US3806355A

US3806355A - Electrostatic printing apparatus and method

Info

Publication number: US3806355A
Application number: US00235930A
Authority: US
Inventors: A Kaufman
Original assignee: Individual
Current assignee: Individual
Priority date: 1972-03-20
Filing date: 1972-03-20
Publication date: 1974-04-23
Anticipated expiration: 1991-04-23
Also published as: JPS496927A

Abstract

AN ELECTROSTATIC PRINTING APPARATUS IN WHICH ELECTROSTATIC FIELD LINES CORRESPONDING TO THE INDICIA TO BE PRODUCED ARE DIRECTED THROUGH A DIELECTRIC SHEET. A RECORD SHEET IS BROUGHT INTO CLOSE PROXIMITY WITH THE DIELECTRICSHEET TO RECEIVE THE FIELD LINES PASSING THROUGH THE DIELECTRIC SHEET. A SUITABLE TONER IS APPLIED TO THE ADJACENT SURFACE OF THE SHEETS IN A SMALL GAP BETWEEN THE TWO SHEETS TO FORM A VISIBLE IMAGE ON THE RECORD SHEET. IN ANOTHER EMBODIMENT, DRY TONER IS APPLIED TO THE OUTER SURFACE OF THE RECORD SHEET TO FORM A VISIBLE IMAGE.

Description

April 23, 1974 A. 1.. KAUFMAN 3,806,355

ELECTROSTATIC PRINTING APPARATUS AND METHOD Filed March 20, l972 4 Sheets-Shoot 1 April 23, 1974 A. L. KAUFMAN 3,806,355

ELECTROSTATIC PRINTING APPARATUS AND METHOD Filed March 20, 1972 4 Shoots-Shoot 2 A 1 il23,1974 L KAUFMAN 3,806,355

. ELECTROSTATIC PRINTING APPARATUS AND METHOD Filed Maz 'ch 2o, 1972 f 4 Shuts-Shut 3 April 23, 1974 A. L. KAUFMAN ELECTROSTATIC PRINTING APPARATUS AND METHOD 4 Sheds-Shoot 4 Filed March 20, 1972 \MAGE RESOLUTION AS A FUNCTION OF SHEET RESISTIVITY AND THICKNESS IN LINE PAIRS PER MM w MR A TP. SA mm m FT w $0 S IE LR TH/ c/r/vess //v M/LS United States Patent 3,806,355 ELECTROSTATIC PRINTING APPARATUS AND METHOD Arthur L. Kaufman, 26 High Point Road, Westport, Conn. 06880 Filed Mar. 20, 1972, Ser. No. 235,930 Int. Cl. C03g 13/08, 13/10, 15/08 US. Cl. 117-37 LE Claims ABSTRACT OF THE DISCLOSURE BACKGROUND OF THE DISCLOSURE It is well known in the electrostatic photocopy art to place an electrostatic charge upon an insulative or photoconductive surface, selectively discharge the said surface in accordance with the indicia to be produced, tone the selectively discharged surface to form a toner image and thereafter transfer the thus formed image to a record sheet to produce a copy of said indicia. Such apparatus has been described in United States Letters Patent No. 2,297,691; 2,264,652 and others.

Another prior art device employs an intermediate insulating webin contact with an exposed photoconductive drum. The powder (toner) image is developed on the web while in contact with the drum and the tackified image then transferred to a record sheet. Such a device is described in United States Letters Patent No. 3,013,878.

Still another prior art teaching involves the charging and selective exposure of a photoconductive surface to form a latent charge image on the photoconductor. A sheet of dielectric material is brought into contact with the charge image bearing sheet and the said charge image transferred to the dielectric sheet. Thereafter, toner is applied to the dielectric sheet to form a visible image. A typical example of this type of device is disclosed in United States Letters Patent No. 3,147,679.

In many of the prior art devices toner is applied directly to the photoconductive surface and thereafter transferred to a record shet or a stored charge image is transferred to an insulative sheet which is subsequently toned. As a result, the photoconductive or dielectric surface requires cleaning following each cycle to remove the toner which has not been transferred. The cleaning. operation causes abrasion and consequent deterioration of the photoconductor and the images produced thereby. The toner which has not been transferred is wasted. These devices require large and expensive cleaning mechanisms to enable them to produce a suitable number of copies before replacement of the photoconductive surfaces.

In the present invention little or no toner is brought into contact with the photoconductive surface or any other source of electrostatic field lines corresponding to the indicia to be produced. The need for prior art rigorous cleaning mechanisms is consequently eliminated. Moreover, the present invention lends itself to a simplified, lower cost, more compact, electrostatic printing apparatus. Since an intense electrostatic field is established as the insulative and record sheets are brought together in one form of the present device rapid production of copies has been achieved in a very simple mechanism.

By not toning the source of electrostatic charges it is possible with the present invention to produce multiple copies from a single selective charging step.

SUMMARY OF THE DISCLOSURE Electrostatic printing apparatus in accordance with the present invention employ any suitable source of electrostatic charges corresponding to the indica to be produced. The electrostatic field lines emanating from the charge source pass through a sheet of dielectric material adjacent the said charge source to a sheet of record material, such as paper, at a time when the two sheets are in close proximity. A quantity of toner material is introduced between the sheets and is directed on to the surface of the record sheet by the electrostatic field lines to form a visible image. The sheet of dielectric material does not enter into the electrostatic operation of the device, as hereinafter more fully set forth but serves to prevent the toner from reaching the source of electrostatic charges. A toner having a charge of the same sign as that of the electrostatic field lines is employed to produce a positive image. If a negative image is desired a toner of opposite sign is used.

Since only the record sheet is toned, multiple copies can be produced from a single charging cycle.

DESCRIPTION OF THE DRAWINGS In the accompanying drawings forming part hereof identical parts have been given the same reference numerals; in which drawings:

FIG. 1 is a diagrammatic view in side elevation of a complete embodiment of the present invention,

FIG. 2 is a fragmentary view, somewhat enlarged, of the toning station of FIG. 1,

FIG. 3 is a view similar to FIG. 1 showing a reflex exposure embodiment of the present invention,

FIG. 4 is a view similar to FIG. 1 showing another embodiment of the present invention,

FIG. 4a is a view similar to FIG. 1 showing the use of a pin array as the source of electrostatic charges,

FIG. 5 is a view similar to FIG. 2 showing the application of toner on the exit side of the toning station,

FIG. 6 is a fragmentary somewhat diagrammatic view in side elevation of a further embodiment of the present invention using dry toner,

FIG. 7 is a graph showing image resolution as a function of sheet resistivity and thickness in the practice of the present invention,

FIG. 8 is a somewhat diagrammatic view of still another embodiment of the present invention using dry toner.

GENERAL DESCRIPTION Referring to the drawings and particularly to FIGS. 1 and 2; 10 indicates a source of electrostatic charges, such as a drum having a coating of suitable photoconductive material 11 thereon. The photoconductive material may be any of the Well-known photoconductors, such as selenium, zinc oxide, anthracene and the like with or without dyes and resinous binders. A layer of conductive material may be deposited beneath the photoconductive layer 11 in the event the drum 10 itself is not made of conductive material. The drum 10 is journaled upon a shaft 12 and connected to a source of rotary power (not shown) in the well-known manner. Disposed above the drum and indicated by the dashed lines 13 is a source of an incorrect left-right aerial image such as can be produced by selected lenses and mirrors well-known in the art. Light from the areial image is directed upon the surface of the photoconductive material 11 as the drum 10 rotates upon the shaft 12.

The drum in FIG. 1 rotates in the direction indicated by the arrow 14 and passes in front of a corona charging station 15. The corona charging station 15 uniformly charges the photoconductive layer 11 as it passes beneath the said station.

At the drum 10 next passes below the aerial image the uniform charge impressed by the corona charging station is selectively discharged whereupon a latent charge image is left upon the photoconductive layer of the drum. The drum then continues its rotation into the toning section of the copying device generally indicated at 16 in FIG. 1. A source of dielectric material in sheet form is led from a roll 17 into the toning section 16. The dielectric material 17 may be any one of a number of wellknown plastics such as those sold under the trade names of Mylar, Teflon, Saran, Kel-F, etc. These materials have a volume resistivity lying in the range of the order of 10 ohms cm. However, it will be apparent to those skilled in the art that the resistivity of the material selected will be dependent upon the thickness of said materials, the speed with which the said materials pass through the machine, the dielectric constant of the materials etc. Highly satisfactory results have been achieved employing dielectric sheet material having a resistivity of the order of 10 ohms cm. and a thickness of the order of .0015 inch, using dried cellulose acetate as the resistive material.

A sheet of record material such as paper 18 is fed into the toning section 16 in the direction of the arrow 19. The paper 18 may be of any suitable stock, such as bond paper, clay coated paper, and the like. The paper 18 is led over a guide 20 and between the drum 10 and a roller 21 disposed beneath the said drum. The roller 21 is preferably made of a suitably rigid material which will hold the paper 18 tightly against the dielectric material 17 which is also passed beneath the drum 10 and overlies the paper 18. A take up roller 22 serves to store the dielectric material 17 after it has passed through the toning section 16. If desired, the dielectric material 17 may be cleaned of any toner that might have been placed upon it by means of the brush 23 as it is rolled up on the take up roller 22.

Referring particularly to FIG. 2 it will be seen that a supply of toner material 24 such as a liquid toner Well known in the photocopying art is directed into the space between the dielectric material 17 and the record paper 18 by means of an elongated nozzle 25. The nozzle 25 spreads a uniform coating of the toner 24 across the sheet of record paper 18 as it moves into the nip between the drum 10 and the presure roller 21. Sufficient toning time, of the order of one to eight seconds should elapse between the contact of the record and dielectric sheets and the toner before they reach the roller 21.

It will be seen from an examination of FIG. 2 that the latent image resulting from the selective discharge of the corona charged surface produces a number of electrostatic field lines 26 which extend outwardly of the surface of the photoconductive material 11 in the direction of the dielectric material 17 and the record sheet 18. The travel of the dielectric material 17 and the record sheet 18 through the toning section 16 is sufiiciently rapid, being of the order of one inch per second to twenty-eight inches per second, to prevent a buildup of induced image charge within the dielectric material 17 which would prevent the penetration of the original latent image field lines from reaching the record material or paper 18.

As the field lines 26 traverse the toner 24 and reach the record paper 18, toner 24 lying within the electrostatic fields gradient, will be directed toward the record paper 18 to form a visible image thereon corresponding to the original latent charge image on the drum 10, but right reading.

As the paper 18 continues into the nip between the resilient roller 21 and the drum 10, the excess toner 24 will be squeegeed off the surface of the record paper 18 and also off the suface of the dielectric material 17. The emerging record paper 18 and dielectric sheet 17 are therefore relatively dry and may be ejected from the machine and wound up upon the rollers 22 respectively.

In the production of a visible copy, according to the present invention, a latent charge image is produced on the surface of the photoconductive material 11 of a given sign, that is, either positive or negative. The toner employed in the toning section 16 is of identical sign if a positive copy is desired, i.e., if a negative charge image is produced upon the photoconductive material, a negative toner would be employed. Correspondingly a positive toner is employed, if positive copy is desired from a positively charged photoconductor. If a negative is desired a toner of opposite polarity is used.

Referring to FIG. 2 it will be seen that in the present invention, the charge on the photocoductive material 11 is not transferred to the record paper 18, but rather a charge of opposite sign is induced in the record paper 18, thereby attracting the toner which is of the opposite sign to the induced charge. It is important that for the time during which the dielectric material 17 is passing through the toning section 16, it be suificiently resistive to permit the electrostatic field lines to pass through it, so that the toner 24 may be driven towards the record sheet 18.

Referring again to FIG. 1, following the passage of the photoconductive material upon the drum 10 past the toning station, it may be desirable, for certain photoconductors, to subject photoconductive surface 11 to a uniform pre-exposing light 27. The light 27 will assist in erasing any prior latent charge image which may remain upon the drum following the toning step. Alternately, uniformity of selectively discharging the photoconductive surface may be improved by subjecting the said surface to the pre-exposing light at each cycle, i.e. the first copy after a long period during which the photoconductor has become dark adapted or after a repetitive number of cycles.

Since the toner 24 is not brought into contact with the photoconductive material on the drum 10, it is possible with the present invention to make successive copies without going through the exposure step at each cycle. During this process the corona charging station 15 is not used following the first copy nor is the pre-exposing light 27. The latent charge impressed upon the photoconductive surface during the first cycle remains thereon during the successive toning steps and a plurality of copies may be made if so desired.

Referring to FIG. 3, there is shown in somewhat diagrammatic form a photocopy machine made in accordance with the present invention, in which reflex exposure is employed to create the latent image on the photoconductive material 11. In this embodiment, the drum 10 is made of some suitable transparent material, such as glass, and a lamp 28 is carried Within the drum 10. The material to be copied, indicated at 29, is fed, with the image side 30 down, through rollers 31 into the nip between the drum 10 and a large white resilient pressure roller 32. Light from the lamp 28 is directed upwardly through the photoconductive material 11 to the image side 30 of the material to be copied and then reflected back to the photoconductive material 11 in accordance with well-known reflex exposure techniques. The photoconductive material 11 having first passed in front of a corona charging station 15 is thereby selectively discharged to create wrong reading the latent image indicated at 33. The drum 10 continues its rotation as indicated by the arrow 34 until it reaches the toning section 16. Toning of the record paper sheet 18 is accomplished in the manner set forth above in connection with FIGS. 1 and 2. The visible image copy is ejected as indicated at 35 and the cycle may be repeated.

It is within the purview of the present invention to produce electrostatic charges suitable for production of electrostatic copies by means other than selectively discharging a charged photoconductive material. One such apparatus is shown in FIG. 4a wherein a series of metal pins 36, which may be energized by separate pulse circuits, by means of an electron beam, or other means wellknown in the art, are disposed adjacent a dielectric surface 37 upon a drum 38. The metal pins place an electrostatic charge on the said surface corresponding to the indicia to be reproduced, which wrong reading charge is then carried into the toning section 16 by the rotation of the drum 38 and the visible copy produced in the manner herein above set forth in connection with P168. 1 thru 3. In this embodiment, the steps of charging by means of a corona, of course, are unnecessary.

The charged image receiving sheet, as shown in FIG 4a, which is an array of pins 36, may be placed above the dielectric sheet 17 without the use of a photoconduc tive drum, in this embodiment of the invention the spacing of the pins from the dielectric sheet 17 protects the said sheet from damage from the pins 36.

Other means of producing an electrostatic charge latent image upon a surface will be readily understood by those skilled in the art.

It is also within the purview of the present invention, as shown in FIG. 4, to form the visible image while the dielectric sheet 17 is in close proximity to, but not in contact with the charged image receiving surface 37. The advantage of this embodiment is to greatly extend the useful life of the charged image receiving surface by eliminating any abrasive contact with the dielectric sheet and by greatly decreasing the possibility of toner coming into contact with the said image receiving surface.

Since the field lines from the latent image on the drum extend through the sheet of dielectric material 17 and into the record sheet, the said record sheet may be toned either before or after passing through the nip of the roller 21 and the drum 10. Such a device is shown in FIG. 5 in which the toner 24 is deposited upon the surface of the record sheet 18 after it passes through the nip of the roller 21 and drum 10. The toner nozzle 25 is disposed between the dielectric material 17 and the record sheet 18 to direct the toner 24 into the space between the said sheets. After the record sheet has left the toning station, it is squeegeed dry by the opposed rollers 40. In this embodiment of the invention, however, it is essential that the record sheet 18 be of sufficient resistivity, as for example of the order of 10 ohms cm., to hold the charges induced therein from the time when the toner is applied until after the record sheet 18 leaves the squeege rollers 40. In this manner, the toner will be held upon the record sheet 18 so that the squeege rollers 40 do not destroy the toned image.

FIG. 6 is a somewhat diagrammatic view of still another electrostatic printing apparatus 41 according to the present invention. In this embodiment the photoconductive material 11 is carried upon a continuous belt 42. The belt '42 may be formed of some suitable plastic such as Mylar or a metal such as stainless steel, brass, or the like. If a plastic is used a conductive layer (not shown) is deposited upon its surface beneath the photoconductor. The belt 42 is tightly stretched upon spaced

pulleys

43, 44, which are driven by a source of rotary power (not shown), at a rate of between 2 and inches per second. The belt is thus caused to travel in the direction indicated by the arrow 45.

A source of electrons such as the corona charger 46 is positioned adjacent the photoconductor 11. The corona charger 46 distributes a uniform charge of electrons across the surface of the photoconductor 11 as the belt 42 passes by. Negative charges are shown in the drawing, but it will be understood that positive charges may be employed.

As the charged photoconductor 11 continues its travel it is selectively discharged by a right reading aerial image projected thereon in the well-known manner as indicated at 47 to form a latent charge image corresponding to the aerial image.

A roll of dielectric sheet material 48 such as Mylar is disposed at one side of the belt 42, led under the portion of the belt supported by the pulley 44 and on to a take up roll 49. The Mylar sheet 48' may be driven by the belt "42 or by the belt source of rotary power. Relative displacement between the surfaces of the Mylar and the belt 42 is to be avoided in the interest of good imaging and to avoid damage to the photocondctor 11.

The record material 50, which may be paper in sheet form is carried within a box 51 and is fed therefrom by a clutch driven roller 52 through a slit 53. The paper 50 is preferably placed in the box 51 in a dried condition such that it has a relatively high resistivity, of at least 1 10 ohms cm. As the paper 50 is driven out of the box 51 it is directed through a heater 54 which further dries it and brings its resistivity up to at least 1X10 ohms cm. A small fan 55 may be used to carry off moisture and speed the drying process.

A small roller 56 in close proximity to the belt 42 and pulley 44 forms a nip 57a into which the Mylar sheet material 48 and the paper record medium 50 are led.

As the Mylar sheet 48 and record medium 50 pass beneath the pulley 44, the latent charge image field lines, which pass through the dielectric Mylar material, create a charge image in the record medium 50. This charge image is toned by means of the magnetic brush 57 which is rotated within a container 58 having a quantity of dry toner 59 therein. The toner may be selected from any of a number of well-known commercially available types such as Tribofax Positive mixed with iron filings manufactured by Surface Processes of Dallas, Pa.

If the charge image is negative, the toner must be a positive toner in order to produce a visible image corre sponding to the aerial image. However, a negative may be produced if desired by using negative toner where the charge image is negative.

As shown in FIG. 6, the positive toner 59 adheres to the negatively charged portion of the record medium 50 and, if the resistivity of the second medium is at least 1 10 ohms cm., the toner will adhere long enough, of the order of 2 to 4 seconds or longer if not mechanically disturbed to enable the record medium 50 to pass through a heater 60 which fuses the toner to the record medium.

A brush 62 is employed to remove any toner particles which may have become attracted to the Mylar sheet 48 so that the Mylar 48 may be reused. The printing apparatus is then ready for the next cycle.

In the event that multiple copies are to be made from the same latent charge on the photoconductor, the belt 42 is permitted to make successive revolutions past the toning brush 57 while the imaging apparatus 47 is withdrawn from operation. The corona charger 46 is used only to form the initial latent image.

It has been found that image resolution achieved by electrostatic printers made in accordance with the present invention vary as a function of record member sheet resistivity and thickness. These relationships have been plotted and are shown on the graph of FIG. 7.

As a further embodiment of the present invention it has been found possible to produce satisfactory images upon a record member using the embodiment shown in FIG. 6, but without the interposed Mylar sheet 48. Such an embodiment is shown in FIG. 8.

The electrostatic printing apparatus 65, shown in FIG. 8 is similar to the embodiment of FIG. 6 in that it employs a belt 42 having a photoconductive layer 11 thereon which belt is driven by

pulleys

43, 44. A uniform electrostatic charge is placed upon the photocondu'ctive layer 11 by a corona charger 46 and an aerial imaging device 47 selectively discharges the charge to form a latent electrostatic charge on the photoconductor 11 corresponding to the indicia of the projected aerial image.

As the latent image on the photoconductor reaches the nip between the pulley 44 and the small roller 56, a sheet of record material of very high resistivity (at least 1X10 ohms cm. and preferably 1 10 ohms cm.) such as dried paper or a synthetic paper such as are sold under the trademarks Nomex, Tyrek (Du Pont) is fed from the box 51 through the heater 54 into contact with the said photoconductor. As a result of the high resistivity of the record sheet 50, the field lines (not shown) emanating from the latent image on the photoconductive layer 11 pass through the record medium 50. The magnetic brush 57 rotating in the toner 59 carries toner up to the adja cent surface of the record medium 50. The toner, which is a positive toner is attracted to the negative charges traversing the record medium as indicated at 67. The brush 57 holds the record medium 50 in tight contact with the belt along a 1 to 2 inch toning zone.

The record medium 50 with the toner 67 adhering thereto (forming a visible image) is led by guide belt 68 and suction platter 70 past a heater 60 which fuses the toner to the record medium 50 to form a permanent visible image.

As the belt 42 travels past the pulley 44 a brush 66 upstream of the corona charger 61 cleans the belt of any stray toner particles which may have found their way on to the surface of the belt. The apparatus is then ready for the next cycle.

It will be apparent from FIG. 8 that, as in the embodiment of FIG. 6, repeated copies may be made from a single latent electrostatic image upon the photoconductor 11, in which case the corona charger 46 is not used after the first cycle and the imaging device 47 is not used until the completion of multiple copying.

In the operation of the embodiment of FIG. 8, however, it has been found that certain factors must be taken into account in order to achieve success. It is necessary to separate the record member 50 from the photoconductor 11 before heat fixing or fusing the toner. If this is not done, any stray toner on the edge of the record member will become fixed to the photoconductor thereby rendering it unusable in a short period of time.

When the record medium 50 is separated from the photoconductor 11, however, a phenomena known as toner jumping occurs i.e. the toner which was held by the electrostatic field lines coming from the latent image and which forms the visible image jumps off the image area and adheres to the background area instead. The reason for this undesirable and image destroying eifect is that during the time when the insulative record member 50 was in contact with the photoconductor 11, an induced image of opposite polarity began to build up in the record member. If the record member and the photoconductor 11 are in contact long enough and the resistivity of the record member is low enough the induced charge will build up rapidly. The primary field from the latent image is strong enough to attract and hold the toner before separation. Upon separation, however, the primary field is gone and only the induced field (of opposite sign) is present. The tribo charged toner of the same sign as the induced field is thereupon repelled and jumps over into the background area to which it is attracted.

Accordingly, in order to practice the method of copying employed in connection with FIG. '8 certain record medium and time conditions must be observed as follows:

(1) A record member of as high an insulative value as possible must be used p51 X ohms cm. and preferably g1 10 ohms cm. for the following reasons;

(a) The field lines from the latent image extend through the record medium further, and thicker sheets 1.0 to 5.0 mils, or higher resolution is possible for a higher resistivity,

(b) A high resistivity provides the minimum induced charge build up in the record member for any given time interval.

(2) The record member and photoconductor should be held together for a minimum period of time, i.e. just while the record member is being toned, not before and not after.

(3) High record member (through put) speeds should be used, consistent with toner brush rotation rates and satisfactory toner fusing operations. Speeds of the order of 4 to 6 inches per second with a length of contact between the record medium and the photoconductor of the order of 1 to 2 inches have been found satisfactory although speeds up to 10 to 12 inches per second and even 20 inches per second have been accomplished. Accordingly, the longest contact time between the record member and the photoconductor might be:

2 inches 1 .=& second 4 inches per see.

A record member with a p=1 10 ohms cm. e=3

will have an induced charge up time of well under 1 second and therefore likely to produce toner jumping upon separation. A record member with a resistivity of 2:1 X 10 ohms cm. provides an induced charge up time of well over one second. At through-put speeds of 4 inches per second and only a 1 inch contact zone there is very little or no toner jumping with good resolution. Record members of higher resistivities are even better.

It has been found that most plain papers when dried out to the equivalent of 1020% relative humidity conditions have volume resistivities of 1x10 ohms cm. or greater. Some papers such as those made with ion free water and without conductive contaminants show resistivities as high as 1 10 ohms cm. Electrostatic printing is therefore possible with the present invention using plain (without photoconductive or insulative coatings) paper.

Synthetic papers of Nylon, Orlon, etc. (such as Nomex, Tyrek of DuPont etc.) which have 51 X 10 ohms cm. are also useful as the record medium in the apparatus shown in FIG. 8.

It will be apparent from the foregoing that certain modifications of the disclosure can be made without departing from the spirit of the invention. Thus, for example, the interposed dielectric sheet 17 may have values of resistivity significantly lower than 1 1O ohms cm. as for example IXIO ohms em, if the toning step is rapid enough to prevent a bucking induced charge from being built up in the interposed sheet 17 of sutficient magnitude to prevent the primary field lines from the photoconductor reaching through the toner to the record sheet 18.

From the foregoing it will be seen that there has been provided electrostatic printing apparatus and methods in which a dielectric sheet has been interposed between the source of electrostatic charges corresponding to the indicia and the record sheet or paper upon which the printed image is produced. As a result of the interposed dielectric sheet, and the toning of the record sheet between the dielectric sheet and the record sheet, the surface bearing the electrostatic charges is protected from the toner and from abrasion thereby greatly prolonging its useful life. The apparatus and method lends itself to the use of a wide variety of photoconductive materials, electrostatic charging and imaging devices, and a wide variety of uncoated (plain) papers.

Having thus fully described the invention, what is claimed as new and desired to be secured by Letters Patents of the United States is:

1. The method of electrostatically producing visible images comprising the steps of selectively charging an image receiving surface to correspond to the indicia to be produced, bringing the said charged surface into proximity with a dielectric sheet and a record sheet closely adjacent the dielectric sheet on the side opposite the image receiving surface, said record sheet having a resistivity lower than the dielectric sheet; directing the electrostatic field lines coming from said charged surface through the said dielectric sheet to induce a charge of opposite sign upon the record sheet, transporting the said dielectric sheet and record sheet past the image receiving surface at a speed such that the image is induced substantially only in the record sheet, and toning the side of the record sheet facing the dielectric sheet to produce a visible image.

2. The method according to claim 1 which includes introducing the toner between the dielectric sheet and the record sheet.

3. The method according to claim 1 in which the toner is a liquid and includes squeegeeing the toned record sheet and the dielectric sheet dry between two rollers.

4. The method according to claim 3 which includes introducing the toner between the dielectric sheet and record sheet in an area upstream of the rollers and in the path of the electrostatic field lines passing through the dielectric sheet whereby a toning interval step is carried out prior to the squeegeeing step.

5. The method according to claim 1 which includes introducing the toner between the dielectric sheet and the record sheet after they have been brought into proximity with the charged image receiving surface.

6. The method according to claim 5 which includes squeegeeing the toned record sheet.

7. The method according to claim 1 in which the toner is a liquid toner.

8. The method according to claim 1 which vincludes bringing the dielectric sheet close to but spaced from the charged surface when the record sheet is in touching contact with the dielectric sheet.

9. The method according to claim 8 in which the record member has a resistivity of the order of 10 ohms cm.

10. The method according to claim 1 in which the record sheet is toned before an induced charge builds up in the dielectric sheet.

References Cited UNITED STATES PATENTS 2,901,374 8/ 1959 Gundlach 117-175 3,281,241 10/1966 Mihajlov 96-1 R 3,556,784 1/ 1971 Robinson et a1 96-1.4 2,824,813 2/1958 Fauser et a1. 117-17.5 2,890,923 6/1959 Huebner 117-17.5 3,013,878 12/19 61 Dessauer 117-17.5 3,102,045 8/1963 Metcalfe et a1. 117-37 LE 3,551,146 12/1970 Gundlach 96-1 R 3,703,376 11/1972 Gundlach 117-37 LE MICHAEL SOFOCLEOUS, Primary Examiner US. Cl. X.R.

96-1 SD, LY; 117-17.5; 118-637, DIG. 23; 355- 10, 17