US3464818A - Method of photoelectric copying - Google Patents

Description

p 2, 1969 M. WALY 3,464,818

METHOD OF PHOTOELECTRIC COPYING Filed June 22, 1967 5 Sheets-Sheet 1 Arrpeygys Sept. 2, 1969 M. WALY 3,464,818

METHOD OF PHO'I'OELECTRIC COPYING Filed June 22, 1967 3 Sheets-Sheet 2 //V MFA/T02 MOI/STA PHA wALY Sept. 2, 1969 M. WALY, 3,464,818

METHOD OF raowommcmc comm Filed June 22, 1967 s Sheets-Sheet 5 United States Patent Office 3,464,818 Patented Sept. 2, 1969 Int. Cl. G03g 13722; H01j 37/26 US. CI. 96-13 8 Claims ABSTRACT OF THE DISCLOSURE The method of the invention consists in forming an electrostatic image upon a photoconductive surface, in uniformly charging the insulating surface of a nonphotosensitive and nonphotoconductive paper sheet, in bringing said photoconductive surface in the presence of said insulating surface at a distance thereof comprised between 0.01 and 120 microns by subjecting said surface to an electric field comprising a high frequency alternative current component and a direct current component and in developing and fixing the electrostatic image so transferred on said insulating surface. The frequency of the alternative current component is comprised between 1.2 kilohertz and 3000 megahertz.

The present invention essentially relates to a method of photoelectric copying or manifolding; more specifically it is concerned with a new xerographic process for making true copies of documents from opaque or transparent on light and cheap papers; these papers also are a subject matter of the invention in as much as they have been subject to a special preparation to be described hereinafter.

The usual xerographic art presently amounts to two methods which are recalled hereinafter. The first of these methods consists in exposing, through contact or an optical way, to the document to be copied, a metal plate or drum coated With a thin photoconductive layer previously charged with electricity. In most cases, the photoconductive layer of amorphous selenium deposited in a vacuum on an aluminum plate or drum previously suitably treated. This selenium coat retains the electric charges in the darkness and dissipates them when subject to an actinic illumination. When after charging such a surface in the darkness, an optical image of the document to be reproduced or copied is projected thereon, the spots, locations or areas of this surface, corresponding to the opaque Zones of said document (black lines of a letter or image), keep their electric charge whereas the spwts, areas or locations, corresponding to the blanks of the document, loose the charge. Thus an electrostatic image of said document is obtained. While still operating or working in the darkness, said layer is dusted with finely divided electroscopic powder which adheres or sticks to the areas or spots left charged thereby forming an electrostatic image of said document. While still in the darkness, a paper sheet, web, band or tape is brought into contact or engagement with the plate or drum. The particles of the electroscopic powder are transferred to the paper on which the replica, reproduction or copy has to be made with the assistance of an electric charge applied to the back of the latter. These particles temporarily adhere or stick by induction on said paper, and, after treatment with infra-red radiation, a permanent, contrasted and fixed document is achieved on this paper. It should be noted that the photoconductive surface made of amorphous selenium is a P-type conductor, which conductor may also receive a positive charge, and this requires or calls for the use of a negatively charged electroscopic powder.

In the second of these usual xerographic methods, the copying or reproduction of the document is directly elfected on a photoconductive surface supported or carried by a paper sheet. In this case, the photoconductor, generally consisting of zinc oxide, is finely dispersed in a highly insulating varnish applied in the form of a thin coat or layer to an ordinary paper. This zinc oxide containing layer or coat is then charged electrostatically, so as to impart thereto a negative polarity with respect to the backing paper or paper support. This layer or coat is then exposed in a known manner (illumination of the document and projecting of the pattern thereof by an optical system) to the document to be copied or reproduced. The electrostatic image achieved on this layer or coat is then developed by means of an electroscopic substance applied to the photoconductive surface by well known processes, either in a smoke chamber, or within an insulating liquid medium (petroleum, etc.) containing carbon black or flame black dispersed and maintained in suspension, either by means of a magnetic brush or even by directly applying a positively charged electroscopic powder.

Both of these known xerographic methods suffer from significant drawbacks:

In the first one of these methods, the apparatus is delicate and its maintenance which represents a substantial part of the cost price of the copy owning to the fact that the latter is achieved on almost ordinary paper, prohibitively increases this cost price.

A second inconvenience of this method resides in the fact that the drum comprising the photoconductive layer or coat, has to be subject to a periodical cleaning after the making of each reproduction or copy, so as to remove the residual electroscopic powder. In spite of such a cleaning being carried out with rotary brushes having a smooth or soft action, there occurs an attack of the photoconductive layer or coat the thickness of which should have to be maintained strictly constant (about 20 microns); it results therefrom a distortion in the copy or reproduction.

The second one of said methods produces expensive copies, on account of the fact that each one of the latter comprises a photoconductive layer or coat on the one side of a backing paper or a paper support which had to be coated with a thin layer of zinc oxide. For this purpose, this backing paper or paper support is at first finely crushed, ground, pounded or milled and dispersed within a highly insulating binder which is soluble in a hydrocarbon. In order to avoid the penetration of the solvent while spreading the photoconductive layer or coat, the paper is carefully provided on the recto and verso with another suitable varnish, which is insoluble in said hydrocarbon. Consequently, the production on an industrial scale of such a paper, the photoconductive layer thickness of which should be kept constant, to 2 or 3 microns for a useful thickness of about 15 to 20 microns and the accuracy required during coating by means of special spreading machines are as many factors which substantially increase the cost price of this paper and decrease or reduce the efficiency of the method. Moreover, the paper used is very heavy: the combined weight of the support and photoconductive layer or coat amounts to about g./m. whereas the weight of a paper of current use for ofiice work is of the order of magnitude of 60 g./m.

In summary, the effect of the weight and the high cost of this paper is to exclude this second method from the techniques of copying or reproduction on an industrial scale.

The method according to the present invention allows to obviate to the aforesaid drawbacks, by the fact that it is of a low cost price in relation of the use or working of a simplified apparatus of easy maintenance and the use of a cheap and light current paper, to achieve copies or reproductions; in addition, it yields a very true copy of the original, even of the half-tints thereof; it may be adapted to the panchromatic copying and enables ready obtainment of a plurality of identical copies.

This method comprises the steps of forming, from the original to be copied, an electrostatic image on a photoconductive surface carried, supported, or backed by a base conducting static electricity, uniformly charging with static electricity an insulating surface of a nonphotosensitive and nonphotoconductive paper sheet or the like, then bringing or feeding with the exclusion of active light said photoconductive surface into contact or engagement with or in the immediate vicinity of said insulating surface, by subjecting the area or zone wherein both of said surfaces are thus fed, to the action of an electric field having a high frequency A.C. component and a continuous or DC. component, and developing and fixing in a manner known per se, after having separated said photoconductive surface from said paper sheet, the electrostatic image formed on the insulating surface of this paper sheet.

The original to be reproduced or copied may be printed, written in pencil or in ink, be in color or in black and white, and carried or backed by an opaque, translucent or transparent support; this support or backing may be a paper or a plastic film or any other support of suitable nature adapted to comprise characters or types, signs or patterns or designs; this original may consist of a photosensitive or nonphotosensitive paper, film, skin or plate comprising a pattern or design (text and/or drawings), obtained by any photographic or xerographic process or any other means. This pattern or design may be a positive or a negative; it may furthermore correspond to a light image or to an inverted or reversed right-left image.

The photoconductive surface, upon which is formed or built up the first aforesaid electrostatic image, is chiefly made from or contains at least one of the following compounds: zinc oxide, cadmium sulphide, anthracene, the triazoles, the oxaziles, the thiazoles, the triazines, etc.

In the case where the selected photoconductor is zinc oxide, the uniform charge imparted to the photoconductive surface will be a negative charge and it will then be necessary that the uniform charge imparted to the insulating surface of the paper be negative; with other photoconductors, it is a positive charge which should be imparted to the photoconductive surface and it will then be necessary to positively charge said insulating surface. In general, it matters that this insulating surface and the photoconductive surface have been charged with static electricity of constant sign.

The concept of mutual contacting or engagement of both aforesaid surfaces has not to be defined; the concept of the immediate vicinity of these two surfaces means that both of said surfaces are placed at a distance from each other that the copying or reproduction takes place with the same results as if they were in mutual contact, the conditions or requirements of the process remaining besides unchanged; this case may correspond to the presence of a free intermediate space or gap of small dimensions or size, generally occupied by air, between both aforesaid surfaces, then disposed preferably in confronting or opposed relationship; it may also correspond to the juxtaposition of the supports or backings of both surfaces or even to the juxtaposition of one of these surfaces with the other surface; intermediate layers or coats of various natures, not connected or bounded to said backings or supports may also be provided between said surfaces. According to the present invention, the distance between both aforesaid surfaces is in the range between 0.01 and 120 microns, this last value corresponding to the upper limit specifying what is meant by immediate or close proximity.

The continuous or DC. component of said-electric field is preferably obtained from a direct current source connected to a conductive support or backing of the said paper sheet or web; this continuous or DC component may also correspond to the induction effect produced by an electret disposed near the area wherein both aforesaid surfaces are in mutual contact or in close or immediate proximity to each other.

According to the present invention, the frequency of the alternative current component should correspond to a value ranging from 1.2 kilohertz up to 3000 megahertz and, preferably from 15 kilohertz to 3000- megahertz; this frequency may be supplied by any device known per se, in particular by a high frequency oscillator.

The area of both aforesaid surfaces, wherein the latter are subject to the action of the electric field with a high frequency component, may be of a more or less large extent; this area, which has in principle a stationary position in space, may be or not movable with respect to both aforesaid surfaces according as the process used is continuous or not. In the case of a continuous process, both aforesaid surfaces will move with the same speed at this zone or area during a time long enough for the whole pattern or design to be copied (that is the electrostatic image to form on said insulating surface from the electrostatic image on the photoconductive surface) to pass or move in said area or zone; preferably, in the case of such a continuous process, said area will have a small width of the order of a few millimeters, so as to gradually form the final electrostatic image by sweeping (in successive garallel rows) both aforesaid surfaces with the electric eld.

With the term paper or the like or more simply by the term paper used hereinafter, is meant a paper of cellulosic nature or even a film or skin of plastic material of suitable nature or still any other suitable thin support, carrier or backing. In general, a sized or nonsized cellulosic paper of current type will be used for economys sake of the method; it is compulsory or requisite that such a paper has at least one insulating surface for the formation of the electrostatic image under the conditions or according to the requirements of the process of the invention; the presence of this insulating surface may be due to the fact that the paper is itself dry and insulating enough throughout its thickness or even it may be due to an insulating layer or coat applied on this paper by any suitable means or formed within an initial superficial coat or layer of this paper, for example by absorption and/or spreading.

According to a further feature of the present invention, a paper of current type, of the kind used in offices, is used, which will have been treated or processed or the manufacturing method of which will have been conducted in such a manner that it comprises, on at least one of its sides, an outer insulating coat or layer, which may extend throughout the whole thickness of this paper; preferably, this layer or coat will consist of a dispersion of insulating substance in a suitable binder. The paper weight such as used in the method of the invention, may be of the order of 40 to 70 g./m. preferably in the vicinity of 55 g./m.

The invention will be better understood and further objects, features, details and advantages thereof will become apparent as the following description proceeds, with reference to the diagrammatic drawings, given by way of example only and wherein:

FIGURE 1 is a diagrammatic of a first device for carrying out the method of the invention in a continuous operation;

FIGURE 2 is a diagrammatic view at an enlarged scale of the discharging zone of the device according to FIG- URE l, on which view a symbolic illustration of the distribution of charges has been adopted in order to shown the assumed mechanism of the process;

FIGURE 3 is a diagrammatic view illustrating the principle of the invention in the case of a discontinuous process;

FIGURE 4 is a diagrammatic view showing the principle of the invention and corresponding to an alternative embodiment or modification of the device of FIG- URE 3;

FIGURE 5 shows a diagrammatic view of a second device for carrying out the method of the invention;

FIGURE 6 is a cross-sectional view of a scorotron-type device for the uniform and regular electrostatic charging of a surface;

FIGURE 7 is a diagrammatic view of a third device for carrying out the method according to the invention;

FIGURE 8 is a diagrammatic view of a fourth device for carrying out the method according to the invention.

The device of FIGURE 1 comprises two metal drums 3 and 4 with substantially parallel axes, which may drive or carry along, in the direction shown by the arrows f, a paper sheet or web 2 having a photoconductive surface and called hereinafter photoconductive paper as well as a light paper sheet or web 1 with an insulating surface; these drums have like diameters and rotate at the same speed; they are mounted on a common frame 15 made of insulating material; the Spacing of the drums 3 and 4 is such that the distance between the photoconductive surface of the photoconductive paper 2 and the insulating surface of paper 1, which surfaces are disposed in confronting or opposed relation to each other, be comprised between 0.01 and 120 microns.

The axes 16 and 17, respectively, of the drums 3 and 4 are connected, through the medium of frictional or sliding contacts 5 and 6, respectively, with an electronic device which comprises a high frequency circuit and a direct current circuit. The drum 3 is grounded through a resistor 12 mounted in parallel or as a shunt with respect to or across or as a branching off the capacitor 7 and the drum 14 is connected through the capacitor 18 to the high frequency oscillator 9 of the high frequency circuit as well as through the resistor 11 to a source of direct current of negative polarity (negatively or positively boosted, that is stepped down or stepped up, and rectified current of the mains). The cold point of the oscillator 9 as well as the positive pole of the direct current source ltl are grounded.

Between the common point of the high frequency circuit and of the direct current circuit and: the ground, a small neon glow lamp 8 is inserted in series with a resistor 13 across which is connected or branched off a capacitor in parallel relation thereto; this arrangement or assembly acts as a stabilizer for the voltage at the axis 17.

The high frequency path is set up as follows: oscillator 9, capacitor 18; frictional or sliding contact 6, axis 17, mass of the drum 4, capacitor 14, mass of the drum 3, axis 16, frictional or sliding contact 5, capacitor 7, ground. The capacitor 14 consists of the opposed or confronting portions of both drums 3 and 4 as well as by the areas of the paper sheets or webs disposed on these portions and by the air lying between the outer surfaces of these areas.

The path followed by the direct current produced by the source 10 comprises the resistor 11, the frictional or sliding contact 6, the axis, shaft or spindle 17, the mass of the drum 4, the area 14, the mass of the drum 3, the frictional or sliding contact 5, the axis, shaft or spindle 16, the resistor 12. and the ground.

The device of FIGURE 1 operates in the following manner: the photoconductive coat or layer of the sheet or web 2 being formed by the zinc oxide dispersed within an insulating varnish, said layer or coat has been negatively charged at the surface thereof in the darkness by a corona-discharge; then the pattern or design of the document to be copied or reproduced has been optically projected, so as to form a negative electrostatic image, the spots or areas of the photoconductive surface corresponding to the portions in black of the original, having been discharged during the projection of said pattern or design. The paper sheet or web 1 has been uniformly charged with static electricity of negative sign. The sheets or webs 1 and 2 are then secured or fastened upon the drums 4 and 3, respectively; the high frequency oscillator 9 and the source of direct current 10 are then switched on; rotation is imparted to the drums 3 and 4, for example in the direction of the arrows and the peripheral speed of the drums is adjusted to a value lying between 2 cm./ s. and cm./s. When both sheets or webs have travelled or moved in confronting, registering or opposed relationship over their whole length, both drums are stopped and the sheet or web 1 is removed from the drum 4.

The development of this sheet 1 is then effected by the methods known in electrophotography. For example, this sheet or web is introduced into a dust chamber, where it is subject to a cascade or succession of positively charged electroscopic particles or the surface of this paper is sprinkled or sprayed with 'an insulating liquid containing in suspension positively charged flame or lamp black particles; according to this last method, the development occurs through process which is a combination of electrophoresis and dielectrophoresis.

The image is then fixed by methods known per se by subjecting the paper sheet or web, upon which :adhere or stick the particles forming the developer of the electrostatic image under the action of a heat flux, for example an infra-red radiation.

Once the development is effected, an image may be observed or noticed on the paper sheet or web, which image is the true and contrasted reproduction, replica or copy, with the finest details, of the pattern or design of the original document. The electroscopie particles with a positive charge have therefore been attracted at the spots or areas Where the negative charge of the paper sheet or web 1 has not been discharged in the zone or area 14; it may be noted that in addition the half-tints, that is the greys, are truly reproduced; until now, all of these qualities could never be obtained to such an extent in xerography.

FIGURE 2 illustrates and explains in a detailed manner, the assumed mechanism of the selective discharge of paper 1 in the area or region 14. On this figure may be seen that the photoconductive paper 2 consists of a paper base 2a and of a photoconductive layer or coat 2b upon which have symbolically been shown by small circles, the negative charges N constituting the electrostatic image formed on said photoconductive layer or coat during the optical projection of the pattern or design of the original to be copied; these charges N in alternation with the intermediate spaces 211', free or devoid of charges, therefore represent the latent image resulting from said optical projection; these charges N correspond to the blacks of the pattern or design of the original and said intermediate spaces 2b correspond to the whites of this pattern or design.

It should be noted that the paper base 2a is a very good static electricity conductor, whereas the photoconductive layer or coat 2b forms a chain comprising in alternation perfect charged insulators (zones or regions of localisation of the charges N and electricity conductive spaces (intermediate spaces 217); these intermediate spaces 2b are therefore connected by a conductive chain to the high frequency circuit and to the direct current circuit.

The paper sheet or web 1, rolled up round, Wound, curled or wrapped about the metal drum 4, comprises a paper support, backing or carrier 1a, hence a very good static electricity conductor and a thin layer or coat 1b made of a plastic material which forms an insulating material; this insulating layer or coat has no photoconductive or photosensitive quality; in its upstream portion, having not yet reached the area or zone 14, the insulating layer or coat 1b of this paper sheet or web is negatively charged as previously stated; the negative charges N of this layer or coat are symbolically shown by small circles.

Under the influence of the high frequency component of the electric field, the zone or region 14 is subject to an ionization in conditions such that there appears in this zone or area a high frequency plasma, which is a mixture of ions of opposed polarities and of electrons; the continuous or DC. component of the field tends to cause the discharge of the insulating layer or coat 1b in those portions thereof which are confronting, opposite to or registering with the intermediate spaces 21), free or devoid of charges, of the sheet or web 2; the high frequency component enables to activate and to regul-arize this discharge which corresponds to the removal or departure of the negative charges N of the insulating layer or coat 1b; the regularity or regular fashion of this discharge is shown by the truth or faithfulness of the reproduction or copy on the paper sheet or web 1 of the electrostatic image of the sheet or web 2; moreover, this truth or faithfulness warrants that there is no discharge opposite to the charges N when these are located or positioned opposite to the charged insulators formed by the zones or areas of localization of the charges N on the photoconductive surface of the sheet or web 2; the activation of the discharge by the high frequency component consists essentially in the fact that the threshold of the discharge potential or voltage of the insulating surface of the sheet or web 1 is clearly or distinctly lower than the discharge threshold which would be present in the case where no high frequency plasma would have been formed or built up in the space 14 owing to the use of an electric field that does not comprise any high frequency component.

The electrostatic image, which is formed in the zone or area 14 on the insulating layer or coat 1b, is the rightleft inverted or reverse one (that is the figure symmetrical with respect to a plane) of the electrostatic image present on the photoconductive layer or coat 2b; this inversion or reversal may be automatically compensated during the optical projection of the pattern or design of the original document, so that to an original, comprising a noninverted or nonreversed pattern or design, an identical noninverted or nonreversed copy or reproduction may be made to correspond directly; it is conceivable that this compensation also allows to obtain an inverted or reversed copy or reproduction from an original comprising an inverted or reversed pattern or design; on the contrary, the absence of compensation produces the right-left inverted or reversed copies or reproductions from originals comprising noninverted or nonreversed patterns or designs and conversely; it should moreover be noted that the method according to the invention makes directly correspond a positive copy or reproduction to a positive original and a negative copy or reproduction to a negative original; the terms negative and positive used hereinabove, have the same meaning as in photography or in chemical-photocopying and are not related to the presence of electric charges.

It should be noted that the action of the high frequency component of the field decreases quickly beyond the space 14 corresponding to the zone of closest approach of the drums 3 and 4; beyond this zone or region, there is no more ionization of the air and no discharge takes place.

It should be noted that the discharge of the insulating surface is effected in the conditions mentioned hereinabove which are requisite for the sharpness and other aforesaid qualities of the reproductions or copies obtained or achieved by the method of the invention, due to the presence of the tank or storage of charged particles of differing polarities, formed by the high frequency plasma in the space 14; it is just this ion storage tank which leads to the obtainment or achievement of a uniform distribution of all the ionization states which are favourable to a selective and homogeneous discharge of the negatively charged parts of the insulating layer or coat 1b lying in front of or oppositely to the conductive intermediate zones 2b of the photoconductive layer er coat 2b; all these secondary effects, such as Lichtenberg figures, trees, stars, etc. encountered with the charge transfer processes, are removed.

Such transfer processes are for example illustrated by the US. Patent No. 2,982,647 of May 2, 1961, where is described a transfer effected from a photoconductive surface to an insulating surface; but until now, only poor and not industrializable results have been obtained, on account of a large number of secondary ionization effects which make it impossible to achieve an acceptable copy from an original.

The distribution of negative charges N of the photoconductive surface of the sheet or web 2 is not substantially modified or altered after passage in the zone 14; therefore, there is no charge transfer from one surface to the other, but a selective discharge of the charged insulating surface of the sheet or web 1, by the inducing effect produced by the charge distribution over the photoconductive surface of the sheet or web 2, owing to the action of said high frequency component.

This absence of modification or alteration of the photoconductive surface 2 is shown by the two following tests:

The sheet or web 2 is developed and fixed, in a fashion known per se in xerography, after passage in the area or zone 14; an image is obtained which is identical or like that achieved by development and fixation of the paper sheet or web 1, but inverted or reversed right-left with respect to this latter;

The charges deposited on the respective surfaces of both paper sheets or webs are measured as well as the remanent charges before and after copying and it is found or verified that these have not been modified or altered during the operation of the method.

It is therefore possible to obtain a great number of high quality copies or reproductions from an initial electrostatic image formed on the photoconductive layer or coat 212.

It should be noted in addition that the same photoconductive paper 2 may serve for the obtainment or achievement of reproductions or copies of new documents which are different from the preceding ones, provided that the first electrostatic image, formed on this paper, be at first effaced by dissipation of the remanent or residual charges after printing off or developing; it sufl'ices therefor to subject the photoconductive paper to the action of an infra-red radiation, and then for a few moments, to darkness, i.e., exclusion of active light.

On FIGURE 3 are seen two electrodes consisting of metal plates between which are maintained in contact with each other, a photoconductive paper sheet or Web 2 and a paper sheet or Web 1 coated with an insulating layer, the photoconductive layer 2b of the sheet or web 2 being in contact with or engaging the insulating layer 1b of the paper sheet or web 1; an electrostatic image has been formed, from an original document, on the photoconductive layer 2b in the manner described hereinabove; the insulating layer 112 has been uniformly charged with static electricity of the same polarity as that of said electrostatic image; the electrode 42 is grounded and the electrode 43 is connected to a high frequency oscillator 44 on the one hand and to a direct current source 45 on the other hand; the detail of the electronic diagram is for instance that shown on FIGURE 1.

The achievement of the electrical connection between the electrodes 42 and 43 enables the simultaneous formation, on the insulating layer 1 b of the sheet or web 1, of all the portions of the electrostatic image of the sheet or web 2; the operation is carried out as previously in the darkness, i.e., with the exclusion of active light; the upper or top electrode is rendered removable owing to a suitable sliding or pivoting arrangement not shown, a flexible or yieldable spring-like electric contact 46 allowing such a removable condition; after printing off or developing, the electrical connection between said circuits is broken or interrupted and the removable electrode 43 is withdrawn or removed so as to space or move the sheet or web 1 away from the sheet or web 2; the development and fixation of the latent electrostatic image formed on the insulating layer 1a gives or produces an image which is the true and suitably contrasted reproduction or copy of the pattern or design of the original document.

The discharge potential or voltage threshold, referred to with respect to FIGURE 2, is practically zero owing to the presence of the high frequency component; however, for a zero potential or voltage (absence of the continuous of DC. component), the reproductions or copies are often too pale, so that in practice, one should contrive to have the potential of the electrode 43 with respect to the electrode 42 (FIGURE 3) or of the drum 4 with respect to the drum 3 (FIGURE 1) be of the order of at least a few volts, preferably of the order of a few hundred volts.

It should be noted that in the charge transfer process of the US. "Patent No. 2,937,943, reference is made to the application of a critical transfer voltage. The graphs, charts or curves of the drawings of this patent show for each distance between two conductors, the breakdown or disruptive voltage of the ambient air. It is interesting to note here that, due to the fact that the voltage in question is the voltage of migration of the charges under a critical stress or constraint of the ambient air, these curves or charts show an extrapolation of the Townsend curve. When there is a breakdown, there is a formation of an ion avalanche due to the multiple collisions with the gas atoms (air), of the electrons accelerated by the field applied to the terminals of the electrodes.

In the method which is the subject matter of the present invention, the curves, charts or graphs shown in the American patent referred to hereinabove do not play any part since a method of discharging in a high frequency plasma is used under conditions such that the discharge begins gradually when the least induction takes place.

The absence of discharge potential threshold (critical breakdown or disruptive voltage) may be shown by the fact that the method is still applicable while yielding the same results when the continuous component of the field is produced, in the absence of any direct current source, by the discharge of an electret; on FIGURE 4, which illustrates this alternative embodiment, an electret 47, consisting of an elastomeric silicone sheet wherein cadmium sulphide has been dispersed, is mover towards or drawn near the electrode 43' of a device besides like that of FIGURE 3, but comprising no source of direct current such as 45; the discharge of the insulating surface previously uniformly charged of the paper sheet or web 1' is operated by progressive or gradual sweeping by parallel rows, since as the high frequency oscillator 44' has been at first connected with the electrode 43', one side of the electret 47 is at first applied to this latter electrode which is then gradually covered by the electret due to a movement of continuous deformation of the latter, allowing its discharge by successive parallel rows, when these rows are put into contact with said electrode 43.

The device of FIGURE 5 comprises a storage or reserve of photoconductive paper which is stored in the form of a reel or roll 26 on a mandrel or the like 27. This paper advances or is fed in the direction of the arrow and passes at first below a charging station 20 (of the scorotron type) the conductor 1% of which is brought by a high voltage source 19a to a voltage of 3,500 to 12,000 volts with respect to the envelope, shell or casing 190 connected to the ground. The paper 2 thus charged moves past an exposure station comprising a lens 29 and illumination or lighting means 24 and 25 past or in front of which the original .30 moves in a direction opposite to that of motion of the paper 2. After having passed around the rotary drum 3 where it is put in the presence of a paper 1 driven or carried along by the rotary drum 4 under the conditions described with respect to FIGURE 1, the paper 2 is exposed to a uniform illumination owing to a light source 23 in order to efiace and to dissipate the remanent or residual charges of its photoconductive layer, then it is rolled up or wound on and about a mandrel or the like 28. The copies or reproductions are made on the paper 1 the insulating surface of which has been previously charged by the charging station 22 of the scorotron type (the conductor 21a of which is brought to a high voltage with respect to the ground by means of a high voltage source 21b). After having exhausted the paper of the reel or roll 26, for example after having achieved about one hundred copies of the size 21 x 27 cm. which is usual with papers for ofiice purposes, the reel or roll 33, upon which the whole paper 2 is now rolled up or coiled, is substituted for the reel or roll 26 by permutating or exchanging the mandrels 27 and 28 or even the paper 2 is reeled again or rewound in the direction reverse of or opposite to unrolling or unwinding through the medium of the drum 3, upon taking care to avoid the formation of reproductions or copies at the locations of the drums 3 and 4 which could then be spaced or moved away from each other by an adjusting or control device not shown, during this period of re- Iwinding. If the reel or roll 26 initially comprises a 60 meter storage or reserve of paper 2, about two hundred reproductions or copies of the format 21 x 27 cm. can be effected.

The scorotron type device, shown on FIGURE 6, is the one used in the device of FIGURE 5 at 20 or at 22. It comprises a metal envelope, shell or casing 210 in the form of an uncomplete cylinder or barrel having a diameter of the order of 20 cm. and grounded, and an axial conductor 21a, consisting of a tungsten wire having a diameter of micromillimeters, brought to a potential or voltage of several thousands of volts by the source 2112; the envelope, shell or casing 21c is extended by a meshed element or member 21d made from insulating material which plays an essential part for the obtainment or achievement of the uniformity of the precharging of the photoconductive paper or of the paper upon which are effected the reproductions or copies; it is assumed that this element 21d forms a barrier for the too fast ions moving towards the surface of said papers, thereby avoiding any overcharge thereof; at the same time, this element forms a trap for the possible few ions of a sign opposite to that of the ions produced within said envelope 21, said first ions being caught or trapped and/or neutralized by this element; this latter is preferably of low mass per surface area unit, so as to then form a veil or thin foil which may be advantageously made of synthetic material such as polyethylene terephthalate (Mylar) or nylon; ittis assumed that said barrier effected is obtained due to the charge taken by this element in presence of oxygen ions of the ambient air; the meshes of said veil are preferably three to four in number per linear millimeter.

The device of FIGURE 7 enables to render the process absolutely continuous; it derives directly from that of FIGURE 5, so that the identical or like elements of both of these figures are designated by the same reference characters. The support, backing or carrier of the photoconductive layer is formed here by an endless tape, strip, band or belt 48. In order to regenerate this photoconductive layer or coat for effecting a new reproduction or copy, this endless belt 48 passes or moves at first past or in front of a light source 23 which enables the dissipation of the remanent or residual charges, then in front of or past the stations 31 and 32 the etfect of which is to restore the sensitiveness of the photoconductive layer with a view to allow quick recycling or reprocessing of the band, tape or belt. The station 31 is a heat source, preferably a source of infrared radiations and 32 is a scorotron type charging station, modified as described with reference 11 to FIGURE 6, the high voltage source of this station being of a polarity opposite to that of the sources 1% and 21b.

In the device of FIGURE 8, the photoconductive layer of the endless belt 49 is disposed in parallel relation to the paper 1 over a length corresponding to the spacing of both pairs 5011-5012 and 51a-51b of rotary cylinders. The electric field with the high frequency component is provided or built up by means of conductive grids 35 and 36 having rotary bars loosely mounted on their axes or journals, said grids or grates being supported by the metal boxes or casings 37 and 38, respectively, wherein an air depression is produced in order to apply the endless belt 49 and paper 1 flat against the grids 35 and 36, respectively. For the sake of clarity of the figure, the showing of the charging stations for the endless belt and paper as well as the optical projection system for the pattern or design of the original to be copied or reproduced have been omitted. Likewise have been omitted the regenerating stations which treat or process the endless 'belt 49 after achievement of the copies or reproductions.

The latent images, formed on the paper 1 of FIGURES 7 and 8, are developed and fixed in a manner known per se in xerography.

As to the constitution of the endless tape, band or belt 48 (FIGURE 7) or 49 (FIGURE 8), it should be noted that it is composed of at least two layers: a first lower or bottom layer made from a suitable material which will impart thereto the physical qualities required for the high number of successive processes or operations or unrollings during the printing off of a large number of copies and a photoconductive upper or top layer. Said material may be natural or synthetic rubber comprising suitable charges or additions which determine its degree of elasticity and/or charges or additions which render it more or less conductive (carbon black issued from the incomplete combustion of kerosene or lamp-oil for example). The rubber used Will be for example a butyl rubber or a silicone elastomer. This material may also be a synthetic plastic material or substance such as a butadiene-vinyl chloride copolymer, a butadiene-styrene copolymer, an acrylonitrile-vinylidene chloride copolymer, a polyvinyl-butyral polyester, the pliolite 3, 4, or 6, etc. The photoconductive layer should contain, dispersed in an insulating synthetic resin, a photoconductive substance such as zinc oxide with sensitizing agents for the panchromatic reproduction or copying, such as for example the orange acridine, the fluoresceine, the eosine, the Bengale pink and the methyleneblue. The upper or top layer may also contain cadmium sulphide as the only or single photoconductive substance or as a photoconductive substance associated with said zinc oxide, or cadmium sulphide doped with manganese and oxygen to obtain a high sensitivity. The photoconductive layer may also contain organic photoconductive substances such as anthracene, the triazoles, the oxaziles, the thiazoles and the triazines. This layer may have any own or proper coloration without any inconvenience for the quality of the copies or reproductions. The paper 1 is preferably a usual or current paper weighing about 40 to 70 g./m. and comprising a thin insulating layer consisting for example of lithium, calcium or aluminum stearate or palmitate dispersed in a binder such as methyl polymethacrylate, a butadiene-styrene copolymer, an acrylonitrile-vinylidene chloride copolymer, butyl rubber, an acrylonitrile-vinylidene chloride copolymer.

It is understood that the present invention should not be construed as limited to the forms of embodiments described and shown herein which have been given by way of examples only, as many modifications, changes and alterations may be effected by those skilled in or conversant with the art without departing from the scope of protection as defined in the appended claims.

What I claim is:

l. A method of photoelectric copying comprising the steps of forming, from an original to be reproduced, an electrostatic image upon a photoconductive surface carried by a static electricity conductive base, uniformly charging with static electricity the insulating surface of a nonphotosensitive and nonphotoconductive paper sheet comprising a conductive support and an insulating surface, then bringing with the exclusion of active light, said photoconductive surface in the presence of said insulating surface at a distance thereof comprised between 0.01 micron and microns while subjecting the zone wherein both of said surfaces are thus in reciprocal contact to the action of an electric field comprising an alternative current of frequency ranging from 1.2 kilohertz to 3000 megahertz and a direct current component, the potential of which is at least equal to a few volts, and developing and fixing the electrostatic image formed on the insulating surface of said paper sheet after having separated the photoconductive surface from said paper sheet.

2. A method according to claim 1, wherein said direct current component is obtained by the induction caused by an electret placed adjacent the conductive support of said paper sheet, on the face thereof opposite to said insulating surface.

3. A method according to claim 1, wherein the uniform charge of said insulating surface of said paper-sheet is obtained by putting the latter in the presence of a scorotron. V

4-. A method according to claim 3, wherein that part of said scorotron which faces said insulating surface consists of a meshed element made from insulating material.

5. A method according to claim 1, wherein said continuous direct current component is obtained by a direct current source connected to said conductive support of said paper sheet. 1

6. A method according to claim 1, wherein said electrostatic image is a reverse right-left image of the original to be reproduced.

'7. A method according to claim 1, wherein said insulating surface is negatively charged.

8. A method according to claim 1,- wherein said insulating surface is positively charged.

References Cited UNITED STATES PATENTS 2,879,395 3/1959 VVaikup 250-495 2,982,647 5/1961 Carlson et a1. 961

NORMAN G. TORCHIN, Primary Examiner J. C. COOPER III, Assistant Examiner US. Cl. X.R.

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