present invention, an electrostatic potential difference between an unexposed portion not exposed to image light and an exposed portion exposed to image light is caused. An image carrier on which an electrostatic latent image is formed, and a developing roller to which a toner for developing the electrostatic latent image is attached on the surface and a developing bias voltage is applied between the developing roller and the image carrier. A toner that is formed on the image carrier by adjusting the electrostatic latent image on the image carrier to a sufficient size and supplying the toner to the developing unit between the image carrier and the developing roller. In the image forming apparatus of the type that determines the amount, the latent image intensity that is (developing bias voltage-exposure portion potential) and the developing bias voltage that determines the reverse bias voltage that is (non-exposure portion potential-developing bias voltage) are , With increasing reverse bias The lower limit is a value that does not cause fogging due to the injection of electric charges of opposite polarity to the toner in the image portion, and the upper limit is a value that does not cause fogging due to a decrease in repulsive force of the charged toner to the image carrier due to a decrease in reverse bias. The configuration is such that it is set to

[0016]

As described above, since the developing bias voltage is optimized by setting the upper and lower limit values, the occurrence of fog can be suppressed and the print density can be stabilized. That is, good printing can be obtained.

[0017]

Embodiments of the present invention will be described below with reference to FIGS.

In the present invention, a developing bias voltage is applied between the image carrier on which the electrostatic latent image is formed and the toner for developing the electrostatic latent image is attached to the surface and the image carrier. This is applied to an electrophotographic apparatus in which the amount of toner adhered to the image bearing member is uniquely determined by the amount of toner supplied from the developing roller to the image bearing member. It is intended to be realized.

To achieve this object, the developing bias voltage that determines the reverse bias voltage (developing bias voltage-exposure portion potential) and (non-exposure portion potential-developing bias voltage) increases the reverse bias. The lower limit is the value that does not cause fogging due to the injection of electric charges of opposite polarity to the toner in the developing section, and the upper limit is the value that does not cause fogging due to the decrease in repulsive force of the charged toner on the image carrier due to the decrease in reverse bias. Set to do. Next, the reverse bias voltage is calculated according to the model diagram of FIG. 1 and the optimum condition in which fogging does not occur is examined.

In FIG. 1, a photosensitive drum (image carrier) 1
When 01 and the toner 100 are considered to be a dielectric and the developing roller 107 is considered to be a resistor, the following equation is established. −∫ ₀ ^d (σ _o / ε _o ε _d ) dy−∫ _d ^{d + x} [{σ ₀ + σ _m + ρ (y−d)} // ε _o ε _t ] dy = V _B ………… (1 ) Where ε _o : constant ε _d : relative permittivity of photosensitive drum y: distance d: thickness of photosensitive layer x: gap between photosensitive drum and developing roller ∫ ₀ ^d : ₀ integration of _d ∫ _d ^{d + x} : 0 ~ (D + x) integral

From the equation (1), the dielectric charge σ _{o of the} photosensitive drum
_{Is, σ o = {- (Xε} d / dε t) V S -V B - (ρX 2 / 2ε o ε t)} / {(d / ε o ε d) + (X / ε o ε t)} (2) However, V _S is the surface potential of the photosensitive drum and is expressed by the following equation. _{V S = (σ m d /} ε o ε d) ................................. (3)

The electric field E ₁ acting on the surface of the photosensitive drum is expressed by the following equation. E ₁ = (σ _o + σ _m ) / ε _o …………………………… (4) Substituting equation (2) into equation (4) and replacing ρ with q / m, transforming The following equation is obtained. _{E 1 = [V S -V B} - {βδ (q / m) X 2} / 2ε o ε t] / {(d / ε d) + (X / ε t)} .................. (5 )

Now, the force acting on the toner particles during development will be examined. As shown in FIG. 2, in addition to the electrostatic force F _e received from the electric field E _o and the force F _r that the developing roller grips the toner, a mechanical adhesion force F _b acts on the toner particles. The conditions under which fogging does not occur for each of these forces are:
Sum mechanical adhesion force F _r Komu addition the electrostatic force Fe F _b
When it becomes larger, it becomes the following formula. F _e + F _r ＞ F _b ………………………………………… (6)

Further, in order to clarify what kind of factor each force is, each force is formulated. F _e = (q / m) E _o · δ · (4/3) πr ³ (7) F _r = (Ar / 6D ² ) (1 + A / 6πD ³ H) (8) However, r : Toner radius, δ: Toner particle density q / m: Toner specific charge, A: HAMAKER constant (1
× 10 ^-19 ) D: Closest distance between toner and developing roller, H: Hardness of developing roller

Further, assuming that the mechanical adhesive force F _b is a constant value that can be exerted by the force for pressing the developing roller against the photosensitive member, the following equation is obtained. F _b = const …………………………………… (9) Substituting equation (7), equation (8), and equation (9) into equation (6) gives the following equation. _{| H S -H B |> [} {F b - (AR / 6D 2) (1 + A / 6πD 3 H)} / {(4/3) πr 3 δ (q / m)}] [(d / ε p ) + (X / ε _t )] + {βδ (q / m) X ² / 2ε _o ε _t } ... (10) where ε _t : relative permittivity of toner, ρ: volumetric charge density of toner σ _m : surface charge on the photosensitive drum, σ _o : induced charge of the photosensitive drum

First, focusing on the equation (10), the electric field E _O
Is proportional to the reverse bias voltage, which is the difference between the uniform charging potential (potential of the non-exposed portion) V _S of the photosensitive drum and the developing bias voltage V _b , so that when the reverse bias voltage decreases, the electric field E _o also weakens and the toner The electrostatic force F _e received from the electric field is reduced, and fogging easily occurs. Further, in Expression (10), when the developing roller hardness H increases, the gripping force F
_r becomes small and fogging easily occurs.

Further, regarding the toner specific charge q / m,
When q / m decreases, the first term on the right side increases, and conversely q
Even if / m increases, the second term on the right side increases. That is, the reverse bias voltage to remove the head (V _S -V
There will be q / m that minimizes _B ). Therefore, the relationship between the toner specific charge and the reverse bias voltage, which greatly affects the fogging, will be examined.

Here, considering the influence of the hardness of the developing roller, which is treated as expansion of the fogging margin,
Expression (10) can be rewritten as the following expression. _{| V S -V B |> (} 2βδT p r t 2 / 2ε o ε t) + {Fa / (4/3) πr t 3 T p} {(2r t / ε t) + (d p / ε p )} (11) Expression (11) indicates that the reverse bias voltage for fogging takes a minimum value with a certain toner specific charge. Further, when the toner specific charge T _{p at} which fogging occurs is obtained from the equation (11), the following equation is obtained.

[0029] _{T p <[{ε o ε} t (V S -V B)} / 4βδr t 2] -A 1/2 T p> [{ε o ε t (V S -V B)} / 4βδr t ^{2] + A 1/2 A = [} {ε o ε t (V S -V B)} / 4βδr t 2] 2 - (3ε o ε t F a) / (8πδ 2 βr t 5) {(2r t / ε _t ) + (d _p / ε _p )} (12) Therefore, regarding the relationship between the toner specific charge T _p and the reverse bias voltage (V _S −V _B ) when various photoconductors are used. consider.

The results are shown in FIG. The data in this figure is OPC
It is a calculated value obtained from the equation (12) using the value of the electrostatic capacity of the photoconductor. From the figure, in the case of the organic photoconductor, the reverse bias voltage is 7 when the toner specific charge is 7 to 35 μc / g.
It can be seen that fogging does not occur at 0V. This is the lower limit of the reverse bias voltage. The values of each parameter used in the calculation are as follows.

Toner particle size r _t = 6 μm Toner layer relative permittivity ε _t = 2.2 Toner density δ = 1111 kg / m ³ Toner layer thickness d
_t = 24 μm Toner layer filling rate β = 0.6 Toner supply ability θ
= 2.0 photoconductor mechanical adhesion between the film thickness d _P = dielectric constant of 25μm photoreceptor epsilon _p = 3.0 toner and the photosensitive member F _a = 4.2 × 10
^-8 N Transfer charge amount σ _c = 320 μc / m ² Adhered toner amount M = 8 g / m ² (development density 1.3) Assumed reverse bias voltage V _S -V when obtaining fog density
_B = 100V

Next, the upper limit value of the reverse bias voltage will be examined. The upper limit of the reverse bias voltage is
Electric charges are injected into the toner from the surface of the developing roller, and instead of the regular polar charge of the toner, toner with an opposite charge amount is created, which is attracted to the background potential of the photoconductor surface and adheres to the background. Will not occur.

Therefore, FIG. 4 shows the fog density when the developing bias voltage is changed. From this figure, it is understood that if the upper limit value of the reverse bias voltage is set to 350 V, fogging due to charge injection does not occur. Further, the lower limit value of the above-mentioned reverse bias voltage also substantially matches the experimental value.

FIG. 5 shows a structural outline of an image forming apparatus (monochrome system) to which the present invention is applied. In the figure, 102 is a corona charger, 103 is a one-component developing device, 104 is a corona transfer device, 105 is a cleaner, and 106 is a fixing device. The developing device 103 includes a developing roller 107, a blade 108 that regulates the toner layer thickness on the developing roller 107, and the developing roller 1
A reset roller 109 that collects the toner on 07 and supplies new toner, a paddle 110 that conveys the toner toward the developing roller 107, and an agitator 111 that does not make the toner in the developing device uniform.

At the time of recording, the photosensitive drum 101 is moved to the position shown in FIG.
Is rotated in the clockwise direction indicated by an arrow to uniformly charge the surface thereof by the corona charger 102, and then exposed by a predetermined pattern by an exposing unit (not shown). This allows
An electrostatic latent image is formed on the surface, and the electrostatic latent image is developed by the developing roller 107 to become a toner image. Recording paper 12
0 indicates the transfer unit 1 at the same timing as the toner image formation.
The toner image is transferred to the recording paper 120 by the corona transfer device 104.

After that, the recording paper 120 is sent to the fixing section, where the toner image is fixed by the fixing device 106. After the fixing is completed, the recording paper 120 is discharged to a stacker (not shown). Further, the surface of the photosensitive drum 101 after the transfer is completed is cleaned by the cleaner 105 and discharged by the static eliminator 113.

[0037]

As described above, according to the present invention, by optimizing the developing bias voltage, it is possible to suppress the occurrence of fog and stabilize the print density, and it is possible to realize good printing. become.

[Brief description of drawings]

FIG. 1 is a diagram illustrating the principle of contact development according to an embodiment of the present invention.

FIG. 2 is a diagram illustrating a force that acts on toner during development according to the exemplary embodiment of the present invention.

FIG. 3 is a diagram illustrating a relationship between a toner specific charge and a reverse bias according to an exemplary embodiment of the present invention.

FIG. 4 is a diagram illustrating the relationship between reverse bias voltage and fog according to the embodiment of the present invention.

FIG. 5 is a side view showing a structural outline of an image forming apparatus to which the present invention is applied.

FIG. 6 is a schematic diagram of a conventional electrophotographic process.

FIG. 7 is a schematic explanatory diagram of a non-magnetic one-component developing method.

[Explanation of symbols]

100 toner 101 photosensitive drum (image bearing member) 107 developing roller V _S unexposed portion potential V _B developing bias voltage V _L exposed portion potential

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Mototsuba Shibano 1015 Kamiodanaka, Nakahara-ku, Kawasaki City, Kanagawa Prefecture, Fujitsu Limited (72) Inventor Naoyuki Fujimoto 35, Saho, Kato-gun, Hyogo Prefecture Fujitsu Peripherals Co., Ltd. Within

Claims (1)

[Claims] 1. An image bearing member on which an electrostatic latent image is formed, and a developing roller for developing the electrostatic latent image, the electrostatic latent image on the image bearing member having a sufficient size. In an image forming apparatus of a system in which the amount of toner to be formed on the image carrier is determined by the amount of toner supplied to the developing unit between the image carrier and the developing roller, (developing bias voltage-exposure unit) The developing bias voltage that determines the latent image intensity, which is the electric potential), and the reverse bias voltage, which is the (non-exposure portion potential-developing bias voltage), is due to the injection of the reverse polarity charge into the toner in the developing portion with the increase of the reverse bias. Image formation characterized in that the lower limit is a value at which fogging does not occur, and the upper limit is a value at which fogging does not occur due to a decrease in repulsive force of the charged toner on the image carrier due to a decrease in reverse bias. apparatus.