US2877132A - Method for development of electrostatic images - Google Patents

Description

March 10, 1959 E. P. MATTHEWS 2,

METHOD FOR DEVELOPMENT OF ELECTROSTATIC IMAGES Flled Feb. .18, 1955 2 Sheets-Sheet 1 SENSITIZATION TRANSFER T (CHARGING) EXPOSURE DEVELOPMEN OR FIXING POWDER CLOUD CHARGING POWDER CLOUD GENERATOR \O l2 l4 13 ll FIGB lNVENTOR EARNEST PAUL MATTHEWS ATTORNEY March 10, 1959 E. P. MATTHEWS 2,877,132

METHOD FOR DEVELOPMENT OF ELECTROSTATIC IMAGES Filed Feb. 18, 1955 2 Sheets-Sheet 2 FIG. 4

\ POWDER I POWDER CLOUD CLOUD GENERATOR CHARGING lNVENTOR EARNEST PAUL MATTHEWS I ATTORNEW United States Patent METHOD FOR DEVELOPMENT OF ELECTROSTATIC IMAGES Earnest Paul Matthews, Alliance, Ohio, assignor, by mesne assignments, to Haioid Xerox Inc., Rochester, N. Y., a corporation of New York Application February 18, 1955, Serial No. 489,083

4 Claims. (Cl. 117-175) This invention relates to a method and apparatus for the development of electrostatic latent images.

In xerography it is usual to form an electrostatic latent image on a surface. One method of doing this is to charge a photoconductive, insulating surface and then dissipate the charge selectively by exposure to a pattern I of activating radiation. Other means of forming electrostatic latent images are set forth in U. S. 2,647,464 to James P. Ebert. Whether formed by these means or any other, the resulting electrostatic charge pattern is conventionally utilized by the deposition of an electroscopic material thereon through electrostatic attraction whereby there is formed a visible image of electroscopic particles corresponding to the electrostatic latent image. Alternatively, the electrostatic charge pattern may be transferred to an insulating film and the electroscopic particles deposited thereon to form the visible image. In any case, this visible image, in turn, may be transferred to a second surface to form a xerographic print.

The process of depositing the electroscopic powder on the electrostatic image to render the electrostatic image visible is called the development step and is one of the most critical steps of the entire process. The step is of particular importance both on machines designed for continuous operation with any type of copy and in processing continuous-tone images. Now, in accordance with the present invention, a method and an apparatus are provided for substantially improved development of an electrostatic latent image.

Fig. 1 of the attached drawings is a block diagram showing the position of the development step in an overall xerographic process which results in a visible image.

Fig. 2 is a diagrammatic side elevation in cross sec tion of apparatus according to one embodiment of the invention showin gthe initial step in development using this embodiment.

Fig. 3 is a diagrammatic side elevation in cross section of the same embodiment showing the second step in development.

Fig. 4 is an isometric drawing of the same embodiment with a cutaway section.

Fig. 5 is a diagrammatic side elevation in cross sec- 'tion of apparatus according to another embodiment or the invention.

Fig. 6 is an isometric drawing of the embodiment shown in Fig. 5 with a cutaway section.

As shown in Fig. 1, the general xerographic process involves the formation of an electrostatic latent image. This is generally, although not always, preceded by a treatment to sensitize the surface on which the electrostatic image is to be formed. The electrostatic latent ice vclopment step permits. About the coarsest type of image reproduced by a xerographic process requires a resolution of at least about 50 lines per inch. Commercial, line-copying machines generally have a resolution power of about to 250 lines per inch. The process used in obtaining this resolution is set forth in U. S. 2,618,552 and involves the use of a finely-divided, colored material called a toner deposited on a slightly more coarsely-divided material called a carrier. This twocomponent developer is cascaded across the electrostatic latent image areas. The control of the concentration of toner in the two-component developer becomes extremely diificult in thecontinuous operation of such line-copy machines. When applied to continuous-tone development where resolutions of about 50 or more lines per millimeter are often desired, it has been found impossible to obtain this high quality of reproduction using such a system. Accordingly, a system known as powder cloud development is preferred. This is the system incorporated in Fig. 1.

As shown in Fig. 1, a powder cloud is generated. The cloud so produced is then charged, the charging step constituting either a separate step or utilizing the inherent charge on the cloud in the case of a cloud of dry particles. A cloud so generated and charged is then contacted with the electrostatic latent image in the development step. The visible image so produced may be used as such, permanently afiixed to the plate, or many be transferred to another material as a sheet of paper or plastic, as is well-known to those skilled in the xerographic art.

The instant invention relates to a process and apparatus therefor which represent a substantial improvement in the art of powder cloud development. Many of the methods of powder cloud development used heretofore have been characterized by various difficulties, thus: When the powder cloud in its flow travels over an area that should be white and then over an area that should be dark, the leading edge of the dark area is not developed. On the developed image the dark area appears to be torn so that a white background shows through. Because of this appearance the flaw is referred to as tearing. Another difiiculty occurs when the powder cloud travels over a large, dark area and then over a large area that should remain white. Although the white area is void of charge, it does not remain free of powder. Developer particles deposit in streaks through this area in amounts roughly proportional to the length of the dark area that immediately precedes the white area. This difiiculty is termed streaking. Still another difficulty is that developed images are not uniformly developed end-to-end nor does extending the development time remedy this defect, as apparently part of the charge in the image areas at one end of the plate are erased in the development process. The process of the instant invention represents substantial improvement on all these. In general, the present invention accomplishes these objectives by reversing the flow of the powder cloud over the image areas during the development step. The proc ess of the invention and apparatus for its accomplishment will now be set forth in more detail.

According to the present invention, a cloud of electroscopic particles is generated in a suitable cloudforming device; for example, as by agitation of a powder mass in a closed container with a rotating brush, as in U. S. Patent No. 2,357,809 to C. F. Carlson, or a rotating air mass as caused by laterally-located nozzles. If a cloud of liquid droplets is desired, it may be produced by any means known to those skilled in the art as by spraying through an atomizer. The cloud by whatever means produced is then charged. Where a cloud of dry particles is used, any method of generating the cloud will almost necessarily produce a charge thereon. For some purposes, the charge so produced will be adequate. If it is desired to produce a more uniform charge, however, other devices, such as triboelectric charging (as by passing the electroscopic powder through anarrow tube of suitable material in turbulent flow), corona charging or other charging device, may be used. In the case of a cloud of liquid droplets, charging means such as induction charging, corona charging, and so forth, may be used. The charged cloud is then optionally passed into a collecting zone and then through suitable valving means into the development zone alternately from each end.

The development zone is the area between the development electrode and the'image-bearing surface. The development electrode draws the field of force of the image externally'above the image-bearing surface. To do this, the development electrode should be no further from the image-bearing surface than Mr" and, desirably, should be no further than .4:". For high quality resolution, the spacing should be no more than ,4 course extend over the entire image surface. The charged electroscopic particles are passed over the imagebearing surface within the field of force. Passing the powder cloud once from each end through the development zone is sufiicient to realize the advantages of the instant process. If desired, however, the direction of flow may be alternated several times in the course of development with some improvement resulting from the greater number.

The general nature of the process of the invention having been set forth, specific embodiments of the invention will now be described. The invention, however, is not limited to these embodiments which are presented necessarily for purposes of illustrating means of working the instant invention.

As shown in Fig. 2, the apparatus illustrated contemplates a support plate having entrance holes 13 and 14 and exit holes 11 and 12. Superimposed on the support plate in movable relation thereto is a base plate 15 also having entrance holes 18 and 19 and exit holes 16 and 17. Positioned thereon are spacing means 20 defining left and right chambers 21 and 22. 7 On top of the spacer is the development electrode 23 having slots therein, 24 and 25, which may serve as either entrance or exit slots. Positioned closely adjacent to the development electrode by means of spacers 26'is an electrostatic image- 'bearing member comprising a photoconductive insulating layer 29 coated on a conductive backing 27. Stop plate 28 aligns the assembly.

In operation, as shown in Fig. 2, which depicts the first step of development, the powder cloud flows through entrance slots 13 and 18 into right chamber 22 where it expands and then flows through slot 24 into the development zone 48 between the development electrode and the image-bearing member. leaves by means of slot where the cloud passes into chamber 21 and then through the exit slots 17 and 12. This position is maintained for no more than one-half the development cycle.

Then the assembly is shifted as shown in Figure 3. This may be done by holding the support plate stationary and moving the rest of the assembly (as was done here), or by moving the support plate (which requires flexible tubing to carry the powder cloud to and from the apparatus) and holding the rest of the assembly stationary. In this position the powder cloud now enters through entrance holes 14 and 19 into chamber 21 where the cloud fans out, flows through slot 25 into development zone 48, leaves the development zone through slot 24, enters chamber 22, and leaves through exit holes 16 and 11. This cycle may be repeated as often as desired in the course of the development of the electrostatic image. By this means the'direction of flow of the powder cloud with respect to the xerographic 'plate being developed may be reversed as often as desired in the course 'of development.

It should, of

Passing through this zone, it

ase'a se H In this embodiment of apparatus according to the invention the left and right chambers 21 and 22 afford additional advantages beyond those resulting from the process of alternate development from each end of the development zone. Thus these chambers provides a smoother and more uniform powder cloud as it is introduced into the development zone with resultant improvement in print quality. In addition, the chambers in combination with the slot exhaust promotes a more uniform development.

Fig. 4 shows an isometric view of the same development apparatus.

Fig. 5 shows another embodiment of the invention wherein valving means are substituted for physical move ment of surfaces.

In the apparatus shown, the powder cloud is generated 42, charged 46 and fed to a two-way valve 31 by a tube 43. The valve diverts the powder cloud to one of the two exit tubes, say, the right-hand tube 3. The powder cloud flows through this tube to the entrance slot 44 where the air pressure exerted by the cloud forces up gate means 32 thereby permitting the powder cloud to flow through the development zone 33 in roughly laminar flow leaving through exit means 34. When no more than half the development time has elapsed the valve position is changed sending'the powder cloud through the left-hand tube 35 thence to the entrance slot 45 where, by means of the pressure exerted by the powder cloud, gate means 36 are forced up permitting the powder cloud to flow through the development zone in approximately laminar flow leaving through exit means 37. In the apparatus shown, 38 is the development electrode and 39 the xerographic plate having an image-bearing surface 40. Spacing means 41 are provided to position the xerographic plate at the correct distance from the development electrode, namely, no more than about%" apart.

Fig. 6 shows an isometric view of the same development apparatus.

The method of the instant invention may also be used on a continuous machine of the rotating-drum type. In this apparatus a photoconductive insulating layer is coated on the curved surface of a cylindrical drum. The various steps of the overall xerographic process as shown in Fig. l are carried out around the periphery of the curvedsurface as the image is moved from step to step by rotating the drum on its lengthwise axis. When using the method of the instant invention, the drum is rotated in one direction (as clockwise) but stepwise rather than at a constant, .uniform speed. Thus, a given arc of the drums periphery, say four inches, is advanced in one step and the drum stopped for a set period, say three seconds, to permit development, then the step is repeated.

On whatever type of machine the process of the invention is used, a preferred method of operation is to use alternate development periods of short duration. For example, if in a total development time of 4 seconds the image is developed for 2 seconds in each direction, tearing and streaking will be definitely reduced. However, if development were for periods ofJ/z second from each direction alternately, a much more complete covering of tears and streaks would be achieved. Hence, for any given overall development period it is preferred to reverse the direction of powder cloud flow through the development zone almost as frequently as the equipment permits within the available time.

Where the electrostatic image is borne by a photoconductive insulating layer coated on a conductive backing as in the embodiments shown in Figs. 2, 3 and 5, desirably. means are provided for applying an electric potential difference between the conductive backing member 27 for the photoconductive insulating layer 29' and'the development electrode 23. These means may include, for example, as shown in Fig.2, a battery 46 or similar D. C. power source connected through a potentiometer 47 to, in this case, the conductive backing 27,. The conductive backing and photoconductive insulating layerare,

of course, insulated from the development electrode in such a system and the electrode itself is grounded as shown. If desired, the potential may be applied to the development electrode and the conductive backing member grounded.

Generally, the powder cloud is charged with a charge of polarity opposite to the polarity of the electrostatic image. However, this is not necessarily so. Thus, reversal development may be efiected by charging the powder cloud with a charge having the same polarity as the image areas. Hence, the powder is repelled from the image. Reversal development may also be obtained by use of the proper bias from potentiometer 47 between the development electrode 23 and the backing member 27. In this system, the potentiometer is adjusted to bias the conductive backing member relative to the development electrode to the highest voltage on the photoconductive insulating layer 23 and of the opposite polarity to the electrostatic image. Where the potentiometer is connected to the electrode and the conductive backing member is grounded, the potential is of the same polarity as the electrostatic image. In either case, the field applied opposes the image field. The powder (charged to a polarity opposite to that to which the photoconductive insulating layer was initially charged) is attracted to the uncharged background areas to develop a reversal print.

Other modifications and variations of the basic invention will, of course, be obvious to those skilled in the art. What ever variation of the basic process is used, however, it is possible by this process to substantially reduce tearing and streaking while greatly improving end-to-end uniformity of the developed xerographic image.

I claim:

1. A process for developing an electrostatic image on an image-bearing surface, said process comprising drawing the field of force of said image externally above the image-bearing surface, generating and electrostatically charging a cloud of powder particles, and passing the powder cloud along the image-bearing surface within the field of force alternately from each of two opposite ends of the image-bearing surface.

2. A process for developing an electrostatic image on an image-bearing surface, said process comprising drawing the field of force of said image externally above the imagebearing surface, generating and electrostatically charging a cloud of powder particles, and passing the powder cloud along the image-bearing surface within the field of force more than once from each of two opposite ends of the image-bearing surface.

3. A process for developing an electrostatic image on an image-bearing surface, said process comprising drawing the field of force of said image externally above the image-bearing surface, generating and electrostatically charging a cloud of powder particles, passing the powder cloud along the image-bearing surface within the field of force from one end of the image-bearing surface for about one-half of the development time, then reversing the flow of the powder cloud and passing the powder cloud along the image-bearing surface within the field of force for the remainder of the development time from the opposite end of the image-bearing surface.

4. A process for developing an electrostatic image on an image-bearing surface, said process comprising positioning said image-bearing surface at a distance of no more than about /4 inch from a conductive electrode to form a development zone, applying a potential to the said electrode equal to the highest potential on the imagebearing surface and of the same polarity as the image, generating and electrostatically charging a cloud of powder particles, passing the powder cloud along the image-bearing surface within the development zone alternately from each of two opposite ends of the image-bear ing surface.

References Cited in the file of this patent UNITED STATES PATENTS Re. 22,419 Smyser Jan. 11, 1944 2,550,724 Sabel et al. May 1, 1951 2,618,552 Wise Nov. 18, 1952 2,659,670 Copley Nov. 17, 1953 2,684,656 Ransburg July 27, 1954 2,711,481 Phillips June 21, 1955 2,716,826 Huebner Sept. 6, 1955 2,725,304 Landrigan et a1. Nov. 29, 1955

Claims (1)

1. A PROCESS FOR DEVELOPING AN ELECTROSATIC IMAGE ON AN IMAGE-BEARING SURFACE, SAID PROCESS COMPRISING DRAWING THE FIELD OF FORCE OF SAID IMAGE EXTERNALLY ABOVE THE IMAGE-BEARING SURFACE, GENERATING AND ELECTROSTICALLY CHARGING A CLOUD OF POWDER PARTICLES, AND PASSING THE POWDER CLOUD ALONG THE IMAGE-BEARING SURFACE WITHIN THE FIELD OF FORCE ALTERNATELY FROM EACH OF TWO OPPOSITE ENDS OF THE IMAGE-BEARING SURFACE.

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