JPS5832375B2 - Development method - Google Patents

Description

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

Conventionally, as an electrophotographic development method using a -component developer, there is a powder cloud method in which toner particles are sprayed, and a single layer of uniform toner formed on a toner support member such as a web or sheet is used as an electrostatic image. A contact development method in which development is carried out by bringing the toner into contact with the holding surface, a jumping development method in which the toner is selectively flown onto the holding surface by the electric field of the electrostatic image without directly contacting the toner layer with the electrostatic image holding surface, and a conductive method. A known method is the magnetry method, in which a magnetic brush is formed using a magnetic toner and brought into contact with an electrostatic image holding surface for development.

Among the above-mentioned various component development methods, 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 to which the toner should originally adhere) and the non-image area (the area where the toner should originally adhere). Since the toner contacts without distinguishing between the toner and the toner (where it should not be attached), the toner adheres to the non-image areas as well, making it impossible to avoid the occurrence of so-called background smearing.

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, since development is performed with a gap between the toner layer and the electrostatic image holding surface without contact.

However, since the development utilizes toner flight development due to the electric field of an electrostatic image, 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 jumping development, the toner only flies when the electrostatic field overcomes the restraining force on the toner support.

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

Therefore, 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 is retained. Toner adhesion to surfaces occurs.

However, the binding force of the above-mentioned toner to the support is approximately constant, even if the toner is manufactured and mixed according to a certain recipe, although the value varies depending on the individual toner or depending on the toner's thread diameter, etc. It is thought that the threshold value of the electrostatic image surface potential at which the toner flight occurs is also narrowly distributed around a certain value. .

Since there is a threshold value when toner flies from the support, toner adhesion occurs in image areas with a surface potential exceeding this threshold value, but conversely, toner adhesion occurs in image areas with a surface potential below the threshold value. This results in almost no toner adhesion, resulting in a crystal in which only an image with a so-called γ (gamma - slope of the characteristic curve of image density versus electrostatic image potential) and poor gradation is obtained.

The present invention was made to eliminate the problems of the various component development 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 electrostatic image developing method and apparatus that make it possible to do this.

In order to achieve the above object, the present invention is characterized by the following features.

(1) An electrostatic latent image holding member and a developer carrying member carrying a dry developer layer are arranged to face each other in a developing section with a development gap larger than the thickness of the developer layer, and the development gap is A step of forming an alternating electric field to cause transfer of developer from the developer carrier to the electrostatic latent image carrier, and a step of causing reverse transfer of developer from the electrostatic latent image carrier to the developer carrier. A developing method characterized in that development is carried out by alternately repeating the above steps to cause adhesion of developer according to the potential of the latent image due to the difference between transfer and countertransference.

(2) The developing method according to item 1, wherein transfer and counter-transition are caused in a non-image area of the electrostatic latent image holder.

(3) The developing method according to item 1, characterized in that transition and countertransition are caused in the image area of the electrostatic latent image holder.

(4) The developing method according to item 1, wherein transfer and countertransference are caused in an image area and a non-image area of the electrostatic latent image carrier.

(5) A transition occurs in the image area of the electrostatic latent image holder,
2. The developing method according to item 1, wherein no reverse transition occurs, and both transition and reverse transition occur in the non-image area.

(6) The electrostatic latent image carrier and the developer carrier carrying the dry developer layer are arranged to face each other in the developing section with a development gap larger than the thickness of the developer layer, and the development gap is Forming an alternating electric field to cause transfer of the developer from the developer carrier to the electrostatic latent image carrier and causing reverse transfer of the developer from the electrostatic latent image carrier to the developer carrier. The strength of the alternating electric field is attenuated from that in the first process, and the image area is unilaterally developed from the developer carrier to the image area of the electrostatic latent image holder. Development is carried out by performing a second process of causing transfer of the developer and, in the non-image area, unilaterally causing reverse transfer of the developer from the non-image area of the electrostatic latent image carrier to the developer carrier. A developing method characterized by:

(7) The developing method according to item 6, characterized in that transfer and counter-transition are caused in a non-image area of the electrostatic latent image holder.

(8) The developing method according to item 6, characterized in that transition and countertransition are caused in the image area of the electrostatic latent image holder.

(9) The developing method according to item 6, characterized in that transfer and countertransference are caused in an image area and a non-image area of the electrostatic latent image holder.

00) A transition occurs in the image area of the electrostatic latent image carrier,
7. The developing method according to item 6, wherein no reverse transition occurs, and both transition and reverse transition occur in the non-image area.

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

Figures 1A and 1B were drawn to explain the principle of the developing method according to the present invention. The principle of prevention and gradation improvement will be explained.

In FIG. 1A, the horizontal axis represents the electrostatic image potential, and the vertical axis represents the amount of toner transferred from the developer carrier (hereinafter also referred to as toner carrier) to the electrostatic image holding surface (in the 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 a non-image area potential (1) (usually the surface potential of the area corresponding to the bright area of the image, which is the minimum value as an electric potential).

) and image area potential■. (Usually, it is the surface potential of the area corresponding to the dark part of the image, and is the maximum potential value.

) is expressed as the potential at both ends.

Note that the surface potential of a halftone portion of an image including halftones takes an intermediate potential between vD and vL depending on the degree of the gradation.

In FIG. 1B, the voltage waveform applied to the toner carrier is plotted with potential on the horizontal axis and time on the vertical 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 t1, a bias voltage of minimum voltage ■min is applied to the toner carrier with reference to the back electrode of the electrostatic image holder, and at time t2, a bias voltage of minimum voltage ■max is applied. This is a periodic alternating waveform to which a bias voltage of is applied.

Image area potential■.

may take a positive potential or a negative potential depending on the electrostatic image forming process used, and the same holds true for the non-image area potential VL.

However, from the perspective of making it easier to understand, here, first, ■.

The case where is a positive potential will be explained below, taking as an example in particular.

Of course, this is for illustrative purposes only, and the invention is not limited thereto.

When VD>0, of course the relationship with the non-image area potential ■L is VD
>VL.

Now, here, the above maximum voltage ■ to be applied to the toner carrier
When the relationship between maX, minimum voltage Vmin, and VL is set to satisfy the following, the bias voltage Vm acts to transfer the toner particles from the toner carrier toward the electrostatic image carrier during the time interval t1. Therefore, this stage is called the toner transfer stage.

Furthermore, in the time interval t2, the bias voltage max acts to reverse the tendency of the toner transferred to the electrostatic image carrier in the time interval t1 to be returned to the toner carrier, so this stage is called the toner reverse transfer stage. .

In FIG. 1A, the amount of toner transfer at tl and the degree of toner reverse transfer at t2 are plotted as a model in accordance with the electrostatic image potential.

The term “toner reverse transfer degree” is used here because
At t2, unlike in reality, a state in which the toner adheres as a uniform layer to both the image area and the non-image area of the electrostatic image carrier is assumed, and from this state the bias voltage vrn
This indicates the amount of toner that is reversely transferred toward the toner carrier when aX is applied, and the term "reversal capacity" is used to express 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 as shown by curve 1 shown by a broken line in FIG. 1A.

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, resulting in poor halftone image reproduction and poor gradation.

The second dashed curve 2 shown in FIG. 1A represents the probability of the above-mentioned toner backtransference during the toner backtransference stage.

The developing method according to the present invention is characterized in that such a toner transfer step and a toner reverse transfer step are alternately repeated.

As a result, a visible image with high gradation from bright areas to dark areas, including the intermediate tones of the image, can be obtained.

In this process, electric fields alternate in the non-image area,
Therefore, it is essential to have a configuration in which the toner is once attached to the non-image area, and for this reason, it is necessary to actively attach the toner also to the halftone image area having a density adjacent to the non-image area. By stripping off the once attached toner (reverse transfer) according to the potential of the non-image area, such an intermediate tone can be obtained. There is an advantage of being able to serve someone (41).

Furthermore, in the latter half of the development process, the strength of the electric field acting in the gap between the toner carrier and the electrostatic image holder, that is, the development gap, is
By changing the electric field strength in a specific manner using the method described below, in other words, by adjusting the electric field strength, the transfer of the toner is controlled, and finally the toner is transferred and attached to the surface of the electrostatic image carrier. The amount of toner transferred that contributes to development is focused according to the potential of the electrostatic image, and the amount of toner transferred is determined as shown in Fig. 1A.
As shown as curve 3, the slope is small and VL
It was possible to obtain a development in which the amount of toner transfer changes almost uniformly from VD to VD.

Therefore, in the non-image area, there is almost no toner adhesion in practical terms, and on the other hand, toner adhesion to the halftone image area is an excellent microscope image with extremely high gradation according to the surface potential. can get.

There are two methods for adjusting the electric field strength in the development gap: the first method is to gradually converge the applied alternating voltage to an appropriate constant DC value, and the second method is to increase the development gap itself in accordance with the development time. The following methods can be considered.

Each method will be explained in detail below.

First, the developing process in the first method is shown in FIG.

FIG. 2A shows an example of the temporal change in the waveform of the applied alternating voltage in the case of the first method described above, in the order of ■, ■, and ■.

Of course, either continuous change or intermittent change is possible, and in the case of continuous change, the symbol (■) in the illustrated example indicates a state in the middle of the change.

Figures B and C illustrate the toner transfer and toner counter-transfer in the image area and non-image area of the electrostatic image holder, respectively, along with changes in development time.

In the figure, the direction of the solid arrow indicates the electric field in the toner transfer direction.
The length of the arrow represents the strength of the electric field.

Further, the broken line indicates the electric field in the direction of toner reverse transfer, and the length of the arrow indicates the intensity of the electric field.

In FIGS. 2A to 2C, the first process is called the first process, and the process (2) from the intermediate stage (described in more detail later) to the end is called the second process.

(2) indicates the end time, at which time the alternation of the applied voltage ends and converges to an appropriate DC constant value (Vo) between VD and VL.

It is important that the effects of toner transfer and countertransference in the image area and non-image area in the first process and the second process change.

This pattern will be explained phenomenologically.

First, in the image part, as illustrated in FIG. 2B,
In the first process (2), since Vmax>VD>vmin, a relatively strong toner transfer electric field occurs from the toner carrier toward the image area of the electrostatic image carrier during the period tl (applied voltage ■). Toner arrives at the image area and adheres there.

On the other hand, during the period t2 (applied voltage max), a relatively weak toner reverse transition electric field occurs from the image area of the electrostatic image carrier toward the toner carrier, and a portion of the toner from the image area returns to the toner carrier. be returned.

Each time the periods t1 and t2 are repeated in this way, toner transfer and counter-transfer occur between the toner carrier and the image area.

This is the applied voltage ■min1Vmax and the image area potential ■.

In this first process, the amount of toner transferred from the toner carrier to the image area is much larger than the amount of toner reverse transfer, so the toner reverse transfer is the toner transfer, That is, deterioration of the development effect does not pose a practical problem.

Next, when the amplitude of the applied bias voltage decreases continuously or intermittently to a predetermined value as shown by ■ in FIG. The amount of reverse transfer to the toner carrier side becomes 0.

Here, 1.sup.th, r is the minimum absolute potential difference between the electrostatic image forming surface and the surface of the toner carrier at which the toner can be separated from the electrostatic image forming surface and reversely transferred to the toner carrier.

Furthermore, if , the countertransference no longer occurs, but the period t1
An electric field is generated which promotes toner transfer from the toner carrier to the electrostatic image holder, although the amount is smaller than the amount of toner transfer at the time of .

Therefore, when the applied voltage reaches a state that satisfies the attenuation relationship, this process is called the second process in the image section.

This phenomenon in the image area is caused by the alternating component of the applied voltage (
It progresses while becoming quantitatively smaller until it converges to a constant DC value, and ends, reaching state (3).

Next, the process of toner movement in the non-image area (potential 7L) of the electrostatic image holder will be explained with reference to FIG. 2C.

First, in the first process shown as ■, Vmax>VL>
Since Vmln, the period t1 (applied voltage V ・)
In this case, a relatively weak toner transfer electric field occurs from the toner carrier to the non-image area of the electrostatic image holder, and the toner adheres to the non-image area.

On the other hand, during the period t2 (applied voltage Vmax), a relatively strong toner reverse transition electric field occurs from the non-image area toward the toner carrier, and the toner is returned from the non-image area to the toner carrier again.

It is considered that each time such periods t1 and t2 are repeated, toner transfer and reverse transfer occur between the toner carrier and the toner, and the toner performs reciprocating motion between them.

This is the applied voltage min, Vmax and the non-image area potential vL.
Since the relationship is set as , it is considered that the amount of reverse transfer of toner is more likely than the amount of transfer.

In this case, of course, more toner than has actually adhered will not be reversely transferred.

Next, as shown by ■ in FIG. 2A, when the amplitude of the applied bias voltage decreases continuously or intermittently to a predetermined value, the toner is transferred from the toner carrier to the electrostatic image carrier during period t1. The amount becomes O.

Here, Ijh-rl is the minimum absolute potential difference between the electrostatic image forming surface and the toner carrier at which the toner can be separated from the surface of the toner carrier and transferred to the electrostatic image forming surface.

This value varies depending on the developer and its conditions.

Then, such a transition no longer occurs, but instead an electric field that promotes the tendency of the toner to reverse transfer from the electrostatic image carrier toward the toner carrier, although it is smaller than the toner reverse transfer during period t2, is generated. begins to occur.

Therefore, when the applied voltage is attenuated and in this case Vrr1in becomes a dog), this process is called the second process in the non-image area.

This phenomenon in the non-image area progresses until the alternating component of the applied voltage gradually decreases and converges to a constant DC value, and then ends.

In other words, although the phenomenon of toner adhesion to the non-image area occurs in the first process, this toner blur is eliminated in the second process.

The second diagram shows a modification of the bias voltage application shown in FIG. be.

The bias voltage application in the case of the second diagram is Vmin<VL
＜VmaX is satisfied, and VmaX〈■D + l ■
The condition of th-r l is added.

In the case of applying such a bias voltage, compared to the case of applying the bias voltage shown in FIG. 2A, there is no phenomenon of toner reverse transfer in the image area, and the phenomenon in the non-image area is as shown in FIG. 2C. There is no substantial change in condition.

In the image area, as shown in FIG. 2E, there is no reverse transfer of toner in the first process (2), and the same is true in the second process (2).

In this case, the boundary between the first and second processes is when Vmin=VL IVth-fl in the non-image area, and it is considered that when Vmin becomes dog, the process shifts to the second process.

The above has simply described the extreme cases of image areas (dark areas) and non-image areas (bright areas), but for intermediate tones, the final result depends on the amount of toner transfer depending on the potential and the amount of reverse transfer. The amount of toner transferred to the electrostatic image surface is determined.

Therefore, the curve of the amount of toner transfer aligned with the electrostatic image potential has a slope that is relatively smaller than curve 1, as shown in curve 3 in FIG. It changes almost uniformly.

As a result, a visible image with high gradation from bright areas to dark areas, including the intermediate tones of the image, can be obtained.

In the first step of the first method described above, it is essential that the electric field is alternated in the non-image area so that the toner is once attached to the non-image area. Toner can be actively deposited even in halftone image areas with density adjacent to the image area, and once adhered toner can be peeled off (reverse transfer).
By performing this in accordance with the potential of the non-image area, there is an advantage that a developed image with high developability and rich tonality in the intermediate tone area can be obtained.

Next, an example of the developing process in the second method is shown in FIG.

As shown in FIGS. 3A and 3B, the electrostatic image holder 4 moves in the direction of the arrow, and during this time passes through the development areas
leading to.

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 described above.

As will be described later, in this second method, rather than attenuating the voltage itself, the main objective is to make the development gap smoother and, as a result, to reduce the electric field strength.

As shown in FIG. 3C, as the bias voltage Vma
Although X and Vmin are repeatedly applied at time intervals t1 and t2, it goes without saying that the applied voltage waveform is not limited to that shown.

As mentioned above, assuming the condition of VmaX>VL>Vmjn, and in FIG. 3C, lVmax −VL l 〉l
VL Vmin l and IVmaX-VDl<I
A condition of VD-Vminl is set.

As a result, in the image area, as shown in FIG. 3A, in the development area, both the l/ner transition and the reverse transition occur alternately.

This phenomenon has been explained in detail with reference to FIG.

Therefore, in this development area where the development gap is small, the first process of development is occurring.

Next, when the development gap expands and enters the development area (2), the second process described above occurs.

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 expansion of the gap, and the reverse transition electric field becomes below the threshold required for reverse transition.
Although toner transfer is possible, countertransference does not occur.

When moving to the development area (3), the gap widens to such an extent that both toner transfer and reverse transfer no longer occur, and development ends there.

In the case of the non-image area shown in FIG. 3B, areas ■ and ■ correspond to the first process and second process, respectively.

In region (3), as described above with reference to FIG. 2, both toner transfer and countertransference occur.

Therefore, soil failure will occur in this area.

When moving to region ■, the electric fields due to the voltages ■maX and ■min both weaken 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, in this area (2), the soil-borne yellowtail is sufficiently removed.

Next, when moving to the development area ■, the toner transfer,
No countertransference occurs, and development is completed.

Therefore, with this method, substantially the same effect as changing the applied bias voltage can be obtained, and not only can ground blur be removed, but also the amount of toner transfer depending on the surface potential for halftones. The final amount of toner transferred to the electrostatic image carrier is determined by the magnitude of the amount of reverse transfer and the amount of reverse transfer, and as a result, the curve of electrostatic image potential versus the amount of toner transfer is as shown in curve 3 in FIG. 1A. The result will be something with high gradation.

What is important here is that there is an upper limit to the frequency of the applied alternating electric field.

That is, as will be described later in Example 2, as the frequency is increased, the γ value will gradually increase, and the effect of tightening the gradation from high will fade, and as it will be described later in Example 2, the effect becomes weaker when the frequency is higher than IKHz. Most of it disappears.

The reason for this is thought to be as follows.

During the development process in which an alternating electric field is applied, when toner repeatedly transfers and reversely transfers between the toner carrier surface and the electrostatic image forming surface, a finite response time is required to ensure the reciprocating movement. be.

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 half-tone 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, be reliably transferred within a half cycle 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 a low frequency region.

Embodiments of the apparatus according to the present invention will be described below.

Embodiment 1 The embodiment shown in FIG. 4A has a configuration in which the alternating bias applied voltage is attenuated, and the power supply voltage formed by superimposing a DC component on a low frequency AC voltage is attenuated using a mechanical sliding electrode. Figure B shows a modified part that uses an electric circuit to attenuate the noise.

In FIG. 4A, 10 is a zinc oxide photosensitive paper on which an electrostatic image is formed at another location (not shown), transported by rollers 13, 13 to the development location shown, stopped during development, and then Transported for fixation.

12 is a toner carrier made of a conductive rubber belt, and is driven by metal rollers 14, 14.

The zinc oxide photosensitive paper 10 as an electrostatic image holder and the toner carrier 12 are sent to a developing area by intermittently driving rollers 13 and 14 by motors 21 and 22, and are stopped during the developing process. , transition before the next development starts.

The toner carrier rotates half a turn and stops again.

Reference numeral 15 denotes an insulating toner stored in a container 7, which consists of styrene resin, 3% carbon black, and 2% positive charge control agent (all percentages by weight).

Additionally, 0.2% by weight of colloidal silica is externally added to improve fluidity.

The toner is conveyed by the carrier 12, and the coating thickness is regulated to 100 to 200 microns by a member 16 that is in sliding contact with the carrier, and a corona charger 18 applies a positive charge to the toner before development.

The gap between the electrostatic image carrier 1 and the toner carrier 2 is maintained at 500 μm.

Reference numeral 14a denotes a sliding electrode that contacts the core metal of the rotating roller 14, and alternately applies voltage to the toner carrier 12 by the power source 9.

20 is a fur brush for stirring the developer and applying it to the toner carrier 12.

The dark potential of the electrostatic image formed on the electrostatic image carrier 10 is
-450V, and the bright area potential was -40V.

The applied voltage is 1200 AC with a frequency of 10 to 1000 Hz.
A DC voltage of -200 V is superimposed on Vpp, and only the AC voltage is attenuated to O at a time constant of 0.5 seconds after 0.2 seconds from the start of development.

The configuration of the power supply 9 that achieves such attenuation will be explained.

21 is a motor that moves the sliding electrode 26 on the secondary side of the AC transformer 2γ, 24 is an AC power source, 25 is a DC power source, and 23 is a timing signal generation circuit and a power source for driving the motors 21 and 22.

After 0.2 seconds have passed after the start of development, the sliding electrode 26
Move from position to position B after 0.5 seconds at constant speed.

When the sliding electrode 26 moves to the B position, the motor 22 is driven,
The toner carrier 12 rotates half a rotation, and during that time the sliding electrode
Return to position.

FIG. 4B shows a power supply 9' that uses a well-known RLC attenuation circuit instead of using sliding electrodes, so that after the start of development, the
．． After 2 seconds have elapsed, turn the switch from position A' to position B'.

The time constant of this attenuation circuit is set to 0.5 seconds (.

The switching of the switch can be performed in a timing manner using a known means such as a relay.

In this way, the development according to the first method described above can be applied, and the obtained image has virtually no ground blur, and the gradation of the image is particularly excellent in the region where the alternating frequency f of the applied alternating voltage is low, and f≦ Good images were obtained at 1000 Hz.

Example 2 This example illustrates a developing method based on the second method described above, and will be described with reference to FIG. 5.

31 is an electrostatic image carrier having an insulating layer on a CdS photoconductive layer, and 32 is a conductive developer carrier.

36 is a power source that applies a low frequency AC voltage to the toner carrier.

Reference numeral 34 denotes a motor that drives the electrostatic image holder 31 via rollers 33 so as to separate it from the toner carrier, and the driving of this motor is controlled by a timing circuit 37.

The electrostatic image carrier 31 and the toner carrier 32 are initially held with a gap of 300 μ to 500 μ, and after 0.2 seconds have elapsed, the electrostatic image carrier 31 is held by the motor 34 with a gap of 0.2 μ.
It is pulled up at a constant speed until the gap becomes 17IITt in seconds, at which point the development ends.

During this time, the positively charged electrostatic image area (+350V) is developed by the negatively charged developer 35.

The components of this negatively charged toner are the same as those of the other examples.

An external alternating voltage is applied between the back electrode 38 of the electrostatic image holder 31 and the toner carrier 32, and as explained in detail with reference to FIG. 3, in this example, VmaX=
500V, Vmin--300V1 alternating frequency f=5
It was 0Hz.

In this case, the maximum potential VD of the image area was +350V, whereas the potential of the non-image area was 2L--50V.

Thus, as explained with reference to FIG. 3, toner did not ultimately adhere to such non-image areas, and on the other hand, good images with high gradation depending on the potential were obtained in the image areas.

In this example, it will be exemplified that the frequency of the applied bias voltage is particularly low, which brings about preferable results, but this result is, of course, not limited to this example.

In FIG. 6, the horizontal axis represents the electrostatic image potential ■, the vertical axis represents the image density (reflection density) D, and the development gap between the electrostatic image carrier 31 and the toner carrier 32 at the initial stage of development is shown. When set to 300 μ, the alternating frequency f of the bias applied voltage is 50 Hz, 8
It shows the experimental results of V-D curves with the frequency changed to 00 Hz, I KHz, and 2 KHz.

As can be seen from this group of curves, the lowest frequency, 50Hz
In this case, the slope of the V-D curve is the minimum, and an image density that faithfully corresponds to the electrostatic image potential can be obtained.

It can be seen that the slope of this curve gradually increases as the frequency increases, and when it reaches 2 KHz, the slope becomes substantially equal to the slope of the V-D curve when no bias voltage is applied.

With such a high-frequency bias, as described above, the γ value becomes irregular, the reproducibility of halftone images deteriorates, and the gradation quality deteriorates.

Therefore, when the applied alternating voltage is set to a low frequency voltage of IKHz or less, an extremely good effect is brought about.

Embodiment 3 This embodiment, like Embodiment 2, is an implementation of the above-mentioned second method of developing by changing the development gap according to the development process, and will be described with reference to FIG.

Reference numeral 41 denotes a selenium photosensitive belt, on which an electrostatic image is formed at another location (not shown), developed at the location shown, and fixed or transferred at the next location (not shown).

42 is a toner carrier made of a conductive rubber belt;
It is driven by a metal roller 43.

45 is an insulating toner stored in a container 47, and its components are polyester resin, 2% carbon black, and 2% negative charge control agent.

Additionally, 0.1% colloidal silica is externally added to improve fluidity.

The toner is conveyed by the carrier 42, but the toner is conveyed by the roller 43.
The coating thickness can be adjusted from 50 μm to 15 μm using the elastic member 46 that can be pressed against the
It is regulated to 0μ, and a negative charge is applied by a corona charger 48 before development.

The electrostatic image holder 41 maintains a minimum gap of 300 μm between the electrostatic image holder 41 and the toner carrier 42 using a metal roller 51 in the developing section.
is maintained.

Further, at a point approximately 30 mm away from that position, the distance between the members 41 and 42 is maintained at approximately 2 degrees by the metal roller 52 (adjustable).

53 is a rotating member that adjusts the position of the metal roller 52.

In this way, after the members 41 and 42 pass through the closest position, the gap between them gradually becomes larger (the shape is taken).

Note that the members 41 and 42 are moving at the same speed and in the same direction at a speed of 200
Proceeds at mm/sec.

49 is a power source for applying alternating voltages.

The image portion potential of the electrostatic image formed on the member 41 is 800■
, the non-image area potential is 200V.

The applied voltage is 400 V DC superimposed on 1000 Vpp AC with a frequency of 200 Hz.

In this way, a good image with high gradation and no background blur was obtained.

Regarding this developing action, especially the first and second processes, the third
As detailed in the figure.

Embodiment 4 FIG. 8 shows still another embodiment of the developing device according to the present invention, which employs the above-mentioned second method.

61 is a photosensitive drum with a radius of 40 mm and has a CdS layer and an insulating layer; 62 is a non-magnetic sleeve with a radius of 15 mm that includes a permanent magnet 63; both members 61 and 62 have a circumferential speed of 10 mm;
Rotates in the same direction at a constant speed of 0 mm/sec.

Reference numeral 65 is an insulating magnetic toner, and its components are 60% by weight of styrene resin, 35% by weight of magnetite, 3% by weight of carbon black, and 2% by weight of a negative charging control agent.

Further, 0.3% by weight of colloidal silica is externally added to improve fluidity.

The toner is conveyed by a sleeve 62, and a magnetic blade 66 close to the sleeve reduces the coating thickness to approximately 70 μm.
regulated by.

Further, the toner is given a negative charge by frictional charging with SuIJ-Pro2.

Member 67 is a toner container.

The gap between the member 61 and the member 620 is maintained at a minimum of 200 μm, but as the members 61 and 62 rotate, the moving speed of both members and the width of the gap are set so as to satisfy the conditions described above with reference to FIG. 3.

The member 62 and the member 66 are maintained in an electrically conductive state,
A power source 69 connects the conductive support member of member 61 to
Alternating voltages are applied.

The alternating voltage was a sine wave, the frequency was 200 Hz, and the relationship between the voltage value and the electrostatic image potential was as shown in FIG.

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

The alternating voltage shown in FIG. 9 is a sine wave with an amplitude of 400 V (800 Vpp) and a DC voltage of +200 V superimposed thereon.

Based on the above configuration, in addition to the developing action described in detail with reference to FIG. 3, a clear image with high gradation could be obtained due to the low frequency.

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

Note that the above shows the relational expression especially when the image part charge is positive, but when the image part charge is negative, (2) to (9)
The formula is expressed as follows.

As explained in detail above, the method of developing by making the electrostatic image bearing member and the toner bearing member face each other with a required minute gap is as follows: First step: The toner bearing member and the non-image area A process of applying a low frequency alternating electric field to the threshold gap in order to alternately repeat the transfer of toner particles to the non-image area and the reverse transfer to the toner carrier.

Second process: Following the first process, toner is unilaterally transferred from the toner carrier to the image area in the gap between the toner carrier and the image area, and the toner is transferred between the toner carrier and the non-image area. A process of applying a low-frequency alternating electric field having a different strength from the electric field in the first process to cause a unilateral reverse transfer of toner from the non-image area to the toner carrier.

By having this, it 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 soil burrs, but there is no problem because this soil burrs will be 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 toner is completely adhered to a portion having a density close to a bright portion of a halftone 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 step, 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, which has the effect of producing a faithful image reproduction with good gradation and no background blur.

In an electrophotographic development method, an electrostatic image carrier and a toner carrier are made to face each other with a gap between them, and a constant high-frequency pulse bias (frequency of 10 kilocycles/second to 30 kilocycles/second) is applied to this gap.
There is a known technique in which toner is applied to the image area but not to the non-image area by applying 00 kilocycles/second (for example, U.S. Pat. No. 389
0929 specification).

In this known example, there is no technical idea of applying 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 can be adjusted or controlled in the developing process.
By changing it, the first and second processes as mentioned above are realized, and by the comprehensive action of both processes, l-ner is once added to the non-image area, and the development of the low potential area is also emphasized, There is no description of the technical idea of subsequently stripping off the toner according to the electrostatic image potential and reproducing faithful gradation.

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. 3,8934.18, etc.), but all of them use high-frequency pulses, etc. For the same reason as mentioned above, the technical idea is different from the present invention.

[Brief explanation of the drawing]

1A and 1B are graphs explaining the principle of the developing method according to the present invention and diagrams showing an example of the applied voltage waveform, and FIG. 2A-
F is a process explanatory diagram schematically showing the first and second steps in the first method of the developing method according to the present invention, changes in applied voltage, and movement of developer in the state at the end of development; Figures A to C 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 second developing method according to the present invention. , FIG. 4A, B, FIG. 5, FIG. 7, and FIG. 8 are explanatory diagrams of each embodiment embodying the developing method according to the present invention, and FIG. 6 is an illustration of the embodiment shown in FIG. FIG. 9 shows an example of the waveform of the voltage applied in the embodiment shown in FIG. 8.

Claims (1)

[Scope of Claims] 1. An electrostatic latent image carrier and a developer carrier carrying a dry developer layer are arranged to face each other in a developing section with a development gap larger than the thickness of the developer layer, A step of forming an alternating electric field in the development gap to cause transfer of the developer to the developer carrier or the electrostatic latent image carrier, and a reverse transfer of the developer from the electrostatic latent image carrier to the developer carrier. A developing method characterized in that development is carried out by alternately repeating the step of causing the developer to adhere in accordance with the potential of the latent image due to the difference between transfer and countertransference. 2. The developing method according to claim 1, wherein transfer and counter-transition are caused in a non-image area of the electrostatic latent image holder. 3. The developing method according to claim 1, characterized in that transition and countertransition are caused in the image area of the electrostatic latent image holder. 4. The developing method according to claim 1, wherein transfer and countertransference are caused in an image area and a non-image area of the electrostatic latent image holder. 5. Development according to claim 1, characterized in that in the image area of the electrostatic latent image holder, a transition occurs but no reverse transition occurs, and in the non-image area, both transition and reverse transition occur. Method. 6. An electrostatic latent image holding member and a developer carrying member carrying a dry developer layer are arranged to face each other in a developing section with a development gap larger than the thickness of the developer layer, and alternately forming an electric field to cause a transfer of the developer from the developer carrier to the electrostatic latent image carrier, and causing a reverse transfer of the developer from the electrostatic latent image carrier to the developer carrier In the first process, the strength of the alternating electric field is attenuated from that in the first process, and the developer is unilaterally transferred from the developer carrier to the image area of the electrostatic latent image carrier in the image area. Development is performed by performing a second process of causing transfer of the developer in the non-image area and unilaterally causing a reverse transfer of the developer from the non-image area of the electrostatic latent image holding member to the developer carrier. A developing method characterized by: 7. The developing method according to claim 6, characterized in that transfer and counter-transition are caused in a non-image area of the electrostatic latent image holder. 8. The developing method according to claim 6, characterized in that transition and counter-transition are caused in the image area of the electrostatic latent image holder. 9. The developing method according to claim 6, characterized in that transfer and countertransference are caused in an image area and a non-image area of the electrostatic latent image holder. 10. Development according to claim 6, characterized in that in the image area of the electrostatic latent image holder, a transition occurs and a reverse transition does not occur, and in a non-image area, both a transition and a reverse transition occur. Method.