US3713422A - Apparatus for developing an electrostatic latent image by liquid development - Google Patents

Abstract

A development device of an electrostatic latent image wherein rotatable and cylindrical development electrodes comprising the conductive center portion which has a small radius and will face to an electrostatic latent image on a flexible electrophotographic material, and two flange portions which have a large radius respectively and will support the both ends of said material are placed such that differences between said radius of the center portion and said radius of flange portions become smaller with the proceeding of the development, thereby a print can be made with extremely small halos, streaks, fogs and edge effects.

Description

United States Patent 1 Sato et al.

[ 51 Jan. 30, 1973 [54] APPARATUS FOR DEVELOPING AN ELECTROSTATIC LATENT IMAGE BY LIQUID DEVELOPMENT Inventors: Masamichi Sato; Osamu Fukushima,

both of Saitama, Japan Fuji Photo Film Kanagawa, Japan Filed: March 10, 1971 Appl. No.: 122,692

Co., Ltd.,

Assignee:

[30] Foreign Application Priority Data March 10, 1970 Japan ..45/20362 U.S. Cl. ..118/637, 117/37 LE, ll8/DlG. 23, 355/10 Int. Cl ..G03g 13/00, B05b 5/02 Field of Search ..1 18/637, DIG. 23, 411; 117/37 LE, 93.4; 36/1; 204/300; 355/10 References Cited UNITED STATES PATENTS Sato et al ..ll7/37 LE 3,627,557 l2/l97l Savit et al ..1 18/637 3,556,050 1/l97l Trachtenberg.... ..1 18/637 3,601,092 8/1971 Toyokazu ....l 18/637 3,186,326 6/1965 Schmidt ..118/637 Primary Examiner-Mervin Stein Assistant Examiner-Leo Millstein Attorney-Gerald .l. Ferguson, Jr.

[57] ABSTRACT A development device of an electrostatic latent image wherein rotatable and cylindrical development electrodes comprising the conductive center portion which has a small radius and will face to an electrostatic latent image on a flexible electrophotographic material, and two flange portions which have a large radius respectively and will support the both ends of said material are placed such that differences between I said radius of the center portion and said radius of flange portions become smaller with the proceeding of the development, thereby a print can be made with extremely small halos, streaks, fogs and edge effects.

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APPARATUS FOR DEVELOPING AN ELECTROSTATIC LATENT IMAGE BY LIQUID DEVELOPMENT This invention relates to a development apparatus for electrophotography and more specifically to one which can rapidly provide a high quality image preventing the formation of i. background toner deposition due to the residual potential,

ii. a toner deficient region at a low density area contiguous to a high density area (such defect is sometimes referred to as halo),

iii. streak which appears as a drainage of toner along the developer flow towards downstream, and

iv. also the edge effect enabling a uniform solid area coverage by toner.

A photoconductive insulating coating employed in electrophotography always exhibits a more or less residual potential after exposure to light, that is, when the coating is uniformly charged at darkness, subjected to image exposure, the surface charge is neutralized at the illuminated area, but retained at the non-illuminated area; the extent of neutralization depends on the amount of exposure thus forming an electrostatic latent image. However, as the amount of exposure is increased, the efficiency of charge neutralization gradually lowers, and thus a small level of residual potential is generally observed at the high light region of an light image. When one tries to realize the perfect neutralization of charge at the high light region with a sufficient amount of radiation energy, then the potentials of other areas where neutralization should proceed only some extent, fall down to undesirably low levels, making it impossible to form a faithful reproduction of the original image. Therefore it is a common practice to adjust the exposure amount so as to leave a certain low residual potential at the high light region and prevent the toner deposition thereon at the development procedure. ln other words, if a toner deposits accurately proportionally to the potential distributed in the latent image, background will be uniformly developed providing an image of unacceptable quality.

conventionally, in order to prevent background toner deposition due to the residual potential, a method has been proposed which is characterized by the application of a dc. voltage to the development electrode from an external source equal to the residual potential. This method, is however, hardly applicable to an automatic development apparatus through which an electrophotographic material is driven in advance. The reasons are as follows: the electrostatic charge comprising an electrostatic latent image gradually leaks during the passage in the developer liquid facing the development electrode; and the residual potential will also reduce its value as the material first faces, passes along and finally leaves the electrode. Accordingly, the residual potential will not be cancelled by the application of a constant voltage onto the development electrode and thus a faithful reproduction of an original image will be impossible. Moreover, the value of the A phenomenon referred to as halo is regarded as specific in electrophotography. Halo occurs where two areas having markedly different charge densities are contiguously present and the charge density gradient is large at the border between them. The name halo is given to the phenomenon in which the low charge density area is not developed proportionally to its charge density but left as toner deficient area at the border adjacent to the higher density area. Halo becomes more noticeable as the spacing between the development electrode and the surface to be developed is reduced, which might seem to conflict with the theory which, however, will be shown afterwards to support the experimental results.

A still further defect intrinsic to electrophotography is streak in the developed image. Streaks take plate along the direction of the flow of the developer liquid blurring at the downstream side of a toner-deposited area. Such streaks look like a trail of a comet. Streaks which are easily distinguishable tremendously deteriorate the quality of a developed image. The most direct factor to cause streak is a relative velocity component of developer flow parallel to the surface to be developed. Perfect exile of this parallel component is practically impossible and also disadvantageous as for the ample supply of the developer liquid. Therefore one has to regard and solve this problem on the assumption of the existence of the parallel component. Though the mechanism by which streak generates is not clarified as yet, many methods of reducing streak have been found. As empirical rules, the streak becomes more noticeable (i) with a toner having a smaller electrostatic charge, (ii) with a steeper change of charge density in an electrostatic latent image, (iii) as the relative velocity between the surface to be developed and the developer liquid increases, and (iv) as the development electrode comes closer to the surface to be developed so as to absorb more effectively the electric lines of force emanating from the latent image (at a constant relative velocity). The development apparatus in accordance with the present invention is for use in developing an electrostatic latent image formed on an insulating coating provided on a flexible conductive backing, and comprises development electrodes which are faced with close spacings to the coating surface in the presence of a developer liquid comprising a finely-divided charged toner dispersed in an insulating liquid. The apparatus can achieve development of an electrophotographic recording material with an electrode placed far from said material at the initial stage and subsequently with another electrode placed close to said material.

A most representative construction of the development electrode is a drum having different diameters at the middle and edge portions thereof.

Practical mode of decreasing the spacing between the surface to be developed and the electrode primarily depend whether the recording material to be treated is driven continuously or intermittently. The essential feature of the present invention is to provide an apparatus which performs development comprising an initial development with little aid of a development electrode, followed by a successive development with the full use of an electrode: And details of the change of the spacing through the overall development little affect the result; the most important thing is that the electrode spacing should gradually decrease (in other words, the effect of the electrode should monotonously increase) from the commencement to the end of the development and a minor change of the effect during the development is not so significant. For example, as a minor modification, the electrode spacing may be made practically infinite in the course of development,

the electrode may or may not have a ground potential;

etc. To prevent background toner deposition, an electrode potential different from earth potential is sometimes preferable. Also, the situation is similar for the conductive backing of the recording material.

Development with an electrodes spacing which is small from the start of development will result in a faithful visualization of the slight residual potential to form a considerable background, and will give a toner image accompanied by halos at the regions where steep charge density changes occur, and by streaks. Accord-- ing to the invention, one may advantageously carry out development with a large electrode spacing at the commencement of the development so as to permit the residual potential to leak away, preventing toner deposition on the background area. Besides, a high density solid area adjacent to a low density area is subjected, to an edge development, at the commencement of the development, allowing the originally steep charge density gradient at the edge to become gentle. When the thus halfway developed image is subjected to the second development with a development electrode with a small spacing between the electrode and the image surface, the background with a negligible residual potential will hardly attract toner, while the areas with abrupt charge density changes, by virtue of the already deposited toner, exhibit milder charge density gradient which is accompanied with a weaker repulsive electric field. Accordingly, the development brings about little halo at the low-density area adjacent to the high density area.

As described above, a toner image of greatly improved quality can be obtained with a first development an emphasized edge development combined with a second development which does not cause any edge effect. Streak, which is prone to occur with a close electrode spacing at the area where the gradient of charge density change is large, is also effectively prevented with the use of a large electrode spacing at the first development and by a reduced charge density gradient by virtue of the edge development owing to the first development at the second development.

Now the theoretical background of the invention will be given with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWING FIG. I is a cross-sectional view of an electrophotographic material of which half of the surface thereof is uniformly charged electrostatically.

FIG. 2 shows the distribution of electrostatic charge on the surface of an electrophotographic material electrostatically charged as shown in FIG. 1.

FIGS. 3 and 4 show the distribution in the absence and presence of developing electrode respectively of electric field perpendicular to and in the proximity of the surface of an electrophotographic material provided with the electrostatic charge distribution as shown in FIG. 2.

FIG. 5 shows the distribution of electrostatic charge obtained when electrostatic charging similar to that in FIG. 1 is applied to the surface of an electrophotographic material provided with uniform retentive charge.

FIG. 6 shows the distribution of electric field in the proximity of the surface provided with the charge distribution shown in FIG. 5 when the developing electrode is placed close thereto.

FIG. 7 shows the distribution of electrode field when said distance is large.

FIG. 8 shows a distribution of electrostatic charge in which a uniform low density area and a uniform high density area are adjacent.

FIG. 9 shows the distribution of electric field in the proximity of the surface of an electrophotographic material provided with the charge distribution shown in I 1 FIGS.

FIG. 10 shows the distribution of electrostatic charge showing gradual change from low charge density to high charge density.

FIG. 11 shows the distribution of electric field generated by the charge distribution shown in FIG. 10.

FIG. 12 is a plan view showing the state of developing observed when liquid developer is made to flow on the surface of an electrophotographic material provided with stepwise distribution of electrostatic charge in a direction perpendicular to said stepwise distributron.

FIG. 13 is a perspective view of developing electrode used in the apparatus of this invention.

FIGS. 14 and 15 show longitudinal cross-sections of this invention.

FIG. 1 illustrates an electrophotographic material 10 on one half surface of which is uniformly charged. The material comprises a photoconductive insulating layer 11 and a conductive substrate 12. The former may comprise amorphous selenium, a homogeneous mixture of photoconductive zinc oxide and an insulating resin, while suitable materials for the latter include metal plate, plastic films treated with conductive agents, and paper imparted with a suitable conductivity by special treatments. During development procedure, the substrate 12 is usually kept at ground potential. The electrostatic latent image in this figure is composed of positive charge distributed on the right hand half of the surface 11 with a uniform density 0,. An equivalent amount of charge of the opposite polarity is induced at the interface between 11 and 12. The positive charge distribution is shown graphically in FIG. 2, in which the ordinate corresponds the lateral distance from the border dividing the charged and not-charged regions. The charge density is assumed uniform throughout the region; a 0' FIG. 3 and FIG. 4 illustrate the electric field configuration arising from the above charge distribution, observed near the charged surface. The

curves correspond to the fields vertical to the charged plane.

FIG. 3 shows the field configuration without or with a development electrode placed quite apart from the plane. In this case it is noted that the field configuration is quite different from that of charge which is the origin of the field. This fact is, as is well known, closely related with the so-called edge-effect. It should also be noted that a region of filed with reversal direction exists in the left hand half of the plane adjacent to the border. (Taking the field upward positive, this inverted field is negative.) Hereinafter, the electric field opposite to that existing above the charged region will be referred to as inverted field. The inverted field has its maximum near the very edge from which the plane is charged.

FIG. 4 illustrates the field configuration associated with the charge pattern shown in FIGS. 1 or 2 with a development electrode placed parallel to the plane with different spacings. The three curves, A, B and C correspond to an extremely small (several microns), medium (1mm), and a relatively large (about 10mm) spacing respectively. The spacing or gap will be designated g. With a small g., the field configuration becomes quite similar to that of charge. However, still the inverted field remains which has a maximum near the edge, rapidly decreasing to the far left. With the increase of g., the inverted field becomes remarkable, while the positive field strength decreases.

FIG. 5 illustrates a charge distribution with a uniform residual charge with a density 0,, on the left half of the plane. The charge distribution in FIG. 2 is an ideal one with 0' 0 at the left half, while the charge distribution which really results form a uniform charging and light exposure may be schematically such as is shown in FIG. 5. FIG. 6 shows the electric field component perpendicular to the plane associated with the charge distribution shown in FIG. 5 in the presence of an electrode placed thereabove with a relatively small g. As is seen from this figure, the field on the left half is positive except the small region adjacent to the highly charged region. A small minimum of negative or inverted field still remains owing to the edge effect. When a developer liquid (here, comprising a negatively charged finely divided toner dispersed in an insulating liquid) is fed on such plane, the toner will deposit where a positive field exists, thus forming a background on the left half though this region is not desired to attract any toner.

FIG. 7 shows the field configuration above the latent image shown in FIG. 5 with an electrode fairly separated from the plane. With a large g., the positive field due to the residual charge 0 is cancelled with the negative field caused by the uniformly charged area (right half) not leaving a net positive field on the left half of the plane. Accordingly, any toner will not deposit thereon.

Then, next to the background toner deposition, explanation will be given concerning halo which occurs in the conventional procedures.

FIG. 8 illustrates another example of charge distribution comprising two regions each having a uniform high and low density 0-,, and respectively. An abrupt change of density occurs at the border of the two regions. The solid line in FIG. 9 shows the field configuration associated with the distribution shown in FIG. 8 in the presence of an electrode with a small g. value. When a developer liquid is applied on the plane under the electrode separated by the small g., toner will deposit in accordance with the field configuration shown by the solid line in FIG. 9. When the development is stopped on the way before the complete neutralization of the charge, the remaining charge on the plane left unneutralized will be expressed schematically by the broken line in FIG. 8. This means that due to the small g., charge neutralization proceeds substantially uniformly except around the border of the two regions. The field will then become such as shown by the broken line in FIG. 9, which retains all the essential features of the original configuration with reduced absolute value. From the figure one may see that the inverted field still remains outside the high charge density area showing that the use of an electrode closely placed above the plane from the commencement of and throughout the development leaves the inverted field even after toner deposition has proceeded to a considerable extent, causing halo which designates the toner deficient region corresponding to the inverted field area.

conventionally, a small gap has been thought better, which, however, is not true unless the extreme condition, g=0 is realized. From the practical point of view, the lower limit for 3 may be several 10 microns, which will increase to about 100 microns for an automatic developing apparatus. For 3 around such value an inverted field will be formed to give rise to halo.

FIG. 10 illustrates a residual charge distribution which results from a halfway development with the aid of an electrode located relatively far (g=5 to 10 mm) from the plane which had an original distribution shown by the solid curve in FIG. 8. Due to a large value of g, the toner deposition has proceeded according to the field configuration shown in FIG. 7, whereby a high field region near the border preferentially attracted toner and has beenneutralized. As a result, the inner portion of a solid area having a uniform charge density is scarcely developed (neutralized). Strictly speaking, during the development with a large g, the charge density will gradually decrease due to leakage in the developer liquid throughout the plane. Consequently, the distribution of the remaining charge to be neutralized by further development will be such as shown in FIG. 10, showing a gradual change of density across the border. The field configuration associated with such charge distribution is free from an inverted field as shown in FIG. 11 due to the disappearance of the abrupt change of charge density.

The charge distribution shown in FIG. 10 will give rise to a field configuration shown in FIG. 11' in the presence of an electrode with a very small gap. Now that even ifg is kept extremely small, it does not attend on any inverted field as shown in FIG. 9, an extremely small gap is permitted. It should be emphasized that development of areas with gradually changing charge densities'is little accompanied with halo of streak.

To maintain the electrode spacing large through the overall period 'of development is not preferred because For example, on an electrophotographic paper provided with a zinc oxide/binder layer an extremely high quality image free of halo, background, and streak could be obtained by a three-step development, comprising a first step without electrode (g for 24 seconds, a second one with an electrode 20 mm apart from the paper surface for 14 seconds and the final one with g 0.] mm for 14 seconds.

Developer liquids for suitable use in an apparatus of the present invention may be those comprising a finelydivided pigment as toner with a particle diameter ranging from about 0.01 to 1 micron dispersed in an insulating carrier liquid selected from various non-polar hydrocarbons or mineral oils. The developers may include a variety of charge controlling agents which are effective to improve the dispersion stability of the pigment particles and to control the electrostatic charge thereon. Control of charge may be accomplished by dissolving or dispersing resinous materials in the insulating liquid.

FIG. 12 illustrates a developer optical wedge showing the effect of g. on streak.

This image resulted from development of an electrostatic latent image corresponding to an optical stepwedge. The development was carried out by maintaining the electrode gap very narrow and by feeding a developer liquid in the direction shown by the arrow Experiments have disclosed that there are two types of streak, one stretching far but weak along the flow direction of the developer and the other being short but dense. As the gap increases, the latter disappears, while the former remained. The long-range streak disappears by reducing the flow rate of the developer, which true for large and small values of g. But the short streak is removed only when the gap is increased. It was also recognized that development beginning with a large g. which reduces on way does not yield the short range streak.

Our experiment has revealed that a first development without electrode for only 0.3 to 5 seconds effectively prevents the formation ofa high density short streak.

One may fear if an edge effect might remain at a large solid area in the final print because of the first development without electrode. It will be easily shown that such fear does not fit the case.

In case where an electrostatic latent image illustrated in FIG. 5 is developed with a closely placed electrode from the start of development, toner deposition will proceed in accordance with the field shown in FIG. 6. At the points A and B, the electric field still differ from each other which will give rise to an image with a slight edge effect. However, such rather slight density difference is often overlooked in a high density solid area, while in the adjacent low density area an easily observable halo occurs. On the other hand, when development is carried out according to the invention, that is, combining a first 'step with a large gap, and a second step with a small gap, the toner deposition at the edge portion will modify the field configuration from one shown in FIG. 7 to that in FIG. I1, and the end result will be toner deposition free from the edge effect as well as with faithful solid area coverage.

As has been described, the essential advantages feature of the present invention will be summarized as follows:

I As for background:

In electrophotography a slight residual potential inevitably exists. Such potential will attract toner to form-undesirable background when development is carried out with the use of an development electrode with a small electrode spacing. To avoid this background toner deposition, the electrode may be placed apart from the image plane at the commencement of development (as an extreme case the spacing is made infinite) allowing the residual potential to leak away. After the residual potential has sufficiently reduced, the electrode is permitted to come closer to the plane.

2. As for halo and edge effect:

At those areas where an abrupt change of charge density occurs, a toner deficient area, which is referred to as halo, is formed at the border in the low-charge density region, while an edge development takes place near the border in the high density region. The perfect resolution of these disadvantageous performances is theoretically impossible as long as a finite electrode spacing is employed.

According to the invention, the edge effect is made as little as possible by the second-step development following the first development.

3. As for streak:

When a region with a steep charge density gradient is developed with a developer which has a relative velocity against the plane to be developed, a streak flows from the high density area towards the low density area. Such streak becomes more noticeable as the electrode spacing becomes smaller, and with a steeper charge density gradient (for a constant flow rate of developer). If one carried out an initial development in which the development electrode is set apart enough to cause an edge development, the steep gradient originally present will become milder. Thus, a subsequent development with an electrode facing the plane with a narrow spacing will not tend to cause streaks.

Practical embodiments of the invention will be shown referring to the drawings in which FIG. 13 is a bird eye view of an electrode roller suitable for the apparatus of the present invention; FIGS. 14 and 15 are the cross-sectional view of developing devices of the invention.

The electrode roller shown in FIG. 13 comprises a middle portion 20 and flanges at both ends of the roller with a larger diameter than that of the middle cylinder which is made of an electrically conductive material. The middle portion works as development electrode, and the flanges 21 are for holding the edges of a flexible electrophotographic material to be processed. The difference of the diameters of the two portions determines the spacing between the cylinder and the developed surface.

In an apparatus in accordance with the invention, as is shown in FIG. 14, a plurality of such electrode rollers are provided in a processing tank along the path of the electrophotographic material in such a manner that the spacing or diameter difference decreases from roller to roller as the material advances. The middle portion diameters of the three electrode rollers 22, 23 and 24 are expressed by dotted lines. The electrophotographic material changes its advancing direction by winding itself around the supplementary rollers 25 and 26. As the material is driven in the direction shown by arrows with its lightssensitive side up, the electrode spacing becomes smaller and due to the mechanism explained above an image free of edge effect, halo and streak results.

In case where the electrophotographic material is driven intermittently, the dimension of the path is designed so that one image frame comes to a standstill just facing to any one of the electrode and that the boundaries between the frames rest at the supplementary rollers. Such arrangement is not necessary for a constantly advancing material.

The electrode rollers may be rotated to drive the electrophotographic material or the material may be transported by other suitable means.

FIG. illustrates another embodiment in which electrode rollers provided in a processing tank (not shown in the Figure) as shown in FlG. 13 are again employed. In this apparatus however, the middle portion of each roller has the same diameter as others but the flange diameter is different decreasing from the first roller 28 to the last one 31. Rollers 32, 33, 34 and 35 rotate the electrode rollers by conductive endless belts suspended between the corresponding electrode and the driving roller. Rollers 36, 37 and 38 turn the direction of the web to be developed; the developer liquid is filled up to the level shown by 39.

In the past it has been widely accepted that the smaller the electrode spacing is, the better result is obtained. Practically, however, one can only realize a finite spacing which will give rise to another defect, halo in the developed image.

In the present invention, at the start of development an electrophotographic material carrying an electrostatic latent image is applied a developer liquid without employing any electrode or with one placed apart from the material, the electrode being characterized by a roller comprising a middle conductive portion and flanges having a noticeably larger diameter than the middle portion, and then after sufficient time of such development with electrodes which are placed gradually nearer to the material by the use of electrode rollers which have smaller diameter differences in their arranged order. And the resulting developed image is a faithful reproduction of the original free of edge development, halo and streak.

What is claimed is:

1. A liquid development apparatus for developing an electrostatic latent image formed on a flexible electrophotographic material, comprising a plurality of rotating electrode rollers with a middle conductive portion which the latent image bearing surface faces and flange portions at the both ends of the rollers having a larger diameter than the middle portion, said flange portions supporting the image bearing surface by direct contact,-said electrode rollers arranged in the path of said carried material and the difference in the diameters of said flange and middle conductive portions of said electrode rollers progressively decreasing along said path.

2. An apparatus according to claim 1 wherein the diameters of said flange portions are equal in all electrode rollers.

3. An apparatus according to claim 2 wherein said electrode rollers are provided in a processing tank storinglitxrid developer.

n apparatus according to claim 2 wherein said flexible electrophotographic material is intermittently carried and when stopped each frame of the latent image on the material faces a middle conductive portion of each electrode roller.

5. An apparatus according to claim 1 wherein the diameters of said middle conductive portions are equal in all electrode rollers.

6. An apparatus according to claim 5 wherein said electrode rollers are provided in a processing tank storing the liquid developer.

Claims (6)

1. A liquid development apparatus for developing an electrostatic latent image formed on a flexible electrophotographic material, comprising a plurality of rotating electrode rollers with a middle conductive portion which the latent image bearing surface faces and flange portions at the both ends of the rollers having a larger diameter than the middle portion, said flange portions supporting the image bearing surface by direct contact, said electrode rollers arranged in the path of said carried material and the difference in the diameters of said flange and middle conductive portions of said electrode rollers progressively decreasing along said path.

1. A liquid development apparatus for developing an electrostatic latent image formed on a flexible electrophotographic material, comprising a plurality of rotating electrode rollers with a middle conductive portion which the latent image bearing surface faces and flange portions at the both ends of the rollers having a larger diameter than the middle portion, said flange portions supporting the image bearing surface by direct contact, said electrode rollers arranged in the path of said carried material and the difference in the diameters of said flange and middle conductive portions of said electrode rollers progressively decreasing along said path.

2. An apparatus according to claim 1 wherein the diameters of said flange portions are equal in all electrode rollers.

3. An apparatus according to claim 2 wherein said electrode rollers are provided in a processing tank storing liquid developer.

4. An apparatus according to claim 2 wherein said flexible electrophotographic material is intermittently carried and when stopped each frame of the latent image on the material faces a middle conductive portion of each electrode roller.

5. An apparatus according to claim 1 wherein the diameters of said middle conductive portions are equal in all electrode rollers.

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