US2930711A - Electrostatic printing - Google Patents

Description

United States Patent ELECTROSTATIC PRINTING Virgil E. Straughan, Euclid, Ohio, assignor, by mesne assignments, to General Dynamics Corporation, San Diego, Calif., a corporation of Delaware No Drawing. Application November 3, 1955 Serial No. 544,815

4 Claims. (Cl. 117 38) This invention relates to electrostatic printing. More particularly, it relates to an improvement in the manner in which a latent electrostatic image is converted into a permanent visual record in an electrostatic printing procedure in which a liquid is employed for the development of a visual image of the latent electrostatic image.

Processes for forming a latent electrostatic image existing as an electrostatic charge pattern in a layer of material having a high resistance and for subsequently converting the latent electrostatic image into a visual image are known. One such process involves the use of a finely divided opaque solid in the form of a suspension in air as described in United States Patent 2,221,776. In the processes described in copending patent applications Serial No. 380,285, filed September 16, 1953, Serial No. 484,215, filed January 26, 1955, and Serial No. 486,995, filed February 8, 1955, all of which are assigned to the same assignee as the instant application, another type of development comprising bringing the surface bearing a latent electrostatic image into contact with a body of liquid developer, either byimmersion of the image bearing surface or by flowing the liquid developer over the surface is described. In this method, a developer comprising a finely divided opaque solid dispersed in a liquid having a high electrical resistance is employed.

The advantages of liquid developers are described by Metcalfe in the Journal of Scientific Instruments published by the Institute of Physics (London), volume 32, Number 2, February 1955, pages 74 and 75, and are stated to be greater simplicity and speed of operation coupled with improved control and versatility in the development process, and they are said to be especially valuable when an immediate visual inspection of the image is desired and the image need only persist for a short time.

The present invention constitutes an improvement in the manner in which the visual image is converted into a permanent record image. I have found that when attempts were made to transfer the visual image to an absorbent medium such as newsprint or other paper, the suspending liquid was often readily absorbed by the transfer medium without transporting a sufiicient quantity of the finely divided opaque powder to produce high contrast. Less absorbent record media were tried, such as an adhesive coated sheet. To transfer the visual image, pressure was exerted on the permanent record medium at the point, line, or plane in contact with the visual image, in an effort to press the finely divided opaque particles into the adhesive, whereby they would become permanently affixed. This approach was not entirelysatisfactory since the suspending liquid was not absorbed by veloper, and which is adapted, in addition, to produce a permanent visual image from the visual image first obtained.

In accordance with my invention the visual image produced by developing the latent electrostatic image with a liquid developer is first contacted by an absorbent means whereby a major fraction of the suspending liquid is separated from the visual image and then the visual image is contacted with a thermoplastic material which has been softened by heat, and the thermoplastic material then takes up a substantial amount of the finely divided opaque powder forming the dried visual image and, after the thermoplastic material has cooled'and set, it retains the opaque powder image in the form of a permane'nt visual record.

One liquid developer found to be economical, relatively stable and one which is simply prepared from readily available commercial products comprises a suitable concentration of a finely dividedopaque particulate material, preferably between 0.01 micron and 50 microns in size in a high resistance liquid in which the particlesmay be dispersed and with which they do not react to any appreciable extent. One such liquid developer, preferred by me, consists of between 5 and 25% by weight of powdered charcoal of 1 micron average particle size suspended in kerosene as the high resistance liquid. Other opaque solid particles and other high resistance liquids may constitute the liquid developer without departing from the invention herein disclosed.

In operation, the high resistance liquid carries the opaque powderparticles to the device bearing a latent electrostatic charge pattern and the opaque particles are deposited, along with' accompanying liquid, on portions of the charge pattern as is well known, to produce an impermanent visual i'mage. I I y In order that the resulting visual image may be conve'rted into a permanent record image, in accordance with my invention, the visual image is first contacted with an absorbent means. I have found that ordinary newsprint possesses the desired absorbency, and even though as much as 20% of the' carbon particles comprising the visual image may be transferred during the blotting step, the

or it may take the form of a porous roller designed to squeeze off excess liquid. When the latent electrostatic image is developed into an impermanent visual image on a flat plate-like element, the absorbing means may take the form of a sheet blotter or a blotting roller passed over the surface. Whatever the form taken by the absorbing means, it merely functions to remove as much of the suspending liquid as possible from the impermanent visual image while removing as little as possible of the opaque powder constituting the image forming means. Once the impermanent visual image has been suitably conditioned for transfer to form a permanent record, it is contacted by the medium on which the permanent record is to be formed. Two approaches have been found suitable. In the first, a paper coated or impregnated with a thermosoftening material such as polyethylene was heated sufficiently to soften the polyethylene, and then was me-= chanically pressed against the image. When the thermo-. softening material hardened it fixed the carbon or other record the suitably preparedblottedvisual image was brought into contact with adhesive covered transfer media such as paper or transparent films. The adhesive removed the opaque powder from the means supporting it and the visual image was then imprisoned in the adhesive itself.

I claim:

1. In the method of converting a latent electrostatic image into a permanent visual record in an electrostatic printing process which includes forming a latent electrostatic charge pattern in a high resistance material and developing a visual image of said charge pattern by bringing the surface of said high resistance material into contact with a developer comprising a solid opaque particulate material dispersed in a liquid possessing a high electrical resistance thereby establishing an impermanent visual image on the surface of said high resistance material, said image consisting of deposited solid particles electrostatically attracted to and held by the latent electrostatic charge pattern and a film of said liquid in excess of that permitting transfer of the solid particles to a non-absorbent permanent record medium, the improvement which comprises: bringing a porous solid absorbent member, adapted to absorb liquid, into physical contact with the film of liquid on the surface of the high resistance material bearing the impermanent visual image consisting of eleetrostatically-held solid particles and the film of liquid associated therewith, thereby absorbing a substantial portion of the liquid and a minor proportion of the solid particles While leaving a substantially liquid-free image formed of the remainder of the solid particles and thereafter transferring the liquid-free image formed of solid particles to a non-absorbent record medium.

2. The method of claim 1 wherein the developer consists of a dispersion in kerosene of between 5% and 25% by weight of solid charcoal particles having a particle size between 0.01 micron and 50 microns.

3. In the method of converting a latent electrostatic image into a permanent visual record in an electrostatic printing process which includes forming a latent electrostatic charge pattern in a high resistance material and developing a visual image of said charge pattern by bringing the surface of said high resistance material into contact with a developer comprising a solid opaque material dispersed in a liquid possessing a high electrical resistance thereby establishing an impermanent visual image on the surface of said high resistance material, siad image consisting of deposited solid particles electrostatically attracted to and held by the latent electrostatic charge pattern and a film of said liquid in excess of that permitting transfer of the solid particles to a non-absorbent pennanent record medium, the improvement which comprises: bringing a porous solid absorbent member, adapted to absorb liquid, into physical contact with the film of liquid on the surface of the high resistance material bearing the impermanent visual image consisting of electrostaticallyheld solid particles and the film of liquid associated therewith, thereby absorbing a substantial portion of the liquid and a minor proportion of the solid particles while leaving a substantially liquid-free image formed of the remainder of the solid particles, preparing a permanent record medium comprising a paper coated with a thermosoftening material by heating sufficiently to soften the said material and thereafter transferring the liquid-free image formed of solid particles to the heated, softened thermosoftening coating.

4. In the method of converting a latent electrostatic image into a permanent visual record in an electrostatic printing process which includes forming a latent electrostatic charge pattern in a high resistance material and developing a visual image of said charge pattern by bringing the surface of said high resistance material into contact with a developer comprising a solid opaque particulate material dispersed in a liquid possessing a high electrical resistance thereby establishing an impermanent visual image on the surface of said high resistance material, said image consisting of deposited solid particles electrostatically attracted to and held by the latent electrostatic charge pattern and a film of said liquid in excess of that permitting transfer of the solid particles to a non-absorbent permanent record medium, the improvement which comprises: bringing a porous solid absorbent member, adapted to absorb liquid, into physical contact with the film of liquid on the surface of the high resistance material bearing the impermanent visual image consisting of electrostatically-held solid particles and the film of liquid associated therewith, thereby absorbing a substantial portion of the liquid and a minor proportion of the solid particles while leaving a substantially liquidfree image formed of the remainder of the solid particles and thereafter transferring the liquid-free image formed of solid particles to a permanent record medium comprising a heated polyethylene paper.

References Cited in the file of this patent UNITED STATES PATENTS 2,221,776 Carlson Nov. 19, 1940 2,576,320 Vlad et al NOV. 27, 1951 2,633,796 Pethick Apr. 7, 1953 2,647,464- Ebert Aug. 4, 1953 2,784,109 Walkup Mar. 5, 1957 OTHER REFERENCES Atkinson et al.: Ferrography, Journal of the Franklin Institute, volume 252, No. 5, November 1951, pages 373 to 377.

Metcalfe: Liquid Developer for Xerography, Journal of Scientific Instruments, volume 32, February 1955, pages 74 to 75.

Claims (1)

1. IN THE METHOD OF CONVERTING A LATENT ELECTROSTATIC IMAGE INTO A PERMANENT VISUAL RECORD IN AN ELECTROSTATIC PRINTING PROCESS WHICH INCLUDES FORMING A LATENT ELECTROSTATIC CHARGE PATTERN IN A HIGH RESISTANCE MATERIAL AND DEVELOPING A VISUAL IMAGE OF SAID CHARGE PATTERN BY BRINGING THE SURFACE OF SAID HIGH RESISTANCE MATERIAL INTO CONTACT WITH A DEVELOPER COMPRISING A SOLID OPAQUE PARTICULATE MATERIAL DISPERSED IN A LIQUID POSSESSING A HIGH ELECTRICAL RESISTANCE THEREBY ESTABISHING AN IMPERMANENT VISUALL IMAGE ON THE SURFACE OF SAID HIGH RESISTANCE MATERIAL, SAID IMAGE CONSISTING OF DEPOSITED SOLID PARTICLES ELECTROSTATICALLY ATTRACTED TO AND HELD BY THE LATENT ELECTROSTATIC CHARGE PATTERN AND A FILM OF SAID LIQUID IN EXCESS OF THAT PERMITTING TRANSFER OF THE SOLID PARTICLES TO A NON-ABSORBENT PERMANENT RECORD MEDIUM, THE IMPROVEMENT WHICH COMPRISES: BRINGING A POROUS SOLID ABSORBENT MEMBER, ADAPTED TO ABSORB LIQUID, INTO PHYSICAL CONTACT WITH THE FILM OF LIQUID ON THE SURFACE OF THE HIGH RESISTANCE MATERIAL BEARING THE IMPERMANENT VISUA IMAGE CONSISTING OF ELECTROSTATICALLY-HELD SOLID PARTICLES AND THE FILM OF LIQUID ASSOCIATED THEREWITH, THEREBY ABSORBING A SUBSTANTIAL PORTION OF THE LIQUID AND A MINOR PROPORTION OF THE SOLID PARTICLES WHILE LEAVING A SUBSTANTIALLY LIQUID-FREE IMAGE FORMED OF THE REMAINDER OF THE SOLID PARTICLES AND THEREAFTER TRANSFERRING THE LIQUID-FREE IMAGE FORMED OF SOLID PARTICLES TO A NON-ABSORBENT RECORD MEDIUM.