Method And Apparatus For Producing Halftone Electrographic Prints

Abstract

Method and apparatus are disclosed for producing halftone electrographic prints from continuous tone charge image patterns deposited on the charge-retentive surface of an electrographic recording medium. The continuous tone image patterns are developed in the halftone rendition by developing the charge image pattern in the presence of a closely spaced development electrode having a development surface, facing the charge image pattern to be developed, constituted of an array of projections in the form of a halftone screen pattern. The projections concentrate the electric field over the surface of the continuous tone charge image pattern into a halftone image pattern which is developed by the pigment particles of the electroscopic ink used for developing the charge image patterns.

Description

DESCRIPTION OF THE PRIOR ART

Heretofore, halftone electrographic prints have been obtained from continuous tone charge image patterns by rolling a gravure roller over the continuous tone charge image patterns for removing some of the charge leaving a halftone dot pattern charge image to be developed. Such an arrangement is disclosed in U.S. Pat. No. 3,248,216 issued Apr. 26, 1966. While this method may be suitable for use with electrofax-type recording paper, it has been found that electrographic recording paper, which employs a dielectric charge-retentive film supported upon a conductive paper backing, provides an extremely stable charge image pattern from which it is very difficult to extract charge with the gravure roller to produce the halftone pattern. Moreover, it would be desirable to provide a processing method for producing halftone images which did not require separate development and halftone-producing operations or steps.

Other prior art workers have produced halftone electrographic prints by exposing a photoconductor layer through a screen pattern of uniformly distributed, finely divided transparent and opaque areas to superimpose the halftone screen pattern upon the continuous tone pattern of the subject to produce a composite halftone image charge pattern of the subject to be printed. Such a method and apparatus is disclosed in U.S. Pat. No. 2,598,732 issued June 3, 1952. The problem with this method and apparatus is that it requires the use of a separate mask or screen for projecting a screened halftone image pattern upon the photoconductive layer. It would be desirable to provide a method and apparatus for producing halftone prints which did not require the use of the screen for superimposing the halftone image pattern upon the continuous tone image pattern.

Others have employed porous element electrodes for developing continuous tone image patterns, such porous development electrodes being saturated with liquid electroscopic ink for development of the image pattern. Such development electrodes have had a smooth surface adjacent the image pattern to be developed and have acted in the normal manner of a development electrode for enhancing development of continuous tone images. However, such porous development electrodes have not, heretofore, produced a halftone development of the continuous tone image. Such a porous development electrode developing method is disclosed in U.S. Pat. No. 3,096,198 issued July 2, 1963.

SUMMARY OF THE PRESENT INVENTION

The principal object of the present invention is the provision of an improved method and apparatus for producing halftone electrographic prints from continuous tone charge image patterns.

One feature of the present invention is the provision of a development electrode for developing the charge image pattern on the recording medium, such development electrode having a development surface disposed adjacent the image to be developed constituted of a halftone screen pattern of projections for concentrating the electric field over the charge image pattern to be developed into a halftone screen pattern, whereby the image is developed into a halftone electrographic print.

Another feature of the present invention is the same as the preceding feature including the provision of forcing liquid electroscopic ink under pressure through the screen pattern in the development electrode surface and against the charge image to be developed, whereby relatively large area continuous tone image patterns are developed at relatively high speed into halftone prints.

Other features and advantages of the present invention will become apparent upon perusal of the following specification taken in connection with the accompanying drawings wherein:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view, partly schematic, of an electrographic camera,

FIG. 2 is a fragmentary plan view of a recording web,

FIG. 3 is a sectional view of the structure of FIG. 2 taken along line 3--3 in the direction of the arrows,

FIG. 4 is a plot of charge density versus transverse distance for the electrographic charge image produced by the camera and subject of FIG. 1,

FIG. 5 is a schematic sectional view of a development apparatus for developing the electrographic prints derived from the camera of FIG. 1,

FIG. 6 is a plot of pigment particle density versus transverse distance depicting a halftone electrographic print derived from the printer of FIG. 5,

FIG. 7 is an enlarged fragmentary sectional view of a portion of the structure of FIG. 5 delineated by line 7--7,

FIG. 8 is a fragmentary plan view of a development electrode incorporating features of the present invention,

FIG. 9 is a sectional view of the structure of FIG. 8 taken along line 9--9 in the direction of the arrows,

FIG. 10 is a fragmentary plan view of an alternative development electrode of the present invention,

FIG. 11 is a sectional view of the structure of FIG. 10 taken along line 11--11 in the direction of the arrows,

FIG. 12 is a fragmentary plan view of an alternative development electrode structure of the present invention,

FIG. 13 is a sectional view of the structure of FIG. 12 taken along line 13--13 in the direction of the arrows, and

FIG. 14 is a schematic line diagram depicting an alternative development electrode structure of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to FIG. 1 there is shown an electrographic camera 1 for depositing continuous tone charge image patterns of a subject being photographed onto the charge-retentive surface of an electrographic recording medium 2. The electrographic recording medium 2 is best seen in FIGS. 2 and 3 and comprises an electrographic recording web or paper 2 having a conductive paper or web 3 supporting a thin dielectric charge-retentive film 4 from one side thereof.

The electrographic camera 1 includes an optically transparent substrate plate member 5, as of glass, onto which is deposited a transparent conductive electrode 6. A photoconductive layer 7 is deposited over the electrode 6. The electrographic recording web 2 is positioned adjacent the photoconductive layer 7 with the charge-retentive surface 4 facing the photoconductor 7. A conductive backing electrode 8 is disposed adjacent the conductive paper web 3. A suitable source of operating potential 9, as of -600 volts relative to ground, is connected intermediate the conductive backing electrode 8 and the transparent conductive electrode 6 via a timing switch 11. A wedge-shaped partially transparent object 12 to be photographed is disposed between a light source 13 and the transparent substrate 5. The timer switch 11 is actuated for a suitable exposure time, such as a fraction of a second, to apply the voltage derived from the source 9 across the photoconductor 7 and electrographic recording paper 2 to cause an electric charge image pattern to be formed upon the charge-retentive surface 4 of the recording paper 2 in accordance with the intensity of the light image falling upon the photoconductor 7 as transmitted through the slightly light-absorptive wedge-shaped transparency 12. The resultant charge image pattern has a charge density taken across the charge-retentive surface in the direction d as shown in FIG. 4, i.e., the charge density corresponding to the thickest part of the object 12 has the lowest charge density, whereas the charge density is highest corresponding to the thin edge of the transparency 12. In this manner, a continuous tone charge image pattern having a relatively large area is deposited upon the charge-retentive surface 4 of the recording paper 2.

Referring now to FIG. 5, there is shown an electrographic developer 14 of the present invention for developing the continuous tone charge image pattern on the charge-retentive surface 4 of the recording paper 2. The developer 14 includes a conductive baseplate 15 onto which is formed the electrographic recording paper 2 with the conductive paper backing 3 disposed facing the conductive electrode 15 and with the image-bearing surface 4 facing a development electrode structure 16. The development electrode structure 16 includes a development surface portion 17 disposed adjacent the charge-retentive surface 4 of the recording paper 2. The development surface 17 is constituted of a halftone screen pattern of conductive projections 18 projecting from the surface toward the charge-image-retentive surface or layer 4 of the recording paper. The projections 18 are preferably formed by embossing the surface of a porous metallic plate 19 through which liquid electroscopic ink is pumped under pressure from a reservoir 21 via line 22 and pump 23 to a chamber 24 disposed at the back side of the porous development electrode plate 19.

The halftone screen pattern of projections 18 causes the pigment particle density, as deposited from the electroscopic ink onto the charge image pattern, to be as shown in FIG. 6, thereby producing a halftone rendition of the original continuous tone charge image pattern of the type as depicted in FIG. 4. The halftone rendition is believed to occur due to the concentration of the electric field at the projections 18 of the development surface 17, as shown in FIG. 7. More particularly, a thin film of electroscopic liquid ink is formed at the charge-retentive surface 4 between that surface and the halftone screen surface 17 of the development electrode 16. This thin film of developer prevents electrical contact being established between the projections 18 and the charge image pattern formed on the recording paper 2. Some experiments have shown that even if electrical contact is established charges do not migrate from the charge-retentive surface 4 to the electrode. At any rate, it is believed that the projections 18 produce localized concentrations of the electric field causing the charges deposited on the charge-retentive surface 4 to migrate into clusters conforming to the halftone screen pattern of the projections 18. Toner particles are then attracted to the halftone pattern of charge images on the recording web 2, thus producing the halftone print as indicated in FIG. 6.

The projections 18 can be formed on the development surface 17 any convenient way such as by embossing the surface as by knurling, photoetching, grooving or the like.

Referring now to FIGS. 8 and 9, there is shown one of the possible surface configurations 17 for the development electrode 16 wherein the surface 17 is embossed with an array of projections 31 as of rectangular cross section which can be conveniently formed by an orthogonal array of intersecting grooves. Such grooves can be conveniently formed by machining or by photoetching.

Referring now to FIGS. 10 and 11, there is shown an alternative embossed surface 17 of the development electrode 16 wherein an array of elongated land-type projections 32 are formed in surface 17 of the development electrode 16.

Referring now to FIGS. 12 and 13, there is shown an alternative halftone pattern of projections formed on the surface 17 of the development electrode 16, wherein the halftone pattern is formed by a woven screen mesh 33 affixed to the porous plate 19 as by spot welding or sintering. The projections are formed at the crossover points for the interlaced wires of the mesh 33. As described above, a variety of halftone screen patterns may be employed for the embossed surface 17 of the development electrode 16. The projections may be of circular, triangular, or rectangular cross section. They may be disposed in a regular pattern or in a random distribution and may be of various sizes.

Any one of several halftone screen sizes may be employed, the finer sizes generally giving the more natural or higher quality results in the halftone rendition of the continuous tone image. Thus, while a coarse halftone screen having 50 and 60 dots or lines to the linear inch will be useful for some purposes, finer screens such as those having 100, 200, 300, 400 and even more dots or lines to the inch will give more nearly a continuous tone appearance to the finished halftone print. For purposes of comparison, newspaper printing utilizes halftone patterns ranging from about 60 to about 100 times per linear inch and a pattern of approximately 150 dots per linear inch produces an image in which no dots are discernible to the naked eye.

Referring now to FIG. 14, there is shown a high-speed rotary developer 35 incorporating features of the present invention. More particularly, the developer 35 includes a rotatable drum-shaped development electrode 36 made of a porous metallic substrate and embossed on its outer surface with a halftone pattern of conductive projections, as above described. Liquid electroscopic development ink is fed through a hollow axle 37, on which the drum rotates, to a trough 38 disposed adjacent the inside surface of the drum 36 via transverse feed pipes 40. A curved skirt 39 is affixed to the trough 38 and extends in the direction of rotation of the drum 36 from the trough in closely spaced relation, as of 0.010 inch, from the inside of the drum 36. Electroscopic ink is fed via pressure into the trough 38 and is trapped by hydrostatic pressure between the skirt 39 and the inside of the drum 36 to cause the electroscopic ink to be forced under pressure through the porous drum to the outside thereof. Electrographic recording paper 2 having continuous tone charge image pattern deposited on the lower side thereof is fed onto the drum 36 between an elastic belt 41 and the drum 36 such that the recoding paper is pulled into nominal contact with drum 36 as the drum rotates about the axle 37. Belt 41 is driven at the same speed as the drum such that the ink is forced through drum against the charge-image-bearing surface or of the paper for developing same. The developed print is peeled from the drum 36 via a comb structure 42 and thence fed through a pair of squeegee rollers 43 for removing the excess ink. The print is then dried by an air blower, not shown. This type of rotary developer is disclosed and claimed in copending U.S. application Ser. No. 858,044, filed Sept. 15, 1969, assigned to the same assignee as the present invention.

The advantage of combining the halftone image production step with the inking step, as obtained with the method and apparatus of the present invention, is that the separate step of producing the halftone image is eliminated, thereby reducing the number of steps required for the production of halftone prints. Moreover the method is applicable to the development of electrographic recoding papers which comprise merely a dielectric charge-retentive film disposed upon conductive backing and which do not include a photoconductive layer as a part of the paper. Thus, the present method is applicable to the halftone development of both electrofax and electrographic papers, whereas the prior art method of removing charge in a halftone pattern by rolling with a conductive grounded gravure roller is not applicable to electrographic papers which do not include a photoconductive layer as a part thereof.

Since many changes could be made in the above construction and many apparently widely different embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

Claims

What is claimed is:

1. An apparatus for developing a charge image on the charge-retentive surface of a recording medium to form a visible image, the combination comprising:

a development electrode having a development surface positioned adjacent to the charge-retentive surface of the recording medium;

halftone development projections formed on the development surface of the development electrode and projecting toward the charge-retentive surface of the recording medium to a position closely adjacent to the charge-retentive surface for causing the electric field extending between the development electrode and the charge image on the charge-retentive surface to become concentrated about each projection; and

means for applying liquid electrographic ink containing pigment particles into the space between the development surface of the development electrode and the charge-retentive surface of the recording medium, the electric field concentrations causing the pigment particles to deposit on the charge-retentive surface of the recording medium in a halftone pattern.

2. The apparatus of claim 1 wherein the means for applying liquid electrographic ink saturates the space between the development surface of the development electrode and the charge-retentive surface of the recording medium.

3. The apparatus of claim 2 wherein the development electrode is pervious to the electrographic ink, and the means for supplying electrographic ink forces the ink through the pervious development electrode into the space between the development electrode and the charge-retentive surface.