US3240596A - Electrophotographic processes and apparatus - Google Patents

Description

March 15, 1966 H. c. MEDLEY ETAL ELECTROPHOTOGRAPHIC PROCESSES AND APPARATUS 2 Sheets-Sheet 1 Filed July 28, 1961 12 [V, ii J T 14 20 (0) DIRECT H POTENTIAL o SUPPLY T K \1 FIG 1 36 -J I 24 O 24 v 26'+ +J+ 20' DIRECT POTENTIAL SUPPLY 60 2o DIRECT T POTENTIAL FIG. 3

SUPPLY I I P P EXL F UWLY 59' \@;-4e

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HAROLD C. MEDLEY ROLAND MSCHAFFERT ATTORNEY March 15, 1966 Filed July 28 1961 H. C. MEDLEY ETAL ELECTROPHOTOGRAPHIC PROCESSES AND APPARATUS 2 Sheets-Sheet 2 CAP VOLTAGE BREAKDOWN VOLTAGE (VOLTS) GAP WIDTH (MICRONS) FIG. 2

United States Patent 3,240,596 ELECTROPHOTOGRAPHIC PROCESSES AND APPARATUS Harold Clinton Medley, San Jose, Roland Michael Schalfert, Saratoga, (Ialifi, assignors to International Business Machines Corporation, New York, N.Y., a corporation of New York Filed July 28, 1961, Ser. No. 127,725 5 Claims. (Cl. 961) The invention relates to electrophotography and electrostatic printing, and it particularly pertains to the transfer of latent electrostatic images from an electrophotographic plate or other medium to dielectric material.

The transfer of latent electrostatic images from one surface to another, as for example from an electrophotographic plate to a dielectric surface, provides a method of electrostatic printing or copying free from the steps of plate and drum cleaning, thereby eliminating the need for cleaning devices, and consequently improving the life of plates and drums and reducing the maintenance thereof. However, the prior art processes of electrostatic image transfer have not been found suitable for practical applications of the teaching. Despite some rather remarkable developments in the field, processes known in the art for the transfer of electrostatic images (an art at times referred to by the acronym tesi) have not found practical application in commercial electrophotographic or electrostatic printing as yet. The reasons for this situation apparently reside in certain critical relationships and/ or disadvantages pertinent to the prior art techniques, among which are critical spacing of surfaces at small but definite distances, critical voltage regulation, relatively high voltage operation required, and low and non-uniform resolution and contrast attained.

It is known to those versed in the art that the inherent resolution and sharpness of electrostatic images, such as those formed by the electrophotographic process on amorphous selenium plates, are extremely high. However the prior art methods of electrostatic image transfer require migration of electric charges across a relatively wide air gap which, due to image spread, results in a transferred electrostatic image with poor resolution and lacking in sharpness. Furthermore, these present methods require maintaining the width of the air gap within closely controlled critical limits. This is extremely diificult and almost impossible to achieve in practice, since variation of a few microns will result in non-uniform charge transfer. Thus, the transferred image will be non-uniform in strength resulting in density variations and generally poor quality of the developed image. Also, the use of prior art methods results in rather low contrast of the transferred image, since electric charges are transferred in background areas as well as in image areas, resulting in a charge pattern on the dielectric of the same polarity for both the background and the image charges. This condition results in poor contrast with high background when the transferred image is developed with fine electroscopic particles.

An object of the invention is to transfer a latent electrostatic image from an electrostatic image-bearing surface (such as a photoconductive-insulating surface) to a dielectric surface while these surfaces are in virtual contact, thereby attaining greater accuracy of reproduction of the original electrostatic image with a high degree of resolution and sharpness, and eliminating the necessity of maintaining an air gap of critical dimensions.

Another object of the invention is to transfer a latent electrostatic image to a dielectric surface whereby the resultant image areas on the dielectric surface are of one polarity of electric charge and the background areas are Patented Mar. 15, 1966 of the opposite polarity of electric charge, thus producing greater contrast and cleaner development of the image.

Another object of the invention is to provide a method for electrostatic image transfer resulting in a stable latent image-bearing medium which can be developed at a station removed from the immediate vicinity of the image transfer station.

A further object of the invention is to provide methods and apparatus for latent electrostatic image transfer where developing can be done under full illumination and under continuous visual inspection.

Still another object of the invention is to provide a record medium chemically and physically stable, unaffected by light or aging, and adaptable for spot processing so that variable data of a record may be changed from time to time without also reproducing the constant data.

A still further object of the invention is to provide a process of temporary electrostatic image transfer useful in producing images for projection display or for a pluralstep image reduction process utilizing low sensitivity dry processing film.

A still further object of the invention is to develop a process for transferring an electrostatic image to a hard copy medium, such as paper coated with dielectric film.

According to the invention, latent electrostatic images are produced on dielectric material by electric charge transfer from surfaces of photoconductive insulating material or another dielectric material upon which electrostatic images have been formed by known techniques. In one known method of forming the initial latent electrostatic image, a photoconductive insulating layer is sensitized in darkness by electrically charging the surface uniformly to a predetermined potential of given polarity with respect to the conductive surface backing element, and an image of the desired subject is optically projected onto the photoconductive insulating layer to produce a charge pattern corresponding to the image of the desired subject. Other methods of forming initial electrostatic images are known, such as pulsing shaped characters located closely adjacent to a dielectric surface with high voltage. However, the manner in which the initial electrostatic image is formed is not a part in itself of the invention.

According to the invention, the layer of dielectric material to which the electrostatic image is to be transferred is backed with a conductive element and the dielectric surface electrically charged uniformly to a potential of polarity opposite to the polarity of the charge of the initial electrostatic image. The charge pattern is then transferred by placing the charged layer of dielectric material in contact with the surface layer containing the initial electrostatic image in darkness, and applying a direct potential from an external source between the two conductive backings of the respective layers. The potential is applied between the conductive backing elements such that the lead attached to the conductive backing of the dielectric layer is of the same polarity as the charge originally placed on the dielectric layer.

The direct potential applied between the conductive surface elements after the materials have been placed in contact is preferably obtained from a variable source, so that it can be adjusted from zero to a relatively high value, up to several thousand volts.

Apparatus for automatically and continuously carrying out a process according to the invention comprises in one form a rotatable drum having a peripheral conductive surface element over which there is a layer of photoconductive material, conventional means for uniformly charging the photoconductive layer at given polarity to a predetermined potential of given polarity with respect to the conductive element, conventional means for projecting an optical image on the charged layer to create a latent electrostatic charge pattern on the layer. Another drum having a peripheral conductive surface over which a dielectric film material is laid, provides the conductive surface backing for the dielectric film material necessary during the processing. Conventional means are employed for charging the surface of the dielectric film material substantially to a predetermined potential of polarity opposite to the polarity of the charge on the layer of photoconductive material. The conductive surface elements of the drums electrically isolated and connected to opposite terminals of a charge transfer aiding potential source in accordance with the invention which is adjustable from zero to several thousand volts direct potential. The apparatus also comprises conventional means for discharging the photoconductive layer after transfer of the image therefrom to the dielectric film material and conventional means for developing the image transferred to the dielectric material.

In order that full advantage of the invention may be readily obtained in practice, a preferred embodiment thereof is described in detail hereinafter with reference to the accompanying drawings forming a part of the specification, and in which:

FIG. 1, sections (a), (b), (c) and (d) illustrate apparatus as arranged for particular steps in the transfer of an electrostatic image according to the invention;

FIG. 2 is a graph providing data and indicating conditions important in an understanding of the invention; and

FIG. 3 illustrates apparatus for carrying out the process of the invention in an automatic and continuous mode of operation.

FIG. 1 depicts the essentials of apparatus necessary for carrying out a method according to the invention for forming a latent electrostatic image on dielectric material. An electrostatic image, corresponding to a desired document is formed by conventional means on a known medium, for example, a xerographic plate comprising a conductive substrate 12 coated with photoconducting material 14 such as amorphous selenium. A conventional corona charging unit 16, energized by electric connections made through a polarity reversing switch 18 to a direct potential supply 20 capable of delivering between 4000 and 9000 volts, is swept across the surface of the selenium layer 14 to place a uniform positive charge thereon as shown in FIG. 1(a). This must be done in darkness as otherwise any light striking the selenium layer 14 will discharge the electric charge laid down. As shown in FIG. 1(b) the image of a document 22, illuminated by photo flood lamps 24 or other suitable photographic illuminating lamps, is projected by means of a lens system suggested by a symbolic lens 26 onto the xerographic plate 10 within some conventional arrangement (not shown) for excluding ambient light. The light areas 28 of the document 22 are projected on to the selenium layer 14 discharging the positive charge immediately there above and leaving the positive charges only in the areas corresponding to dark areas of the document 22.

Referring again to FIG. 1(a), a desired dielectric material 30 is backed by a conducting backing surface element 32 and sensitized by charging negatively to a uniform potential by sweeping with another conventional corona charging unit 36 connected through the electric reversing switch 18 to the direct potential supply 20. According to the invention the charge placed on the dielectric material 30 is always of opposite polarity to the charge placed on the plate 10. In the arrangement shown amorphous selenium is given as an example of photoconductive material. While amorphous selenium can be charged negatively and electrostatic image transfer accomplishcd wording to the invention, his well known that this material functions best when positively charged. It should be understood, however, that it is clearly within the scope of the invention to form an electrostatic image with either polarity and transfer the same to a dielectric material charged to the opposite polarity.

As shown in FIG. 1(a), the image record plate 10 which has been charged in accordance with the desired image is superimposed in darkness over the dielectric material 30 with conductive backing surface element 32 in place. The backed selenium 14 of the plate 10 and the backed dielectric material 30 are brought into contact without any direct electric connection between the conductive backing elements 12 and 32. The conductive backing elements 12 and 32 are then electrically interconnected, as by throwing an electric transfer switch 38. The image is transferred according to the invention when the conductive backing elements 12 and 32 are maintained at a predetermined direct potential obtained from a transfer aiding potential supply 39 capable of supplying direct potential in a range from zero to several thousand volts as will be described hereinafter. In the special case of interconnection at zero potential the switch 38 may be thrown to a contact bypassing the supply 39, if desired.

It is obvious, of course, that these methods can also be used to transfer images from negatively charged electrostatic images, in which case the dielectric material would be charged to a positive polarity. Most dielectric materials function about equally well with either polarity although polyethylene functions better with negative charging than with positive charging.

Laboratory tests of the above described methods have indicated that this technique is less sensitive to surface defects than prior known techniques of electrostatic image transfer.

Examples of dielectric material which have been used successfully: Polyethylene glycol terephthalate, which is a polyester most commonly known by the registered trademark Mylar; polystyrene; polyethylene; styrene butadiene copolymers. Aluminized Mylar film has been used as a material having a conductive backing surface integral with the dielectric with and without additional coating; aluminum plates; and conductive glass slides more commonly known by the registered trademark NESA have also been used for conductive backing surfaces.

The mechanism of transfer of electrostatic images from one surface to another has been explained on the basis of gaseous discharge phenomena. In practice, the gas usually will be air at atmospheric pressure. This gas, or air, fills the gap between the electrostatic image surface and the surface of the dielectric material to which the image is to be transferred. As the two surfaces are brought together, the gap between decreases from a relatively large valve to a very thin film even when the surfaces are brought into virtual contact. The gas film thickness at contact will depend upon the degree of surface smoothness. Surfaces polished to optical standards and placed in contact are still separated by gaps of the order of 3 X 10 cm. The gas film between a smooth amorphous selenium surface in contact with a smoother dielectric film is estimated to be in the range of 0.5 to 1.0 micron.

V Paschens law for the breakdown of gas in an electric field states that the breakdown voltage in a linear function of the product of gas pressure and distance between the electrodes. This law holds for values for the distancepressure product which are greater than about 5 mm. X mm. of mercury in air. Below this value, the breakdown voltage increases sharply for gases at low pressures. However, for relatively high pressures (for example, normal atmospheric pressure), the sharp upward turn of the breakdown curve is not observed. At these relatively high pressures, the gaps (below the Paschen minimum) p ating the surfaces are very small, and, discharge: is

due primarily to field emission because of the very high electric fields. For instance, the breakdown curve in air for very small gaps (less than about 8 microns for air at 760 mm. of mercury) is a downward sloping curve due to electron emission from the surfaces forming the gap. Such a curve is very useful in practice when it is desired to transfer charges across extremely small gaps.

It will be appreciated that charge transfer is preferred across a small gap to avoid the image spread which occurs when the charges are transferred across a relatively Wide gap. In all of the techniques of electrostatic image transfer, the image surface and the dielectric surface are brought together and then, subsequently, separated. During these manipulations it is desired that conditions be avoided that produce air breakdown discharges when the gap between the surface is large. Spark discharges are to be avoided under any conditions and according to the invention, transfer is made under conditions that will produce silent discharge when the surfaces are very close together; generally less than 3 microns apart. Preferably according to the invention while the surfaces are in contact, conditions are altered, as by closing the transfer switch 38, to provide a voltage across the gap which will produce discharge by field emission.

Reference is now made to FIG. 2 in which the heavy solid black curve M represents the minimum voltage for discharge as a function of gap width for air at atmospheric pressure. The nature of this curve M will be different for different gases and different pressures; however, the slope of the critical field emission line will be independent of the type of gas, depending only on the nature of the surfaces forming the gap. The plateau portion of the curve (at approximately 360 volts) is an extension of the Paschen minimum; the upward sloping line to the right of the plateau which is the lower portion of the normal Paschen curve for breakdown of the gaseous discharge; and the steep sloping line portion to the left of the plateau is the critical field emission line, which in FIG. 2 has been drawn to correspond to a slope of 10 volts per cm. Shown also in FIG. 2 are curves representing gap voltage versus gap width for several different conditions of applied voltage, charge voltage of the initial electrostatic image, charge voltage of the dielectric surface, and thickness and dielectric constant of the dielectric materials. The light solid curves A, B, C, and D indicate gap voltages in the regions of the electrostatic images and the the light broken curves A, B, C, and D are for gap voltages in the regions of the background areas.

Several examples of image transfer according to the invention will now be described in order to more clearly set forth a manner of practicing the invention.

Example 1 Applied voltage volts 1500 Charge voltage of electrostatic image do +500 Charge voltage on the dielectric layer do +1000 Thickness of image layer 14 microns 12.6 Thickness of dielectric layer 30 do 36 Dielectric constant of layer 14 6.3 Dielectric constant of layer 30 3.0

These conditions are illustrated by curves A and A in FIG. 2. It will be noted that both curves A and A are above the critical field emission line portions of the curve M. Thus, electric charges will transfer while the surfaces of layers 14 and 30 are in virtual contact. Since the curve A for the image area is higher than that of curve A for the background area, a greater amount of charge will transfer in the image area than in the background.

A process for accomplishing image transfer under the conditions set forth above is as follows:

A photoconductive layer 14 is given a positive surface charge of 500 volts with respect to the conductive base 12 as shown in FIG. 1(a). An electrostatic image is produced in the layer 14 as shown in FIG. 1(b). The dielectric layer 30 is then given a negative surface charge of 1000 volts with respect to the conductive base 32 as shown in FIG. 1(a). Layers 30 and 14 are then brought into virtual contact and the switch 38 in FIG. 1(c), is connected momentarily to the positive pole of a 1500 volt direct potential source. The switch 38 is then disconnected and the two surfaces of the layers 14 and 38 are separated.

It is found after the above procedure, that an electrostatic charge pattern corresponding to the initial image has been formed on the dielectric layer such that the image and background charges are of opposite polarity as shown in FIG. 1(d) the image areas having a positive polarity charge of approximately 260 volts, and the background areas having a negative polarity charge of approximately volts. This condition is particularly desirable for attaining high contrast in development, for example, with a developing technique such as the powder cloud method where the image surface is subjected to the cloud of aerosol particles electrically charged with negative polarity.

Furthermore, it will be noted that charge transfer took place while the surfaces were in virtual contact with the gap width in the range of 0.5 to 1.0 micron. Thus, this procedure is capable of achieving high electrostatic resolution in the transferred image, and there is very little loss of resolution during the transfer step. It is known that the maximum resolution attainable in electrostatic image transfer is an inverse function of the gap width, being approximately proportional to the reciprocal of the gap width. It has been found that when transfer takes place across a gap width of one micron, a maximum resolution of 200 lines per mm. is possible.

Example 2 Applied voltage volts +1000 Charge voltage of image do +700 Charge voltage on dielectric do 300 Thickness of dielectric layer 14 microns 25 Thickness of dielectric layer 30 do 25 Dielectric constant of layer 14 6.3 Dielectric constant of layer 30 3.0

These conditions are represented by the curves B and B in FIG. 2. It will be noted that as in the case of Example 1, the image charge is transferred by field emission while the surfaces are in virtual contact. Thus, a high maximum resolution is attainable in the transferred image. It will also be noted from curve B that only a small amount of charge is transferred in the back'gnound areas under these conditions.

The procedure for transferring electrostatic images with the conditions of Example 2 is essentially the same as for Example 1 except that the voltages applied are of different values.

It is found that after separation of the surfaces, an electrostatic image corresponding to the initial image is formed on the dielectric layer 30, such that the image areas are charged to a positive polarity of approximately volts, and the background areas are charged to a negative polarity of approximately 275 volts.

Example 3 Applied voltage 0 Charge voltage of image volts +800 Charge voltage of dielectric do 200 Thickness of layer 14 microns 25 Thickness of layer 30 do 25 Dielectric constant of layer 14 6.3 Dielectric constant of layer 30 3.0

These conditions give rise to curve C and C in FIG. 2. It will be noted that with these conditions charge will transfer in the image areas by gaseous discharge at about 6.7 microns whereas no discharge will take place in the background areas as can be seen from curve C, which curve C does not cross the minimum curve M.

The process for transferring electrostatic images under these conditions is somewhat different from the process used in Examples 1 and 2. In this case, the image layer 14 is charged positively to 800 volts and an electrostatic image formed on this layer in the usual manner. A negative charge of 200 volts is then applied to the dielectric layer 30. The surfaces are then brought together, and switch 38 is connected to the right-hand terminal of FIG. 1(0) to electrically interconnect the conductive backing surfaces 12 and 32 at substantially zero potential, and preferably at ground potential as shown. This connection is maintained as the surfaces are separated.

It is found that with this procedure and these conditions an electrostatic image is formed on the layer 30 such that the image areas are charged to a positive polarity of 195 volts, and the background remains charged to a negative polarity of 200 volts.

Since transfer takes place at 6.7 microns, the maximum resolution attainable for electrostatic image is transferred in this manner is about 30 lines per mm.

Example 4 Applied voltage Charge voltage of the image 'volts +700 Charge voltage of the dielectric layer 30 do 500 Thickness of layer 14 microns 50.4 Thickness of layer 30 do 48.0 Dielectric constant of layer 14 6.3 Dielectric constant of layer 30 3.0

These conditions result in curves D and D' in FIG. 2, from which it will be noted that transfer in the image areas takes place by gaseous discharge at a gap width of 11.0 microns, whereas no charge is transferred in background areas.

The procedure here is the same as Example 3, except that the voltages applied to the surfaces are different. After separating the surfaces it is found that an electrostatic image has been formed on the dielectric layer 30 such that the image areas are essentially neutralized whereas the background areas remain charged to a negative polarity of 500 volts. The maximum resolution attainable in this case is about 18 lines per mm.

Examples 1 and 2 are particularly suited for transferring electrostatic images of micro-image size, whereas Examples 3 and 4 are suitable for transferring images of intermediate or macro-image size, for example, images normally readable without optical aids.

The above examples all utilize initial electrostatic images of positive polarity. It will be appreciated that images of negative polarity could be transferred by similar procedures and processes, in which case the dielectric layer 30 would be charged to positive polarity, instead of negative as shown in and described hereinbefore.

Thus far the techniques of the invention have been described as a stepwise process performed with flat surface structural elements. Continuous processing, using an image retaining drum instead of the electrophotographic plate, is possible with the techniques and one such arrangement according to this invention is shown in FIG. 3. The essentials are shown in this illustration, it being understood that conventional methods and structures for transporting the various components of :the apparatus, shielding the charged areas from light or electrostatic fields, and the like, are readily apparent to those skilled in the art.

A drum conveniently completely metallic but at least having a conductive peripheral surface element 12' maintained at ground potential, as shown, and having a charge image recording layer, for example, of amorphous selenium, 14' thereon is arranged in a light tight housing 40, the upper part of which is hinged so that it may be 8 opened wide for access in feeding documents and servicing the unit.

Images on a continuous web of documents 22' (or on single documents inserted and removed by hand, one after the other, into a feeding slot 42), illuminated by a synchronized slit exposure system indicated only schematically by a pair of lamps 24' are produced in succession as charge images on the selenium drum 10' in more or less conventional manner. A web 30 of dielectric material unwinding from a feed reel 44 and winding on a takeup reel 46 is carried over guide rollers as necessary. At a point where the web 30 passes over a conductive drum 47 which is maintained at fixed reference potential, preferably ground, the web is given a uniform negative charge by means of a corona charging unit 36'. A pair of direct potential supplies 20', 20 are arranged to energize the corona charging units 16 and 36' with potentials of opposite polarity. A suitably housed lamp 60 is arranged to discharge the image charge remaining on the selenium coating 14' after charge transfer.

Insulating rollers 48 urge the web 30 into contact with the xerographic drum 10. Another drum 49 is arranged to back the web 30' at the point on the xerographic drum 10 where it is desired that the charge transfer process take place. This backing drum 49 has at least a peripheral conductive surface element 32' forming the backing conductive surface element of the dielectric web 30 during the transfer process and preferably connected to the shaft on which the backing drum 49 rotates as shown. The conductive surface element 32 on the backing drum 49 is electrically insulated from the remainder of the structure and connected, as exemplified by the electric charge transfer switch 38, to an aiding potential source 39', delivering from zero to several thousand volts direct potential. In the special case wherein it is desired to work at zero potential between the interconnected backing surfaces, as exemplified by the electric switch 38' in the right-hand position, the backing surface element 32 is at ground potential. More often it will be desired to use transfer aiding potential in which case the connection in FIG. 3 would be as exemplified by the electric switch 38 being thrown to the left-hand position. It should be understood that the electric switch 38' serves only to connect the aiding potential supply for the mode of operation desired; the switching during the charge transfer process being effected by the translation of the web 30' with respect to the drums 10 and 49.

The resulting charge pattern is developed by a conventional developing means; shown as a powder charge unit 50 comprising a powder 51 in a hopper 52 cascading down onto the web 30 as at point 53. The overflow powder is caught in a bin 54 by suitable arrangement (not shown) returned to the hopper 51 for later use. The image developed on the web 30' is then fixed by means of a heat fusing unit 58 according to known techniques.

By suitable arrangement, the lens 26 can be shuttered; the corona discharge unit 16 can be disconnected; and the discharge lamp 60 can be extinguished, so that the residual charge on the selenium layer 14' can be used to transfer additional images as desired. Conceivably the arrangement can be so shuttered in known fashion that an image on the selenium coating 14' can be discharged in a local area only and a new portion inserted thereat for spot updating of the information recorded.

The invention claimed is:

'1. A method of transferring an electrostatic image on a photoconductive insulator material to dielectric material,

0 comprising the steps of placing a uniform charge on one face of said dielectric material of polarity opposite to that on one face forming said electrostatic image on said insulator material,

placing said one face of said charged dielectric material in virtual contact with said one face of said charged insulator material, electrically interconnecting said dielectric material and said insulator material at a predetermined direct potential difference for electron flow therebetween,

disconnecting said electric connection between said materials, and

separating said materials,

thereby transferring said image to said dielectric material with the image thereon at one polarity and the background at opposite polarity.

2. A method of transfering an electrostatic image on a face of photoconductive material backed by a conductive surface element to dielectric material, comprising the steps of backing said dielectric material with a conductive surface element,

placing a uniform charge on the face of said dielectric material of polarity opposite to that forming said electrostatic image on the face of said insulator material,

placing said face of said charged dielectric material in virtual contact with said face of said charged insulator material,

electrically interconnecting said backing surface elements at a predetermined direct potential difference, disconnecting said electric connection between said elements, and

separating said materials,

thereby transferring said image to said dielectric material with the image thereon at one polarity and the background at opposte polarity.

3. A method of transferring an electrostatic image on a face of photoconductive insulator material backed by a conductive surface element to dielectric material, comprising the steps of backing said dielectric material with a conductive surface element,

placing a uniform charge on the face of said dielectric material of polarity opposite to that forming said electrostatic image on the face of said insulator material,

placing said face of said charged dielectric material in virtual contact with said face of said charged insulator material,

electrically interconnecting said backing surface ele-.

ments at zero direct potential difference, disconnecting said electric connection between said elements, and

separating said materials,

thereby transferring said image to said dielectric material with the image at one polarity and the background at opposite polarity.

4. A method of transferring an electrostatic image on a face of photoconductive insulator material backed by a conductive surface element to dielectric material, comprising the steps of backing said dielectric material with a conductive surface element,

placing a uniform charge on the face of said dielectric material of polarity opposite to that forming said electrostatic image on said insulator material, placing said face of said charged dielectric material in virtual contact with said face of said charged insulator material, electrically interconnecting said backing surface elements at a predetermined direct potential difference of substantially less than five hundred volts, disconnecting said electric connection between said elements, and separating the said materials, thereby transferring said image to said dielectric material with the image at one polarity and the background at a polarity opposite to that of said image. 5. A method of transferring an electrostatic image ona face of photoconductive material backed by a conductive surface element to dielectric material, comprising the steps of backing said dielectric material with a conductive surface element, placing a uniform charge on the face of said dielectric material of polarity opposite to that forming sa d electrostatic image on said insulator material, placing said face of said charged dielectric material in virtual contact with said face of said charged insulator material, electrically interconnecting said backing surface elements at a predetermined direct potential difference ranging between zero and five hundred volts, disconnecting said electric connection between said elements, and separating said materials, thereby transferring said image to said dielectric material with the image represented by charge of one polarity opposite to that of the background.

References Cited by the Examiner UNITED STATES PATENTS 2,909,971 10/1959 Barber 961.7 2,937,943 5/1960 Walkup 96-1 2,946,682 7/1960 Lauriello 961 2,947,625 8/1960 Bertelsen 961 2,968,553 1/1961 Gundlach 96-1 2,982,647 5/1961 Carlson et al. 96-1 2,984,163 5/1961 Giaimo 1.7 3,062,110 11/1962 Shepardson et al. 951.7 3,147,679 9/1964 Schafrert 95-1.7

FOREIGN PATENTS 607,290 10/1960 Canada. 807,079 1/ 1959 Great Britain. 855,727 12/1960 Great Britain.

NORMAN G. TORCHIN, Primary Examiner.

HAROLD N. BURSTEIN, Examiner.

Claims (1)

1. A METHOD OF TRANSFERRING AN ELECTROSTATIC IMAGE ON A PHOTOCONDUCTIVE INSULATOR MATERIAL TO DIELECTRIC MATERIAL, COMPRISING THE STEPS OF PLACING A UNIFORM CHARGE ON ONE FACE OF SAID DIELECTRIC MATERIAL OF POLARITY OPPOSITE TO THAT ON ONE FACE FORMING SAID ELECTROSTATIC IMAGE ON SAID INSULATOR MATERIAL. PLACING SAID ONE FACE OF SAID CHARGED DIELECTRIC MATERIAL IN VIRTUAL CONTACT WITH SAID ONE FACE OF SAID CHARGED INSULATOR MATERIAL, ELECTRICALLY INTERCONNECTING SAID DIELECTRIC MATERIAL AND SAID INSULATOR MATERIAL AT A PREDETERMINED DIRECT POTENTIAL DIFFERENCE FOR ELECTRON FLOW THEREBETWEEN, DISCONNECTING SAID ELECTRIC CONNECTION BETWEEN SAID MATERIALS, AND SEPARATING SAID MATERIALS, THEREBY TRANSFERRING SAID IMAGE TO SAID DIELECTRIC MATERIAL WITH THE IMAGE THEREON AT ONE POLARITY AND THE BACKGROUND AT OPPOSITE POLARITY.

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