US 3669073 A
Description (OCR text may contain errors)
United States Patent Savit et a1.
ELECTROSTATIC DEVELOPING SYSTEM Inventors: Joseph Savit, Glencoe; Rudolph P. Guzik,
Chicago; Harry A. Wayne, Evanston, Arvind R. Saklikar, Park Forest; Jack M. Van Eek, Chicago, all of 111.
American Photocopy Equipment Company, Evanston, 111.
Filed: April 4, 1969 Appl. No.: 813,531
U.S. Cl. ..1 [8/637, 1 17/37 LE Int. Cl. ..G03g 13/10 Field of Search ..118/637, 637 L, 629, 630, 257,
118/258, 259, DIG. 23; 117/93, 93.4, 37 LE References Cited UNITED STATES PATENTS McFarlane ..l17/l7.5 X 18/637 ..1 18/261 118/637 L UX Marlor et a]. Goldsmith Schaffert 51 June 13, 1972 3,237,277 3/1966 Gallino ..1 18/259 X 3,367,791 2/1968 Lein, ..118/637 X 3,368,526 2/1968 Matsumoto et al ..1 18/637 L UX FOREIGN PATENTS OR APPLICATIONS 565,379 1l/l923 France ..l18/257 OTHER PUBLICATIONS Developing Electrostatic Charge Patterns; T. M. Crawford; IBM Technical Disclosure Bulletin; Vol. 8, No. 4, September 1965; page 527.
Primary Examiner-Mervin Stein Attorney-Richard G. Smith, Richard J. Lustig and Lowell G. Wise 57 ABSTRACT Liquid developer system for electrostatic printing. Latent images carried on a sheet or web are developed by distributing a liquid carrier containing toner particles over a foraminous flexible belt unit. The latent image surface is contacted with the liquid developer carried on the foraminous flexible belt, and excess liquid is removed from the contact area through small openings in the belt surface.
9 Claims, 10 Drawing Figures PATEMEMR 13 m2 A SHEET 1 BF 4 INVENTORS PH SAVIT RU LPH P. GUZIK HARRY A. WAYNE ARVIN LIKAR BY JACK NECK ATTORNEY JOSE 00 D SAK PATWQJUH 33 m2 3. 669.073
sum 2 or 4 F ig. 7
RELATIVE ABSORPTION SHEET 30F 4 WHI E O MAXIMUM ACCEPTABLE BACKGROUND L 1 5 IO I5 20 25 3O TONER CONCENTRATION (CC foner/ liter) 3 669,073 SHEET W 4 mo -BLACK Y E Du G O 5 TWO ZOCQKOWmQ MEFQ EK HI TE PAPER SPEED (cm/sec) Fig. IO 7 ELECTROSTATIC DEVELOPING SYSTEM BACKGROUND OF THE INVENTION This invention relates to the art of electroprinting. In particular, it provides apparatus and processes for developing electrostatic latent images with a liquid developer having solid particles suspended in -a carrier. The advantages of liquid developing have been recognized, especially in process control, development time, compactness of equipment and image quality.
Such printing systems are used in photocopy machines, data printers, facsimiledevices, multi-color printers, etc. Since a common utility for liquid developing of electrostatic patterns is found in office copy machines, most of the description is devoted to these types of machines; however, numerous applications for the invention exist and the teachings of this disclosure may be applied to other systems by a skilled worker.
A typical liquid electroprinting apparatus comprises (1) a photoconductive element which is capable of retaining an electrostatic charge on its surface which charge is removed from selected areas by exposure to electromagnetic radiation, such as a paper sheet or web coated with zinc oxide in a resin binder; (2) means for creating a uniform electrostatic charge over the photoconductive surface, e.g., a corona; (3) means for altering the charge in desired areas, such as an optical exposure system to form a latent image; (4) means for directing the surface bearing a latent electrostatic image in contact with a liquid developer composition; and (5) means for separating the developed surface'from the liquid developer. The present invention is directed specifically to the developing step in which the latent electrostatic image iscontacted with a liquid carrier having finely divided opaque particles which are attracted to the charged areas of the latent image and deposited on the surface.
The common types of liquid developing machines use an immersion technique in which a copy paper bearing the latent image is dipped into a tank containing a volatile hydrocarbon liquid carrier with suspended carbon particles. Total immersion of the copy paper results in excessive pickup of developer liquid, and longer drying time is needed to remove the volatile carrier from the developed copy. Also, contacting the unimaged back side-with liquid developer is objectionable because of retention of developer particles which contact the back side. In previous liquid developer systems using the im mersion tank method, the use of a sealing coat has been necessary to provide a barrier for preventing particle accumulation on the back side.
While electrostatic printing can be applied to numerous combinations of background and toner colors, this discussion and the description of the invention will be directed primarily to the development of latent images on a white paper'with black toner particles. A goal of many workers in the electroprinting or electrophotography field is to produce an office copy machine or the like which is capable of reproducing a variety of materials. Line copy such as typewritten material can be reproduced with sufficient fidelity by many electrostatic systems; but half-tones or continuous tones, such as present in photographs, are a much greater challenge to the art.
Deposition of particles onto an electrostatically charged surface is effected by having opposite electrical charges between the imaged areas and the particles in suspension. In a typical liquid developer system an inert liquid carrier of high .resistivity has small particles of opaque material dispersed in it. These toner particles usually include a pigment with a resin, which functions as a charge director and adhesive fixer. An electrical charge is developed due to surface coating phenomena as well as differences in the dielectric properties of the carrier and toner materials. When an electrically insulating surface receives an electrostatic charge of opposite polarity to the toner particles; as byv passing the surface through a corona, the toner particles are attracted to those portions of the surface which retain the electrostatic charge.
This attraction results in deposition of particles on the surface, with most particles being deposited in those areas having a greater electrostatic charge. Some particles are adsorbed in unchanged areas and toner particles will mechanically adhere to completely uncharged sheets. In addition, the depositable particles in present commercial developers exhibit some of the properties of dipoles and consequently the shape of the field or, the field gradient, is as important as'the average field strength of the latent image areas.
At the edge of a large latent image area the change in the field is at its maximum, and toner particles are ordinarily deposited more heavily in such areas. This phenomenon is known as edge enhancement, which represents a difficult problem in obtaining good copies for those patterns having large black areas. To render the charged area equally attractive to particle migration, an intensifier can be placed in contact with developer liquid and in close proximity to the charged surface during development. This intensifier is an electrically conductive structure which shapes the filed by creating separate. fields between the intensifier and each of the charged points on the latent image surface. The fields become geometrically normal to the surface. The intensifier can be grounded, biased with a direct current potential, or insulated electrically from the rest of the developer unit.
A second major problem in liquid developing of electrostatic latent images is that of local depletion of toner particles in the area of heavy deposits. In order to prevent excessive accumulation of particles on the uncharged or weakly charged background on the copy paper, liquid toner compositions are usually rather dilute. Typical developers contain only a few grams of depositable particles per liter of liquid. Accordingly, increasing the developer concentration to compensate for local depletion and increase the black density of copies is usually attended by increased particle deposition in the background areas.
SUMMARY OF THE INVENTION It has been discovered that electrostatic images can be developed by contacting the surface bearing the latent image with a liquid carrier having suspended toner particles carried on a moving foraminous surface and having an integral or closely adjacent intensifier. In the apparatus and process of this invention developer liquid is distributed evenly between the image-bearing surface and a developing area having a network of small holes which permits excess developer carrier and particles to be removed from the interface between the charged surface and the foraminous surface while providing a controlled volume of liquid developer in reservoirs adjacent the developing copy. The spacing of the intensifier from the charged surface can be closely controlled to obtain even deposition throughout large black areas. This is accomplished by constructing the developer system with a conductive foraminous surface or supporting a non-conductive porous surface with the intensifier structure. The foraminous contact surface is constructed to move along with the sheet or web of copy paper by providing a continuous band in the form of a belt. This system results in high developing speed, good black density, even distribution of toner particles in large black areas, and low background levels.
THE DRAWING FIG. 1 is a schematic side view of a developing system, showing a belt-type liquid contact section, copy feed and separation units;
FIG. 2 is a cross-section view of an alternative drum-type developer system;
FIG. 3 is a side view of the developer drum of FIG. 2, partially cut away;
FIG. 4 shows a further modification in cross-section of a drum-type developer having internal liquid distribution;
FIG. 5 is an enlarged scale cross-section view of a latent image-bearing paper in contact with a wire mesh foraminous surface during developing;
FIG. 6 a cross-section view similar to FIG. 5, showing a perforated plate intensifier and integral foraminous support;
FIG. 7 is a similar cross-section view to FIG. 5, showing a non-conductive foraminous cloth carrying liquid developer over a metal intensifier plate. The dimensions in several figures are exaggerated to display the inventive concept more clearly. i V
FIG. 8'is a schematic side view of another embodiment of a belt-type developer system;
FIG. 9 is a graphic plot of light absorption vs. toner concentration; and
FIG. 10 is a graphic plot of absorption vs. developing speed. DESCRIPTION OF TI-IE INVENTION The primary'functions of the liquid developing systems according to the present invention include the shaping of the electrostatic field during particle deposition to effect a controlled point-to point accumulation of toner solids in which adjacent charged areas in the latent ir'nage have a minimum influence on one another, and providing a sufficient reservoir of dispersed toner particles to compensate for local depletion at the interface between the liquid developer and heavilycharged image areas. In order to achieve these objects, a careful consideration of the properties of eachcomponent of the system is valuable.
The developer liquid itself has two'main components, a carrier liquid and suspended solid opaque particles. In a typical system the carrier liquid consists essentially of an inert organic material having high electrical resistivity (low conductivity); the more common carriers, such asmineral spirits, have a resistivity value in the range of about 10 to 10" ohm-cm, with l to I0 ohm-cm'beingpreferred. In order to promote high electrophoretic mobility of the charged toner particles during the deposition process, low viscosity is also desirable, especially for high-speed printing systems. Numerous desirable carriers exist which have viscosities in the thinner ranges, usually from less than a centipoise to several centipoises. Another property'which should be considered is the wetting of the image-bearing copy surface and foraminous surface by the carrier. This is usually no problem where ordinary liquid carriers and materials of construction are used, but there should be sufficient affinity. between the carrier liquid and foraminous surfaces to assure an adequate supply of developer liquid in the reservoirs of the foraminous surface adjacent the latent image.
The developer particles for use with high resistivity organic carriers are preferably mixtures of solid pigments and charge directing resins with a particle size up to a few microns. Stable dispersions of carbon blackparticles in mineral spirits can be formed with the aid of dispersants. The preferred particle sizes are less than one micron average size; but, particles larger than one micron can be tolerated if no plugging of the system results. .Typical developer compositions including carbon black, asphalt and resin, are described in U. S. Pat. No. 3,399,140. Other finely divided opaque particles may be used provided they give the necessary deposition density and can migrate electrophoretically in a liquid carrier. The high electrical resistivity of .the liquid carrier permits suspended colloidal particles to retain electrical charge, and these suspended particles migrate toward the oppositely charged areas of the latent image by electrophoresis. Also, the high resistivity of the carrier prevents discharge of the latent image areas. This attraction between toner particle and image surface is sufficient to hold the deposited particle until it is fixed. Even when the developed image is subject to rapid movement of fluid, most of the deposited particles adhere in the desired areas.
The advantages of this invention can be achieved with several modifications of the foraminous band. Since the image-bearing surface is usually copy paper having thin cellulosic substrate and a photoconductive coating, the flexibility of the 'copy can be used in the design of mechanical components. The continuous geometric form can be a cylindrical drum or a flexible belt. The desired-control of the electrostatic field shape can be achieved either with a non-conductive band in combination with a conductive intensifier or-with an in tegral structure in which the foraminous band is constructed of an electrically conductive material, such as metal, low-resistance elastomers or carbon fibers. The foraminous surface can be provided by weaving the belt or drum, or by a perforated sheet. The opening size is importantbecause excess liquid developer having toner particles is removed from theinterfacial area between the copy and the band. by passing through the pores of the band. The openings must not be so large as to prevent'the formation of the liquid reservoirs in the openings of the band. The precise maximum size-is determined by the surface tension of the liquid on the band material, thickness of the foraminousstructure and superficial ratio of solid areas to open areas. For most developer compositions, a toner particle size is about 5 to 20 microns, but high quality liquid toners have mostly submicron particles; It has been found that image quality deteriorates when-theopenings are larger than about 450 microns. For a drum-type developer system, the preferred pore size is 35 to 400 microns, which corresponds to about 400 to.40 mesh size (U. S. Sieve). However, for a belt-type system, the preferred range is about 35 to 150 microns. The ratio of open area to closed area is usually about 9:1.to 1:9.
Referring to the drawing, in FIG. 1, a liquid developing system is shown where a copy paper l0 bearing an electrostatic latent image is fed into the developing section by means of a vacuum plenum roll 15 and guide plate 16. The imagebearing paper enters housing 18 and advances along a foraminous belt 20. Means for driving the foraminous band or belt 20 co-directionally with the advance of the image-bearing surface is provided by cylindrical rolls 22 and 22',.which drive the belt at a linear speed at least that of theadvancing paper 10. A metal intensifier plate 25 contacts the interior of belt 20 closely adjacent to the path of the latent'image-bearing surface. The intensifier may be flat or convex and itslength is substantially coextensive with the paper-liquid contact area of the developer system. The belt is wider than the extremities of the sheet or web 10, to assure development of the entire image. A developer tank 30 is positioned under the foraminous belt 20 and a desired amountof liquid developer composition is maintained in the tank,by liquid level sensing or other means for controlling quantity. Liquid developer is distributed onto the foraminous surface 20 by a'flow control system comprising a pump 34 and spray header 35 having a plurality of holes spaced laterally across the width of the belt 20. The fluid pumping rate need not be closely controlled pro vided there is sufficient liquid developer provided to completely develop the electrostatic images. One of the main advantages of this invention is the inherent regulation of liquid developer which results from this system. The even distribution of carrier and toner particles is achieved by the flow of excess liquid developer through the pores of the belt 20. This is observed when the liquid flows over the band surface and is removed by gravity or by pressure of the copy paper on the band. In the embodiment of FIG. 1, the liquid developer is introduced between the nip of the copy paper 10'and the belt 20. If a large excess of liquid is delivered, further removal can be obtained by perforating the intensifier plate 25. While there is no critical relationship beyond the minimum amount of developer liquid supplied, a flow rate of about 4-12 milliliters of liquid developed per minute per square cm. of band area is satisfactory for a typical system using a mesh foraminous surface and a linear developing speed of 10-30 cm/sec.
The proper amount of liquid developer adheres to the belt, forming liquid reservoirs in the intersticial areas between the solid portions of the belt and creating a thinfilm of liquid at the interface between the image-bearing surface 10 and belt 20.
When local depletion of the particles from the film adjacent areas of heavy electrostatic change occurs, the toner particles migrate electrophoretically from the reservoirs in fluid communication with the thin film and reconstitute the particle concentration in the film.
The developed copy is separated from the belt and liquid developer by exit rolls 40 and 40. The exit rolls maybe constructed of metal, rubber or combinations of these. These rolls co-act to squeeze the remaining liquid from the developed copy 10. A resilient wiper'42 and trough 44 collect the excess liquid developer and return it to the developer tank 30. Toner concentrate and carrier are added to the tank to maintain the proper quantity and concentration of developer. The level of tank 30 may be adjusted below roll 22 so that the entire supply of developer liquid is distributed through the circulation system 34, 35 or a higher level can be used to provide a pickup of carrier and toner particles by the foraminous surface of belt 20 as it passes through the tank in its rotation cycle. After squeezing, the developed copy is deflected upwardly by a hot air stream from blower 46 and dried by infrared heaters 50. FIG. 1 shows the development of sheets, but a continuous web of electrostatically charged material can be handled as well.
An advantage of the belt configuration for the continuous band is the ability of this developertohandle rigid image-bearing surfaces. Stiff sheetsof paper stock or any non-conductive substrate capable of carrying an electrostatic charge can be developed by arranging the belt and feed means in a straight line which can be vertical, horizontal, or inclined. A relatively long contact time is provided, even at high speeds. Development speeds of about 1 cm. to 1 meter per second can be achieved with the apparatus and process of this invention.
A second important embodiment of the invention is shown in FIGS. 2 and 3, where a flexible copy paper 10 is fed to the developer section by guide rolls ISA and 158. The imagebearing surface enters the development contact zone tangential to a foraminous surface A, comprising a woven metal screen held on a support roll 23. A helical groove 23 between the body of inner roll 23 and screen 20A permits excess liquid developer 32 to be removed from the interfacial development zone. The flexible copy paper 10 is guided around the cylindrical drum 20A by a plurality of support ribs 38. These ribs are located within tank 30 and are spaced apart from the drum surface 20A. The spacing can be varied by adjusting the distance between the ribs and the foraminous roll surface. Liquid developer 32 is distributed over the surface of the drum surface through header 35. The position of the liquid distributor is not critical, but best results were obtained when the liquid stream contacted the drum surface along a lateral line about midpoint above the radial center and periphery of the drum. A'portion of liquid 32 flowstoward the interior of foraminous screen 20A, along a channel formed byhelical groove 23, and over the side edge of the paper. Reversal of direction for the flexible paper as it passes through the development section provides a liquid trough, and a portion of the liquid developer is retained on the outer surface of the drum 20A and the excess flows laterally over the sides of the trough into the tank 30. The exit squeeze rolls 40 and 40' are arranged to maintain a contact angle of about 90 to 190 around the lower halfofthe drum.
Eccentricity in the drum surface can cause uneven deposition due to the alterations in the electrostatic field caused by variable intensifier distance from the charged paper. Surface deviations of more than one-half percent of the roll diameters should be avoided.
The support roll 23 can be constructed of a perforated metal tube having large holes (0.2-1 cm. diameter) uniformly spaced around the cylindrical surface to permit the flow of liquid from the mesh cover 20A. Thetype of configuration shown in FIG. 2 and 3 is useful for supporting 90 mesh screens.
A further modification of the invention is shown in FIG. 4, where a self-supporting'wire mesh tube 20A is formed into a cylindrical drum. Liquid developer 32 is distributed to the foraminous band surface by forcing a spray from header 35 mounted internally parallel to the axis of rotation of the drum. Unlike the structures in FIG. I and 2, this light weight drum requires no independent drive means. The frictional drag of the paper passing around the curvature of the support ribs, as in FIG. 2, carries the drum at about the same speed as the image-bearing surface. To prevent clogging of the wire mesh, filtering of the liquid developer supply is sometimes necessary to remove lint, etc. Other than the omission of a drive mechanism, this structure can be used in apparatus similar to FIG. 2.
The enlarged cross-section view in FIG. 5 shows a woven wire mesh foraminous surface 20A supported on a grooved support 23, 23' as in FIG. 2. A quantity of liquid developer 32 is carried by the wire mesh 20A in the intersticial volumes between the wires, with excess liquid being removed through grooves 23. An image-bearing surface is in direct contact with the liquid developer. A paper sheet 10, is coated with a layer 11 of photoconductive material, such as zinc oxide in a binder. The interconnected points on the wire mesh serve as an integral intensifier.
The liquid carrying capacity of the wire mesh was measured for the roll configuration shown in FIG. 2, 3 and 5. A support tube was cut with a helical groove (No. l lNClhaving a flat land spacing of about 0.2 mm. between grooves. A mesh stainless steel screen was laid over the grooves. The drum diameter was about 5 cm. and the length was about 25 cm.
This drum structure was immersed in a typical liquiddeveloper diluted with mineral spirit carrier. The mesh plus grooves held about 50 ml. of liquid, while the mesh held about 15 ml. after draining the helical groove. Dividing this value by the superficialsurface area of the foraminous surface gives a unit liquid carrying capacity of about 0.04 ml/cm Another metal surface is shown in FIG. 6, where a perforated plate has closed land areas 208 and open holes 20C. This type of structure may be formed as a shell or belt, depending on the relative flexibility of the material. Excess liquid developer 32 flows throughthe holes 20C and is collected below. The liquid wets the outer surface ofthe plate and the electrostatically charged surface 11 is borne on a thin liquid film. If the concentration of toner particles is depleted in the film adjacent heavy deposition areas, electrophoretic migration of particles from the reservoirs'20C offsets this local depletion.
The modification shown in enlarged cross section in FIG. 7 is the combination of a non-conductive belt and metal intensifier plate depicted in FIG. 1 of the drawing. A woven fabric 20 such as wool or a synthetic fiber rides over a separate intensifier 25. The liquid carrying capacity is determined primarily by the woven fabric 20. A close spacing is maintained between the charged surface 11 and the intensifier 25 by the combination of fabric thickness and liquid film. The relative thicknesses of elements shown in FIGS. 5-7 are not shown to scale.
The developer system shown in FIG. 8 employs a belt type foraminous band 20 and a multiple-stage liquid distribution means 35A, 35B adapted to supply toner liquid at spaced points along the travel path of the copy paper 10 during the development step. The flexible belt 20 of a woven or knitted porous material is supported by an intensifier plate 25 across the major portion of the effective development zone. The copy paper 10 is fed to the developer by rolls 17,17 and travels co-directionally with the rotary movement of the foraminous belt 20. A portion of the liquid toner is applied to the under-side of the belt 20 by a first liquid distributor means 35A, located adjacent the forward end of the development zone. The liquid is applied to the belt surface by forcing the toner liquid through a header with spaced holes, upward through openings in the intensifier plate 25. The fluid pressure should be such that the liquid toner emerges through the upper surface of the belt 20. As the copy paper 10 advances across the development zone, a second liquid supply stage 353 replenishes the developer liquid with fresh toner. The excess liquid is removed by squeeze rolls 40, 40. The flexible belt 20 is guided by rolls 22 and 22' in cooperation with a third roll 23, which can be adjusted to give the proper tension to the belt. The rotary drive power can be supplied through any or all of these rolls. The multiple stage liquid distribution system can use a number of different fluid handling conduit arrangements. If a very high development speed is desired, the dwell time can be controlled by using a relatively long contact zone having several entry points for the developer liquid.
A series of experiments were conducted to compare the operation of the foraminous carrier and intensifier developing system 'to that of a standard immersion tank developer. These tests were conducted under standard conditions using two commercial electrostatic office copy machines (Apeco Super-Stat) and commercial toner concentrate, diluent, and electrostatic copy paper. The only'modification made in the.
machines was to replace the standard immersion tank developer with a cm. dia. developer roll mechanism and tank as shown in FIG. 2. In the machines the electrostatic image is formed by passing the coated paper, through a corona dischargeand exposing the uniformly charged surface to light in selected areas by projecting an optical image onto the charged surface. After-developing and dryingthe copies, measurements were made on a light absorption meter to determine how the copy background and image black density for the foraminous band system compared with the background and black of the original. The relativeabsorption is a measure of the light absorption changes from a white copy paper (0 percent) to a completely coated blackarea 100 percent).
These tests were made using a standard paper speed of 10 .cm/sec. Line A shows the increase in background deposition for the improved foraminous belt system while the higher line A shows background for the conventional immersion system. The maximum acceptable background (A',) is reached at cc/l for the conventional system, while the improved system can use a toner concentration of cc/l (A to retain this maximum, and has a lower background (A at the 15 cc/l concentration. I
The measurement of absorption at the center of large black areas 1X 2 cm.) demonstrates the uniformity with which such patterns are developed in the improved system. At the 15 cc/l concentration, absorption for areas, developed by the improved system (point B on line B) is about 50 percent greater than for thecorresponding absorption (point B for the immersion' system. This improved uniform density may be explained by the availability of toner particles in the intersticial areas of the foraminous band adjacent the latent image. The
reduction in background deposition is due to the direction of toner to the charged areas and to increased turbulence at the surfaceof the copy. I
An experimental device was constructed to establish the advantagesof the improved developer for changesinlinear paper velocity. The copies were charged and illuminated at standardoperating speed and then released to fall through a guide into either a standard immersion developer tank with paper feed rollers, or the improved foraminous band developer mechanism as illustrated in FIG. 2.
Both developers were capable of driving the copy paper at speeds betweenScm/sec. and 40 cm/sec. and the developer path length was the same. The toner concentration was adjusted for each system to obtain equal background levels. Since the improved foraminous band system produces low background levels, the toner concentration was about twice that of the conventional immersion developer to produce equal background levels. In FIG. 10, this level is shown as being equal for both the immersion system and improved system with the background level corresponding to point A of FIG. 9. After developing and drying the copies, the image area and background levels were measured with a light absorption meter. FIG. 10 illustrates the results of increasing the paper speed through the developer tanks. Both the standard developer and the improved developer show similar rates of decrease in black density as speed increases.
Line E shows the relative absorption for line copy made by the improved roll-type developer (FIG. 2). Line E shows that deposition is considerably less for the conventional immersion type developer. Line E" represents a similar test of the belttype improveddeveloper system (FIG. 8 although line E" was taken from copy developed in a longer development zone where the dwell time was about twice that of the copies for lines E and E. Lines F and F represent the relative absorption at the center of large areas for the improved and immersion systems, respectively; In each of these tests, the improved system produces about the same shape of the curve for absorption vs. linear speed, but the improved systems result in greater deposition of toner particles.
The standard prior'art immersion system has a practical limit to paper speed at about 20 to 25 cm/sec. This is the result of the mechanical difiiculties of forcing the paper through a curve while immersed in fluid. In the improved developer, the foraminous carrier assiststhe paper andcontrols its path as well, which allows paper speeds in excess of 40 cm/sec. to be practically attainable.
In addition to the advantages illustrated graphically in FIGS. 9 and 10, the improved developer allows the copy to be completely dry on the reverse side. This means that the copy carries only half of the fluid normally'adhering to the copy and, therefore, only half as much fluid needs to be evaporated in drying. This also contributes to a distinct improvement in efficiency. 1 V a While the invention has been described with reference to specific examples, there is no intent to limit the inventive concept except as set forth in the following claims.
We claim: I I
1. In a liquid developing system wherein latent electrostatic images are developed by contacting a latent image-bearing surface with a liquid developer composition having developer particles suspended in a liquid carrier, the improvement which comprises:
a continuous flexible belt liquidlcontact means having substantially uniform foraminous surface with openings sufficiently large to pass developer particles and with reservoirs for carrying liquid developer;
means for advancing the image-bearing surface into direct contact with liquid developer carried on the foraminous surface;
means for rotating the belt co-directionally with the advance of the image-bearing surface;
means for distributing liquid developer on the foraminous surface to provide developer particles suspended in the reservoirs adjacent the image-bearing surface;
electrostatic field shaping means in electrical contact with developer closely adjacent to the image-bearing surface and substantially co-extensive with the effective area of contact between the image bearing surface and developer liquid distributed in the reservoirs of the foraminous surface;and
means for separating the developed image-bearing surface from the flexiblebelt and removing excess liquid.
2. The liquid developing system of claim 1 wherein the flexible belthas openings of about 35 to l 50 microns average size.
3. The liquid developing system of claim 1 including means for collecting excess liquid developer from the continuous band in a developer tank below the flexible belt, means for maintaining a predetermined amount of liquid developer above the flexible belt between the foraminous surface and the image-bearing surface.
4. The liquid developer system of claim 1 wherein the foraminous surface consists essentially of a electrically conductive structure having electrically interconnected land areas with discrete openings to provide liquid reservoirs in the interstitial areas between the land areas.
5. The liquid developer system of claim 4 wherein the superficial ratio of land area to open area is about 1:9 to 9:1.
6. The liquid developer system of claim 4 wherein the foraminous surface consists essentially of a woven metal-containing cloth fabric of about 40 to 400 mesh.
7. An electroprinting process comprising the steps of:
a. providing a flexible belt of porous material having a uniform pattern of small openings;
b. introducing a liquid developer composition comprising a comprises:
a continuous flexible belt liquid contact means having a substantially uniform foraminous surface with openings of about 35 to 450 microns average size forming reservoirs high-resistivity carrier with toner particles dispersed for carrying liquid developer, said flexible belt comprising therein onto the flexible belt; a woven fabric of non-conductive fibers;
c. feeding copy sheet having an electrost'atically charged means for advancing the image-bearing surface into direct surface along the flexible belt with the charged surface in COHtaCt i h qui de lope rri On he f raminous direct contact with liquid developer carried on the flexi- Surface; ble belt; means for moving the flexible belt co-directionally with the d. rotating the flexible belt substantially codirectionally with advance of the image-bearing Surface;
the copy sheet surface at a speed at least as great a the means for distributing liquid developer on the flexible belt copy surface; to provide developer particles suspended in the reservoirs e. removing excess liquid developer between the charged adjacent the image-bearing Surface;
surface d fl ibl b l to create a i m f liquid aconductive intensifier in electrical contact with developer developer and maintaining a reservoir of liquid developer Closely adjacent to the image'bearing surface and i h ll openings f h fl xibl belt in liquid Contact stantially co-extensive with the effective area of contact i h h hi fil between the image-bearing surface and developer liquid f migrating particles h h h thin fil and distributed in the reservoirs of the foraminous surface, trophoreticany depositing the panicles on the charged said intensifier contacting the nterior portion of the belt f along a flat area opposite the image-bearing surface durg. controlling an electrostatic field shape during particle mg developmenfiand deposition with an electrically conductive intensifier; and means for SePafatmg the develop ed E F Q E Surface h. separating the developed copy sheet from the flexible belt from flexlble l n removing excess liquid and liquid developer. 9. The liquid developing. system of claim 8 wherein liquid developer is applied directly into reservoirs on the moving belt immediately before the image-bearing surface contacts the liquid developer at the surface of the moving belt.
8. In a liquid developing system wherein latent electrostatic images are developed by contacting a latent image-bearing surface with a liquid developer composition having developer particles suspended inaliquid carrier, the improvement which