JPH0424710B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for developing an electrostatic image, and more specifically, to a method for developing an electrostatic image using a one-component developer.
In particular, the present invention relates to an electrostatic image developing method and an apparatus thereof, which make it possible to obtain a visible image with no background fog, excellent image clarity, and rich gradation.

Conventionally, electrophotographic development methods using a one-component developer include the powder cloud method, in which toner particles are sprayed, and electrostatic imaging, in which a uniform toner layer is formed on a toner support member such as a web or sheet. A contact development method in which development is performed by bringing the toner layer into contact with a holding surface, 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;
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 various one-component development methods mentioned above, powder and
In the cloud method, contact development method, and MagneDry method, toner is applied to the electrostatic image holding surface to distinguish between image areas (areas to which toner should originally adhere) and non-image areas (areas on the ground to which toner should not originally adhere). Because of the non-image contact, toner adhesion also occurs in non-image areas, 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. This is an extremely effective method in this respect. 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.

The first problem is that the sharpness decreases at the edges of the image portion. 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 the developer support 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 charges induced in the non-image area, and wraparound occurs.
The adhesion of flying toner is extremely uncertain; some may barely stick, while others may not. As a result, the resulting image lacks sharpness and is unclear at the edges of the image portion, and when developing a line image, there is a problem that the image is developed with a narrower 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 surface of the developer support must be made sufficiently small (for example, 100μ or less), and in practice, the gap between the two It is easy to cause pressure-contact accidents due to developer or foreign matter mixed in between the parts. Moreover, 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 based on the Van der Waals 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, the potential of the electrostatic image is a certain value (hereinafter referred to as
Only when the toner transfer threshold (referred to as the toner transfer threshold) is exceeded, and the resulting electric field exceeds the toner restraining force, toner flight occurs and toner adhesion to the electrostatic image holding surface occurs. However, the binding force of the above-mentioned toner to the support is approximately constant, although the value varies depending on the individual toner or the particle size of the toner, even if the toner is manufactured and mixed according to a certain prescription. 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. In this way, when toner flies from the support, there is a threshold value, so toner adhesion occurs on image areas with a surface potential exceeding the threshold value, but conversely, toner adhesion occurs on image areas with a surface potential below the threshold value. The result is that almost no toner adhesion occurs, and only images with a so-called γ (gamma=gradient of the characteristic curve of image density with respect to electrostatic image potential) and poor gradation 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 provide excellent reproducibility at the edges of images, eliminate background fog, and improve 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.

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

The principle of the present invention will be explained 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 t ₁ and a bias voltage of magnitude Vmax is applied during time interval t ₂ . The magnitude of Vmin and Vmax is determined by assuming that the image area charge formed on the image surface is positive and that this is to be developed with negatively charged toner, the image area potential is V _D and the non-image area potential is V _L. When Vmin＜ _VL ＜ _VD ＜Vmax......(1) is selected. With this choice, during the time interval t ₁ the bias voltage Vmin acts with a tendency to promote development and is called the toner transfer stage. In the time interval t ₂ , the bias voltage Vmax inhibits development and tends to return the toner transferred to the latent image surface in the time interval t ₁ to the toner carrier, which is called a toner reverse transfer stage.

Vth・ and Vth・r in FIG. 1 are potential thresholds for toner to transfer from the toner carrier to the latent image surface and from the latent image surface to the toner carrier, respectively, and the curves shown in the figure It is considered that the potential value is extrapolated by a straight line from the point where the slope of the rise is the largest. The upper part of Figure 1 shows the amount of toner transfer at t ₁ and
The degree of toner backtransition at t ₂ is plotted against the latent image potential as a model.

In the toner transfer stage, the amount of toner transferred from the toner carrier to the electrostatic image holder is as shown by curve 1 shown by a broken line in FIG. 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. 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, The amount of toner transferred and attached to the surface of the electrostatic image holder and contributing to development is controlled by controlling the toner transfer.
The electrostatic image is focused according to the potential, and the toner transfer amount is developed with a small slope and an almost uniform change in the toner transfer amount from V _L to V _D , as shown as curve 3 in Figure 1. That's what I was able to obtain. 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. is obtained.

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 a suitable constant DC value, but the present invention proposes a method of increasing the development gap itself 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,
pass through and reach. 5 is a toner carrier. Therefore, the gap between the electrostatic image holding surface and the toner carrier increases from the closest position in the developing section to the next stage. Figure A is the image area of the electrostatic image holder, Figure B is
indicates the electric field of the transition and countertransition from the toner carrier in the non-image area. Also, Figure c shows the waveform of the alternating voltage applied to the toner carrier, and when the electrostatic image charge is positive, |Vmax−V _L |>V _L −Vmin|, |
It is set as Vmax−V _D |<|V _D −Vmin|……(2). 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.

A first process in development is occurring in the area, and a second process is occurring in the area. 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. As the development gap becomes farther apart, 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). Now, neither transfer nor countertransference occurs, and development is completed.

In the case of the non-image area shown in FIG. 2B, both toner transfer and reverse transfer occur in the area. Therefore, ground fog occurs in this area. In this case, both the transfer and counter-transition electric fields become weak, and although toner counter-transition is possible, the transfer electric field (below the threshold value) sufficient to cause the transition disappears. Therefore, ground fog is removed in this area. In this case, neither transfer nor countertransference occurs anymore, and development is completed. Regarding midtones,
The final amount of toner transferred to the latent image surface is determined by the magnitude of the amount of toner transfer and the amount of reverse transfer depending on the potential, and the slope is ultimately small as shown in the curve in FIG. Therefore, it has high gradation.

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 a 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 to a layer thickness of approximately 60 μm on a non-magnetic sleeve containing a magnet, and a negative charge is imparted to the toner by friction between the toner and the sleeve surface. Figure 3A shows the results when the minimum development gap between the electrostatic image forming surface and the magnetic sleeve was maintained at 100μ, and Figure 3A shows the results when the minimum development gap between the electrostatic image forming surface and the magnetic sleeve was maintained at 300μ.
Shown in Figure B. 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 (peak-to-peak 800V) plus a DC voltage.
200V is superimposed. In Figure 3, the alternating frequencies of this applied voltage are 100Hz, 400Hz, 800Hz, 1KHz,
V-D curve for 1.5KHz (Figure 3 A only),
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 bias electric field.

From these results, it can be seen that when no external electric field is applied, the slope of the V-D curve, the so-called γ value, is very large, but by applying a low-frequency alternating electric field, the γ value decreases and becomes extremely large. It can be seen that the gradation becomes higher. As the frequency of the external electric field is increased, the γ value gradually increases, and the effect of tightening the gradation from high to high fades.If the gap is 100μ, the effect becomes extremely weak when the frequency exceeds 1KHz under the above amplitude. If the gap is 300μ, the frequency will be 800μ.
The effect decreases at around Hz, and becomes extremely weak when it exceeds 1KHz. 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 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 γ value becomes considerably large at about 800 Hz, and when it exceeds 1 KHz, it becomes almost the same as the γ value when 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 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, the frequency is 40Hz.
Until then, generally good images were obtained, but below that, unevenness appeared in the visible images. It has been found that the lower limit of the frequency for preventing unevenness in such a microscopic image is particularly dependent on the development conditions, especially the development speed (also called process or speed, Vpmm/sec). In this experiment, the moving speed of the electrostatic image forming surface was
Since it was 110mm/sec, the lower limit of frequency is 40/110×Vp0.3×Vp. It has been confirmed that any waveform of the applied alternating voltage, such as a sine wave, a rectangular wave, a sawtooth wave, or an asymmetric wave thereof, is effective.

In this way, applying alternating bias has a remarkable effect on improving gradation, but
The voltage value must be set appropriately.
That is, if |Vmin| of the alternating bias is set too large, the amount of toner adhering to the non-image area during the toner transfer stage is too large, and in the second process of development,
The toner may not be removed sufficiently, causing fog and stains on the image. On the other hand, if |Vmax| is set too large, the pullback of toner from the image area becomes large, and the density of the so-called solid black area decreases. In order to avoid these developments and to sufficiently improve the gradation effect, it is necessary to keep VmaxV _D + |Vth・r| ...(3) VminV _L + |Vth・| ...(4) is reasonable. Vth・,Vth・
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;
In the toner reverse transfer stage, a proper 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 emitted from the end of the latent image wrap around the back electrode of the latent image forming surface, as shown in FIG. 4A, and cannot reach the surface of the toner carrier. Therefore, the toner starting from the toner carrier is difficult to adhere to the edges of the image. For this reason, the resulting image tends to have thin lines and poor edges.

Therefore, in this system, if alternating bias is applied and its Vmin is set sufficiently low, the lines of electric 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 the embodiment of the present invention, the advantage of using magnetic toner as the developer and a sleeve containing a permanent magnet as the developer carrier is mainly to solve this problem. By appropriately setting the magnetic substance content in the developer and the magnetic field strength of the permanent magnet, it is possible to uniformly increase the force of binding the toner onto the sleeve, and therefore, the value of Vth can be made sufficiently large. . As a result, Vmin
could be set low.

In this manner, by applying an alternating bias during jumping development using magnetic toner, it has become possible to obtain an image with high gradation, clear edges, and no fogging.

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 methods, one extremely advantageous method is to use magnetic toner as the developer, transport it through a sleeve, and apply an electric charge through frictional charging between the sleeve surface or application member and the toner. Conceivable.

In addition, as a means of applying this magnetic toner onto the sleeve, there are two methods: pressing an elastic body onto the sleeve, or placing a magnetic body facing the magnetic pole position of the permanent magnet inside the sleeve and keeping it out of contact with the sleeve surface. Therefore, a method of regulating the coating thickness of magnetic toner can be considered. The sleeve and the electrostatic image holder are made to face each other,
When developing by rotating in the same direction at approximately the same speed, in normal jumping development, the state of toner application on the sleeve directly affects the image quality, and when applying with the former method, the state of application is It is detailed 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 toner is significantly prevented from being charged.

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 toner coating state on the sleeve surface is rough with scattered toner particles, and as a result, after development The image quality directly reflects this state and becomes coarse as shown in FIG. 5A.

However, by applying the alternating bias repeatedly mentioned 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 broken up into individual particles, and as shown in FIG.
This allows the toner to adhere densely.

Specific details will be shown below using examples.

In FIG. 6, 21 is a photosensitive drum having an insulating layer on a CdS layer, 22 is an aluminum sleeve, and the members 21 and 22 rotate in the same direction at substantially the same circumferential speed of 400 mm/sec. The diameters of the photosensitive drum 21 and sleeve 22 are 200 mm and 50 mm, respectively, and a minimum gap of 300 μm is maintained between the two.
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.

23 is an isotropic permanent magnet that is fixed within the sleeve; 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.

Polyester resin ferrite carbon black colloidal silica 73 wt% 25 wt% 2 wt% 0.3 external addition 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 22 maintained at 250μ. . The magnetic field at the tip of the blade 25 is about 750G. The magnetic toner 24 is regulated to a thickness of about 120μ by the blade 25.
It is conveyed to the developing section while being supplied with a negative charge due to friction with the surface of the sleeve 22. The development position is opposite between the poles of the magnet in the sleeve. That is, as shown in the figure, N and S poles of opposite polarity and adjacent to each other are arranged such that the closest position between the sleeve 22 and the photosensitive drum 21 is sandwiched between them. 27 is a toner container.

Sleeve 22 and blade 25 are maintained in electrical continuity to prevent electrical discharge between them.
6, alternate voltages are applied to the conductive support members 21. The frequency of the alternating voltage is 400Hz, and it is applied in the form of a DC voltage +200V superimposed on a sine wave with an amplitude of 600V (1200Vpp). Further, the electrostatic image potential is +350V in the image area and -20V in the non-image area.

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

In the above description, the case where the image area potential V _D 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 positive direction of is small,
The same can be applied by increasing the negative direction. Therefore, if the image area charge is negative, the above-mentioned (1) ~
(4) is expressed as the following (1)′ to (4)′.

Vmax＞V _L ＞V _D ＞Vmin ……(1)′ ｜Vmin−V _L ｜＞｜V _L −Vmax｜ ｜Vmin−V _D ｜＜｜V _L −Vmax｜ ……(2)′ VminV _D − |Vth・r| ...(3)' VmaxV _L + |Vth・| ...(4)' The present invention supports a magnetic developer in a developing section that develops an electrostatic latent image formed on an image carrier. A hollow developer carrier to be conveyed, a magnet fixedly arranged inside the hollow developer carrier, and a thickness of the developer layer carried and conveyed by the developer carrier to the developing section are A developer layer thickness regulating member regulates the thickness so that the thickness is thinner than the minimum gap between the carrier and the developer carrier, and an alternating member forms an alternating electric field in the developing section that repeatedly attaches and detaches the developer from the image carrier. and an electric field forming means, wherein the magnet has two adjacent magnetic poles having mutually different polarities and is positioned with the minimum gap position between the developer carrier and the image carrier in between. Since the developing device is fixedly arranged so that fogging is further suppressed, a denser image with higher density and less roughness, especially in solid areas, can be obtained. Images with good gradation can be obtained.

[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. FIGS. 3A and 3B, which are process explanatory diagrams schematically showing the movement of the agent and the applied voltage waveform, show 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. Figures 4A and 4B are explanatory diagrams of electric lines of force generated from electrostatic images,
FIGS. 5A and 5B are explanatory diagrams illustrating the movement of developer;
FIG. 6 is an explanatory diagram of each embodiment embodying the developing method according to the present invention. Electrostatic image carrier...4, 21, developer carrier...
5,22.

Claims (1)

[Scope of Claims] 1. A hollow developer carrier that carries and conveys a magnetic developer to a developing section that develops an electrostatic latent image formed on the image carrier; and regulating the thickness of the developer layer carried and conveyed by the developer carrier to the developing section so that the thickness is thinner than the minimum gap between the image carrier and the developer carrier in the developing section. It has a developer layer thickness regulating member, and an alternating electric field forming means for forming an alternating electric field in the developing section that causes the developer to repeatedly adhere to and detach from the image carrier, and the magnets have different polarities and are adjacent to each other. A developing device in which two matching magnetic poles are fixedly arranged so as to sandwich the minimum gap position between the developer carrier and the image carrier.