US3741761A

US3741761A - Non-electrostatic printing electrography

Info

Publication number: US3741761A
Application number: US00870405A
Authority: US
Inventors: C Cantarano
Original assignee: Individual
Current assignee: Individual
Priority date: 1969-12-08
Filing date: 1969-12-08
Publication date: 1973-06-26
Anticipated expiration: 1990-06-26
Also published as: DE2060355A1

Abstract

A METHOD FOR PRODUCING ELECTROGRAPHIC IMAGES FROM CONDUCTIVITY CHARACTERISTICS RANGING FROM AN AREA EXHIBITING MAXIMUM CONDUCTIVITY, TO AN AREA EXHIBITING MINIMUM CONDUCTIVITY, SAID CONDUCTIVITY PATTERN BEING AFFIXED TO AN INSULATING BACKING MEMBER, COMPRISING THE STEPS OF COATING SAID CONDUCTIVITY PATTERN WITH A THIN LAYER OF DEVELOPER POWDER CAPABLE OF RECEIVING AN ELECTRIC CHARGE, DISPOSING A FLUID INSULATING LAYER AGAINST SAID COATED CONDUCTIVITY PATTERN SO THAT SAID COATED CONDUCTIVITY PATTERN IS ELECTRICALLY INSULATED BETWEEN SAID INSULATING BACKING MEMBER AND SAID FLUID INSULATING LAYER, SAID INSULATING BACKING MEMBER AND SAID FLUID INSULATING LAYER BEING DISPOSED BETWEEN FIRST AND SECOND ELECTRODES, AT LEAST ONE OF SAID ELECTRODES BEING A GRID ELECTRODE WITH SAID GRID ELECTRODE BEING IN SPACED RELATIONSHIP WITH SAID COATED CONDUCTIVITY PATTERN AND BEING JUXTAPOSED TO SAID FLUID INSSULATING LAYER, GENERATING AN ELECTRIC FIELD OF SUFFICIENT STRENGTH BETWEEN SAID ELECTRODES AND ACROSS SAID FLUID INSULATING LAYER AND SAID INSULATING BACKING MEMBER TO CHARGE SAID POWDER FROM SAID CONDUCTIVITY PATTERN AND TO ELECTRICALLY REMOVE A PART OF THE COATING POWDER, WHEREBY A PORTION OF SAID POWDER IS SUFFICIENTLY CHARGED AND REMOVED FROM SAID CONDUVTIVITY PATTERN AND THE REMAINDER OF SAID POWDER IS SUFFICIENTLY CHARGED SO THAT IT CONTINUES TO COAT SAID CONDUCTIVITY PATTERN THREBY PRODUCING A STABLE ELECTROGRAPHIC IMAGE ON SAID CONDUCTIVITY PATTERN BY THE REMAINING PART OF THE COATING POWDER SAID PORTION OF SAID POWDER REMOVED FROM SAID CONDUCTIVITY PATTERN MIGRATING THROUGH THE OPENINGS IN SAID GRID ELECTRODE AND BEING REMOVED FROM SAID ELECTRIC FIELD. THIS FIELD CAN BE ALTERNATIVELY MODULATED.

Description

June 26, 1973 c. M. CANTARANO NON-ELECTROSTATIC PRINTING ELECTROGRAPHY 4 Sheets-Sheet 2 Filed Dec. 8, 1.969

June 25, 1973 c. M. CANTARANO 3,741,761

NON-ELECTROSTATIC PRINTING ELECTROGRAPHY Filed Dec. 8, 1969 i 4 Sheets-Sheet i3 lave/liar: F, avsfAmmo XAKCW QmARAuo J 1973 c. M. CANTARANO NON-ELECTROSTAT IC PR IN'I'ING ELECTHOGHMHY 4 Sheets-Sheet 4 Filed Dec. 8, 1969 //7 ran for CbSTA v'n U0 AA ficus CAuTARANo y: mammal:

United States Patent Olfice 3,741,761 Patented June 26, 1973 US. Cl. 961 R 6 Claims ABSTRACT OF THE DISCLOSURE A method for producing electrographic images from conductivity characteristics ranging from an area exhibiting maximum conductivity, to an area exhibiting minimum conductivity, said conductivity pattern being aflixed to an insulating backing member, comprising the steps of coating said conductivity pattern with a thin layer of developer powder capable of receiving an electric charge, disposing a fluid insulating layer against said coated conductivity pattern so that said coated conductivity pattern is electrically insulated between said insulating backing member and said fluid insulating layer, said insulating backing member and said fluid insulating layer being disposed between first and second electrodes, at least one of said electrodes being a grid electrode with said grid electrode being in spaced relationship with said coated conductivity pattern and being juxtaposed to said fluid insulating layer, generating an electric field of suflicient strength between said electrodes and across said fluid insulating layer and said insulating backing member to charge said powder from said conductivity pattern and to electrically remove a part of the coating powder; whereby a portion of said powder is sufliciently charged and removed from said conductivity pattern and the remainder of said powder is insufficiently charged so that it continues to coat said conductivity pattern thereby producing a stable electrographic image on said conductivity pattern by the remaining part of the coating powder said portion of said powder removed from said conductivity pattern migrating through the openings in said grid electrode and being removed from said electric field.

This electric field can be alternatively modulated.

The present application is a continuation-in-part of my application S.N. 631,792 filed on Apr. 18, 1967 and now abandoned This invention relates to the production of electrographic images from an original provided with a conductivity pattern, to the production of an electrographic image resulting from the composition of different electrographic images, and to the transfer of the obtained images on to sheet or webs of copy material.

As used herein, the term conductivity pattern is to be understood as including any virtually plane surface formed by parts having different electric conductivities.

In the following specification, the term insulating is to be understood as defining the quality of having an electric conductivity lower than mho/cm. and the term non-insulating as defining the quality of having an electric conductivity superior to 10" mho/ cm.

In the actual art, a feature of electrographic methods resides in the use of an original provided with a conductivity pattern including high insulating parts which will selectively hold electric charges to form a latent electrostatic image; thus an electrographic image may be developed by an electrically responsive powder which.

adheres to the charged parts of the latent image. This electrographic image will not be obtained in a stable way because of the passage of electric charges even through the high insulating parts of the original causing the effacement of at least a part of the latent image during the step of the development. A typical original of actual electrography consists in a photoconductive insulating layer provided with a conductivity pattern resulting from an exposure to a light image; such a photoconductive layer will be a high insulator in the dark in order to obtain a conductivity pattern including the non-illuminated high insulating parts serving to develop an electrographic image according to existing methods. These photoconductive insulating layers are slow in their response to successive different exposures to the light and, consequently, they may not be used to afford high speed processes to produce successive different electrographic images. Furthermore, these insulating layers having a very low sensitivity to the light, the enlarging of a document is still diflicult to obtain in electrography.

I have found, however, that a stable electrographic image may be formed and simultaneously developed in the absence of a latent electrostatic image; to this end an original is used which is provided with a pattern of conductive and less conductive parts aflixed to an insulating backing material, the conductivity pattern of the original is coated with a thin layer of developer powder, an insulating layer is placed against the layer of powder, and an electric field is generated to charge the powder from said conductivity pattern and to electrically remove a part of the coating powder; whereby a stable electrographic image is formed on the least conductive parts of said pattern by the remaining coating powder which is never sufficiently charged to be removed. One of such form of electrographic method is disclosed in my co-pending application S.N. 631,792, filed Apr. 18, 1967. The present invention thus relates to the production of stable electrographic images from an original provided with a conductivity pattern aflixed to an insulating backing material, to the development of stable electrographic images from an original consisting in a photoconductive layer excited by a light image, to the development of a stable electrographic image from a photoconductive layer successively excited by different light images, and to the transfer of the obtained images on to sheets or webs of copy material.

A feature of the present invention resides in the use of an original provided with an insulating backing material carrying a conductivity pattern including non-insulating conductive parts, the minimal conductivity of this pattern being not critical to develop a stable electrographic image according to the invention. Furthermore, I have found that a stable electrographic image may be developed from an original consisting in an insulating backing material carrying a photoconductive layer provided with a conductivity pattern including illuminated non-insulating parts resulting from an exposure to alight image. Such a photoconductive layer can be called photoconductive non-insulating layer because of its electric conductivity superior to 10- mho/cm. when it is illuminated. These photoconductive non-insulating layers are to be distinguished from the photoconductive insulating layers of classic electrography. Another object of this invention is to provide methods and means for the production of electrographic images on a member of copy material by using a photoconductive non-insulating layer. In carrying out this invention, a photoconductive non-insulating layer may be used which has a virtually instantaneous response and a high sensitivity to the light as, for example, a layer of cadmium sulfide or cadmium selenide, or other high sensitive layers commonly used in the photoresistive cells.

According to the present invention, the conductivity pattern of the original is coated with a thin layer of developer powder, an insulating layer is placed against the layer of powder and an electric field is generated to charge the powder from said conductivity pattern; because of the insulation of the coated conductivity pattern between the insulating backing of the original and said insulating layer, the coating powder will receive electric charges having maximum values in proportion with the conductivities of said pattern; under the action of the electric field a part of the coating powder will be electrically attracted away from the original, while the powder coating the least conductive parts of said pattern is never sufficiently charged to be removed and it develops a stable electrographic image thereon. This method is Well adapted to produce the dense large areas as well as the half-shadow areas of the electrographic image. By using an original provided with a pattern having high differences in conductivity, this method is well adapted for the high speed production of stable electrographic images; to this end a direct electric field may be generated to obtain each stable electrographic image in less than 1 millisecond; by using an original consisting in a photoconductive noninsulating layer, this stable image is generally obtained in less than milliseconds, this lack of time including the light and dark responses of the layer when it is exposed to successive different light images. On the other hand, by generating an alternatively modulated electric field transferring alternating charges from the pattern of the original to the coating powder, this method is well adapted to produce stable electrographic images from an original provided with a pattern having low differences in conductivity; the amplitude of the modulated field is then adjusted to attract particles of powder having successive opposite polarities away from the most conductive parts of said pattern while the powder coating the least conductive parts of said pattern is never sufficiently charged to be removed; because of the opposite charges of these particles, the removal of the powder may be prosecuted to electrically attract away all the powder coating the most conductive parts of said pattern, while the remaining part of the coating powder develops a stable electrographic image of high density on the least conductive parts of said pattern.

According to another embodiment of the invention, an original is used which consists in a photoconductive noninsulating layer, the photoconductive layer is exposed to a first light image and a first stable electrographic image is developed according to the above mentioned method by using a first non-insulating developer powder, and, thereafter, the first electrographic image bearing photoconductive layer is exposed to a second light image and a second electrographic image is developed according to said method by using a second non-insulating developer powder; because of the high conductivity of the powder forming the first electrographic image, the second powder will selectively form a second image on the least insulating parts of the layer which are not covered by the powder of the first image. According to this embodiment of the invention, the successive developments may be prosecuted to obtain an electrographic image from successive different exposures; colored electrographic images of satisfactory quality may be produced according to this method of the invention.

According to a further embodiment of the invention, an original is used which consists in a photoconductive non-insulating layer provided with a conductivity pattern resulting from an exposure to a light image, the photoconductive layer is coated with a thin layer of developer powder, an insulating layer is placed against the layer of powder, an electric field is generated to charge the powder from the conductivity pattern and to remove a part of the coating powder leaving a stable electrographic image on the least conductive parts of said pattern, and, thereafter, a sheet of copy material is placed against the powder of the electrographic image, the photoconductive non-insulating layer is excited to a uniform high conductivity by a uniform exposure to a light of high intensity,

and a second electric field is generated to charge powder from the excited photoconductive non-insulating layer; whereby, under the combined action of the electric field and of the light, and charged powder image is transferred on to the sheet of copy material.

An object of this invention is to improve electrographic methods and to provide means and devices for use in electrography.

Another object of this invention is to provide methods and means for the advantageous use of photoconductive non-insulating layers in electrography.

Other objects of this invention will be apparent from the following description and accompanying drawings taken in connection with the appended claims.

In the drawings:

FIG. 1 is a sectional view showing a development device comprising an original between two electrodes;

FIG. 2 is a schematic representation showing the electrographic image developed in the device illustrated in FIG. 1;

FIG. 3 is a sectional view showing a development and transfer device comprising an original and a sheet of copy material between two electrodes;

FIG. 4 is a schematic representation showing two grains of developer powder against the original of the device illustrated in FIGS. 1 and 3;

FIG. 5 is a schematic representation showing two grains of developer powder against a photoconductive layer exposed to a light image;

FIG. 6 is a sectional view showing a development device comprising a photoconductive layer exposed to a light image;

FIG. 7 is a schematic representation showing a development device comprising an original and a powderer;

FIG. 8 is a schematic representation showing grains of powder on the photoconductive layer of the development device of FIG. 7;

FIG. 9 is a sectional view showing a transfer device comprising a uniformly excited photoconductive layer and a sheet of copy material, between two electrodes;

FIG. 10 is a schematic representation of an apparatus serving to the development and the transfer of electrographic images;

FIG. 11 is a schematic representation of another embodiment of the apparatus illustrated in FIG. 11.

In the arrangement shown in FIGS. 1 to 4, for producing electrographic images an original 1 provided with indica 2 having another electric conductivity than the surface 3 of the backing material 11 is disposed between two

electrodes

6 and 7. Owing to the differences of electric conductivity between the materials of the

parts

11 and 2 of original 1, the latter is provided with a conductivity pattern formed by the areas 2 of the indicia and by the blank surface 3 of the backing 11. The indicia 2 may be of different types as typewriting, China ink or pencil traces, for example. Furthermore, if continuous tone-electrographic images are to be produced, an original 1 will be used which is provided with diiferently conductive indicia 2 forming dense areas and half-shadow areas as like as a photographic picture. On the other hand, as FIGS. 5 to 7 show, an original 1 may be used which consists in a photoconductive layer 24 provided with a

conductivity pattern

2, 3 resulting from the exposure of layer 24 to a light image; the

pattern

2, 3 is then formed by the illuminated conductive parts 2 and the low illuminated low conductive parts 3 of the layer 24. In order to produce the light image on the layer 24, a transparent electrode 7 is used which consists, for example, in a thin layer of NESA, a high conductive transparent varnish sold by Pittsburgh Plate Glass Co., Pittsburgh, Pa. The layer of NESA may be supported by a transparent glass plate 17, for example. The light sources 41 illuminate a document 21 to be reproduced; the light is reflected by document 21 toward objective 31 and is transmitted across the transparent electrode 7 and the transparent backing 44 of layer 24 to form the optical image of document 21 on the photoconductive layer 24. Document 21 may be a sheet of paper carrying printed or typewritten matter, or drawing, for example, although other things may be photographed such as I i-dimensional objects, for example. Alternatively, other radiations than light may be used to form the

pattern

2, 3 such as, for example, X-rays or gamma-rays; furthermore, any other means inducing in the layer 24 a pattern of conductive parts 2 and low conductive parts 3 may be used to produce electrographic images according to invention. On the other hand, when, for example, an

X-ray image

2, 3 is formed on the layer 24, a sheet of aluminium may constitute the transparent electrode 7, for example.

In the preferred form of this invention a layer 24 of a photoconductive non-insulating material is used which has a high sensitivity and a virtually instantaneous response to the light. Alternatively, many photoconductive noninsulating materials may be used such as, for example,

metallic selenium, thallium sulfide, cadmium sulfide, cad

mium selenide, lead sulfide. In general, the sensitivity to the light of the layers of these non-insulating materials is from the 200 microamp/lumen of layers of metallic selenium to the 100 milliamp/lumen of cadmium selenide layers. The spectral response of metallic selenium is from the ultra-violet to the red part of the spectrum with maximum sensitivity in the ultra-violet, cadmium sulfide has virtually the same spectral response than human eye with maximum sensitivity between yellow and green, lead sulfide has maximum sensitivity in the infra-red from 2 to 3.5 microns of wave length. Alternatively and for example, lead sulfide, lead telluride or lead telenide layers may be used according to the invention to photograph objects emitting invisible light from 2 to microns of wave length. The use of layers having maximum sensitivity in the visible part of the spectrum permit to reduce the losses in the transmission of light across the lens, mirrors etc. of the optical devices serving to form the light image to be reproduced. Moreover, cadmium selenide is well adapted for the high speed production of copies from successive different light images, the responses of this material to the light and to the dark being shorter than 15 milliseconds.

The above mentioned photoconductive layers may be produced, for example, by evaporating under vacuum the photoconductive material to deposit a thin uniform layer on the surface of a transparent backing. Alternatively and for example, a plate of insulating or conductive glass from 1 to 5 mm. of uniform thickness or a sheet of Mylar (registered trademark) having a uniform thickness from to 250 microns has been found useful to constitute the backing material of the photoconductive layer. The receiving surface of the backing may be rendered rough to improve the adherence of the photoconductive layer. Cadmium sulfide and cadmium selenide are evaporated under vacuum to form a layer having a uniform thickness from 15 to 25 microns. Suitable cadmium sulfide layers are sold under the trademarks CDSX7 and CDSH 35, and cadmium selenide layers under trademarks CDSEX7 and CDSEH35, by Acova Co., Paris. Furthermore, in order to produce a metallic selenium layer afiixed to an insulating flexible sheet, amorphous selenium is evaporated under vacuum to form a layer of about microns thickness on the polished surface of a glass plate, thereafter a sheet of Mylar is affixed by a transparent glue to the selenium layer, and then, while a pressure from 2 to 50 Kgr./cm. is exerted between the sheet and the glass plate, the selenium layer is heated at a temperature from 200 to 216 C. for a period from a few minutes to one hour to develop the crystalline structure of the metallic variety of selenium; the Mylar sheet and its metallic selenium layer are then easily detached from the polished surface of the glass plate. On the other hand, when less than 4 electrographic images per second are to be produced from a high contrastful light image, a usual in the art photoconductive insulating layer affixed to a backing electrode may be used, in spite of the low sensitivity of this type of photoconductive layer; although, a photoconductive insulating layer may be used which is constituted by a thin metalliclayer of about 5 microns of gold or tellurium afiixed to an insulating backing and by a photoconductive layer of 50 microns of amorphous selenium evaporated on said thin metallic layer.

As FIGS. 1 and 6 show, the

conductivity pattern

2, 3 of originals 1 is coated with a developer powder 5. If the grains size of powder 5 is from 1 to 20 microns, the thickness of the layer of powder 5 will be about 50 microns, for example. For the uniform application of the powder 5, classic spraying or cascading devices may be used, provided that a thin uniform layer of powder 5 is formed rather than a particular amount of grains. In carrying out the invention it is expedient to use a developer powder 5 having an electric conductivity between the maximum and the minimum conductivities of the parts forming the

pattern

2, 3 of the original, although the exact conductivity of the powder 5 is not critical in order to produce satisfactory electrographic images. Alternatively, metallic or semi-conductive or thermoplastic powders have been found useful. By way of example, charcoal, stannous oxide, lead sulfide, cadmium selenide as well as other colored materials may be powdered to be used as developers. The grains size of the powder may be between 1 and 40 microns, for example. For instance, a suitable developer powder can be produced by oxidizing at a temperature of about 700 C. a commercial bronze to obtain a black powder containing copper bioxide and other metallic oxides; this powder is then passed thorugh sieves to reduce the grains size between 2 and 10 microns, for example. The electric conductivity of copper bioxide is generally between about 10- and 10- mho/cm. It is moreover possible to use commercial bronze colored powders having a conductivity from 10- to 10- mho/cm. Other types of developer powders, such as thermoplastic powders may be used; by way of example, a conductive black thermoplastic powder may be obtained from a solid polystyrene resin; to this end, for example, two parts of the resin are melted at a temperature of about C. to be intimately mixed with one part of pure carbone, and thereafter the mixture is cooled and powdered to obtain a developer having grains from 1 to 10 microns, for example; the conductivity of these thermoplastic powders may be varied by changing the carbone ratio in the mixture. Furthermore, other colored thermoplastic powders may be used; these powders may be rendered conductive, for example, by evaporating a metal to form a conductive coat on their grains. Other usual developer powders, such as, for example, developers constituted by conductive micro-capsules containing colored liquids, may be used in carrying out the invention.

In the arrangement shown in FIGS. 1 and 6, a powdercoated original 1 is disposed between the electrode 7 and a second electrode 6 in the form of a grid. The thin layer of developer powder 5 is insulated from the grid 6 by a fiuid dielectric consisting, for example, of an air layer 4. The grid 6 may be made of brass and have a mesh width of about 0.5 mm., for example. The layer of powder 5 is applied loosely-adhering to the

pattern

2, 3; for example, this adherence of powder 5 may be obtained by previously coating the

pattern

2, 3 with a slight adhesive material as well as through the use of a powder 5 the grains of which are rendered slight adhesive by a thin zinc stearate or aluminium stearate coat, for example. Any other means to obtain a loosely-adherence of powder 5 to the

pattern

2, 3 may be useful in carrying out the present invention. Under the influence of an electric field generated between

electrodes

6, 7, the powder 5 is electrically charged and removed from the conductive parts 2, while the powder coating the low conductive parts 3 is never sufiiciently charged to electrically overcome its adherence to the parts 3 and thus it develops a stable electrographic image thereon. The intensity of the electric field cannot exceed 3.3 v./micron in the layer of air 4 to avoid a sudden electric discharge between electrode 6 and layer of powder 5; which would reduce the electric field serving to the development of the image. Instead of this, the quality of the electrographic image is improved by generating between

electrodes

6 and 7 an electric field having, in the air layer 4, a gradient between 2.5 and 3.1 v./micron to obtain a silent ionizin discharge in the air 4 simultaneously with the development of the electrographic image; in this manner the powder 5 will be electrically charged from the slight conductive air 4 to adhere to the conductivity pattern, while the electric field remains sufliciently intense, in the air 4, to electrically charge and remove the part of the coating powder toward the electrode 6 to develop an electrographic image by the remaining part of the coating powder on said pattern. Instead of the air 4, other insulating gas as well as an insulating liquid may be used as fluid dielectric 4. What matters is that the coating powder 5 is insulated from the electrode 6 and that the layer 4 permits the passage of the grains of powder attracted away from the original 1 during the development; these grains thus migrate through the openings of the grid 6 and they are definitively removed from the electric field. It is to be noted that the electric field between the

electrodes

6 and 7 must have an intensity lower than the electric rigidity of the fluid in the layer 4. Thus, in order to increase the intensity of the electric field, a fluid having a high electric rigidity may be used; in practice this rigidity may be as high as 25 v./micron by using, for example, a silicone oil. Furthermore, in accordance with the present invention, when a direct field is generated between

electrodes

6 and 7, the

conductivity pattern

2, 3 is to be insulated from electrode 7 to prevent any direct electric currents filtering through the low conductive parts 3 from electrically charging and removing away even the part of the powder which serves to develop the stable image over the original. The insulation of the

pattern

2, 3 may be constituted by the insulating

backing

11, 44. If, on the contrary, the backing of the original is made of a low insulating material such as, for example, a sheet 11 of ordinary paper, a dielectric is to be arranged between the sheet 11 and electrode 7. This dielectric may be constituted by a sheet of Mylar having a thickness of 125 microns, for example.

According to an embodiment of the invention, electrographic images are produced by applying an alternating voltage to

electrodes

6 and 7; to this end the terminals 9 are connected, for example, across the secondary coil 29 of an electric transformer 19. The amplitude of the alternating voltage may be adjusted by means of the potentiometer 69 (FIG. 1), for example. Referring to this embodiment, FIGS. 4 and 5 schematically show two

grains

12, 13 of the coating powder 5 placed against the

conductivity pattern

2, 3 of the original 1. The adherence of the grains to the

pattern

2, 3 is indicated by the arrows b. Because of the different conductivities of the

parts

2 and 3, the Contact conductance r between grain 12 and the conductive part 2 is higher than the contact conductance r between the grain 13 and the low conductive parts 3. By generating an alternating electric field between

electrodes

6 and 7,

grains

12 and 13 will receive alternating electric charges having dilferent maximum values according to the different contact conductances r and r under the action of the field, the charged

grains

12 and 13 are repelled from original 1 by modulated forces having maximum values a and a in substantial proportion to the contact conductances r and r respectively; the amplitude of the alternating voltage is then adjusted to apply, to grain 12, a force a more intense than its adherence b to the original 1, whereby the grain 12 is electrically attracted through grid 6, while, because of the alternating character of the charges of powder 5, the electric force a is never sufliciently intense to overcome the adherence of the grain 13 to the low conductive part 3. A stable electrographic image is thus obtained by the powder 13 on the parts 3 of original 1. Generally, a satisfactory image is developed by generating two or three complete periods of the alternating field although, the electrographic image being obtained in a stable way, its good quality is irrespective of a longer duration of the electric field and of a slight electric conductivity of the parts 3. By way of example, if a photoconductive insulating layer 24 of amorphous selenium having a conductivity of about 10* mho/cm. is used as original 1, an alternating field from 0.2 to 4 Hz. may be generated to obtain stable images; when a photoconductive insulating layer 24 is used which has a conductivity from 10 to about 10* mho/cm., the frequency of the field will be from 5 to 60 Hz. If a photoconductive layer 24 is used which has an electric conductivity superior to those cited above, the frequency of the alternating field will be higher than Hz. In this case the spacing between electrode 6 and the coating powder 5 is reduced, for example, at 0.5 mm. to avoid that the charged grains 12 fall again on original 1, which would deteriorate the electrographic image during its development. On the other hand, instead of grid 6, a compact electrode 6 may be used which is coated with a high insulating material such as a polyvinyl resin, for example; by using this arrangement of parts the charged grains 12 will electrically adhere to the insulating coat of electrode 6 in spite of the electric action of the successive opposite polarities of the alternating field.

According to another embodiment using the devices of FIGS. 1, 3, 6 and 7, an original 1 is used which is provided with an insulating

backing

11, 44 having an electric conductivity lower than about 10 mho/cm., such as a sheet of Mylar, for example. By applying an alternating or an alternatively modulated voltage to terminals 9, the coating powder 5 receives, from the

pattern

2, 3, alternat ing electric charges having maximum values in proportion to the conductivities of said pattern; the amplitude of the alternating modulation of the voltage is adjusted to electrically attract the powder 12 away from the conductive parts 2 while the powder 13 develops a stable electrographic image on the low conductive parts 3. According to this embodiment, it is expedient the use of an original 1 provided with parts 2 having an electric conductivity higher than 10 mho/ cm. and a thickness superior to 10 microns. Because of the insulation of the

pattern

2, 3 from the electrode 7, electric current filtering through the low conductive parts 3 are avoided and thus a low frequency of the field is not critical in order to produce stable images from a photoconductive non-insulating layer 24. This frequency may be as low as 10 or 60 Hz., for example. The amplitude of the modulated field is then adjusted to attract particles 12 of powder having successive opposite polarities away from the most conductive parts 2 of original 1, while the powder 13 coating the least conductive parts 3 is never sufliciently charged to be removed; because of the opposite charges of the particles 12 attracted away from the original 1, the removal of powder 12 may be prosecuted to electrically remove all the powder coating the most conductive parts 2, while the remaining part 13 of the coating powder develops a stable electrographic image of high density. This method is well adapted to produce satisfactory electrographic images from an original 1 provided with a

pattern

2, 3 having low differences in conductivity such as, for example, a CDSEX7 layer 24 of cadmium selenide excited by a light image having a maximum intensity of about 0.6 lux and a minimum intensity of about 0.2 lux; the light image induces in the CDSEX7 layer a maximum conductivity about 6 orders in magnitude higher than its minimum conductivity. Moreover, the best quality of continuous tone electrographic images is obtained by using a layer 24 which has a photoelectric linear character such as, for example, the above mentioned layer CDSH35 of cadmium sulfide; the linear feature of this layer residing in the proportionality between its electric conductivity and the intensity of the exciting light. Contrastful electrographic images may be obtained when the CDSH35 layer 24 is exposed, for example, to a light image rendering the parts 3 about orders in magnitude less conductive than the parts 2, the minimum illumination of parts 2 being 0.1 lux, for example. On the other hand, a direct voltage may be applied to terminals 9 to produce stable images from a contrastful original 1 provided with parts 2 at least 30 orders in magnitude more conductive than low conductive parts 3, such as, of example, a sheet of insulating paper carrying China ink traces; an electrographic image may be obtained by applying an impulsion of direct voltage during 0.1 or 1 millisecond, for example. Although, the image being developed in a stable way, a longer duration of the electric field is not critical in order to obtain satisfactory results. By applying said direct voltage, electrographic images of good quality may be produced from a CDSEX7 layer 24 excited by a light image having a minimum intensity of about 2.5 lux and a maximum intensity of 20 lux, for example; the response of the CDSEX7 layer to light (20 lux) is about 4 milliseconds, its response to the dark (2.5 lux) about milliseconds. Furthermore, the sensitivity to light of layer 24 may be improved by applying a high electric potential of a suitable polarity to this layer; thus the sensitivity of a selenium layer 24 may be improved by applying, for instance, 1000 volts of positive potential to layer 24; to this end, the electrode 7 may be grounded, an electronic valve 49 and a condenser 39 are used to apply said positive potential to layer 24 through the electrode 6 and the ionized air 4, and an electric transformer 19 induces an alternating signal in the secondary coil 29 to alternatively modulate the direct voltage, in order to develop satisfactory electrographic images according to the invention.

The devices illustrated with reference to FIGS. 1 and 6 can be used to develop two stable electrographic images simultaneously from the same original 1. To this end, as shown in FIG. 3, an insulating sheet of copy material 8 is placed against the layer of powder 5 to intercept the powder 12 electrically removed from the conductive parts 2 during the development of the powder image 13; whereby a first stable electrographic image is developed on the sheet of copy while a second electrographic image 13 is developed on the low conductive parts 3 of original 1. For example, a sheet of paper may be used as copy material 8. The original 1 may be constituted, for instance, by a photoconductive layer exposed to a light image accordingly to the method described with reference to FIG. 6. Satisfactory electrographic images may be developed by using an original 1 provided with a

pattern

2, 3 having a maximum conductivity at least 30 orders in magnitude higher than its minimum conductivity.

Referring now to FIG. 7, an electrographic image may be developed by blowing a cloud of powder 5 against the photoconductive layer 24 simultaneously to the exposure of this layer to a light image and to the application of an alternating or an alternatively modulated electric voltage to

electrodes

6 and 7. A cloud generator 35 is used to blow the powder; alternatively, rotating brushes or a spraying device may be used as well as any other means for gently blowing a cloud of powder 5 against the

pattern

2, 3 of original 1. The amplitude of the modulated voltage is adjusted to electrically attract away the powder 12 which comes in contact with the conductive parts 2 of layer 24, while the powder 13 adheres to the least conductive parts of this layer. The layer 24 may be previously coated with a slight adhesive material to insure the adherence of the powder image 13. Moreover, a slight adhesive powder 5 may be used. The simultaneous application of the powder and of the electric voltage are prosecuted to obtain a uniform coat of powder 13 thus forming an electrographic image of high density on the least conductive parts of the photoconductive layer 24.

The duration of the development depends on the density of the powder cloud, although a stable image of good quality is generally obtained by applying from 1 to 5 complete periods of the modulated field. An excess of development will not change the obtained image, if a non-insulating powder 5 is used which has an electric conductivity superior to that of the least conductive parts of the

pattern

2, 3. The other features of this method are substantially the same of those of the above described embodiments of the invention.

According to the invention, an electrographic image may be developed from a colored document 21. To this end the layer 24 is exposed to the optical image of document 11; instead of this, a 3-dimensional colored object may be photographed, for example. It is preferred to use, as layer 24, a photoconductive non-insulating layer having a high sensitivity to the light, such as, for example, the above mentioned CDSH35 layer of cadmium sulfide having a spectral response similar to that of the human eye. As it has been described above with reference to the methods of the invention using a photoconductive layer, successive electrographic image may be speedily developed with powders of different colors by successively exposing the photoconductive layer to the optical image passing across different filters; these image are transferred to a copy material to form the colored image.

Referring now to FIG. 8, the photoconductive layer 24 is exposed to a first light image inducing a first conductivity pattern in this layer. By using one of the above described methods of the invention, a first stable electrographic image of non-insulating powder 13 is developed on the parts 3 of the first conductivity pattern. Thereafter, the photoconductive layer is exposed to a second light image inducing a second conductivity pattern in the layer 24. A second electrographic image of powder 112 is then developed by the same method; under the action of the electric field between 6 and 7, a part of the powder 112 will selectively adhere to the low conductive parts 3 of the second conductivity pattern While the remaining powder 112 is removed from both the part 2 of the second conductivity pattern and the conductive area 13 of the pattern formed by the non-insulating powder of the first image. A composite image of the first and the second conductivity pattern is thereby developed by the

powder

13 and 112. According to this embodiment, a composite electrographic image may be developed with different powders from successive differents exposures of the photoconductive layer.

Referring now to FIG. 9, a sheet of copy material 8 is placed against the powder 13 of an electrographic image carried by the photoconductive layer 24 affixed to a transparent backing material 44. The sheet 8 and the layer 24 are interposed between a first electrode 6 and a transparent second electrode 7. The light of the sources 54 uniformly illuminate the photoconductive layer 24 to induce a uniform electric conductivity therein. An insulating layer 18 may be interposed between the sheet 8 and electrode 6 as well as a transparent second insulating layer (not shown) may be disposed between the backing material 44 and electrode 7. By generating an electric field between

electrodes

6 and 7, the powder 13 is charged from the uniformly illuminated layer 24 and electrically transferred on to the sheet 8. For example, an electric field may be generated which has an intensity from 3 to 30 v./micron in the gap 15 between the original 1 and the sheet 8. The satisfactory transfer. of the powder image 13 is obtained by using a layer 24 uniformly excited to be at least 50 orders in magnitude more conductive than the material of the sheet 8. Thus, for example, by using a layer 24 of amorphous selenium, a sheet 8 having an electric conductivity lower than 10 mho/cm. will be used, such as a sheet of paper coated with polyvynil chloride, for example. 0n the other hand, in accordance with the invention, an electrographic image is developed on a photoconductive non-insulating layer 24 (FIG. 6) in order to obtain, in the device of FIG. 9, the satisfactory transfer of the powder image from the non-insulating layer 24 on to a sheet of ordinary paper of copy; this type of paper is often constituted by a low insulating material having a conductivity from 10- to l mho/cm. One of the above mentioned photoconductive non-insulating layers may be used; in order to obtain the satisfactory transfer, this type of layers may be excited to a conductivity of at least mho/cm. by an illumination of 10 lux, for example.

For carrying out the invention as described above, an apparatus of the type illustrated in FIG. 10 may be used. This apparatus comprises a development station 100 to form electrographic images from an original 1 provided with a

conductivity pattern

2, 3 and a station 200 to transfer the electrographic images on to a web of copy material 8. In the example of FIG. 10, the original 1 is constituted by an

endless belt

24, 44 formed by a transparent and flexible backing 44 on to which a photoconductive layer 24 is afiixed. The apparatus comprises four rollers 10 over which the

endless belt

24, 44 travels in the direction of arrow 110. A transparent insulating plate 47 is made of glass, for example, and it serves to guide the

belt

24, 44. The transparent electrode 7 and the gridelectrode 6 are connected to the terminals of a voltage generator. A microfilm projector comprises a light source 41, an objective 31 and a film unroller 51 of which the unrolling direction is reversed that the

endless belt

24, 44, as indicated by the

arrows

210 and 110, respectively. In operation, the photoconductive layer 24 affixed to its flexible transparent support 44 is driven by rollers 10. The film 21 moves in the direction indicated by the arrow 210 whereas the photoconductive layer 24 and its support 44 moves in opposite direction with a synchronous movement capable of immobilizing, in relation to the photoconductive layer 24, the

optic image

2, 3 formed on the latter. The

belt

24, 44 moving in the direction of arrow 110, the developer powder 5 of the container 25 uniformly coats the photoconductive layer 24 and a layer of powder 5 is driven by the upward movement of the latter in the electric field generated between

electrodes

6 and 7. In order to insure the adherence of powder 6 to the layer 24, a slight adhesive powder 5 may be used, such as, for example, a powder 5 the grains of which are coated with zinc or aluminium stearate. Moreover, any other means having similar slight adherent qualities of holding the uniform layer of powder 5 may be used. Under the action of the electric field, the powder 12 coating the illuminated parts 2 of layer 24 is electrically attracted through the grid-electrode 6 and falls again in the container 25, while the powder 13 coating the low illuminated parts 3 forms a stable electrographic image thereon. If an excited layer 24 is used which is provided with a

pattern

2, 3 having low differences in conductivity, an alternatively modulated voltage will be applied to

electrodes

6 and 7. In this case, the time of passage of layer 24 under the grid-electrode 6 is two or three complete periods of the alternative modulation of the electric field. On the other hand, if the

pattern

2, 3 has high differences in conductivity such as, for example, the above mentioned CDSEX7 layer excited by a light image from 2.5 to lux, a direct electric voltage may be applied to

electrodes

6 and 7; in this case the development of the stable electrographic image occurs in about 5 milliseconds, this lack of time including the response of layer 24 to the light (20 lnx). By using a CDSEX7 layer, satisfactory electrographic images will be obtained at the maximum speed of 20 m./sec. if the optical image on layer 24 has a length of about 0.1 m. in the direction of the movement of the

belt

24, 44. At the transfer station 200, the electrographic image 13 is transferred on to the web of copy 8. For example, the web 8 may be a web of copy paper driven against the layer 24 by the two roll ers 20. An electric field is generated between the electrodes 26 and a second transparent electrode 27; the light sources 54 uniformly illuminates the layer 24 across the electrode 27 and the backing 44. Under the combined action of the illumination of layer 24 and of the electric field generated between

electrodes

26 and 27, the powder 13 is electrically charged from layer 24 and it is transferred on to the copy material 8.

According to another embodiment of the invention, an apparatus of the type illustrated in FIG. 11 may be used to produce two copies from the same optical image forming a

conductivity pattern

2, 3 in the photoconductive layer 24. The layer 24 is aflixed to a flexible transparent I support 44 and it is driven by rollers 10 in the direction indicated by the arrow 110. The apparatus of FIG. 11 comprises a powderer 400 to coat the layer 24 with a thin uniform layer of powder 5, a first development and transfer station 300' to develop a first stable electrographic image on a first material of copy 28 and a second stable electrographic image 13 on the least conductive parts of layer 24, and a second transfer station 200 to transfer said second image of powder 13 on to a second material of copy 8. The

endless belt

24, 44 moving in the direction of arrow 110, the developer powder of powderer 25 coats the layer 24 and the powder is driven by the upward movement of the latter between a grid-electrode 36 and a transparent electrode 3 7. Under the action of an electric field generated between

electrodes

36 and 37, the excess of powder 5 is charged from the remaining part of the layer of powder and it is electrically attracted through the grid-electrode 36 to fall again in the container 25; a thin uniform layer of powder 5 is thus obtained on the layer 24, rather than a particular amount of grains. Proceeding from powderer 400' to the first development and transfer station, the thin uniform layer of powder 5 is sandwiched between the layer 24 and the first material of copy 28. This material 28 may consist, for example, in a web of ordinary paper. The web 28 is driven against layer 24 by two rollers 30. Under the action of an electric field generated between the

electrodes

6 and 7, the powder 12 coating illuminated parts 2 of layer 24 is electrically transferred on to the first web of copy 28 to form a first stable electrographic image thereon, while the powder 13 forms a second stable electrographic image on the least conductive parts of layer 24. This second electrographic image is then transferred on to the second web of copy 8 at the second transfer station 200. The other features of the apparatus of FIG. 11 are the same as those above described with reference to the apparatus of FIG. 10.

While the present invention, as to its objects and advantages, has been described herein as carried out in specific embodiments, thereof, it is not intended to be limited thereby but it is intended to cover the invention broadly within the spirit and scope of the appended claims.

What I claim is:

1. A method for producing electrographic images from a conductivity pattern formed by areas having differing electric conductivity characteristics ranging from an area exhibiting maximum conductivity, to an area exhibiting minimum conductivity, said conductivity pattern being affixed to an insulating backing member, comprising the steps of coating said conductivity pattern with a thin layer of developer powder capable of receiving an electric charge, disposing a fluid insulating layer against said coated conductivity pattern so that said coated conductivity pattern is electrically insulated beween said insulating backing member and said fluid insulating layer, said insulating backing member and said fluid insulating layer being disposed between first and second electrodes, at least one of said electrodes being a grid electrode with said grid electrode being in spaced relationship with said coated conductivity pattern and being juxtaposed to said fluid insulating layer, generating an electric field of suflicient strength between said electrodes and across said fluid insulating layer and said insulating backing member to charge said powder whereby a portion of said powder is sufficiently charged and removed from said conductivity pattern and the remainder of said powder is insufficiently charged so that it continues to coat said conductivity pattern thereby producing a stable electrographic image on said conductivity pattern said portion of said powder removed from said conductivity pattern migrating through the openings in said grid electrode and being removed from said electric field.

2. The method of claim 1 which includes alternatively modulating said generated electric field to transfer alternating electric charges from said conductivity pattern to said powder and to electrically attract grains of powder having successive opposite polarities away from said conductivity pattern through said grid electrode, proceeding in the application of said alternatively modulated electric field to remove a part of the coating powder away from said conductivity pattern and through said grid electrode whereby a stable electrographic image of high density is formed on said pattern by the remaining part of the coating powder.

3. The method of claim 1 wherein coating said conductivity pattern with a thin layer of developer powder comprises blowing a cloud of said developer powder onto said conductivity pattern thereby coating the same and, simultaneously generating an alternatively modulated electric field between said electrodes to transfer alternating electric charges from said conductivity pattern to said powder and to electrically attract grains of powder having successive opposite polarities away from said conductivity pattern through said grid electrode, proceeding in the simultaneous blowing of said coating powder and generation of said modulated electric field to remove a part of the coating powder away from said conductivity pattern and through said grid electrode whereby a stable electrographic image of high density is formed on said pattern by the remaining part of the coating layer of blown powder.

4. A method for producing electrographic images on members of copy material, comprising the steps of aflixing a thin photoconductive non-insulating layer to an insulating backing member, coating said photoconductive non-insulating layer with a thin layer of developer powder capable of receiving an electric charge, disposing a fluid insulating layer against said coated photoconductive noninsulating layer so that said coated photoconductive non-insulating layer is electrically insulated between said insulating backing member and said fluid insulating layer, exposing said coated photoconductive non-insulating layer to a pattern of electromagnetic radiation to form a conductivity pattern in said coated photoconductive noninsulating layer, said conductivity pattern comprising areas having differing electric conductivity characteristics ranging from an area exhibiting maximum conductivity to an area exhibiting minimum conductivity, said insulating backing member and said fluid insulating layer being disposed between first and second electrodes, at least one of said electrodes being a grid electrode with said grid electrode being in spaced relationship with said conductivity pattern formed in said coated photoconductive non-insulating layer and being juxtaposed to said fluid insulating layer, generating an electric field of suflicient strength between said electrodes and across said fluid insulating layer and said insulating backing member to charge said powder whereby a portion of said powder is sufficiently charged and removed from said conductivity pattern and the remainder of said powder is insufiiciently charged so that it continues to remain in said conductivity pattern thereby producing a stable electrographic image on said conductivity pattern, said portion of said powder removed from said conductivity pattern migrating through the openings in said grid electrode and being removed from said electric field, transporting said electrographic image bearing photoconductive non-insulating layer from said electric field to a transfer station and thereafter, at said transfer station, placing a copy material against the powder of said electrographic image, uniformly exposing said photoconductive non-insulating layer to an electromagnetic radiation inducing a uniform high electric conductivity in this photoconductive non-insulating layer, generating an electric field across said copy material and said photoconductive non-insulating layer to charge said powder from said uniformly conductive photoconductive non-insulating layer whereby said electrographic image is electrically transferred onto said copy material.

5. An electrographic device comprising a thin photoconductive non-insulating layer, an insulating backing member onto which said photoconductive non-insulating layer is affixed, means for coating said photoconductive non-insulating layer with a thin layer of developer powder capable of receiving an electric charge, a fluid insulating layer in juxtaposition to said photoconductive non-insulating layer coated with said developer powder so that said coated photoconductive non-insulating layer is electrically insulated between said insulating backing member and said fluid insulating layer, means for exposing said coated photoconductive non-insulating layer to a pattern of electromagnetic rediation to form a conductivity pattern in said coated photoconductive non-insulating layer, said conductivity pattern comprising areas having differing electric conductivity characteristics ranging from an area exhibiting maximum conductivity to an area exhibiting minimum conductivity first and second electrode means between which are disposed said insulating backing member and said fluid insulating layer, at least one of said electrode means being a grid electrode, said grid electrode being in spaced relationship with said conductivity pattern formed in said coated photoconductive non-insulating layer and being juxtaposed to said fluid insulating layer, means for generating between said electrodes and across said fluid insulating layer and said insulating backing member an electric field of sufiicient strength to charge said powder whereby a portion of said powder is sufficiently charged and removed from said conductivity pattern and the remainder of said powder is insufliciently charged so that it continues to coat said conductivity pattern thereby producing a stable electrographic image on said conductivity pattern said portion of said powder removed from said conductivity pattern migrating through the openings in said grid electrode and being removed from said electric field.

6. A method for producing electrographic images, comprising the steps of affixing a thin photoconductive non-insulating layer to an insulating backing member, coating said photoconductive non-insulating layer with a thin layer of developer powder capable of receiving an electric charge, disposing a fluid insulating layer against said coated photoconductive non-insulating layer so that said coated photoconductive non-insulating layer is electrically insulated between said insulating backing member and said fiuid insulating layer, exposing said coated photoconductive non-insulating layer to a pattern of electromagnetic radiation to form a conductivity pattern in said coated photoconductive non-insulating layer, said conductivity pattern comprising areas having differing electric conductivity characteristics ranging from an area exhibiting maximum conductivity to an area exhibiting minimum conductivity, said insulating backing member and said fluid insulating layer being disposed between first and second electrodes, at least one of said electrodes being a grid electrode with said grid electrode being'in spaced relationship with said conductivity pattern formed in said coated photoconductive non-insulating layer and being juxtaposed to said fluid insulating layer, generating an electric field of suflicient strength between said electrodes and across said fluid insulating layer and said insulating backing member to charge said powder whereby a portion of said powder is sufliciently charged and removed from said conductivity pattern and the remainder of said powder is insufliciently charged so that it continues to remain in said conductivity pattern thereby producing a stable electrographic image on said conductivity pattern said portion of said powder removed from said conductivity pattern migrating through the openings in said grid electrode and being removed from said electric field.

References Cited UNITED STATES PATENTS Moncrieff-Yeates 95-13 Yeh 95-17 Gundlach 117-175 Mihajlov 96-1 LY Goffe 96-13 Mihajlov 96-13 Lehmann 117-37 LE 'Kaprelian 117-175 Huebner 117-175 Moncriefl-Yeates 117-17.5 Van Wagner 118-638 1 6 Gundlach 117-175 Carlson 1l7-17.5 Bryne 117-17.5 Gundlach 96-l.4 Rheinfrank 117-175 Jarvis 117-175 Carlson 117-175 Zabiak 117-175 Nail 117-17.5 Matkan et a1 96-14 Bixby 117-175 Hunter 11717.5 Oliphant et a1 117-37 15 MURRAY KATZ, Primary Examiner M. SOFOCLEOUS, Assistant Examiner U.S. Cl. X.R.

20 96-12, 1.3, 1.4, 1 LY; 117-175, 37 LE; 355-3, 10, 17