JPS6321187B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an electrophotographic developing method and apparatus,
More specifically, it relates to an electrophotographic developing method and apparatus using a one-component developer, and in particular, an electrophotographic development that makes it possible to obtain visible images with excellent image clarity, rich gradation, and no background fog. METHODS AND APPARATUS.

Conventionally, in an electrophotographic development method using a one-component developer, toner particles are sprayed into a powder. Cloud method, Contact development method in which a uniform toner layer formed on a toner support member such as a web or sheet is brought into contact with an electrostatic image holding surface, and development is performed by bringing the toner layer into direct contact with the electrostatic image holding surface. First, a jumping development method in which toner is selectively flown onto a holding surface using the electric field of an electrostatic image, and a magnetic brush is formed using conductive magnetic toner.
A magneto-dry method and the like are known in which the image is developed by bringing it into contact with an electrostatic image holding surface.

Among the various one-component development methods mentioned above, in the powder cloud method, contact development method, and MagneDry method, toner is applied to the electrostatic image holding surface in the image area (the area where the toner should originally adhere) and the non-image area (the area where the toner should originally adhere). Since the toner comes into contact with the toner without distinguishing between the parts of the background (where it should not be attached), toner adheres to non-image areas as well, making it impossible to avoid the occurrence of so-called background fog. However, in the jumping development method (for example, the method described in Japanese Patent Publication No. 41-9475), development is performed with a gap between the toner layer and the electrostatic image holding surface without contact, which prevents background fogging. In this respect, it is an extremely effective method. However, since the toner flight phenomenon caused by the electric field of the electrostatic image is utilized during development, the resulting visible image generally has the following drawbacks.

That is, the main problem is that images obtained by the jumping development method generally lack gradation. In the jumping development method, the toner first flies when it overcomes the restraining force on the toner support due to the electric field of the electrostatic image. The force that binds the toner to the toner support is the Van der Waals force between the toner and the toner support, the adhesion force between the toners, and the toner support based on the fact that the toner is electrically charged. It is the resultant force between the body and the reflected force. Therefore, toner flight occurs 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 adhesion to the image holding surface occurs. However, the binding force of the above-mentioned toner to the support is almost constant, even if the toner is manufactured according to a certain recipe, because the value varies depending on the individual toner or the particle size of the toner. Correspondingly, 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. Since there is a threshold value during nato flight from the support, toe adhesion occurs in image areas with a surface potential exceeding this threshold value, but conversely, toe adhesion occurs in image areas with a surface potential below the threshold value. The result is that almost no toner adhesion occurs, and only images with poor tonality and a so-called r (gamma = slope of the characteristic curve of image density with respect to electrostatic image potential) are obtained.

The present invention was made to eliminate the problems of the various one-component developing methods described above, and its main purpose is to obtain a visible image with excellent image reproducibility and rich gradation. An object of the present invention is to provide an electrophotographic developing method and apparatus that make it possible to do this.

In order to achieve the above object, the present invention is characterized by a developing method and apparatus.

A feature of the present invention is that when applying a superimposed alternating electric field obtained by superimposing first and second alternating electric fields of different frequencies,
The transfer of the developer from the developer carrier to the electrostatic image holder and vice versa can be made more reliable, improving the gradation level and ensuring that the developer is actively attached to the non-image area. Since it can be recovered from the non-image area of the electrostatic image carrier, the antifogging effect is high.

Embodiments and Examples of the electrophotographic developing method and apparatus according to the present invention will be described in detail below with reference to the drawings.

FIGS. 1A and 1B are drawn to explain the principle of the electrophotographic developing method and apparatus according to the present invention. We will explain the principles of preventing background fog and improving gradation of images.

In FIG. 1A, the electrostatic image potential is plotted on the horizontal axis, and the developer carrier (hereinafter also referred to as toner carrier) is plotted on the vertical axis.
Amount of toner transferred from to the electrostatic image holding surface (positive direction),
Alternatively, it is a graph showing the degree of toner reverse transfer (in the negative direction, the degree of transfer will be described later) at which the toner attached to the electrostatic image holding surface is peeled off to the toner carrier. The electrostatic image potential includes the non-image area potential V _L (usually the surface potential of the area corresponding to the bright area of the image, and the lowest potential value) and the image area potential V _D
(Usually the surface potential of the area corresponding to the dark part of the image,
This is the maximum potential value. ) is expressed as the potential at both ends. Note that the surface potential of the halftone portion of an image including halftones varies depending on the degree of the gradation.
Takes a potential between V _D and V _L.

In FIG. 1B, the voltage waveform applied to the toner carrier is plotted with time on the vertical axis and potential on the horizontal axis. Although a rectangular wave is illustrated, the waveform is not limited to this waveform, as will be described later. In the illustrated rectangular wave, at time interval t ₁ , the minimum voltage Vmin applied to the toner carrier with reference to the back electrode of the electrostatic image holder is applied, and at time interval t ₂ , the same maximum voltage is applied.
This is a periodic alternating waveform to which a bias voltage of Vmax is applied.

The image portion potential V _D may be a positive potential or a negative potential depending on the electrostatic image forming process used, and the same holds true for the non-image portion potential V _L.
However, here, from the perspective of making it easier to understand,
First, the case where V _D is at a positive potential will be specifically explained below. Of course, this is for illustrative purposes only.
The present invention is not limited to this. If V _D > O,
Of course, the relationship with the non-image area potential V _L is V _D >V _L.
Now, if the relationship between the maximum voltage Vmax and minimum voltage Vmin applied to the toner carrier and V _L is set to satisfy Vmax > V _L > Vmin (1), then at time interval t ₁ ,
This step is called the toner transfer step because the bias voltage Vmin acts to transfer the toner particles from the toner carrier toward the electrostatic image carrier.
In addition, in the time interval t ₂ , the bias voltage Vmax tends to return the toner transferred to the electrostatic image carrier in the time interval t ₁ to the toner carrier, so this stage is referred to as a toner reverse transfer stage. call.

In FIG. 1A, the amount of toner transfer at t ₁ and the degree of toner reversal at t ₂ are plotted as a model against the electrostatic image potential. The term "toner back transition degree" is used here because at t ₂ ,
Unlike reality, we assume that the toner is attached as a uniform layer to both the image area and the non-image area of the electrostatic image holder, and from this state, we apply a bias voltage.
This indicates the amount of toner that is reversely transferred toward the toner carrier when Vmax is applied, and the term ``degree of reverse transfer'' was chosen because it represents the probability of toner reverse transfer.

Now, the amount of toner transferred from the toner carrier to the electrostatic image holder in the toner transfer stage is shown in Figure 1A.
It will look like curve 1 shown by the broken line in . 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 a value intermediate between V _L and V _D , resulting in poor halftone image reproduction and poor gradation. Figure 1A
The second broken line curve 2 shown in FIG. 2 represents the probability of the above-mentioned toner backtransference at the toner backtransference stage.

One of the features of the developing method and apparatus according to the present invention is that such toner transfer stage and toner reverse transfer stage are alternately repeated, and a second feature is that the second half of the developing process By changing the strength of the electric field acting on 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 strength. ,
By controlling the toner transfer, the amount of toner transferred and attached to the surface of the electrostatic image holder and contributing to development is converged according to the potential of the electrostatic image, and the amount of toner transferred is As shown as curve 3 in FIG. 1A, we were able to obtain a phenomenon in which the toner transfer amount changes with a small slope and is almost uniform from V _L to V _D. Therefore, in the non-image area, there is almost no toner adhesion in practical terms, while toner adhesion to the halftone image area results in an excellent visible image with extremely high gradation in accordance with the surface potential. can get.

As a method of adjusting the electric field strength in the development gap, there is a method of gradually converging the applied alternating voltage to an appropriate constant DC value, but in the present invention,
A method is adopted in which the development gap itself is increased in accordance with the development time. The method will be described in detail below.

An example of the developing process in this developing method will be explained with reference to 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,
pass through and reach. 5 is a toner carrier. Figure A shows the electric field of the image area of the electrostatic image carrier, and Figure B shows the electric field of the toner transfer and countertransference from the toner carrier 5 in the non-image area. Further, C in the same figure shows the waveform of the alternating voltage applied to the toner carrier, and shows the first process. As will be described later, this development method focuses on increasing the development gap as the development time elapses, and as a result, reducing the electric field strength, rather than attenuating the voltage itself.

As shown in FIG. 2C, bias voltages Vmax and Vmin are repeatedly applied at time intervals t ₁ and t ₂ , but it goes without saying that the applied voltage waveform is not limited to that shown. As mentioned earlier, Vmax>
Assuming the condition of V _L > Vmin, and in Figure 2 O, |Vmax−V _L |>|V _L −Vmin| and |Vmax
-V _D |<|V _D -Vmin| Set the condition.

In this way, in the image area, as shown in FIG. 2A, in the development area, both toner transfer and counter-transfer occur alternately. In the figure, the arrows schematically indicate the moving direction of the toner, and the length thereof schematically indicates the strength of the electric field. Therefore, in this development area where the development gap is small, the first process of development is occurring. A second process then occurs as the development gap widens and enters the development zone. In this development region, the development gap widens, so even though the applied voltage value does not change, the electric field weakens in inverse proportion to the gap expansion, and the reverse transition electric field
The value is below the threshold required for countertransference, and toner transfer is possible, but countertransference does not occur. When the toner moves to the development area, the gap widens to such an extent that neither toner transfer nor reverse transfer occurs, and development ends there.

In the case of the non-image part shown in FIG. 2B, the area,
correspond to the first process and the second process, respectively.
In this region, both toner transfer and countertransference occur. Therefore, ground fog will occur in this area. When moving to the area, Vmax, Vmin
The electric field due to the voltage decreases in inverse proportion to the expansion of the development gap, and although reverse transfer of toner is possible, a transfer electric field sufficient to cause toner transfer is not generated.
Therefore, ground fog is sufficiently removed in this area.

Next, when moving to the development area, neither toner transfer nor reverse transfer occurs, and development is completed.

Therefore, this method is practically equivalent to changing the applied bias voltage, that is, the electric field strength acting on the development gap is changed, and not only background fog can be removed, but also halftones can be improved. The final amount of toner transferred to the electrostatic image carrier is determined by the amount of toner transfer and the amount of reverse transfer depending on the surface potential, and as a result, the curve of the electrostatic image potential versus the amount of toner transfer is As shown by curve 3 in Figure 1A, the gradation becomes high.

In addition, when the image part charge is positive |Vmax−V _L |>
｜V _L ―Vmin｜, ｜Vmax－V _D ｜＜｜V _D ―Vmin
The condition for | is that when the image area charge is negative, |Vmin−
V _L ｜＞｜V _L ―Vmax｜, ｜Vmin－V _D ｜＜｜V _D ―
Vmax｜.

The important thing here is that the frequency of the applied alternating electric field _{is 1}
(Hz) has an upper limit. That is,
As will be described later, as the frequency is increased, the r value gradually increases, and the effect of tightening the gradation from high to high fades, and the effect almost disappears at 1 KHz or higher. The reason for this is thought to be as follows. When the toner repeatedly transfers and reversely transfers between the toner carrier surface and the electrostatic image forming surface during the development process when an alternating electric field is applied, in order to ensure that the toner moves back and forth,
A finite response time is required. In particular, toner that transfers when subjected to a weak electric field requires a long time to ensure the transfer. On the other hand, in order to reproduce halftone density, it is necessary that toner subjected to an electric field of a certain threshold value or more, even if it is a weak electric field, is transferred reliably within a half period of the alternating electric field. For this purpose, it is advantageous to have a low frequency of the alternating electric field, and particularly good gradation can be obtained in the low frequency region.

Experimental results in which the effects of the present invention were clearly demonstrated are shown in FIGS. 3A and 3B. This is a measurement of the image reflection density D with respect 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-D curve. The experiment was conducted under the following configuration. A positive electrostatic latent image is formed on the cylindrical electrostatic imaging surface. The toner used is the magnetic toner (magnetite content
30%) is applied onto a magnetic sleeve to a layer thickness of about 60 μm, and the friction between the toner and the sleeve surface imparts a negative charge to the toner. The minimum development gap between this electrostatic imaging surface and the magnetic sleeve is
The results when the temperature was kept at 100μ are shown in Figure 3A.
The results when the temperature was maintained at 300μ 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, the electrostatic image forming surface gradually moves away from the toner carrier after passing through the minimum gap in the developing section. The alternating electric field applied to this sleeve is a sine wave with an amplitude of 400V (800V peak-to-peak) and a DC voltage of +200V superimposed on it. Figure 3 shows the alternating frequency of this applied voltage.
V for 100Hz, 400Hz, 800Hz, 1KHz, 2KHz
-D curve and a V-D curve when the back electrode of the electrostatic image forming surface and the sleeve are electrically connected without applying an external electric field.

These results show that when no external electric field is applied, the slope of the V-D curve, the so-called r value, is very large, but by applying a low-frequency alternating electric field,
It can be seen that the r value becomes smaller and the gradation becomes extremely high. As the frequency of the external electric field is increased, the r value gradually increases, and the effect of tightening the gradation from high to high fades.If the gap is 100μ, the frequency is 1KHz.
If it exceeds the
In the case of 300μ, the effect decreases when the frequency becomes about 800Hz, and becomes extremely weak when the frequency exceeds 1KHz.
The reason for this is thought to be as follows. When toner repeatedly adheres and detaches between the sleeve surface and the latent image forming surface during the developing process where an alternating electric field is applied,
A finite amount of time is required to reliably perform the reciprocating motion. In particular, toner that transfers when subjected to a weak electric field requires a long time to ensure the transfer. On the other hand, in order to reproduce the density of halftones, it is necessary that toner subjected to an electric field of a certain threshold value or more is transferred reliably 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 a 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 to which the toner is exposed decreases, and therefore the toner transfer speed decreases. Furthermore, since the flight distance becomes longer, the transfer time becomes longer. In fact, as is clear from FIG. 3B, the r value becomes considerably large at about 800 Hz, and when it exceeds 1 KHz, it becomes equal to the r value when almost no alternating voltage is applied.
Therefore, in order to produce the same effect as when the gap is narrow in terms of gradation improvement, it is preferable to further 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 formation image passes through the developing section, and the image will likely develop unevenness due to alternating voltages. As a result of the above experiment, the frequency is 40Hz.
Up to this point, generally good images were obtained, but below that point, unevenness occurred in the visible images. It has been found that the lower limit of the frequency that does not cause unevenness in such a microscopic image is particularly dependent on the development conditions, especially the development speed (also called process speed, Vpmm/sec).

In this experiment, the moving speed of the electrostatic image formation image was 110
mm/sec, the lower frequency limit is 40/110×Vp ≒ 0.3×Vp.

It has been confirmed that any waveform of the applied alternating voltage, such as a sine wave, a rectangular wave, a triangular wave, a sawtooth wave, or an asymmetric wave thereof, is effective.

Applying a low-frequency alternating electric field has a remarkable effect on improving image quality as described above, but
At the toner transfer stage, since toner is also attached to non-image areas, this may not be completely removed and background fog may occur. Therefore, in the present invention,
A high frequency alternating electric field (its frequency ₂
A third feature is to superimpose Hz). As a result, it was possible to obtain a clear image with no fog in the non-image area and with rich gradation. For example, when this high frequency voltage is applied to the developer carrier side,
It is as shown in FIG. 4, and is shown superimposed on a low frequency voltage.

This high-frequency electric field has the effect of keeping the toner in an oscillating state at all times during the development process and loosening the toner from its strong embrace on the developer carrier or the latent image forming surface. As a result, the toner transferred to the non-image area in the toner transfer stage of the first development process described above can easily return to the toner carrier side in the second process to obtain a fog-free image.

Furthermore, depending on the method of applying the toner to the toner carrier, the adhesive force between the toner and the toner carrier or the cohesive force between toner particles may become large, making it difficult for transfer to occur. In this case as well, superimposing a high frequency electric field has the effect of releasing the toner from adhesion and cohesive force, and therefore has the advantage of widening the permissible range of toner application conditions. For example, if a gravure roller whose surface is strongly coated with toner is used as a toner carrier, toner adhesion and cohesive force may increase, resulting in a decrease in development density. In such a case, developing density and image quality can be guaranteed by applying the alternating electric field.

As described above, since the purpose of the high-frequency electric field is to vibrate the toner, it is preferable that the electric field does not completely transfer the toner from the toner carrier to the latent image forming surface or vice versa. For this purpose, the frequency of ₂ Hz needs to be sufficiently high, at least 600 Hz, preferably at least 2 KHz.

It was also revealed that at frequencies above 50KHz, the response to the electric field deteriorated and no significant effect was observed.

Furthermore, the amplitude of the high-frequency electric field is desirably equal to or greater than the electric field threshold at which the toner can separate from the toner carrier or the electrostatic image forming surface in order to provide a sufficient toner oscillation effect. .

Next, an embodiment of the developing device according to the present invention will be described with reference to the drawings.

Embodiment 1 This embodiment implements the above-mentioned method of developing by changing the development gap according to the development process, and will be described with reference to FIG. Reference numeral 21 denotes a selenium photosensitive belt on which an electrostatic image is formed at another location (not shown), fixed at the location shown, and fixed or transferred at the next location (not shown). 22
is a toner carrier made of a conductive rubber belt, and is driven by a metal roller 23. 25 is
It is an insulating toner stored in a container 27, and its components are polyester resin and carbon black 2.
%, and 2% of a negative charge control agent (both weight %). Additionally, 0.1% colloidal silica is externally added to improve fluidity. The toner is conveyed by a carrier 22, and the coating thickness is regulated to 50 μm to 150 μm by an elastic member 26 pressed against a roller 23, and a negative charge is applied by a corona charger 28 before development. .

The electrostatic image holder 21 is maintained at a minimum gap of 300 μm with the toner carrier 22 by a metal roller 31 in the developing section. Further, at a point approximately 30 mm away from that position, the distance between the members 21 and 22 is maintained at approximately 2 mm by the metal roller 32 (adjustable). Development is completed in the area between rollers 31 and 32. 33 is a rotating member that can adjust the position of the metal roller 32. In this way,
The members 21 and 22 have a shape in which the gap between them gradually increases after passing through the closest position.
Usually, members 21 and 22 are at the same speed and in the same direction.
Proceeds at 200mm/sec. 29 is a power source for applying a low frequency alternating voltage. At this time, a voltage is superimposed from the high frequency voltage applying power source 29'. As a result, the relationship between the alternating voltage waveforms to be applied and the latent image potential is shown in Figure 6.The latent image potential is image area potential (V _D ) 700V, non-image area potential (V _L ) 200V, and low frequency voltage is the maximum value 800V. ,
It is a rectangular wave with a minimum value (Vmin) of −100V, and a high-frequency sine wave with a frequency of 5KHz and an amplitude (peak-to-peak) of 600V is superimposed on it.

In this way, it was possible to obtain an image with no fog and high gradation and clarity.

Embodiment 2 The structure of a developing device according to the present invention is shown in FIG. 41
A photosensitive drum 42 having a CdS layer and an insulating layer is a non-magnetic sleeve containing a permanent magnet 43, and both members 41 and 42 rotate in the same direction at a constant circumferential speed of 100 mm/sec. Reference numeral 45 is an insulating magnetic toner consisting of 60% by weight of styrene resin, 35% by weight of magnetite, 3% by weight of carbon black, and 2% by weight of the negative charge control agent spirone. Additionally, 0.1% by weight of colloidal silica is added externally to improve fluidity. The toner is transported by a non-magnetic sleeve 42, while the magnetic blade 4 adjacent to the sleeve
6 regulates the coating thickness to approximately 70μ. Further, the toner is given a negative charge by frictional charging with the sleeve 42. Member 47 is a toner container. Part 4
1 and 42 are kept at a minimum gap of 200μ,
With each rotation, the gap becomes progressively larger.
Members 41 and 42 have diameters of 80 mm and 30 mm, respectively. Members 42 and 46 are maintained in electrical continuity to prevent electrical discharge between them, and a power supply 49 provides electrical connection to the conductive support member of member 41. power supply 4
Alternate voltages are applied in the form of a superimposed high frequency voltage from 9'. For alternating voltage, the low frequency component is the frequency
It is a sine wave with an amplitude of 200Hz, maximum value (Vmax) + 600V, minimum value (Vmin) - 200V, and a frequency of 2KHz and amplitude (peak-to-peak).
A 400V sine wave is superimposed. Also, the latent image potential is
Image section 400V, non-image OV. With this configuration, it was possible to obtain an image with no fog and high gradation.

The above explanation, especially in the developing device adopting the above method, can be applied even if the minimum distance of the gap between the toner carrier and the electrostatic image holder is smaller than the thickness of the toner layer. Since the toner tends to aggregate within the toner layer, it is preferable that the gap be larger than the thickness of the toner layer, but the gap is not necessarily limited to this.

Moreover, although the case where the image portion potential V _D is positive has been described in detail above, the present invention is not limited to this embodiment.
When the image portion potential V _D is a negative potential, the same can be applied by making the potential smaller in the positive direction and larger in the negative direction.

As described in detail above, the present invention provides a method for performing development by confronting a moving electrostatic image carrier and a toner carrier with a required minute gap, in which the electric field in the development gap is at least The present invention provides an electrophotographic developing method for applying a low-frequency alternating electric field to alternate, and further superimposing a high-frequency alternating electric field to the low-frequency alternating electric field, and an apparatus for carrying out the method. As described above, the developing effect of the application is to maximize the reproducibility of the halftone image and thereby prevent background fog from occurring, which has the effect of faithfully reproducing the original light image.

Further, the present invention can be more effective when preferably combined with a developing method having the following two steps.

First process: In the gap between the toner carrier and the non-image area in the development area, the transfer of toner particles to the non-image area and the reverse transfer to the toner carrier are repeated alternately, so the high frequency alternating electric field is applied to the gap between the toner carrier and the non-image area. The process of applying a superimposed low-frequency alternating electric field.

Second process: Following the first process, as the electrostatic image carrier moves, the distance to the toner carrier is made larger than that in the first process to create a gap between the toner carrier and the image area. unilateral transfer of toner from the toner carrier to the image area, and unilateral reverse transfer of toner from the non-image area to the toner carrier in the gap between the toner carrier and the non-image area. A process of applying a low-frequency alternating electric field in which the above-mentioned high-frequency alternating electric field having a different intensity from the electric field in the above-mentioned first process to generate is superimposed.

The present invention further provides that the frequency of the low frequency electric field is 1K.
Hz or less is also a preferable requirement.
The present invention, which has such a process and develops by applying a low frequency voltage and a high frequency voltage superimposed thereon, has the following excellent effects.

In the first process described above, since the configuration is such that toner particles actively perform reciprocating motion (transfer-reverse transfer) between the toner carrier and the non-image area, in this process, the toner particles are transferred to the non-image area. This actively causes toner adhesion. This causes ground fog, but this ground fog is removed in the next second process. On the other hand, in this first process, which allows toner to adhere to non-image areas, the adhesion is further strengthened in image areas that have a potential as an electrostatic image. Therefore, it can be ensured that the toner is completely adhered to a portion having a density close to a bright portion of a half-tone image portion including a so-called half tone, depending on the potential thereof. As a result, a visual image rich in tonality and excellent in reproducibility of halftone images can be obtained.

Next, in the second process, as described above, the toner adhering to the non-image area is returned to the toner carrier, which not only has the effect of completely removing toner adhering to the non-image area. Since it promotes toner adhesion to the image area, the adhesion of the toner to the image area is complete, and there is an effect that a faithful image with good gradation and no background fog can be reproduced.

Furthermore, by applying the low frequency (1 KHz or less) electric field used in the present invention, the movement of the toner in the first and second processes described above can be sufficiently ensured, and the above transition and reverse transition are ensured. This contributes to the formation of visible images with rich gradation and no background fog.

In an electrophotographic development method, an electrostatic image carrier and a toner carrier are placed facing each other with a gap between them, and only a constant high-frequency pulse bias (frequency of 10 kilocycles/second to 3000 kilocycles/second) is applied to this gap. There is a known technique that allows toner to adhere to image areas, but prevents toner from transferring to non-image areas so that toner does not adhere there (for example, US Pat. No. 3,890,929). In this known example, there is no technical concept of applying a high-frequency alternating voltage superimposed on a low-frequency alternating voltage from the viewpoint of improving gradation as in the present invention, and even more so, the applied electric field strength is adjusted in the developing process.・The first and second processes as mentioned above are realized by changing the toner, and by the comprehensive action of both processes, toner is temporarily added to the non-image area, and the development of the low potential area is also emphasized. The technical concept of reproducing faithful gradation by stripping off the toner in accordance with the electrostatic image potential 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 only high-frequency pulses, etc. For the same reason, the technical idea is different from the present invention.

[Brief explanation of the drawing]

FIGS. 1A and 1B are graphs illustrating the principle of the developing method and apparatus according to the present invention, and diagrams showing an example of applied voltage waveforms. 2A to 2C are process explanatory diagrams schematically showing the movement of the developer, the applied voltage, and the applied voltage corresponding to the electric field change in the first and second steps in the developing method and apparatus according to the present invention, Figure 3A,
B is a graph showing the experimental results of applying a low frequency electric field in the present invention, FIG. 4 is a diagram showing superimposed waveforms applied to the developing method and apparatus according to the present invention, and FIG. FIG. 6 is a diagram showing the superimposed applied voltage waveform, and FIG. 7 is an explanatory diagram of another embodiment of the developing method and apparatus according to the present invention. . 21, 41... Electrostatic image holder, 22, 42...
Developer carrier, 29, 49...means for applying a low frequency alternating electric field, 29', 49'...means for applying a high frequency alternating electric field.

Claims (1)

[Scope of Claims] 1. A rotating electrostatic image holder having an electrostatic image formed on its surface and a developer carrying member carrying a developer layer on its surface are brought into close proximity to each other. An electrophotographic developing method in which a first alternating electric field and a second alternating electric field having a higher frequency than the first alternating electric field are superimposed to provide a superimposed alternating electric field in the development gap. Electrophotographic development method. 2. The superimposed alternating electric field alternates at least with respect to the non-image area of the electrostatic image carrier, and after causing the developer to adhere to the non-image area, the attached developer is returned to the developer carrier. The electrophotographic developing method according to item 1. 3 The above-mentioned superimposed alternating electric field alternates at least with respect to the image area of the electrostatic image carrier, and after making the developer adhere to the image area, the superimposed alternating electric field carries the excess adhering developer to the electrostatic image in the image area. The electrophotographic developing method according to claim 1 or 2, wherein the electrophotographic developing method is returned to the body. 4 The first alternating electric field transfers the developer from the developer carrier to the surface of the electrostatic image holder regardless of the electrostatic image, and the second alternating electric field transfers the developer from the developer carrier to the surface of the electrostatic image holder, and the second alternating electric field transfers the developer from the developer carrier to the surface of the electrostatic image holder, and The electrophotographic developing method according to claim 1, wherein the adhered developer is kept in an oscillating state. 5 Developing by supplying developer to the electrostatic image in a development gap formed by bringing a rotating electrostatic image holder with an electrostatic image formed on its surface and a developer carrier carrying a developer layer on its surface close to each other. In the electrophotographic developing device, in the development gap, a first alternating electric field that causes the developer to transfer and reverse transfer, and a second alternating electric field that has a higher frequency than the first alternating electric field that causes the developer to oscillate. An electrophotographic developing device characterized by having means for applying voltage.