US 3220831 A
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Description (OCR text may contain errors)
Nov. 30, 1965 s. M FARLANE ELECTROSTATIC PRINTING METHOD AND APPARATUS USING DEVELOPER POWDER PROJECTION MEANS Filed Aug 6, 1962 BY 6%, 45/14 JMM ATTORNEYS United States Patent 3,220,831 ELECTROSTATIC PRINTING METHOD AND APPARATUS USING DEVELOPER POWDER PROJECTION MEANS Samuel McFarlane, Summit, N.J., assignor to Sun Chemical Corporation, New York, N.Y., a corporation of Delaware Filed Aug. 6, 1962, Ser. No. 214,949 4 Claims. (Cl. 961) This invention relates, in general, to the field of electrophotography and has as an objective the improving of xerographic and electrostatic printing techniques to provide image transfer by a force field rather than through mechanical pressure to yield uniform printing on various materials and even corrugated surfaces and as a continuous printing operation when desired.
In the usual process of xerography as disclosed, for example, in Carlson Patent No. 2,297,691, issued October 6, 1952, a photoconductive insulating material provided with a conductive backing is given a uniform electric charge over its surface and is then exposed to light or radiation through an image to form an electrostatic latent image on the insulating layer. The electrostatic charges making up the image are formed due to the light exposure discharging photoconductive surface areas to conductive backing plate in accordance with the light intensity impinging upon the incremental surface areas. Development of the image is effected by bringing oppositely charged printing particles to the electric charge of the pattern itself such that they are held thereon clectrostatically in the outline of the electrostatic latent image. Thereafter, the developed image is transferred to a suitable material to which it may be fixed, if required. In other embodiments, the electrostatic latent image is transferred from one non-conductive surface to another before it is used for printing purposes.
Owing to the internal electrostatic lines of force established in the latent image due to the charge distribution, the prior art systems have experienced great difficulties with the development step in attempting to overcome concentrated charges, which result in the copies reproduced having hollow centers and emphasized edges. Usually, some form of a development electrode is required. Such electrodes have at least a surface of a conductive material biased slightly or held at the same potential as the plate or backing conductive member to establish electrostatic fields outwardly of the image and away from its backing plate to minimize the distortion.
The present invention essentially avoids these problems by providing a strong development field with definite lines of force established through an air gap between at least the image and the transfer medium or material to be printed. The problem is further minimized by avoiding intermediate transfers of the electrostatic latent image, thereby eliminating handling of the charged images which heretofore caused smudging.
By using an electrostatic latent image, the present invention eliminates stencils and hence permits continuous printing of repetitive patterns or images of printed or pictorial matter. In addition to this advantage, the use of an air gap for image transfer avoids the disadvantages of pressure transfer which requires uniform contact and the attendant use of wetting agents, resulting in more uniform printing without bleeding, even on a corrugated transfer medium.
More particularly, apparatus in accordance with the invention employs, in one embodiment thereof, an electrostatic latent image-fonning carrier or plate in the form of a surface producedby a vast plurality of diminutive needle-like or' rod projections, each tipped or coated on 3,220,831 Patented Nov. 30, 1965 their extremities with a light-sensitive or photoconductive layer. The needles are all set in a conductive member or film which is adapted along with a plate or other conductive film to form an electrostatic field therebetween. The non-conductive coating of the image carrier elements is uniformly charged and a light image focused thereon to produce an electrostatic latent image, defined by charges distributed along the coated tips of the needles or rods. The discharged areas of the carrier lose their charges through the photoconductive insulator coating to their internal needles.
The thus formed electrostatic image is suitably dusted with a pigment in the form of a fine printing powder, building up fairly sizable deposits of printing material along the properly charged coated needle tips in accordance with the image pattern thereon. The dusted image is then placed in or caused to enter a strong electrostatic field developed in an air gap, which space also receives the transfer medium. The printing particles are projected across the air gap and then to the transfer medium being specifically fixed thereto, if necessary, to provide permanent copy. By properly arranging the field polarity relative to the charge of the particles, the particles are actually repelled from the image carrier and attracted by the oppositely disposed field bounding plate, but are intercepted in their flight by the intermediately disposed transfer medium.
The foregoing purpose may be carried out in a continuous manner or in step-by-step fashion for individual image reproductions In the former, a pattern is printed on a fabric or a film of images is reproduced continuously. In the latter, images are reproduced intermittently as in oflice business machines or the like.
The described process is capable of being carried out by hand or partially by machine to produce xerographic plates for use in other arrangements.
One unusual feature presented by the air gap charge transfer arrangement permitting printing on rough or corrugated paper is the facility to create an illusion of depth. Embossed or specially indented paper can be printed in the indentations to present the appearance of a three dimensional scene.
With the foregoing in mind, it is an object of the invention to provide a novel printing method and apparatus capable of continuous printing from projected images ,without pressure contact with the printing material;
A further object is the provision of such an apparatus which minimizes the mechanical elements and relies upon electrostatic transfer through an air gap to the transfer medium without the necessity of wetting agents;
It is a still further object to provide an arrangement wherein electrostatic latent image techniques are employed in a contact-free air gap utilizing an image carrier capable of very heavy printing;
Other objects and advantages of this invention will become apparent to those skilled in the art in view of the following detailed description when considered in light of the attached drawing in which:
FIG. 1 is a schematic view in perspective of an image carrier being uniformly charged;
FIG. 2 is a schematic view of the same carrier being exposed to an image;
FIG. 3 is a further similar schematic view of the soexposed image carrier being powdered with inking material;
FIG. 4 is a view in side elevation showing the plate of FIGS. 1 through 3 actually in use in the printing process;
FIGS. 4a and 4b show detailsof the image carrier or plate and the field of FIG. 4; and f I FIG. 5 is a perspective schematic of apparatus capable of the automatic printing operation described herein.
Referring now to the drawing and particularly to FIGS. 1 through 3, there is shown a novel type xerographic plate 11 which is formed of a backing member 13 embedded with or otherwise holding a plurality of conductive needle-like or rod-like projections 15, all desirably coterminous to form an image receiving surface.
The needles 15 are best illustrated in FIG. 4a wherein it may be appreciated that they. are quite closely spaced, numbering some 250 tothe square'inch distributed over the face of the backing member 13. Each tip is coated to a depth of A inch or so as shown at 17 by a thin layer of photoconductive material, such as, zinc oxide in an organic resin binder, which is commercially available.
In FIG. 1 a fur or plush roller 23 is shown being rubbed lightly and uniformly over the surface. formed by the photoconductive tips 17. This may be done by hand or carried out through an oscillatory motion developed at the .slot 25 through suitable mechanical connections to the roller handle or extension 27.
The so-charged coatings 17 of the plate 11 are now exposed to a light source, schematically illustrated at 31, through an image 33 to be reproduced. This may be carried out by hand or automatically, but in any event should be done in the dark or a semi-lighted area (as in the case of the complete process) in order that the electrostatic latent image pattern formed on the photoconductive tips 17 will not be affected.
Next the electrostatic latent image is developed, as
shown in FIG. 3, by dusting onto it a suitable pigment, such as a fine powdered ink of carbon black, dry dye or other commercially available marking material in fine powder form. The powder particles may be given a charge opposite to that of the electrostatic latent image areas to be printed such that they will cling only to the printing areas to outline the image. The so-developed image is brought into an electrostatic field for printing purposes, as is illustrated in FIG. 4. A potential difference is established between a conductive plate 43 and the conductor 13 through an air gap generally indicated between the plates 43 and 13, into which space the transfer medium, such as the corrugated paper 45 is disposed. The field intensity is quite high relative to the magnitude of the electrostatic charges such that the marking powder is projected across the air space and onto the corrugated paper 43 to reproduce the developed electrostatic image.
An approximately field distribution is shown in FIG. 4b to reveal how accurate but heavy printing is available even on rough or corrugated surfaces. The electrostatic lines of force 51 tend to establish parallel paths from the needles of plate 13 to the opposite conductor 41 when the field is initiated. The charged powdered ink particles tend to follow the electrostatic lines 51 and impact and stick to the transfer medium 43 in fine printing areas in accordance with the electrostatic charges. The provisions of the relatively large area photoconductive surfaces, presented by the depth of coating on the needle tips, permits the accumulation of extra heavy charges of printing material on the tips 17 to provide the heavy printing mentioned.
It should be mentioned that the plate 13 may be of conductive material wholly or it may comprise, for example, a moldable insulator which is coated with a conductive film in electrical contact with the needles 15.
In FIG. there is illustrated schematically suitable apparatus for carrying out the aforedescribed printing process continuously or in intermittent fashion. An endless conveyor 61 is shown comprising a flexible conductive film or strip 63 corresponding to the backing plate 13 previously described. The film is of suflicient rigidity as to support the tipped or coated needles 65 corresponding 'to the needles 15 of FIG. 4a. The endless conveyor 61 is supported on driving sprockets .71 and 73 for endless rotation. A common drive is shown in the form of the motor 75 adapted for control in any suitable conventional manner. 1
In the right hand intermediate portion of FIG. 5, there is shown a fur or plush roller 77 of the type described as roller 23 in connection with FIG. 1. The roller 77 is lightly disposed against the conveyor needles 65 to establish a uniform charge on the photoconductive surface of the tips. Since the rotation of the conveyor 65 is in a counterclockwise direction, the conveyor presents a uniformly charged photo-sensitive surface to a light image of a projection system, generally indicated at 81.
A film strip 83 is shown as an endless loop pattern where continuous printing of fabric or the like is desired. However, it will be appreciated that the pattern 83 could as well consist of a regular film strip or documents of images to be reproduced. A slit exposure mechanism comprising projection means in general, including lens means 95, an image slit 97 and projection slit 89 are provided so that the lens projects through the image slit a focused image of the projection slit in conventional manner.
The pattern or original document or film 83 is passed across the projection slit 89 at the desired rate of speed through the sprocket drive 91 which is, of course, synchronized with the rate of travel of the conveyor 63. Latent electrostatic images are produced on the photoconductive surfaces of the conveyor needles 65. These images are comprised of charges distributed along the surfaces of the non-conductive needle tip coatings such that effectively a relatively thick image charge can be formed. The apparatus of FIG. 5 is disposed in a substantially light-tight container (not illustrated) such that the charge pattern will not be affected during the printing process. 'A dusting chamber 95, provided with a continuously rotating brush 97 for agitating the powder, is shown disposed adjacent to the upper reach of the conveyor 63. The suitably charged inking particles are attarcted to the light exposed areas of the conveyor needles 65 to stick thereto, thereby developing-the electrostatic image. These particles may comprise powdered ink of a thermosetting variety or other similarprinting material as mentioned or commercially used.
The now dusted latent image enters an electrostatic field of a relatively high value compared to the latent image charges. This field is formed between the backing strip 63 (or its conductive film) and the needles 65 of the conveyor 61 and a flat electrode 101, similar to the plate 43 of FIG. 4. A difference of potential (preferably direct current) is established by the voltage source E effective over lead to the plate 101 and lead 104 to roller 106 which is in contact with a flat strip portion of the conductive backing 63 for the conveyor 61. The electrical potential difference is of the correct polarity to project the image particles through the air space (as illustrated in FIG. 4) to the flat conductor 101. However, these particles are intercepted in their flight by the transfer medium, shown as the paper 107, moving in synchronism with the conveyor 61 through the supply roll drive 109 which may also be powered from common drive 75. The paper 107 is maintained in contact with or adjacent to the flat plate conductor and under tension or in a taut condition by the rollers 111.
The driving mechanism for the printing medium 107 and the endless conveyor 65 are synchronized such that the printing surface moves at the same speed through the electrostatic field as the electric image. The spacing in the air gap (of necessity illustrated as grossly exaggerated) is made quite small in order that a relatively small voltage difference may establish .sufiicient electrostatic force lines for detail printing. An air gap of several hundredths of an inch between'the conveyor needles 65 and the printing plate 101 requires a voltage difference of several hundred volts (preferably direct curent) for proper printing. Factors affecting the optimum printing voltage are: width of the gap, humidity in the air gap,
type pigment and paper. The effective gap width may be varied as much as 0.1" for voltage differences of 1500 volts for printing.
It is believed that ionization of this air space is not necessary but that the charged powdered ink particles are simply projected through the air space to impact the printing surface, the latter naturally being taut to maintain a substantially parallel relationship with the tips of the conveyor needles. The parallel relationship is not essential to printing by this method or aparatus but the spacing should be uniform to preclude arcing at a narrow point and the illustrated apparatus provides such uniformity. The uniformity is not critical within the limits of ordinary corrugated paper provided that the voltage difference is set accordingly.
The so-transferred image is then fixed, if necessary, as by the heating coils 103 (when thermosetting ink is used) located along the printing surface path of travel. The paper then exits through a light-tight opening in the housing (not shown).
It may be appreciated that in the embodiment of FIG. 5 either continuous or intermittent operation may be employed, it being only necessary to correlate the mechanical drive mechanism after Well-known principles. The apparatus may function as an office reproducing machine by simply using a holder for the document to be reproduced in the proper location for focusing an image of the entire document on conveyor 61. In such an arrangement, the device is preferably intermittently operable, a button or the like being provided for each reproduction operation, including exposure and printing. The electrostatic latent image is produced during a dwell time, as is also the printed copy.
Thus, the operation of the conveyor is cycled to charge the conveyor image receiving area; to arrest conveyor movement for a light flash of the image onto its charged surface; to start conveyor movement to powder and deliver the image to the field producing area; to establish the field during this dwell time to print, and to move the thus printed copy through the fixing station and out of the machine. Suitable intermittent type drives are well known to achieve the foregoing described operation and are therefore not illustrated here.
While the present invention, as to its objects and advantages, has been described in relation to certain specific embodiments herein, it is understood that it be intended that the invention be construed within the spirit and scope of the appended claims wherein what is claimed 1. The method of electrostatic printing comprising the steps of producing a latent electrostatic charge image on the photoconductive light-sensitive coated tips of a plurality of electrically conductive spaced-apart needles set in an electrically conductive backing member in a distribution of some 200 or more per square inch, wherein the needles are substantially parallel and have their tips terminating in a common surface to define a charge image carrier; powdering the image with fine, granular marking material along the tips on the coating; and, introducing the so-powdered image into an electrostatic field established in the air to project the marking material along the lines of electrostatic force onto a transfer medium spaced from the image in the field for printing the image on the transfer medium, said electrostatic field being characterized by a voltage gradient of approximately 15,000 volts per inch and the spacing between the transfer medium and the powdered image on the coated tips of said needles being of the order of at least several hundredths of an inch.
2. In apparatus for strong development field electrostatic deposition printing, the comhination of an image carrier in the form of an electrically conductive backing member having approximately 200 or more spaced-apart substantially coterminous electrically conductive rod-like projections defining a common surface; an insulating photoconductive coating on the tip of each to a depth of approximately for receiving electrostatic charges; means for forming an electrostatic latent image on the coatings of the projection tips carried by the image carrier; means supporting the carrier for endless rotation; means for powdering the image with marking material; means for establishing an electrostatic field having an air gap bounded on one side by the carrier; and means for introducing the image carrier with the powdered image into the air gap along with a transfer medium in spaced-apart relation whereby the marking material is projected through a portion of the air gap to reproduce the image on the transfer medium; said field being characterized by a voltage gradient of approximately 15,000 volts per inch and said portion of the gap being of the order of at least several hundredths of an inch.
3. In apparatus for electrostatic printing, the combination of an electrically conductive supporting member supporting a plurality of electrically conductive spaced-apart rod-like projections extending outwardly thereof and each having on its outer end a layer of light-sensitive coating thereon; said projections numbering in excess of 100 per square inch and said coating covering approximately ,5 of the outer extremity of each projection; means for producing an electrostatic latent image on said coatings; means for applying granular printing material on the image; a planar conductive member disposed in spacedapart relation with the projection ends of the supporting member; means for introducing a transfer medium into said spacing; and, separate connections to the supporting member and the planar conductive member whereat a potential difference may be applied to project the printing material in the direction of the planar conductive member for interception by the transfer medium to print said image; said potential difference being sufficient to attract the charged granular printing material to the transfer medium across the spacing between the coated projections and the transfer medium.
4. In apparatus for electrostatic printing, the combination of a flexible strip-like conductive backing member supported for rotary movement; a plurality of electrically conductive needle-like projections extending outwardly of the member in spaced-apart substantially parallel alignment; light-sensitive coatings along the outer tip-ends of the projections; said projections numbering in excess of 100 per square inch and said coating covering at least of the outer extremities of each projection; means for producing an electrostatic latent image on said coatings while they are moving; means for applying granular printing material on the image; a planar conductive member disposed in spaced-apart relation with the projections tipends of the backing member; means applying a potential difference between the conductive backing member and the planar member of sufiicient magnitude to project the printing material in the direction of the planar conductive member but onto the transfer medium to print said image.
References Cited by the Examiner UNITED STATES PATENTS 2,599,542 6/ 1952 Carlson.
2,682,478 6/1954 Howse 96-1 2,692,948 10/ 1954 Lion.
2,820,716 1/1958 Harmon et al. 1.7 2,917,385 12/ 1959 Byrne 96-1 3,056,136 9/1962 Macgritf 96-1 FOREIGN PATENTS 745,500 2/ 1956 Great Britain.
OTHER REFERENCES Nicoll, RCA Technical Notes, RCA TN No. 375, June 1960.
NORMAN G. TORCHIN, Primary Examiner.