GB2041790A

GB2041790A - Liq. development of electrostatic images

Info

Publication number: GB2041790A
Application number: GB8000325A
Authority: GB
Original assignee: Savin Corp
Current assignee: Savin Corp
Priority date: 1979-02-23
Filing date: 1980-01-04
Publication date: 1980-09-17
Also published as: GB2041790B; DE3006781A1; CA1156879A; FR2449908B1; JPS55115066A; FR2449908A1; CH641576A5

Abstract

Liq. development of electrostatic images using a roller 20 spaced from the image-bearing surface 4, wherein a field electrode 60 biased to a potential of the same polarity as that of the image or a corona discharge device 82 (Fig. 8) biased to a potential opposite to that of the image is provided downstream of the liq. application zone to aid post-transfer electrophoresis. The roller spacing from the surface 4 is preferably 1.5-3 mils and the developer liq. layer on the roller has a vol. of 10-30x10<9> cubic microns per sq. in., the developer liq. having a conductivity of 50-1000x10<3> pico-mhos and the roller surface a resistivity of 10<7>-10<9> ohm-cm. Following the field electrode 60, the surface is illuminated by a lamp 64 and contacted with carrier sheet 66. <IMAGE>

Description

SPECIFICATION Improved method and apparatus for developing latent electrostatic images BACKGROUND OF THE INVENTION There are a number of methods in the art of developing latent electrostatic images. These latent images may be formed by charging a photoconductive surface and then exposing the charged surface to light or ionizing radiations adapted to render the photoconductor conductive in the areas exposed to light or radiant energy and thus neutralize the areas subjected to light. The latent electrostatic image may be formed on a dielectric sheet by energizing styli to form the desired electrostatic pattern on the dieiectric sheet.The development of the latent electrostatic image was first accomplished by the electrical attraction of fine particles to the latent image areas on the photoconductive surface preferably, particles carrying electrical charges opposite in polarity to the latent image charges.

These particles were usually dry, from which the process derived the name "xerography." Later, it was discovered that particles could be suspended in a dielectric liquid and the latent electrostatic image could be developed by immersing it in the liquid. When dry powder was used as the developing medium, it was usually transferred to plain paper or other carrier sheet and then fixed by fusing. This required heat, and the fine particles were formed of heat-fusible resin. If a carrier sheet, such as paper, was coated with a photoconductive material, such as zinc oxide, no transfer of the electrostatic image after development was required and the development was made with a developing liquid carrying toner particles.

As the art developed, it was convenient, in order to eliminate the necessity of fusing a powder-developed image, to develop the latent electrostatic image with a liquid developer and then transfer the developed image to plain paper.

The developer liquid usually comprised a hydrocarbon carrier, such as Isopar-l;Ei, or the like, in which were dispersed toner particles adapted to render the latent image visible. The immersing of the photoconductive surface carrying the latent electrostatic image in a bath of liquid developer required wetting the entire surface of the photoconductor. The amount of liquid developer remaining on the developing image could be largely removed by a doctor roller. However, there was always a small amount of developer liquid remaining on the entire photoconductive surface.

This remaining liquid, along with the developed image, would be transferred to paper, or other carrier, and had to be dried on the paper. This resulted in a continual evaporation of the small amount of carrier liquid transferred from the photoconductive surface to the carrier paper. This evaporation was undesirable from several standpoints: First, the necessity of drying the sheet not only required some energy, but the time involved was such that the speed of copying was limited by this requirement. Furthermore, the ratio of carrier liquid to toner would constantly vary. If a battery of electrophotographic machines were used in a small chamber, the evaporation of the carrier liquid into the chamber would be objectionable.

The art recognized that it would be desirable if a latent electrostatic image could be developed by applying developing liquid only to the image area, and not to the photoconductive surface on which the latent electrostatic image was formed. If this were possible, since a typical copy is only about five per cent to seven percent image area, the potential evaporation would be correspondingly reduced. If a roller, such as a printing roller, were coated with developer and this roller were allowed to touch the photoconductor bearing the latent electrostatic image, both the photoconductor and the latent image would be covered with the developer liquid.This would leave developer liquid on the non-image areas, as well as the image areas, and reduce the contrast between the image areas and non-image or background areas of the conductive surface, with a resultant unsatisfactory copy. it was then conceived that, if the roller were formed with re-entrant portions and a more concentrated developing liquid were used - that is, the ratio of the toner particles to the carrier liquid were increased, so that there would be more solids in the developing liquid - a gravure roller of this type would tone the latent electrostatic image without leaving developing liquid on the non-image areas. A wiper blade would remove the developer liquid from the land portions, while the reentrant portions would carry the developer liquid.It was found that wiping the land areas clean was more difficult than believed. The wiping blade did not completely wipe the land areas free of liquid, so that there would be stringing, and a sharp image did not result. Furthermore, this stringing increased as the speed of copying was increased, thus limiting the number of copies which could be made in a given time.

The art also attempted to overcome the difficulties experienced in employing an applicator roller in contact with the drum by separating the applicator roller from the photoconductive surface and permitting the electrostatic field of the latent electrostatic image to pul9 the developing liquid from the applicator roller onto the photoconductive drum. These attempts were unsuccessful, for that the toned image was not sharp and free of imperfections owing to the unpredictability of what the developer liquid did when it arrived on the photoconductor.

FIELD OF THE INVENTION Our invention relates to a novel method of developing latent electrostatic images by developing liquid, comprising a carrier liquid having toner particles dispersed therethrough, and avoiding the drawbacks and disadvantages of the prior art which prevented the successful employment of developer-liquid applicator rollers.

DESCRIPTION OF THE PRIOR ART Gundlach 3,084,043 uses a belt having raised portions forming peaks and valleys. A layer of conductive ink is applied to the belt, which is then brought into contact with the photoconductor bearing the latent electrostatic image.

Willmott 3,232,190 discloses passing a film bearing dry toner particles sufficiently close to a latent electrostatic image that the dry particles will jump the gap and tone the image. This requires fusing or fixing of the toner particles after transfer to the carrier sheet.

Cassiers et al 3,383,209 discloses the use of an applicator roller formed with grooves to apply conductive ink to a latent electrostatic image. The applicator roller is in contact with the photoconductor and the toning takes place by selective wetting.

Cassiers et al 3,486,922 discloses the development of a latent electrostatic image with an aqueous conductive developing liquid. The applicator roller is in contact with the surface bearing the electrostatic image. The surface is hydrophobic, so that only the image will be toned, and not the background areas of the water repellant surface.

Damm 3,560,204 discloses the use of a water soluble acid or basic dyestuff as a self-fixing ink.

The surface bearing the latent electrostatic image passes close to but not touching a rotating drum having a layer of the ink. A field exists between the applicator drum and an electrode behind the image, so that an unsupported column of ink having the shape of the image jumps the gap and forms an inked image. Damm also discloses that nonpolar liquids, such as toluene or nitrile silicone, can be used and points out that they do not jump the gap as rapidly as water-based solutions.

Smith 3,667,428 disloses the use of an applicator roller having a regularly patterned surface adapted to transfer developer material to a latent electrostatic image. The applicator surface has a doctor blade for removing excess developer.

The developer is described as comprising polar and nonpolar liquids and dry-powder electroscopic materials. The applicator roller is in contact with the photoconductive surface.

Whittaker 3,772,012 discloses a method of reverse development. This is accomplished by.

biasing the developer applicator to substantially the same potential as the maximum potential on the imaging surface. Development is obtained by placing the applicator surface sufficiently close to the electrostatic imaging surface, such that the polar liquid developer is pulled from the recessed portions of the applicator surface to the imaging surface in reversal image configuration.

Smith et al 3,839,032 discloses the use of applicator rolls in contact with a photoconductive drum bearing a latent electrostatic image for applying a liquid developer to the latent electrostatic image to develop it.

Deshayes eft at 3,886,900 shows the use of an applicator roller to form a dynamic pool, or meniscus, to apply a liquid developer to a latent electrostatic image.

Fantuzzo 3,974,554 discloses the use of a roll for applying fluid to develop latent electrostatic images, formed with a pattern of volute and convex grooves and lands, which roll is in contact with the latent electrostatic image.

Nakano et al Patent 3,991,771 discloses a roller for applying electroconductive ink to a latent electrostatic image, the roller being formed of an electroconductive elastomer and positioned in contact with the photoconductive surface.

Matkan Patent 4,021,586 discloses an applicator roller adapted to reciprocate into and out of a liquid developer bath. A coating electrode in the bath is adapted to electrodeposit toner particles from the bath onto the roller while in the bath. The roller is then lifted out of the developer and into contact with the photoconductive surface bearing the latent electrostatic image which is mounted on a rotating drum. A corona is adapted to depress carrier liquid contained in the toner deposit from the applicator roller before it contacts the photoconductive surface.

SUMMARY OF THE INVENTION In general, our invention contemplates a method and apparatus of employing liquid toners on plain paper copying machines, in which the development of the latent electrostatic image on the photoconductive surface is so controlled as to tone only the charged image areas on the photoconductive surface. We employ a developer liquid with a higher concentration of toner than is feasible in the developing liquids used in copying machines of the prior art in a composition in which the carrier liquid and the toner concentrate, or solids, in such that the layer of developing liquid on the toned image is approximately from three to seven microns thick in the damp state.The developer liquid has a carrier, such as a liquid hydrocarbon, which has a higher vapor pressure at ambient temperature than the carrier liquid hydrocarbons customarily employed in liquiddeveloper copying machines to reduce evaporation of the carrier component of the liquid developer.

Our invention comprises the provision of an applicator roller positioned closely adjacent but not touching a photoconductive surface. The roller is driven in such direction and at such a speed that, at the closest approach of the two surfaces, the relative motion between them is substantially zero. The gap between the applicator roller and the photoconductive surface may vary between one and one-half mils and four mils. The applicator roller is adapted to be wetted with developer liquid to produce a film of developer liquid on its surface. The amount of liquid on the applicator roller must be uniform and controlled within narrow limits as a function of the gap between the applicator roller and the photoconductive surface and the conductivity of the liquid developer. If the film on the applicator roller is too thick, the copies will be quite wet, and if the film is not thick enough, the image will not be properly toned. If the liquid developer does not have sufficient conductivity, it will not jump the gap sufficiently: if it is too conductive, it will partially discharge the latent electrostatic image and impair satisfactory electrophoretic development and result in poor copy quality. The prior art used conductive liquids to tone by jumping the gap. We rely on electrophoresis, which takes place on the photoconductor during and after transfer of the developer liquid from the applicator roller to the surface of the photoconductor. Our improvement is such that columns of developer, corresponding precisely to the form of the latent electrostatic image, do not occur.Small amounts of developer liquid, only approximateiy corresponding to the fields of the latent electrostatic image, jump the gap and electrophoresis continues after the developer liquid has left the gap and arrived onto the photoconductive surface. This post-transfer ejectrophoresis enables us to obtain sharply defined images while subjecting the latent electrostatic image to small amounts of developer liquid for short periods of time. This enables us to achieve rapid operation of apparatus capable of carrying out our invention and thus produce a larger number of copies within a given period of time than is possible with machines of the prior art. Furthermore, the copies produced will be sharp and substantially dry, owing to the fact that the entire surface of the photoconductor has not been wetted with the developer liquid.The amount of liquid on the applicator roller can be controlled by providing the applicator roller with re-entrant portions of a predetermined capacity.

The applicator roller can be wiped with a doctor blade to level the amount of liquid in the reentrant portions of the applicator roller. However, we have found it advantageous to use a smooth applicator roller in combination with a doctor roller which rotates in the same direction as the direction of the rotation of the applicator roller. By varying the speed of the reverse doctor roller, we.

can control the volume of liquid on the applicator roller within narrow limits. Light liquid hydrocarbons are normally dielectric. A polar compound or charge director must be added to the developer liquid for imparting a charge to the toner particles, or the manufacture of the toner particles per se must be such that they have the proper polarity and further such as to render the developer liquid conductive to the desired extent, in order that the developer liquid will'jump the gap. This conductivity must not be so great that the developer liquid will partially discharge the latent electrostatic image after transfer.

The post-transfer electrophoresis is aided by a development electrode adapted to enhance the fields of the latent electrostatic image. The image can be further sharpened before it is transferred to a carrier sheet, such as plain paper, by flooding the developed image with light, as shown in the application of Benzion Landa, Serial No. 958,979, filed November 9, 1978, now Patent No.

Not only is offset to the carrier sheet aided by light fiooding but the image density on the carrier sheet is increased. Alternatively, the post-transfer electrophoresis may be augmented by a corona discharge over the toned image of a polarity opposite to the polarity of the latent electrostatic image.

One object of our invention is to provide a novel method of using liquid developers in plain paper copying machines, and the like, in which the development of the image on the photoconductive surface is so controlled as to tone substantially only the charged image areas of the latent electrostatic image, and to provide novel apparatus for carrying out our method.

Another object of our invention is to provide such apparatus having an applicator roller which serves as a development electrode.

Still another object of our invention is to provide such applicator roller having means for controlling the volume of liquid on the same.

A further object of our invention is to provide a method of developing latent electrostatic images on a photoconductive surface, such that the electrophoresis takes place, not only during transfer of developer liquid from the applicator roller to the photoconductive surface, but also after transfer has been accomplished.

An additional object of our invention is to provide a method of employing liquid developers to develop latent electrostatic images, in which the ratio of solids to a carrier liquid in the liquid developer is higher than had heretofore been employed.

Another object of our invention is to provide a method and apparatus employing liquid developers in which a developing liquid having a carrier-liquid component of iow-vapor pressure may be employed.

Still another object of our invention is to provide a method and apparatus for dynamically controlling the volume of liquid on an applicator roller of an electrophotographic copying assembly employing liquid developers.

A further object of our invention is to provide a toning system for an electrophotographic process and apparatus in which an applicator roller for applying developer liquid is spaced from and does not touch the photoconductive surface bearing the latent electrostatic image.

An additional object of our invention is to provide an electrophotograpic method and apparatus employing a field electrode for enhancing the field of the latent electrostatic image to assist in post-transfer electrophoresis.

A further object of our invention is to employ a corona discharge device to augment development of the latent electrostatic image on the photoconductor.

Other and further objects of our invention will appear from the following description.

BRIEF DESCRIPTION OF THE DRAWINGS In the accompanying drawings, which form part of the instant specification and which are to be read in conjunction therewith, and in which like reference numerals are used to indicate like parts in the various views: FIGURE 1 is a diagrammatic view of a photocopying machine adapted to carry out the method of our invention, shown with parts in section.

FIGURE 2 is a side elevation viewed along the line 2-2 of FIGURE 1, showing the applicator roller and the reverse doctor roller.

FIGURE 3 is a side elevation, similar to FIGURE 2, showing the applicator roller driven from the photoconductive drum with the doctor roller and wiping blade assembly removed.

FIGURE 4 is a diagrammatic perspective view, drawn on an enlarged scale, showing a portion of the surface of an applicator roller provided with reentrant portions formed by knurling.

FIGURE 5 is a fragmentary sectional view, drawn on an enlarged scale, taken along the line 5-5 of FIGURE 1, showing hemispherical reentrant portions on an applicator roller.

FIGURE 5a is a sectional view, similar to FIGURE 5, showing circumferential grooves forming re-entrant portions on the applicator roller.

FIGURE 6 is a top plan view of FIGURE 5.

FIGURE 6a is a top plan view of FIGURE 5a.

FIGURE 7 is a top plan view, drawn on an enlarged scale, showing grooves running axially of the applicator roller to form the re-entrant portions.

FIGURE 8 is a diagrammatic view of a photocopying machine similar to that shown in FIGURE 1, with parts shown in section adapted to carry out a modified method of augmenting electrophoresis in the development of the latent electrostatic image.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT More particularly, referring now to the drawings, a conductive drum 2, provided with a layer 4 of photoconductive material, is mounted by apertured discs 6 upon a shaft 8 and keyed thereto by key 10 for rotation therewith in a counterclockwise direction indicated by the arrow (A). The photoconductive drum and surrounding assembly are mounted in a lightproof housing (not shown), as is known in the art. The shaft 8 may be grounded or biased to any desired direct current voltage from a source Inot shown), as is known in the art. A corona discharge device 12 is adapted to impose a charge on the photoconductor 4 as it passes the same. If the photoconductor is selenium, the charge will have a positive polarity.

If the photoconductive material is resin-bound zinc-oxide, or an organic photoconductive material such as polyvinyl carbazole, or the like the charge will be negative.

After the photoconductor receives a charge, it passes lens 14, fitted in the housing and adapted to prnjectanimage which is desired to be reproduced upon the charged photoconductive surface. Since a photoconductor is an insulator in the dark and a conductor in the light, the impingement of the image pattern of light and shade upon the photoconductor will permit the areas of light to conduct the charge through the photoconductor to the conductive drum 2, thence through the disc 6, to the shaft 8. A latent electrostatic image will thus be formed on the surface of the photoconductor 4. This image will have a high potential in the order of 800 to 1000 volts above that of shaft 8, though it may vary over wide limits. A tank 1 6, formed of insulating material, is adapted to contain a developing liquid 18.An applicator roller 20, which may be made of a conductive resin such as buna N, is mounted adjacent the photoconductor. Buna N, as is known in the art, is a synthetic rubber produced by the copolymerization of butadiene and acrylonitrile.

The applicator roller may be made of synthetic resin or of metal. If made of dielectric synthetic resin, thin lines of the latent electrostatic image will be toned, but large areas will not. With a metal roller, while toning will take place over a wide range of image areas, arcing may occur over areas of high image intensity, resulting in copies showing horizontal white lines where the arcing occurred. Buna N is practical because its conductivity can be closely controlled, and it is resistant to hydrocarbon oils, which are the carrier liquids for the toners used in our process.

The surface of the roller may be smooth or may be textured -that is, provided with re-entrant portions adapted to hold a volume of liquid developer. FIGURE 4 shows a knurled surface; FIGURES 5 and 6 show a surface formed with depressions; FIGURES 5a and 6a show a roller formed with circumferential grooves concentric with the axis of the roller; and FIGURE 7 shows axial grooves parallel to the axis of the roller.

The conductivity of the roller is important and optimum results have been obtained when the surface of the roller is smooth and polished and the roller material has a resistivity of 107 ohmcentimeters to 109 ohm-centimeters. Optimum performance with minimum breakdown occurs when the resistivity of the roller material lies between 108 ohm-centimeters and 5(108 ohmcentimeters). The resistivity of the roller is important, as will be pointed out hereinafter.

The gap between the applicator roller and the photoconductive surface, at the closest approach, indicated by the opposing arrows (B), lies between one and one-half mils and four mils. We have obtained good performance when the physical gap between the roller and the photoconductive surface is approximately two mils.

In our method, the developing liquid jumps from the applicator roller to the latent electrostatic image owing to the electrostatic fields of the electrostatic image. In areas where the fields are intense, breakdown may occur, discharging the latent electrostatic image along a line of closest approach of the applicator roller to the photoconductive surface. We have discovered that the occurrence of this breakdown can be reduced dramatically by making the roller slightly conductive, as pointed out above. The conductivity can be controlled by ioading the buna N rubber, or other hydrocarbon oil-resistant synthetic resin, with disseminated carbon particles to impart a degree of conductance to the material of which the rollers is made.

The quantity of developing liquid which negotiates the gap is important. We have found that the volume of the developing liquid on the applicator roller should lie between 10 billion and 30 billion cubic microns per square inch. This can be obtained by controlling the re- entrant portions if the applicator roller is textured. The textured roller is simply wiped by a doctor blade (not shown), as is known in the art, to level the quantity of liquid remaining in the re-entrant portions of the textured roller. In this manner, we are assured that a predetermined quantity of developer liquid will be present on the applicator roller for transfer over the gap between the applicator roller and the photoconductive for developing the latent electrostatic image on the photoconductive surface.

Preferably, the applicator roller is driven to move at the same speed as the surface of the photoconductor. Precise equivalence of speed is not necessary. It is important that the relative motion between the applicator roller and photoconductive surface be smooth and not varying. The applicator roller can be driven at a slightly greater speed than the velocity of the photoconductive surface bearing the latent electrostatic image. Too great an increase in speed, however, tends to flood the electrostatic image. If the roller is driven at a slower speed, the image tends to be undertoned.

Referring again to FIGURE 1, a doctor roller 22 is positioned to rotate with a shaft 24 by a belt or chain 26 driven by a pulley or sprocket wheel 28 which is acutated by a variable speed motor 30.

The direction of rotation of the doctor roller 22 is the same as that of the rotation of the applicator roller 20. A wiper blade 32 removes excess developer fluid from the doctor roller and keeps it clean. The doctor roller is spaced by rings 34 from the applicator roller 20 by a distance of between one mil and five mils. The amount of liquid on the applicator roller, as pointed out above, should lie between 10 billion and 30 billion cubic microns per square inch. This amount of liquid is easily metered by varying the rotational speed of the doctor roller and the gap (C) between the doctor roller and the applicator roller. A close gap will permit a slower rotational speed of the doctor roller; a wider gap between the applicator roller and the doctor roller requires a greater velocity of rotation of the doctor roller.The doctor roller can be made to rotate at various speeds by the variable speed motor 30. At a higher speed, the volume of developing liquid on the applicator roller is so reduced that the latent electrostatic image on the photoconductive surface is lightly toned and very dry. If the speed of the doctor roller is reduced too much, images that are quite wet are achieved.

The speed should be such that the correct volume of developing fluid is applied to the doctor roller.

This speed lies between 100 rpm and 200 rpm and depends on the gap (B) between the applicator roller and the photoconductive surface being used and the specific developing liquid being employed. The smooth-surfaced applicator roller, together with a variable-speed doctor roller, produces a most flexible combination and has a number of advantages. The quantity of toner on the applicator roller can be easily controlled. A wiper blade 36, bearing against the surface of the applicator roller, keeps the applicator roller clean.

With a textured roller, the volume of the developer liquid is fixed and no adjustment of the roller surface can be made for variations in the developer liquid. Furthermore, there is a tendency for the the re-entrant portions of a textured roller to acquire deposits which change the volumetric capacity of the liquid applicator.

Referring again to FIGURE 1, the applicator roller may be driven from a variable speed motor 38 through a belt or chain 40, driven through pulley or sprocket wheel 39 to rotate at such speed that the relative motion between the surface of the applicator roller and the surface of the photoconductor is close to zero. By driving the applicator roller faster, more developer liquid will be presented for developing the latent electrostatic image. By driving the applicator roller slightly slower, less developer fluid will be presented for negotiating the gap to develop the latent electrostatic image. If a textured roller is used, the control of the speed of the applicator roller enables us to made adjustments for variations in the developing liquid and changes in the capacity of the re-entrant portions owing to collection of sediment in them.

The applicator roller is biased either above ground or below ground by a potentiometer, indicated generally by reference numeral 42, adapted to apply a bias through a conductor 44 to a shaft 46 with which the applicator roller rotates.

Since zero relative motion between the surface of the applicator roller and the surface of the photoconductive drum is usually optimum, we can easily achieve this by the construction shown in FIGURE 3. The applicator roller 20 is formed with flanges 21 at each end. These flanges bear against the photoconductor 4 and are secured to the roller for rotation therewith. The roller is mounted for rotation with the shaft 46, and is insulated from ground so that the bias on shaft 46 can be adjusted by potentiometer 42. It will be seen that the friction between the flanges 21 and the surface of the photoconductor upon which they bear causes the roller to rotate such that the relative motion between the surface of the applicator roller and the surface of the photoconductor on drum 2 is zero.

Referring again to FIGURE 1, the applicator roller 20 is shown as independently rotatable in respect of the photoconductive surface. The shaft 46 of the applicator roller carries end discs 50 which are secured to the inner races 52 of ball bearings, the outer races 54 of which bear against the photoconductive surface, as can be seen by reference to FIGURES 1 and 2. The shaft 46 extends through the bearings 47 loosely supported on the machine frame 51 and carries a pulley or sprocket wheel 53, adapted to be engaged by the belt or chain 40. Springs 49 connected between bearings 47 and the frame urge outer rings 54 into engagement with the surface 4. As pointed out above, the relative speed of the applicator roller to the photoconductive surface may be adjusted by varying the speed of motor 38.

The doctor roller 22 is similarly spaced from the applicator roller by a ball bearing in which the rings 34 form the outer races. The inner rings 33 of the ball bearing assemblies are mounted on reduced diameter end roll portions secured to the shaft 24 of the doctor roller, which extends through the housing 51 and which carries a pulley or sprocket wheel 23. The shaft of motor 30 drives a pulley or sprocket wheel 29. The belt or chain 26 extends between the member 23 carried by the shaft 24 and member 29 carried by the motor shaft. The variable-speed motor 30 is adapted to rotate the doctor roller 22 in the same direction as the direction of rotation of the applicator roller at any desired speed.

Referring again to FIGURE 1, the tank 16 is provided with a pipe 1 5 connected to a source of developing liquid (not shown) to supply developing liquid to the tank. A level control (not shown), known to the art, is adapted to keep the level of developing liquid in the tank at a predetermined level. As can be seen by reference to FIGURE 1, the applicator roller 20 is mounted so that its lower portion dips into the developer liquid in the tank and is adapted to carry liquid from the tank to adjacent the latent electrostatic image on the photoconductor, as heretofore described.

The composition of the developing liquid is important. If it is too conductive, it will partially discharge the latent electrostatic image. If it is not conductive enough, sufficient liquid developer will not negotiate the gap and underdevelopment will result. The developing liquid which we employ differs from developing liquids of the prior art, in that the concentration of toner in the developing liquid is greater than heretofore employed and the carrier liquid is a hydrocarbon oil having a lower vapor pressure than the hydrocarbon liquids used in known liquid-developer copying machines.

Most liquid developers of the prior art use Isopar G as the carrier liquid. This is a narrow cut of isoparaffinic hydrocarbon having an initial boiling point of 319OF, and an end or dry point of 345OF It has a flash point above 1000F. However, it will completely evaporate, if left exposed, in 2200 seconds, as measured by Method 11 of the Federation of Societies for Paint Technology. As one object of our invention, we desired to reduce the evaporation of the carrier hydrocarbon liquids and, accordingly, employ a higher-boiling isoparaffinic hydrocarbon such as lsopar-M, which has a boiling range between 410OF. and 4850F. and a lower vapor pressure.Its flash point is 170OF., and it takes 3500 seconds to evaporate only 10%, as measured by Method 11 of the Federation of Societies for Paint Technology. It is substantially a light mineral oil. Other light mineral oils, such as "Marcol 52" and "Marcol 62", are highly refined white mineral oils, free of unsaturated compounds and possessed of a high degree of chemical stability. They are odorless and tasteless and meet all the standards of internal, medicinal, and cosmetic uses. They are manufactured by Humble Oil and Refining Company and have a viscosity in Saybolt seconds, at 1000F., of 51 and 61 respectively.These oils conform to the requirements of FDA Regulation 124.1146. "Marcol 52" has a specific gravity of .827 at 600F., and "Marcoi 62" has a specific gravity of .834 at 60OF. These light mineral oils are used by us as the carrier liquid in which concentrated toner particles are disseminated.

Development of the image takes place by electrophoresis -- that is, the migration of the toner particles through the carrier liquid to the latent electrostatic image, which is a pattern of charge on the photoconductive surface corresponding to the light and shade pattern projected onto the charged photoconductor. The carrier liquids have very low vapor pressure, and the small amount of carrier liquid which negotiates the gap carries toner particles which develop the image by electrophoresis. What happens is that droplets of toner liquid negotiate the gap and arrange themselves, roughly, according to the fields of the electrostatic image.

This electropnornsis continues after the gap has been jumped and a small amount of liquid carrier remains on the photoconductor, permitting this electrophoresis to occurs When the developed image is transferred to a carrier sheet such as paper, the carrier liquid disperses immediately into the paper and, though the image is damp on the photoconductor, it is substantially dry and smearfree on the paper to which it is transferred, as will be described more fully hereinafter. It is to be understood that while we prefer to employ the carrier liquids herein particularly described, any appropriate dielectric liquid which does not dissolve the toner particles may be used.

In our electrophoretic type of toner, we employ as much as 5% to 30% by weight, in respect of the weight of the carrier liquid, of toner solids. Liquid hydrocarbons are dielectric and will not jump the gap. A polar compound, such as a charge director, must be added to the developer liquid in order to make our process operative with with the applicator roller out of contact with the photoconductor. As described in U. S. Patent 3.660,886, these charge directors are well known to the art. They are polar compounds, soluble in the hydrocarbon carrier liquid, and create an electrostatic charge on the micron pigment (toner) particles which are adapted to migrate by electrophoresis during the development process.

The charge director should be such as to impart a negative polarity when the photoconductor is adapted to be charged positively and to impart a positive polarity when the photoconductor is adapted to be charged negatively. Charge directors need not be used if the pigment particles are of such nature that they render the developer liquid conductive in and of themselves. It is known that the ball milling of long-chain polymers degrades them during milling and they acquire a charge. A charge may also be imparted to longchain polymers by dyeing select polymers or by adding a select comonomer in the polymerization step. The carrier liquid remains dielectric but the charged toner particles make the developer liquid as a whole conductive. The conductivity of the developer liquid is critical for a given gap. For a wider gap, a greater conductivity is required.By way of example, and not by way of limitation, with a two-mil gap, a conductivity of from 50,000 pico mhos to 1,000,000 pico mhos is required. If the conductivity is too great, the latent electrostatic image will be partially discharged. If the conductivity is too low, sufficient developer liquid will not jump the gap and the image will not be properly developed.

Following transfer of the developer from the applicator roller to the photoconductive surface, electrophoretic development, which was initiated during transfer, continues. The toner particles outside the charge of the latent electrostatic image move into and within the boundaries of the latent electrostatic image. This electrophoresis is enhanced, in one form of our method, by the use of a field electrode 60, which is biased by a directcurrent source such as battery 62. The polarity of the bias on the development electrode 60 is the same as the charge on the photoconductor -- that is, the charge of the latent electrostatic image. If the photoconductor is of selenium, the charge will be positive; if an organic photoconductor or cadmium sulphide is used as the photoconductor, the polarity will be negative.We have observed that the field electrode, when grounded, provides a ground plane which is effective in aiding electrophoresis even when the bias is absent. The gap, indicated by reference letter (D), between the field electrode and the photoconductive surface should be as small as practicable. The function of the field electrode is to increase the projection of the electrical field of the latent electrostatic image to a greater distance from the surface of the photoconductor. The gap, however, should not be so close that it will touch the image. Since the developing liquid transferred to the photoconductive drum is never more than one or two mils in depth, if the electrode is positioned between twelve and fifteen mils, there will be no contact of the field electrode 60 with the latent electrostatic image on the photoconductive drum 4.There is another consideration which must be taken into account. The distance between the surface of the photoconductive drum and the field electrode -that is, the gap (D) - should be less than ten times the thickness of the photoconductor, If it is greater than this, the increased resolution produced by the field electrode will be reduced. The optimum spacing is equal to the thickness of the photoconductive layer. This, of course, is not possible since the photoconductive material on the drum is usually quite thin.The function of the field electrode is different from the classical development electrode used in development systems; that is, it is different from the function of the applicator roller 20 which acts as a development electrode during transfer of the developing liquid from the applicator roller to the photoconductive surface.

The development electrodes of the prior art have for their purpose the prevention of the deposit of toner on the background or non-image areas of the photoconductor. This takes place, of course, only when the developing liquid is in contact both with the electrostatic image on the photoconductor and the development electrode. In our method and apparatus, the electrophoresis takes place chiefly after transfer. The development electrode enhances the electric fields, which are normally small fringe fields at the edge of the charged areas forming the latent electrostatic image. The field electrode, in our process, applies a voltage of the same potential as the charged image, but of a higher value than the highest charge of the latent electrostatic image.Our development electrode creates field patterns on the surface of the photoconductor which are conductive to assisting the mobility of the toner particles in the small amount of developing liquid which has been transferred to the photoconductive drum and which lies outside of the charged image patterns. This enables electrophoresis to continue on the photoconductor in an expeditious and efficient manner during the post-transfer period and enables development to continue to a greater extent than otherwise. The development electrode 60 should be as long as possible, since this will enable the electrophoresis to continue to the greatest extent and produce a sharper developed image.

A low-level illumination lamp 64 - such as described in the application of Benzion Landa, Serial No. 958,979, filed November 9, 1978, now Patent No. referred to above - is positioned so that the photoconductive surface and the developed image will be flooded with light after the enhancement electrode 60 has been passed by the image. This not only sharpens the image, as described by Landa, by discharging the background or non-image area of the image, but also enables a more complete transfer of the developed image to the carrier, such as paper, to which the image will be transferred.

The use of a higher-boiling mineral oil leaves a damp developed image on the photoconductor, though the amount of carrier liquid is small and the dampness siight. A carrier material such as paper 66 is fed by rollers 68 past a pressure roller 70. We have found that the pressure roller 70 will enable the transfer of the damp image from the surface of the photoconductor to the carrier sheet 66 if the carrier sheet is absorbent, such as paper.

This transfer will be aided by biasing the roller as is known in the art. If the carrier sheet is a transparent film of synthetic resin which is not absorbent, a corona 72 is adapted to charge the back of the carrier sheet to effect transfer of the image from the photoconductive surface to the carrier sheet. The polarity of this charge should be the same as the charge of the developed electrostatic image. If selenium is used, the charge will be highly positive; if cadmium sulphide or an organic photoconductor is used, the charge will be negative. The developed image will be pulled from the sheet onto the carrier. A pick-off blade 74 ensures that the carrier sheet 66 will leave the photoconductor for feeding by roller 76 to a tray or other copy-handling device, such as a collator or the like.In order to ensure that the surface of the photoconductor is clean, a cleaning roller 78 wipes the surface of the photoconductor drum and a wiper blade 80 completes the drum-cleaning operation.

Referring now to FIGURE 8, in an alternative embodiment of our invention after the developer liquid has been applied to the image by roll 20 and while the image is still damp we subject it to the action of a corona 82 having a polarity which is opposite to that of the electrostatic image to enhance the electrophoretic action which was initiated upon the transfer of the developer liquid from the applicator roll to the latent image on the photoconductive surface. We have discovered that this greatly sharpens the resultant copy.

Further in the form of our invention illustrated in FIGURE 8, after the image has been developed, we transfer it to the sheet of copy material 66 by passing the sheet 66 from the register rolls 68 into the nip between a transfer roller 84 and the surface 4. We form the roller 84 from a deformable conductive material, which may for example be buna N rubber, to which sufficient carbon particles have been added to give it the required degree of conductivity. Any suitable means, such for example as a battery 88, provides a source of biasing potential which is applied to the shaft 86 of the roller 84.Under the combined action of the bias provided by battery 88 and the pressure of roll 84, the developed image is transferred from the surface 4 to the sheet 66, which then is picked off the drum surface by pickoff 74 and deiivered to a tray or other paper handling equipment as is well known in the art.

In operation, the photoconductive drum is rotated by a motor or other drive means (not shown) to rotate in the direction of arrow (A).

When a copy is desired to be made, the drum is charged by corona device 12 and then exposed to an image of the document of which a desired copy is to be made, projected by lens 14 on the surface of the photoconductor. This produces a latent electrostatic image. The applicator roller 20 is rotating at a speed such that there is substantially no relative motion between the photoconductor and the applicator roller. This may be biased as desired further to reduce the amount of developer liquid on the non-image areas of the photoconductor during transfer of developer liquid. Such bias, owing to our post-transfer electrophoretic method, is not as important as is the case in the copying machines of the prior art using liquid developers.The closest portion of the applicator roller 20 is spaced from the surface of the photoconductor 4, carried by the drum 2, to form a gap, preferably by a distance of two mils.

The doctor roller 22, which is wiped by wiper blade 32, is driven at the proper speed by motor 30 and in a reverse direction, so that the volume of developer liquid on the applicator roller is proper to produce a pleasingly developed image having the desired contrast. The applicator roller dips into the tray 1 6 which carries the developing liquid 18, the wiper blade 36 keeping the applicator roller clean. The applicator roller may, if desired, be independently driven by motor 38 at speeds such that the relative motion between the applicator roller and the surface of the photoconductor is not zero, but such that the surface of the applicator roller moves faster than the surface of the photoconductor.The developing liquid is, as described, formed of a carrier liquid having a lower vapor pressure than the carrier liquids of the prior art, and having a concentration of solids, with respect to the carrier liquid, greater than the prior art. The conductivity of the developer liquid is closely controlled by the addition of polar compounds or charge directors, such that it is conductive enough to permit sufficient developer liquid to jump the gap, but not so conductive as to partially discharge the latent electrostatic image on the photoconductive surface. After the developing liquid jumps the gap, development occurs by post-transfer electrophoresis, though some development may take place during transfer.To enhance the electrophoretic development, we employ a field electrode 60, which is biased by the same polarity as the polarity of the latent electrostatic image and positioned as closely as possible to the photoconductive surface, without permitting the developer to touch the field electrode. This field electrode enhances the electrical fields which are normally small-fringe fields at the edge of the charge areas and assists electrophoresis by increasing the mobility of the toner particles in the carrier liquid lying outside the charged image patterns.

After the developed image passes the field electrode 60, the background areas, or non-image areas, are discharged by a low level illumination lamp 64 which floods the developed image. This sharpens the image and enables a denser image to be transferred to the carrier sheet such as paper. If paper or other absorbent carrier is used, a slightly damp image will transfer by pressure alone. The small amount of carrier liquid in the damp image will be rapidly absorbed by the paper, and the copy on the paper will be dry without the necessity of using heat.

The operation of the form of our invention illustrated in FIGURE 8 is the same as that of the form of our invention illustrated in FIGURE 1, up to the point at which the surface 4 leaves the applicator roller 20. Following this point, the image to which the developer has been applied is subjected to the action of the corona 82 which greatly sharpens the image. Subsequently, register rolls 68 feed a sheet of copy material 66 into the nip between roll 84 and the developed image bearing surface 4. Under the combined action of the pressure of roller 84 and the bias provided by battery 88, the image is transferred from surface 4 to the sheet which then is picked off the drum and delivered to the user.

It will seen that we have accomplished the objects of our invention. We have provided a novel method of employing liquid toners in plain-paper- copying machines, in which the development of the image on a photoconductive surface is so controlled as to tone substantially only the charged image areas of the latent electrostatic image without wetting the uncharged or background areas. We have provided a novel apparatus for carrying out our method, which apparatus is equipped with an applicator roller which applies liquid developer to the latent image through a gap. This applicator roller is provided with means for either statically or dynamically controlling the volume of liquid presented by the applicator roller to the latent electrostatic image, separated from the roller by a gap.The static control means may involve a wiper blade and a textured roller having a predetermined cubic volumetric capacity. The applicator roller may be a smooth roller, used in association with a doctor roller adapted to revolve in the same direction as the direction of rotation of the applicator roller and at speeds controlled to dynamically meter the volume of liquid on the applicator roller.

Our method is such that electrophoresis takes place, not only during the period of transfer of the developer liquid from the applicator onto the photoconductive surface, but chiefly after transfer has been accomplished. Our method employs liquid developers having a higher ratio of solids to carrier liquids than has heretofore been employed and a dielectric carrier liquid having a lower vapor pressure than the carrier liquids of the prior art.

In our process, the conductivity of the developer liquid is controlled by employing toner particles which have polar characteristics or charge directors. The conductivity of the developer liquid composition as a whole is such that it is high enough to jump the gap in connection with which our process is employed, but not so high as to partially discharge the latent electrostatic image. Our field electrode, or a post-toning corona, enhances the field of the latent electrostatic image and assists in the post-transfer electrophoresis. We may alternatively employ a post-toning corona to augment electrophoresis.

It will be understood that certain features and subcombinations are of utility and may be employed without refererence to other features and subcombinations. This is contemplated by and is.

within the scope of our claims. It is further obvious that various changes may be made in details within the scope of our claims without departing from the spirit of our invention. It is, therefore, to be understood that our invention is not to be limited to the specific details shown and described.

Claims

1. In a method of developing latent electrostatic images wherein a latent electrostatic image is formed on a photoconductive surface, the steps of applying a developer liquid comprising dielectric liquid hydrocarbons having charged toner particles disseminated therethrough and having a conductivity from 50,000 pico-mhos to 1,000,000 pico-mhos to the latent electrostatic image over a gap whose distance from the surface of the developer liquid to the surface of the latent electrostatic image lies between one-half mil and three mils, said applying step being performed from a layer of developing liquid having a volume between 10 billion cubic microns per square inch and 30 billion cubic microns per square inch, permitting the development of the latent electrostatic image to occur by electrophoresis of the toner particles in the developing liquid which has jumped the gap, increasing the velocity and extent of said electrophoresis by subjecting the initially developed electrostatic image to an electric field of the same polarity as the charge of the latent electrostatic image, then discharging the non-image areas of the charged photoconductor by illuminating the developed image, and then transferring the developed image to a carrier sheet.

2. In a method of developing latent electrostatic images wherein a latent electrostatic image is formed on a photoconductive surface, the steps of forming a layer of developing liquid comprising liquid hydrocarbons having toner particles disseminated therethrough and a minute amount of a polar compound adapted to charge said toner particles whereby to impart a conductivity to the developing liquid between 50,000 pico-mhos and 1,000,000 pico-mhos upon a support having resistivity between 107 ohm-centimeters and 109 ohm-centimeters, controlling the thickness of the layer of developing liquid on said support to between one and two mils, applying said liquid developer on said support to said latent electrostatic image over a gap whose distance from the surface of said layer to said photoconductive surface lies between one-half mil and three mils, permitting the development of the latent electrostatic image to occur by electrophoresis of the toner particles in the developing liquid which has jumped the gap, and increasing the extent of said electrophoresis by subjecting the initially developed electrostatic image to an electric field of the same polarity as the charge of said electrostatic image.

3. In a method of developing latent electrostatic images wherein a latent electrostatic image is formed on a moving photoconductive surface, the steps of applying developer liquid comprising dielectric carrier fluid having charged toner particles disseminated therethrough by a moving applicator whose surface moves at substantially the same velocity as the photoconductive surface, spacing said toner applicator from said photoconductive surface to form a gap therebetween, forming a film of liquid developer on said applicator, metering the thickness of the film on said applicator to a predetermined thickness as a function of the distance of said gap and the conductivity of said developing liquid and the potential of the field of said electrostatic image such that the developing liquid will jump said gap, permitting the development of the latent electrostatic image to occur by electrophoresis of the toner particles in the developing liquid which has jumped the gap, and increasing said electrophoresis by subjecting the developing latent electrostatic image to a corona discharge of a polarity opposite to the polarity of the latent electrostatic image.

4. In a method of developing a latent electrostatic image wherein the latent electrostatic image is formed on a moving photoconductive surface, the steps of applying developer liquid comprising a dielectric carrier fluid having charged toner particles disseminated therethrough from a rotary applicator revolving at a rate such that its surface moves at a relative velocity equal to or slightly greater than the velocity of said moving photoconductive surface, spacing said toner applicator from the photoconductive surface to form a gap therebetween, forming a film of liquid developer on said applicator, dynamically metering said film to a predetermined thickness as a function of the conductivity of the developing liquid the distance of said gap and the potential of the field on said electrostatic image such that the developing liquid will jump said gap, permitting the development of the latent electrostatic image to occur by electrophoresis of the toner particles in the developing liquid which has jumped the gap, and increasing said electrophoresis by subjecting the developing latent electrostatic image to an electric field of the same polarity as the charge of said electrostatic image.

5. In a method developing a latent electrostatic image wherein the latent electrostatic image is formed on a moving photoconductive surface, the steps of applying developer liquid comprising a dielectric carrier fluid having charged toner particles disseminated therethrough from a rotary applicator revolving at a rate such that its surface moves at a relative velocity equal to or slightly greater than the velocity of said moving photoconductive surface, spacing said toner applicator from the photoconductive surface to form a gap therebetween, forming a film of liquid developer on said applicator, dynamically metering said film to a predetermined thickness as a function of the conductivity of the developing liquid the distance of said gap and the potential of the field on said electrostatic image such that the developing liquid will jump said gap, permitting the development of the latent electrostatic image to occur by electrophoresis of the toner particles in the developing liquid which has jumped the gap, and increasing said electrophoresis by subjecting the developing latent electrostatic image to a corona discharge of a polarity opposite to the polarity of the latent electrostatic image.

6. In a method of developing latent electrostatic images with a liquid developer comprising a dielectric carrier fluid having charged toner particles disseminated therethrough, the steps of applying said developer liquid to the latent electrostatic image to develop the same by electrophoresis of the charged toner particles and then subjecting the damp and developed electrostatic image to an electric field of the same polarity as the charge of said electrostatic image.

7. In a method of developing latent electrostatic images with a liquid developer comprising a dielectric carrier fluid having charged toner particles disseminated therethrough, the steps of applying said developer liquid to the latent electrostatic image to develop the same by eiectrophoresis of the charged toner particles and then subjecting the damp and developed electrostatic image to a corona discharge of a polarity opposite to the polarity of the charge of said electrostatic image.

8. In a method of developing latent electrostatic images with a liquid developer comprising a dielectric carrier fluid having charged toner particles disseminated therethrough, the steps of applying said developer liquid to the latent electrostatic image to develop the same by electrophoresis of the charged toner particles, then subjecting the damp and developed electrostatic image to an electric field of the same polarity as the charge of said electrostatic image, and then transferring said developed image to a carrier sheet.

9. In a method of developing latent electrostatic images with a liquid developer comprising a dielectric carrier fluid having charged toner particles disseminated therethrough, the steps of applying said developer liquid to the latent electrostatic image to develop the same by electrophoresis of the charged toner particles, then subjecting the damp and developed electrostatic image to a corona discharge of a polarity opposite to the polarity of the charge of said electrostatic image, and then transferring said developed image to a carrier sheet.

10. In an apparatus for making copies of a document by electrophotography, a rotatable conductive drum, a photoconductor carried by said conductive drum; means for charging the photoconductor; exposing means for subjecting the charged photoconductor to the image of a document being copied to form a latent electrostatic image; the improvement comprising developing means including an applicator roller having a resistivity between 107 ohm-centimeters and 109 ohm-centimeters, means for mounting said applicator roller for rotation about an axis parallel to the axis of rotation of said drum so as to form a gap between the surface of the photoconductor and the surface of said applicator roller at their point of closest approach of between one and one-half mils and four mils, means for rotating said applicator roller so that the relative motion between the surface of the photoconductor and the surface of the applicator is substantially zero, means for applying developing liquid to said applicator roller, means for metering the thickness of the film upon the applicator roller such that it has a volume between 10 billion cubic microns per square inch and 30 billion cubic microns per square inch, a field electrode positioned adjacent the photoconductive drum in a position to subject a developed latent electrostatic image which has been subjected to the action of developer liquid to an electric field, means for imparting a potential to said field electrode of the same potential as the potential of the electrostatic image, and means for transferring the developed electrostatic image to a carrier sheet.

11. Apparatus as in Claim 10 in which said metering means comprises an applicator roller formed with re-entrant portions and a wiper blade for said applicator roller.

12. Apparatus as in Claim 10 in which said metering means comprises a doctor roller, means for spacing said doctor roller from said applicator roller, and means for rotating said doctor roller in the same direction as the direction of rotation of said applicator roller.

1 3. Apparatus as in Claim 10, including illuminating means for illuminating the developed electrostatic image after it has been subjected to the field of said field electrode.

14. In an apparatus making copies of a document by electrophotography, a movable conductive support, a photoconductor carried by said support; means for charging the photoconductor; exposing means for subjecting the charged photoconductor to the image of a document being copied to form a latent electrostatic image; the improvement comprising developing means including movable.

developer applicator means, means for moving said applicator means substantially in synchronism with said photoconductor so the relative motion between the photoconductor and the applicator is substantially zero, means for mounting said applicator means so that the surface of the same at its closest approach to the surface of said photoconductive means lies between one and one-half mils and four mils, means for applying developing liquid to said applicator means, means for metering the thickness of the film on said applicator means, a field electrode positioned adjacent the surface of said photoconductor in a position to subject a developed latent electrostatic image which has been subjected to the action of the developer liquid to an electric field, and means for imparting a potential to said field electrode which is the same as the potential of said electrostatic image.

15. Apparatus as in Claim 14 in which said metering means comprises a doctor roller, means for spacing said doctor roller from said applicator means, and means for rotating said doctor roller in a direction so that its surface moves opposite the direction of movement of said applicator means.

1 6. In an apparatus for making copies of a document by electrophotography having a photoconductor, a station for electrostatically charging the photoconductor, an exposing station for subjecting the charged photoconductor to a light and shade image of the document to be copied to form a latent electrostatic image on the photoconductor, a liquid developing station to make the electrostatic image optically visible, and means for providing relative motion of the photoconductor with respect to the charging station, the exposing staton, and the development station, the improvement which comprises a field electrode for subjecting the developed electrostatic image to an electrical field, means for applying a potential to said field electrode of the same potential as the potential of the electroståtic image, and means for moving the developed image past said field electrode.

1 7. Apparatus as in Claim 1 6 in which said developing station comprises a developer applicator roller and means for spacing said applicator roller from said photoconductor so that the surface of the photoconductor and the surface of the applicator roller at their closest approach lies between one and one-half mils and four mils.

18. In an apparatus for making copies of a document by electrophotography having a photoconductor, a station for electrostatically charging the photoconductor, an exposing station for subjecting the charged photoconductor to a light and shade image of the document to be copied to form a latent electrostatic image on the photoconductor, a liquid developing station to make the electrostatic image optically visible, and means for providing relative motion of the photoconductor with respect to the charging station, the exposing station, and the developing station, the improvement which comprises a corona discharge station, means for moving said optically visible electrostatic image while on the photoconductor past said corona discharge station, and means impressing a potential to the corona discharge station of a polarity opposite to the polarity of the visible electrostatic image.

19. Apparatus as in Claim 18 in which said developing station comprises a developer applicator roller and means for spacing said applicator roller from said photoconductor so that the surface of the photoconductor and the surface of the applicator roller at their closest approach lies between one and one-half mils and four mils.

20. In an apparatus for making copies of a document by electrophotography having a photoconductor, means for charging the photoconductor; exposing means for subjecting the charged photoconductor to a light and shade image of a document being copied to form a latent electrostatic image on said photoconductor; means for electrophoretically developing the latent electrostatic image with a liquid developer; means for transferring the developed image to a carrier sheet; and means for moving the photoconductor past the charging means, the exposing means, the developing means, and the transfer means in succession; the improvement comprising interposing a field electrode between the developing means and the transfer means, and means for impressing a potential on said field electrode of the same polarity as the polarity of the developed latent electrostatic image on said photoconductor.

21. In an apparatus for making copies of a document by electrophotography having a photoconductor, means for charging the photoconductor; exposing means for subjecting the charged photoconductor to a light and shade image of a document being copied to form a latent electrostatic image on said photoconductor; means for electrophoretically developing the latent electrostatic image with a liquid developer; means for transferring the developed image to a carrier sheet; and means for moving the photoconductor past the charging means, the exposing means, the developing means, and the transferring means in succession; the improvement comprising a corona discharge device positioned adjacent the photoconductor between said developing means and said transferring means, and means for impressing a potential upon said corona discharge device of a polarity opposite to that of the developed electrostatic image.

22. In an apparatus for making copies of a document by electrophotography having a rotary conductive support, a photoconductor carried by said support, means for charging the photoconductor, exposing means for subjecting the charged photoconductor to a light and shade image of the document being copied to form a latent electrostatic image, the improvement comprising a rotary liquid-developer applicator roller, means for rotating said applicator roller substantially in synchronism with the photoconductor so that the relative motion between the photoconductor and the applicator roller is substantially zero, means for mounting said applicator roller so that its surface at its closest approach to the surface of the photoconductor lies between one and one-half mils and four mils, a supply of developing liquid, means for dipping a portion of said applicator roller into said liquid-developer supply, a doctor roller, means for positioning said doctor roller closely adjacent but out of contact with the surface of said applicator roller, and means for driving said doctor roller so that its surface moves in a direction opposite to the direction of movement of the surface of said applicator roller whereby to meter the thickness of a film of developed liquid on said applicator roller.

23. Apparatus as in Claim 22 including a wiper blade, and means for positioning said wiper blade to wipe the surface of said applicator roller after it has transferred developing liquid to said photoconductive surface.

24. Apparatus as in Claim 22 including a wiper blade, and means for positioning said wiper blade to wipe the surface of said doctor roller before its surface approaches the surface of said applicator roller.

25. Apparatus as in Claim 22 in which said means for rotating said doctor roller comprises variable-speed driving means.

26. Apparatus as in Claim 22 in which said applicator roller is provided with spacing flanges positioned adjacent opposite ends thereof for spacing its surface from the surface of said photoconductor, said flanges being in contact with the surface of said photoconductor.

27. Apparatus as in Claim 22 in which said applicator roller has a resistivity lying between 107 ohm-centimeters and 109 ohm-centimeters.

28. Apparatus as in Claim 22 in which said applicator roller is formed with a textured surface provided with re-entrant portions.

29. Apparatus as in Claim 28 in which the volume of said re-entrant portions lies between 10 billion cubic microns per square inch and 30 billion cubic microns per square inch.