JPS5832377B2 - developing device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method of developing an electrostatic image, and more specifically to a method of developing an electrostatic image using a one-component developer. The present invention relates to an electrostatic image developing method and an apparatus thereof that make it possible to obtain a visible image rich in color.

Conventionally, as an electrophotographic development method using a -component developer, the powder cloud method uses toner particles in a spray state, and the electrostatic image processing method uses a single layer of uniform toner formed on a toner support member such as a web or sheet. in contact with the holding surface,
A contact development method in which development is carried out, a jumping development method in which the toner layer is not brought into direct contact with the electrostatic image holding surface and the toner is selectively flown onto the holding surface by the electric field of the electrostatic image, and
A known method is the magnetry method, in which a magnetic brush is formed using conductive magnetic toner and brought into contact with an electrostatic image holding surface for development.

Among the above-mentioned various component development methods, the powder cloud method, contact development method, and MagneDry method apply toner to the electrostatic image holding surface in the image area (the area where toner should originally adhere).
Since the toner contacts the non-image area (a region of the background to which toner should not originally adhere) without distinction, toner adheres to the non-image area as well, making it impossible to avoid so-called background fog.

However, jumping development method (for example, Japanese Patent Publication No. 41
The method described in Japanese Patent Publication No. 9475) is an extremely effective method in terms of preventing background fog because development is performed with a gap between the toner layer and the electrostatic image holding surface without contact.

However, since the phenomenon of toner flight caused by the electric field of an electrostatic image is utilized during development, the resulting visible image generally has the following drawbacks.

The first problem is that the sharpness decreases at the edges of the image area.

In the case of an electrostatic latent image formed on an electrophotographic photoreceptor, the electric field of an electrostatic image at the edge of the image is as shown in FIG. 4a.

In other words, if a conductive member is used as a developer carrier near the center of the image area, the lines of electric force will emanate from the image area and reach the toner support, so the toner will flow along the lines of electric force. It flies, attaches to the surface of the photoreceptor, and develops.

However, at the edge of the image area, the lines of electric force do not reach the toner support due to the electric charges induced in the non-image area, and wrap around occurs, so the adhesion of the flying toner is extremely uncertain. Some barely stick, and some don't.

As a result, the resulting image lacks sharpness and is unclear at the edges of the image, and when developing line drawings, there is a problem that the line drawings are developed with a thinner appearance than the original image.

In order to avoid this in the normal jumping development method, the gap between the electrostatic image holding surface and the developer support surface must be made sufficiently small (for example, less than IOOμ), and in practice,
Accidents of pressure contact between the two surfaces with the developer or mixed foreign matter are likely to occur.

Furthermore, maintaining such a small gap often involves difficulties in device design.

The second problem is that images obtained by the jumping development method generally lack gradation.

In jumping development, the toner only flies when the electric field of the electrostatic image overcomes the restraining force on the toner support.

The force that binds the toner to the toner support is due to van der Waalska between the toner and the toner support, adhesion between the toners, and the fact that the toner is electrically charged. It is the resultant force of the reflection force between

Therefore, only when the potential of the electrostatic image exceeds a certain value (hereinafter referred to as the toner transfer threshold) and the resulting electric field exceeds the toner binding force, toner flight occurs and the electrostatic image Toner adhesion to the holding surface occurs.

However, the binding force of the above-mentioned toner to the support varies depending on the individual toner or the particle size of the toner, even if the toner is manufactured and formulated according to a certain recipe, but the value is approximately It is thought that the electrostatic image surface potential is narrowly distributed around a certain value, and accordingly, the threshold value of the electrostatic image surface potential at which toner flight occurs is also thought to be narrowly distributed around a certain value. .

In this way, when the toner flies from the support, there is a threshold value, so the image area with a surface potential exceeding the threshold value is
Toner adhesion occurs, but conversely, toner adhesion hardly occurs in image areas with a surface potential below the threshold, and the so-called γ (gamma - the slope of the characteristic curve of image density according to the electrostatic image potential) ), resulting in an image with poor gradation.

The present invention has been made to eliminate the problems of the various component developing methods described above, and its main purpose is to provide excellent reproducibility at the edges of images, free from blurring, and improved gradation. An object of the present invention is to provide an electrostatic image developing method and an apparatus for the same, which make it possible to obtain a richly visible image.

In order to achieve the above object, the present invention provides a developing method and an apparatus for carrying out the same, which are characterized as follows.

(1) A toner carrier carrying a single layer of magnetic toner; a means for cutting the toner carrier onto an image carrier in a developing section and moving the toner carrier while maintaining a development gap larger than the thickness of the single layer of magnetic toner; and a toner carrier. The image carrier is placed on the opposite side,
A magnetic field generating means for generating a developing magnetic field in a development gap, a step of causing transfer of magnetic toner from a developer carrier to an image carrier, and a step of causing reverse transfer of magnetic toner from an image carrier to a developer carrier. and a means for forming an alternating electric field in the development gap in order to cause adhesion of developer according to the latent image potential due to the difference between transfer and countertransference by repeating the steps alternately. .

(2) The developing device according to item 1, wherein the toner carrier is a nonmagnetic sleeve.

(3) an image carrier; a toner carrier arranged with a development gap in line with the image carrier to carry magnetic toner; and a magnetic toner supply that supplies magnetic toner to the surface of the toner carrier. a doctor blade made of a magnetic material for forming a layer of magnetic toner supplied to the surface of the toner carrier thinner than the development gap; and a doctor blade for cutting the toner carrier. The magnetic pole is placed on the opposite side, and the image carrier is placed on the opposite side with respect to the toner carrier.
A magnetic field generating means for generating a developing magnetic field in a development gap, a step of causing transfer of magnetic toner from a developer carrier to an image carrier, and a step of causing reverse transfer of magnetic toner from an image carrier to a developer carrier. and a means for forming an alternating electric field in the development gap in order to cause adhesion of developer according to the latent image potential due to the difference between transfer and countertransference by repeating the steps alternately. .

Hereinafter, embodiments and examples according to the present invention will be described in detail with reference to the drawings.

The present invention will be described in detail using FIG. 1 as an example.

The voltage waveform applied to the toner carrier is shown in the lower row, and although it is a rectangular wave here, it is not limited to this as will be described later.

A bias voltage of magnitude Vmin is applied during time interval t1, and a bias voltage of magnitude Vmax is applied during time interval t2.

The magnitudes of Vmin and Vmax are determined when the image area charge formed on the image plane is positive and when this is to be developed with negatively charged toner, the image area potential is VD and the non-image area potential is VL. Choose to be satisfied.

If selected in this way, the bias voltage Vm in the time interval t1
in acts on the tendency to promote development and is called the toner transfer stage.

Furthermore, during the time interval t2, the bias voltage Vmax inhibits development, and acts to reverse the tendency of the toner transferred to the latent image surface during the time interval t1 to be returned to the toner carrier, which is referred to as a toner reverse transfer stage.

vth-f and Vth-r in FIG. 1 are the potential thresholds for the toner to transfer from the toner carrier to the latent image surface and from the latent image surface to the toner carrier, respectively, and are as shown in the figure. This is considered to be the potential value extrapolated along a straight line from the point where the slope of the curve is the steepest.

In the upper part of FIG. 1, the amount of toner transfer at tl and the degree of toner reverse transfer at t2 are plotted in a model manner by marking the latent image potential.

At the toner transfer stage, the amount of toner transferred from the toner carrier to the electrostatic image holder is curve 1 shown by the broken line in FIG.
It will be like this.

The slope of this curve is approximately equal to the slope of the curve when no alternating bias voltage is applied.

This slope is large, and the amount of toner transfer tends to be saturated at an intermediate value between vL and VD.
Poor reproduction of halftone images and poor gradation.

The second broken line curve 2 shown in FIG. 1 represents the probability of the degree of toner reverse transition.

One of the features of the developing method according to the present invention is that the toner transfer stage and the toner reverse transfer stage are alternately repeated. By changing the intensity of the electric field acting in the gap between the toner carrier and the electrostatic image holder, that is, the development gap, in a specific manner by the method described below, in other words, by adjusting the electric field intensity, By controlling the toner transfer, the amount of toner transferred and attached to the surface of the electrostatic image carrier and contributing to development is converged according to the potential of the electrostatic image, and the amount of toner transferred is controlled. As shown as curve 3 in FIG. 1, it was possible to obtain a development in which the slope is small and the amount of toner transfer changes almost uniformly from vL to VD.

Therefore, in the non-image area, there is almost no toner adhesion in practical terms (on the other hand, toner adhesion to the halftone image area is excellent visualization with extremely high gradation according to the surface potential). An image is obtained.

As a method for adjusting the electric field strength in the development gap, there is a method in which the applied alternating voltage is gradually converged to an appropriate constant DC value, but the present invention proposes a method in which the development gap itself is increased during the development process. We are hiring.

The method will be described in detail below.

An example of the developing process in this method is shown in FIG.

As shown in FIGS. 2A and 2B, the electrostatic image holder 4 moves in the direction of the arrow, and during this time the developing area is moved.

Pass through ■ and reach ■.

5 is a toner carrier. Therefore, the gap between the electrostatic image holding surface and the toner carrier gradually widens starting from the closest position in the developing section.

Figure A shows the electric field of the image area of the electrostatic image carrier, and Figure B shows the electric field of the transfer and countertransference from the toner carrier in the non-image area.

Further, C in the figure shows the waveform of the alternating voltage applied to the toner carrier, and when the electrostatic image charge is positive, it is set as follows.

The solid line arrow shown in the waveform is an electric field that causes toner transfer, and the broken line arrow is an electric field that causes toner reverse transfer.

The first process in development occurs in area (2), and the second process occurs in area (2).

In the case of the image area shown in FIG. 2A, both toner transfer and countertransference occur alternately in the area depending on the phase of the alternating electric field.

Because the development gap becomes wider, in (2), both the transition and reverse transition electric fields become weaker, and although toner transfer is possible, the reverse transition electric field is no longer strong enough to cause reverse transition (below the threshold).

In case (2), neither transfer nor countertransference occurs anymore, and development is completed.

In the case of the non-image area shown in FIG. 2B, both toner transfer and countertransference occur in area (3).

Therefore, there is a decline in soil fertility in this area.

In case (2), both the transition and reverse transition electric fields become weak, and although toner reverse transition is possible, the transition electric field (below the threshold) that causes transition is no longer present.

Therefore, the ground yellowtail will be removed in this area.

In case (2), neither transfer nor countertransference occurs anymore, and development is completed.

Regarding halftones, the final amount of toner transferred to the latent image surface is determined by the amount of toner transferred and the amount of reverse transfer depending on the potential, and in the end, as shown in curve 3 in Figure 1, the slope is small, and therefore the gradient is It becomes highly tonal.

Figure 3A shows the experimental results in which the effects of the present invention were clearly demonstrated.
, shown in B.

This is a measurement of the image reflection density corresponding to the electrostatic image potential V, and the experimental results are plotted in the figure.

Hereinafter, this curve will be referred to as the V-9 curve.

The experiment was conducted under the following configuration.

A positive electrostatic latent image is formed on the cylindrical electrostatic imaging surface.

A magnetic toner (30% magnetite content), which will be described later, is used as the toner, and is applied to a layer thickness of about 60μ on a non-magnetic sleeve containing a magnet, and the friction between the toner and the sleeve surface creates a negative charge on the toner. Grant.

The results when the minimum development gap between the electrostatic image forming surface and the magnetic sleeve is maintained at 100μ are shown in Figure 3A.
The results when the temperature was maintained at 0μ are shown in FIG. 3B.

The magnetic flux density in the developing section due to the magnet contained in the sleeve is approximately 700 Gauss.

The cylindrical electrostatic imaging surface and the sleeve rotate at approximately the same speed, approximately 110 mm/sec.

Therefore, after passing through the minimum gap in the developing section, the electrostatic body forming surface gradually moves away from the toner carrier (.

The alternating electric field applied to this sleeve is a sine wave with an amplitude of 400 V (peak-to-peak so ov) and a DC voltage +
200V is superimposed.

In Fig. 3, the alternating frequency of this applied voltage is 100Hz,
400Hz, 800Hz, 1KHz, 1.5KH2
The V-9 curve for the case (FIG. 3A only) and the V-9 curve for the case where the back electrode of the electrostatic imaging surface and the sleeve are electrically connected without applying an external bias electric field are shown. There is.

From these results, when no external electric field is applied, V
The slope of the −9 curve, the so-called γ value, is very large, but by applying a low-frequency alternating electric field, the γ value becomes smaller.
It can be seen that the gradation becomes extremely high.

As the frequency of the external electric field is increased, the γ value gradually increases, the effect of increasing the gradation from high to high fades, and the gap becomes 10
In the case of 0 μ, the effect becomes extremely weak when the frequency exceeds IKHz under the above amplitude, and when the gap is 300 μ, the effect decreases when the frequency becomes about 800 Hz.
If it exceeds lKH2, the effect becomes extremely weak.

The reason for this is thought to be as follows.

When the toner repeatedly attaches and detaches between the sleeve surface and the latent image forming surface during the developing process in which an alternating electric field is applied, a finite amount of time is required to reliably perform the reciprocating movement.

Particularly when toner is transferred under a weak electric field, it takes a long time for the toner to transfer reliably.

On the other hand, in order to reproduce half-tone density, it is necessary that toner subjected to an electric field of a certain threshold value or more is reliably transferred within a half period of the alternating electric field, even if the electric field is weak.

For this purpose, it is advantageous to have a low frequency of the alternating electric field, and therefore, as shown in the experimental results, particularly good gradation can be obtained with an alternating electric field of very low frequency.

The validity of this argument can be obtained from a comparison of the experimental results shown in Figures 3A and 3B.

The results shown in FIG. 3B were conducted under the same conditions as the experiment shown in FIG. 3A, except that the gap between the electrostatic image forming surface and the sleeve surface was increased to 300 microns.

When the gap is widened, the electric field strength that the toner receives becomes smaller.
Therefore, the toner transfer speed becomes low.

Furthermore, since the flight distance becomes longer, the transfer time becomes longer.

In fact, as is clear from FIG. 3B, the .gamma. value becomes considerably large at about 800 Hz, and when it exceeds IKHz, it becomes equivalent to the .gamma. value when almost no alternating voltage is applied.

Therefore, in order to achieve the same effect as when the gap is narrow in improving gradation, it is preferable to lower the frequency or increase the intensity of the alternating voltage.

On the other hand, if the frequency is too low, the reciprocating motion of the toner will not be repeated sufficiently while the latent image forming surface passes through the developing section, and uneven development will likely occur in the image due to alternating voltages.

As a result of the above experiment, a generally good image was obtained up to a frequency of 40 Hz, and below that frequency, unevenness occurred in the visible image.

The lower limit of the frequency to avoid unevenness in such a microscopic image is:
It has been found that it particularly depends on the development conditions, especially on the development speed (or process, also referred to as speed, Vp mm/sec).

In this experiment, the moving speed of the electrostatic image forming surface was 110 mm/
sec, the lower frequency limit 0 is XVp2O,3XVp.

The waveform of the alternating voltage to be applied may be a sine wave, a square wave, a sawtooth wave, or
It was confirmed that all of these non-phantom waves were effective.

As described above, applying alternate biases has a remarkable effect on improving gradation, but the voltage value must be set appropriately.

That is, if the alternating bias I Vmin l is set too large, the amount of toner adhering to the non-image area during the toner transfer stage will be too large, and the toner will not be sufficiently removed during the second development process, resulting in fog stains on the image. may occur.

On the other hand, if I Vmax l is set too large, the pullback of toner from the image area becomes large, and the density of the so-called black area decreases.

In order not to cause these developments and to sufficiently improve the gradation effect, it is appropriate to use the following values.

Vth-flVth-r is the potential threshold value already explained.

If the voltage value of the alternating bias is selected in this way, excessive toner will not adhere to the non-image area during the toner transfer stage;
At the toner reverse transfer stage, an appropriate image can be obtained without pulling back too much toner from the image area.

As mentioned above, applying an external alternating voltage between the latent image forming surface and the toner carrier significantly improves the gradation of the image and prevents fogging. By using a sleeve containing a permanent magnet as a developer carrier for magnetic toner and appropriately setting the external alternating voltage value, it is possible to simultaneously improve the reproducibility of line images.

Hereinafter, the description will be given assuming that the electrostatic image forming charge is positive, but the invention is not limited to this.

In the so-called jumping development method, electric lines of force emanating from the end of the latent image can wrap around the back electrode of the latent image forming surface and reach the surface of the toner carrier, as shown in FIG. 4A.
Therefore, the toner starting from the toner carrier is difficult to adhere to the edge of the image.

For this reason, the resulting image tends to have poor edges if the lines are thin.

Therefore, in this system, an alternating bias is applied and the V
If min is set sufficiently low, the electric lines of force in the developing section at the toner dislocation stage will become as shown in FIG. 4B.

That is, the wraparound of the lines of electric force at the edges of the electrostatic image is small, and a parallel electric field is formed.

This makes it possible to reliably apply toner even to the edges.

However, as already mentioned, if Vmin is set too low, fogging and staining will generally occur in non-image areas.

In an embodiment of the present invention, magnetic toner is used as a developer,
The advantage of using a sleeve containing a permanent magnet as a developer carrier is mainly that it solves this problem.

By appropriately setting the magnetic substance content in the developer and the magnetic field strength of the permanent magnet, the binding force of the toner on the sleeve can be uniformly increased, and therefore the value of Vth -f can be made sufficiently large. reactor.

As a result, Vmin could be set low while keeping toner adhesion to non-image areas to a small amount during the toner transfer stage.

In this way, by applying an alternating bias during jumping development using magnetic toner, it became possible to obtain images with high gradation, clear edges, and no stains. .

On the other hand, in general, in jumping development of high-resistance toner, transporting the developer to the developing section and imparting a charge are extremely difficult problems.

Among these, it is considered that one of the most advantageous methods is to use magnetic toner as the developer, to transport the toner through a sleeve, and to apply an electric charge by frictional charging between the sleeve surface or application member and the toner.

In addition, as a means of applying this magnetic toner onto the sleeve, there are two methods: pressing an elastic body onto the sleeve, and placing a magnetic body between the magnetic pole positions of the permanent magnets in the sleeve to keep it out of contact with the sleeve surface, thereby reducing the magnetic force. One possible method is to regulate the coating thickness of magnetic toner.

When developing is performed by rotating the sleeve and the electrostatic image holder at approximately the same speed in the same direction, in normal jumping development, the state of toner application on the sleeve directly affects the image quality, and the former When applied by the above method, the applied state is relatively dense and the image quality is good.

However, since this application method involves strongly rubbing the toner onto the sleeve surface, the resin component of the toner adheres to the sleeve surface, and as a result, the charging of the toner is significantly hindered.

On the other hand, if the latter method is used, the adhesion of toner to the sleeve surface can be suppressed to a minimum, but the state of toner application on the sleeve surface is rough with scattered toner particles, and after the formed image. The image quality directly reflects this state and becomes coarse as shown in FIG. 5A.

By the way, by applying the alternating bias described repeatedly in the present invention in the developing section, the toner particles perform reciprocating motion between the latent image and the sleeve surface, and in the process are loosened into one particle. As shown in FIG. 5B, toner can be densely adhered to the image area of the electrostatic image plane.

Specific details will be shown below using examples.

Example 1 FIG. 6 shows an example of the structure of a developing device for carrying out the developing method according to the present invention.

11 is a photosensitive drum having an insulating layer on the CdS layer; 12
is a non-magnetic (stainless steel) sleeve, and both members 1
1 and 12 rotate in the same direction at a constant speed of 110 mml sec.

Furthermore, the diameters of the photosensitive drum 11 and the sleeve 12 are each 8.
0 mm and 301 m, and a minimum gap of 200 μ is maintained between the two, and a developing section is formed in the vicinity thereof.

As the two rotate, the gap between them inevitably becomes larger after passing the closest position.

13 is a permanent magnet fixed within the sleeve; 14 is a magnetic toner to be described later; and 15 is a magnetic (iron) blade for uniformly applying the toner onto the sleeve.

The components of the magnetic toner used in this example are as follows.

Polystyrene 60wt% magnetite 35wt% carbon black 5wt% negative charge control agent (
Spiron) 2.5% colloidal silica (externally added) toner 0.2% The blade 15 is installed at a position facing the magnetic pole of the magnet 13 with a gap between its tip and the sleeve 120 maintained at 180μ. .

The magnetic field at the tip of member 15 is about 1000G.

The magnetic toner 14 is regulated to a thickness of approximately 70 μm by the blade 15, and is conveyed to the developing section while being negatively charged by friction with the surface of the sleeve 120.

The magnetic field at the development position is 750G.

Sleeve 12 and blade 15 are maintained in electrical continuity to prevent electrical discharge therebetween, and alternating voltages are applied to the conductive support member of photosensitive drum 11 by power supply 16 .

The frequency of the alternating voltage is 200Hz, and the amplitude is 400V (
A DC voltage of +200V is applied in a superimposed manner on a sine wave of 800Vpp).

Further, the electrostatic potential is +500V in the image area and OV in the non-image area.

Further, the member 17 is a plastic toner container.

Based on the above configuration, a clear image with high gradation and no fogging could be obtained.

Example 2 A developing device configuration for carrying out another developing method according to the present invention is shown in the seventh example.
As shown in the figure.

21 is a photosensitive drum having an insulating layer on a CdS layer;
Reference numeral 2 denotes an aluminum sleeve, and members 21 and 22 rotate in the same direction at substantially the same circumferential speed of 400 mm7 sec.

The diameters of the photosensitive drum 21 and the sleeve 22 are 200 mm and 50 mm, respectively, and the minimum gap between the two is 300 μm.
The developing area is formed in the vicinity of the developing area.

As the two rotate, the gap between them inevitably becomes larger after passing the closest position.

23 is an isotropic permanent magnet located within the sleeve and fixed;
24 is a magnetic toner to be described later, and 25 is an iron blade for uniformly applying the toner onto the sleeve.

The components of the magnetic toner used in this example are as follows.

The blade 25 is installed at a position facing the magnetic pole of the magnet 23, with the gap between its tip and the sleeve 220 maintained at 250μ.

The magnetic field at the tip of the blade 25 is about 750G.

The magnetic toner 24 is compressed by the blade 25 to a thickness of approximately 120 mm.
μ is regulated and conveyed to the developing section while being supplied with a negative charge due to friction with the surface of the sleeve 220.

The development position is opposite between the poles of the magnet in the sleeve.

27 is a toner container.

Sleeve 22 and blade 25 are kept in electrical continuity and powered by power source 26 to prevent electrical discharge between them.
Alternate voltages are applied to one conductive support member.

The frequency of the alternating voltage is 400H2, and the amplitude is 600V (
1200Vpp) (7) Sine wave, DC voltage +200
It is applied in the form of superimposing V.

In addition, the electrostatic potential is +350V in the image area and -20V in the non-image area.
It is V.

Based on the above configuration, it was possible to obtain a clear image with high gradation and no capri.

In the above description, the case where the image area potential VD is positive has been described in detail, but the present invention is not limited to this case, and can also be applied when the image area potential is negative. The same applies if the positive direction is small and the negative direction is dog.

Therefore, if the image area charge is negative, the above-mentioned (1)
~(4) is expressed as the following (1)'~C4Y.

As explained in detail above, the present invention has a back electrode,
An electrostatic image carrier on which an electrostatic image has been formed and a developer carrier carrying a magnetic developer layer and having a magnet fixed therein are held in a developing section with a gap equal to or larger than the thickness of the developer layer. To provide a developing method in which development is carried out while applying an alternating electric field of a low frequency, that is, a frequency of IKHz or less, so that the electric field in the development gap is alternated both in the image area and the non-image area, and an apparatus for carrying out the same. Therefore, by applying a low-frequency alternating electric field, in the gap between the developer carrier and the non-image area in the development area, the developer is alternately transferred to the non-image area and back transferred to the developer carrier. This reciprocating movement of the developer enables development with extremely excellent gradation.

Furthermore, since the magnetic developer layer is supported on a non-magnetic carrier containing a magnet, the magnetic developer uniformly increases the binding force of the developer on the carrier by the action of a magnetic field. Thereby, the value of the potential threshold value Vth -f for developer transfer can be set to a sufficiently large value, so that it is possible to suppress the adhesion of the developer to the non-image area to a small amount, and it is possible to minimize the ground force blurring.

In this way, the developing method using the magnetic developer according to the present invention to perform developer transfer and reverse transfer has high gradation by applying a low frequency alternating bias electric field.
A beautiful image with clear image edges and no fogging or staining could be obtained.

In an electrophotographic development method, an electrostatic image carrier and a toner carrier are timed with a gap between them, and a constant extremely high-frequency pulse bias (frequency of 10 kilocycles/clock) is applied to this gap.
A technique is known in which toner is applied to the image area by applying a toner of 3,000 kilocycles/second to 3,000 kilocycles/second, but the toner is not attached to the non-image area during the development process (for example, U.S. Pat. No. 3,890,929). No. Specification).

In this known example, there is no technical idea of subjecting the magnetic developer to the action of a magnetic field (as well as applying a low-frequency alternating voltage to the development gap) in order to improve gradation as in the present invention. By adjusting and changing the applied electric field strength during the development process, the magnetic developer is transferred to the non-image area under the required magnetic restraint, and the development of the low potential area of the image is also emphasized, eliminating the phenomenon of lack of development at the edges of the image. , the technical idea of reproducing faithful gradation is not described.

Other developing methods similar to the above-mentioned known techniques have been described (for example, U.S. Pat. No. 3,866,574, U.S. Pat. No. 3,893,418, etc.), but all of them apply high-frequency pulses, etc. For the same reason, the technical idea is different from the present invention.

[Brief explanation of drawings]

FIG. 1 is a graph explaining the principle of the developing method according to the present invention and a diagram showing an example of the applied voltage waveform, and FIG. 2 A to C are
A process explanatory diagram schematically showing the movement of the developer and the applied voltage waveform in the first and second steps of the developing method according to the present invention;
Figures A and B are diagrams showing the characteristics of electrostatic image potential versus image density in the case of applying a low frequency voltage in the developing method according to the present invention, and Figure 4A. B is an explanatory diagram of electric lines of force generated from an electrostatic image, Figure 5A,
B is an explanatory diagram for explaining the movement of the developer, and FIGS. 6 and 7 are explanatory diagrams of each embodiment embodying the developing method according to the present invention. Electrostatic image carrier...4, 10, 2L developer carrier...5, 12, 22°

Claims (1)

[Scope of Claims] 1. A toner carrier carrying one layer of magnetic toner, and a means for aligning the toner carrier in a developing section with a development gap larger than the thickness of the one layer of magnetic toner between the toner carrier and the image carrier. The image carrier is placed on the side opposite to the toner carrier,
A magnetic field generating means for generating a developing magnetic field in a development gap, a step of causing transfer of magnetic toner from a developer carrier to an image carrier, and a step of causing reverse transfer of magnetic toner from an image carrier to a developer carrier. and means for forming an alternating electric field in the development gap in order to cause adhesion of developer according to the latent image potential due to the difference between transfer and countertransference by repeating the above steps alternately. . 2. The developing device according to claim 1, wherein the toner carrier is a non-magnetic sleeve. 3. an image carrier, a toner carrier disposed with a development gap in line with the image carrier to carry magnetic toner, and a magnetic toner supply means for supplying magnetic toner to the surface of the toner carrier; A doctor blade made of a magnetic material is used to reduce the layer thickness of the magnetic toner supplied to the surface of the toner carrier to form a single layer of magnetic toner thinner than the development gap, and the doctor blade is on the opposite side to the toner carrier. The provided magnetic pole and the image carrier are placed on the receiving side with respect to the toner carrier.
A magnetic field generating means for generating a developing magnetic field in a development gap, a step of causing transfer of magnetic toner from a developer carrier to an image carrier, and a step of causing reverse transfer of magnetic toner from an image carrier to a developer carrier. and means for forming an alternating electric field in the development gap in order to cause adhesion of developer according to the latent image potential due to the difference between transfer and countertransference by repeating the above steps alternately. .