CH632856A5 - METHOD AND APPARATUS FOR FORMING ELECTROSTATIC IMAGES USING ION PRODUCTION. - Google Patents

Description

The present invention relates to a method and an apparatus for the formation of electrostatic images by means of an ion production allowing electrostatic printing and photocopying, in particular at high speed.

Electrostatic printers and photocopiers have a number of common elements, although they perform different operations. Electrostatic printers and photocopiers, which are capable of producing an image on untreated paper, are generally different in the method and means of obtaining a latent electrostatic image on an intermediate element.

The object of the invention is the compatibility of the design of an inexpensive printer and photocopier working at

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high speed by producing an image of excellent quality.

To this end, the subject of the invention is a method for forming an electrostatic image by means of an ion production, characterized in that it comprises:

- the application of an alternating potential of sinusoidal shape, square or triangular, between a first and a second respectively emitting and control electrode separated by a solid dielectric element comprising a surface not covered by the first electrode, from which one causes an aerial breakdown with production of ions between the first electrode and the surface not covered with the dielectric element,

- the extraction of the ions, coming from the aerial breakdown, by a direct tension, determining an electric field, applied to the second electrode,

- the deposition of ions on a dielectric element carried by a conductor to form the electrostatic image with the ions extracted and deposited.

Thus, an aerial breakdown can be caused in a hole in the first electrode coming either from a cutout if it is full, or from its manufacture if it is materialized by a woven screen with open meshes, by an alternating potential of sinusoidal shape, square or triangular, which causes an extraction of the ions coming from said aerial breakdown by a continuous voltage, determining an electric field, applied to the first electrode, and finally a deposit of the ions on another element, like in particular a dielectric layer carried by a conductor, to form an electrostatic configuration with the extracted ions.

It is possible to provide a multiplicity of electrodes forming networks whose intersections which determine a matrix correspond on one side of the solid dielectric element, each to a hole, whence an extraction of the ions from the holes by the application simultaneous electric discharge and an external field, and the formation of a latent electrostatic image on a dielectric layer with said ions extracted before coloring and expansion for posterior transfer.

The method for forming an electrostatic image according to the invention is implemented with an apparatus which is characterized in that it comprises

- a solid dielectric element,

a first emitting electrode on one of the sides of the solid dielectric element with an aerial breakdown zone between itself and a surface not covered with the solid dielectric element,

- a second control electrode on the other side of the solid dielectric element,

means for applying an alternating potential having a sinusoidal shape, square or triangular, of frequency varying between 60 Hz and 4 MHz, between the electrodes leading to an aerial breakdown, and

- Means for creating an electric field applied to the second electrode for the extraction of ions from air breakdown and for transferring them to another element consisting of a dielectric layer carried by a conductor.

Embodiments of the apparatus for implementing the method according to the invention are described, by way of examples, with reference to the appended drawings, in which:

- Figure 1 is a schematic view, in section of an ion generator.

- Figure 2 is a schematic view, in section, of an ion generator and an extractor.

- Figure 3 is a top view of a character generator.

- Figure 4 is a top view of an ion generator matrix for an electrostatic printer of a dot matrix.

- Figure 5 is a partial perspective view of a model of an ion generator.

- Figure 6 is a schematic view of a copier using the ion generator of Figure 5.

- Figure 7 is a sectional view of another ion source produced.

- Figure 8 is a sectional view of an aerosol device loaded for high speed printing of dot matrices using an ion generator.

- Figure 9 is a sectional view of a line printing device along a line using an ion generator.

- Figure 10 is a sectional view of an electrostatic precipitator.

- Figure 11 is a graph illustrating the relationship between the electrode and paper voltages.

- Figure 12 is a perspective view of a colored electrographic image produced by the dot matrix ion generator described in Figure 4 on a dielectric element carried by a conductor.

- Figure 13 is a schematic view, in section of an ion generator according to one embodiment.

FIG. 14 is a schematic view, in section, of an ion generator and an extractor using the device presented in FIG. 13.

FIG. 15 is a schematic view of another circuit which can be used for the generator and the ion extractor of FIG. 14.

Ion emission and extraction

The ion emission and extraction are essentially obtained from two electrodes separated by a solid dielectric element (subsection A), but certain advantages appear by the addition of a third electrode (subsection B).

A. Two-electrode device

FIG. 1 represents an ion generator (100) which determines an aerial breakdown between a solid dielectric element (101) and two electrodes (102-1,102-2). The first electrode (102-1) is placed on one side of the solid dielectric element (101) with an aerial breakdown area between the electrode and the uncoated surface of the dielectric element (101), the second electrode (102-2) being placed on the other side of the solid dielectric element (101). A source (103) is used for the application of an alternating potential having a sinusoidal shape, square or triangular, of frequency varying between 60 Hz and 4 MHz between the two electrodes (102-1, 102-2) leading to a aerial breakdown. When the lateral electric fields (Ea) and (Eb) in the zones (104-a) and (104-b) are greater than the breakdown fields of the air, an electric discharge occurs due to the charge of the dielectric element (101) in the zones (101-a) and (101-b) adjacent to the sides of the electrodes. When the alternating potential from the source (103) is reversed, there is a reversal of the charges in the areas (101-a) and (101 -b). Thus, the generator (100) of FIG. 1 determines an aerial breakdown twice per cycle of the potential of the source (103) and leads to an emission of alternating polarity ions. The extraction of the ions produced by the generator (100) of FIG. 1 is carried out by the generator-extractor (110) of FIG. 2. The generator (110A) comprises a dielec element5

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sheet (111) between the conductive electrodes (112-1) and (112-2). In order to avoid an aerial breakdown near the electrode (112-1), said electrode is shielded by an insulating material (113). An alternating potential is produced between the conductive electrodes (112-1) and (112-2) by a source (114A). In addition, the second electrode (112-2) has a hole (112-h) in which the aerial breakdown occurs relative to a region (111-r) of the dielectric element (111) from which a source of ions.

The ions formed in the hole (112-h) can be extracted by a continuous potential coming from a source (114B) to obtain an external electric field between the electrode (112-2) and an auxiliary electrode (112-3) grounding. For example, a dielectric surface which can receive a charge from the ion source presented in FIG. 2 is a dielectric (electrophotographic) paper (115) consisting of a base (115-p) coated with a thin dielectric layer (115-d).

When an inverter (116) is in position X and is grounded as shown in Figure 2, the electrode (112-2) is also grounded and no external breakdown is present in the area between the generator ion (100A) and dielectric paper (115). However, when the inverter (116) is in the Y position, the potential of the source (114B) is applied to the electrode (112-2), which determines an electric field between the ion reservoir (111- 4) and the back of the dielectric paper (115). The ions extracted from the aerial breakdown zone will then charge the surface of the dielectric layer (115-d).

Many materials can be used for the manufacture of the dielectric (111), namely aluminum oxide, glass enamel, ceramics, plastic films and mica. Aluminum oxide, glass enamel and ceramics lead to difficulties in obtaining a sufficiently thin layer (of the order of 0.025 mm) which avoids excessive drawing on the base source ( 114A). Plastic films comprising polyimides such as kapton, nylon, tend to deteriorate due to exposure to parasitic chemicals from the aerial breakdown in the hole (112-h), namely in particular the 'ozone and nitric acid. Mica avoids these drawbacks and is therefore the preferred dielectric material (111). In particular, we choose H2 KAI3 (Si 04) 3 (muscovite) mica.

The generator and ion extractor (110) of Figure 2 are normally used, for example, for forming characters on dielectric paper for high speed electrographic printing. Typical sources for electrographic character printing are shown in Figures 3 and 4.

In FIG. 3, a character generator (120) is formed using a dielectric element (121) which is arranged between a raised electrode (122-1) and a set of counter electrodes (122-2 ), (122-3) and (122-4).

The raised or hidden electrode (122-1) is presented with the raised characters A, B and C. The lateral fields on the sides of the engraved characters determine a very dense source of ions when a breakdown occurs overhead caused by an alternating potential applied between the raised electrode (122-1) and the counter electrodes. Thus, when it is desired to produce ions for the printing of a determined character, such as for example the letter B, a high frequency alternating voltage source (not shown in the figures) is applied between the electrode in relief (122-1) and the associated electrode (122-3), hence a very dense supply of ions in the region of the dielectric element (121) on the sides of the character B in the relief electrode ( 122-1). The ions are then extracted and transferred to a dielectric surface, for example the paper (115), covered with a dielectric layer shown in FIG. 2, by applying a DC voltage between the back of the paper and the raised electrode. (122-1), hence the formation of a latent electrostatic image B on the dielectric surface of the paper (115).

For the formation of a character matrix materialized by dots on dielectric paper, the ion generator (130) of FIG. 4 can be used. The generator (130) comprises a dielectric layer (131) with a set of aerial breakdown holes formed in electrodes (132-1) to (132-4) on one of its sides and a set of selection bars ( 133-1) to (133-4) on the other side, cooperating with an independent selector (133) corresponding to each hole (135) of each elongated electrode (13 2).

When an alternating potential is applied between a selection bar (133) and the earth, ions are emitted in the holes at the intersections of said bar and elongated electrodes. The ions can only be extracted from the opening if, at the same time, the selection bar receives energy in the form of a high alternating potential, and the elongated electrode receives a continuous potential applied between the elongated electrode and the dielectric counter electrode to be charged. The position (135-23) on the matrix is printed by simultaneously applying a high frequency potential between the selection bar (133-3) and the earth and a continuous potential between the elongated electrode (132-2) and an element dielectric receiver forming a counter electrode held to earth.

By multiplexing a hole matrix in this way, the number of voltage conductors is reduced significantly. If, for example, we want to print a 200 mm hole matrix with 800 holes per dm, 1600 conductors would be required without multiplexing. Using the diagram in Figure 4 with, for example, 80 elongated electrodes can be used with a total number of 100 conductors instead of 1600.

In order to avoid aerial breakdown between the electrodes (132) and the dielectric layer (131) in the areas which do not correspond to the holes (135), it is necessary to shield the sides of the electrodes (132) with an insulating material. An aerial breakdown can be avoided around the electrodes (132) by embedding them.

When constructing and using an ion generator of this type, it is desirable that the different ions created at different crossing points of the matrix be at substantially uniform potential. Variations in the thickness of the dielectric layer (131) result in measurable variations in the ion current, with a lower ion current being obtained at a hole (135) when the dielectric (131) is thicker. A particularly advantageous property of mica lies in its ability to cleave along planes separated by an extremely uniform thickness, which makes it very practical for producing the ion generator presented in FIG. 4. It is remarkable that the uniformity of the The thickness of the layer (131) is much greater than the value of this thickness.

For the formation of a rectangular charging surface, the module 140 presented in FIG. 5 is used. The charging electrodes (142-1) and (142-2) are separated from the electrode (142-3) by an element. dielectric (141), the electrode (142-3) being embedded in an insulator (145). The region between the electrodes (142-1) and (142-2) has a slot in which an aerial discharge occurs when a high frequency alternating current is applied between the electrodes (142-1) and (142-2) , and, the electrode (142-3).

The load surface of Figure 5 can be used in an untreated paper copier to replace the crowns normally found there.

Figure 6 shows schematically a paper copier s

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untreated using load surfaces of the type of Figure 5. A copying drum (151) is loaded by a load member (152-1) having the configuration shown in Figure 5. If the drum is selenium or made of selenium alloy and you want to charge the surface, for example at a potential of +4600 V, the slit electrode (142-1) is maintained at 600 V. After being charged, the drum (151) is discharged with an optical image obtained by scanning at the station (153). The resulting latent electrostatic image is colored at the station (156) and the dye is transferred to an untreated sheet of paper (158) by an ion transfer generator (152-2) with a split electrode maintained from again to positive potential. The residual latent electrostatic image on the surface of the drum and any uncharged dye can be electrically discharged by using a discharge unit (152-3) similar to the charging member (152-1). The split electrode is kept grounded and any residual charge on the surface of the drum or dye determines the extraction of ions by air breakdown in the slot which discharges the surface. A cleaning brush (154) is used to remove any dye remaining on the surface of the drum which is then ready for a new charge.

Figure 6 also shows the load head (155) having a hole matrix (Figure 4), which allows the use of untreated paper. The drum (151) is unloaded at the station (153) and recharged by the charging head (155) comprising a hole matrix, which allows the machine (150) to be used for copying and printing. In addition, the machine (150) can operate simultaneously for copying and printing when an overload is desired. Thus, an ion generator and an extractor according to Figure 2 can be used to form an image.

Figure 7 shows another type of ion generator (160) constructed for charging or discharging an insulating surface. In this figure, the split electrode (142-1,142-2) of FIG. 5 is replaced by a woven screen with open meshes (162-2) comprising longitudinal (162-a) and transverse (162-b) elements. The discharge electrodes (162-1) and (162-2) are separated by a dielectric layer (161) and the overhead breakdown is caused by an alternating potential (163).

Figure 8 shows an apparatus (170) for using a hole matrix load head (171) of the type shown in Figure 4 for high speed printing on untreated paper. The charge head (171) charges an aerosol (175) consisting of a dye dissolved in a suitable solvent which is carried by a low speed air flow flowing in a channel (176). The aerosol particles are charged by the ion generator and enter an electric field established by a continuous potential formed between the electrodes (173) and (174). This field directs aerosol particles loaded onto an untreated sheet of paper (172) which travels through the device at a speed roughly similar to that of the aerosol.

Figure 9 shows a mechanical scan printing along a line. A split electrode (186) is used in cooperation with a dielectric film (185) and with a rapidly moving conductive bed (187) so as to form a region in which the air breakdown travels. The bed (187) mounted on the cable (188) is driven by pulleys driven by a fast electric motor (not shown in the figures). A high frequency source (183) provides the potential for overhead breakdown in the electrode slot (186). In this example, a dielectric paper (181) is charged by a charging potential from an amplifier (184) whose output is connected to the conductive support (182) of the dielectric paper and to the slit electrode (182). The scanning along a line is obtained by a mechanical displacement of the bed (187) and selected areas are printed by the application of a potential between the conductive sheet and the split electrode. As in the previous cases, the latent electrostatic image which is formed can be colored and encrusted according to any known technique. Degraded coloring images can be obtained in this way since the amount of ions extracted by the discharge depends on the extraction potential from the amplifier (184).

Figure 10 shows the use of an ion generator as an electrostatic precipitator (190). A tabular electrode (192) is separated from a segmented electrode (194) by a dielectric (191). An aerial breakdown is produced in the open areas of the segmented electrode (194) by application of a high alternating potential coming from a generator (196). The segmented electrode (194) is also connected to a source of direct potential (198). A ground cable (199) is mounted in the center of the tube (192). Particulate gases or other aerosols can be cleaned by electrostatic precipitation as they pass through the tube. The intense current of ions from the aerial breakdown zones charges the solid particles and accumulates them on the electrode (199).

In general, the relationship between the voltage of the electrode and that of the surface receiving the ions, for example of paper, is of the type of that presented in Figure 11, for charging systems identical to those presented in Figures 2 to 5. The electrode voltage is the direct potential applied between the perforated electrode and the counter electrode of the dielectric surface to be charged. The tension of the paper is the potential of the electrostatic image of the charged electrostatic elements, of the electrographic paper in this case.

The following examples of use of the ion generator show its wide field of application. In addition to the applications described, the method and the apparatus can be used in numerous other applications, not described, such as those relating to electrostatic separation and coatings.

Example 1

A 0.025 mm thick stainless steel sheet is laminated on both sides of a 0.025 mm thick mica (muscovite). The stainless steel sheet is covered with resin and photo-etched in a configuration similar to that shown in Figure 4, with holes or openings of 0.15 mm in diameter in the elongated electrodes, hence a charging head which can be used for the creation of a latent electrostatic dot matrix. The charge only occurs when a potential of -400 V is applied simultaneously to the elongated electrodes with holes and an alternating potential of 2 kV peak at the frequency of 500 kHz between the electrodes and the counter electrode. A space of 0.20 mm is maintained between the print head and the dielectric surface of the electrographic sheet. The duration of the print pulse is 20 msec. Under these conditions, it has been found that a latent electrostatic image of approximately 300 V is produced on the dielectric sheet.

This image is then colored and inlaid to determine a dense image. The ion current, extracted from this charge head, collected by an electrode spaced 0.20 mm from the head, is 1 mA / cm2. The charging head can be used properly for approximately 2000 hours.

Example 2

Example 1 is repeated using a dielectric polyimide rather than mica (muscovite). As before, a 0.025 mm thick stainless steel sheet is laminated on a 0.025 mm thick polyimide (kapton) film. Results equivalent to those of Example 1 are s

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obtained at a potential of 1.5 kV at high frequency. The charging head has a useful life of approximately 50 hours.

Example 3

An electrostatic charge head of the type shown in Figure 3 is fabricated using a 0.025 mm thick sheet of stainless steel, laminated on both sides with a 0.025 mm thick sheet of polyimide. To print fully formed characters on a dielectric surface, 2.5mm characters are etched into the laminate on one side of the sheet while the elongated electrodes covering each character are etched on the other side of the sheet as shown in FIG. 3. In order to establish a conductivity in parts normally isolated from the characters, bridges of 0.025 to 0.50 mm in thickness are not attacked. The font size is approximately 0.15 mm. Printing is carried out by applying the potentials of Example 2 with a pulse of 40 msec. Colorful images have sharp sides and high optical density. The font size is approximately 0.30 mm.

Example 4

Degraded images are determined by extracting a number of ions from the charge head per unit of time in proportion to the ion extraction potential, as illustrated in FIG. 11, where the apparent surface potential on a dielectric surface is carried as a function of the potential difference between the ion-generating electrode and the dielectric counter electrode. The slot of the ion generating electrode is 0.15 mm and the charging time is 50 msec.

B. Three-electrode devices

The image generator (130) presented in FIG. 4 is advantageously incorporated into an electrostatic printing apparatus according to FIGS. 16,17,18 (subsection II). As already seen in connection with Figure 6, an ion generator and extractor (110) like that shown in Figure 2, can be used to both create an electrostatic image on a dielectric surface and to download such an image. Referring to Figure 2, if the inverter (116) is in the Y position, the electrode (112-2) is maintained at a positive potential V and a latent electrostatic image of potential less than V is formed on the surface (115-d). However, if the inverter (116) is in the X position and if a latent electrostatic image previously formed is under the opening (112-h), the generator (110A) behaves like an eraser. This phenomenon is illustrated below with a dot printing matrix disclosed in FIG. 4 and carried under the reference (200) in FIG. 12. At a time (ti), a given hole (135-23) of the generator d ion (130) is activated by a continuous pulse which determines a negative potential at an elongated electrode (132-2) while a high frequency potential is applied to a selection bar (133-3), hence the formation of the electrostatic image of a point, image of negative polarity, occupying regions (203) and (204) on the dielectric layer (201) supported by the counter-electrode (202). At a later time (tî), the hole (135-23) is above the regions (204) and (205), the selection bar (135-3) is still activated, but a load is not desired , no negative pulse is applied to the elongated electrode (132-2). The presence of a negative electrostatic image in the region (204) however extracts the positive ions from the hole (135-23) which erases the previous image.

It has been discovered that the addition of a third electrode to the two-electrode structure presented above eliminates this problem and has additional advantages with respect to controlling the size and shape of an electrostatic image formed by a generator. ions of this type. An ion generator (210) with three electrodes is shown in section in FIG. 13. The ion generator (210) comprises an “emitting” electrode (211) and a “control” electrode (215) separated by a solid dielectric layer (213). A source of alternating potential (212) is used to obtain an aerial breakdown in the hole (214).

A third electrode ("screen" electrode) (219) is separated from the "control" electrode by a second dielectric layer (217). The terminology adopted for the three electrodes comes from that used for vacuum tubes. The terms “emitting” electrode and “control” electrode may be validly extended to the corresponding electrodes of the basic device with two electrodes. The second dielectric layer (217) has an opening (216) which is preferably substantially wider than the hole (214) in the "control" electrode. This is necessary to avoid load effects on the walls. The "screen" electrode (219) has a hole (218) which is at least partially located under the hole (214). In a matrix electrographic printer, for example the “emitter” and “control” electrodes may be the selection bars and the elongated electrodes of FIG. 4, and the “screen” electrodes may consist of other additional elongated electrodes with holes reproducing the configuration of the “control” electrodes or a metal or other plate, regularly perforated, with its holes adjacent to the printing holes. In the latter case, the “screen” electrodes can be embodied in a woven screen with open meshes.

The application of the ion generator presented above to electrostatic printing is illustrated in Figure 14 which shows the ion generator (210) of Figure 13 used in connection with a dielectric paper (220) consisting of conductive base (223) coated with a dielectric layer (221) and supported by an auxiliary electrode (225). When the inverter (222) is in the Y position, there is simultaneously an alternating potential through the dielectric layer (213), a negative potential (Vc) on the "control" electrode (215) and a negative potential. (Vs) on the "screen" electrode (219). Negative ions in the hole (214) are subjected to an accelerating field which leads them to form a latent electrostatic image on the dielectric layer (221) as in the case of two electrodes. The presence of a negative potential (Vs) on the "screen" electrode (219), chosen potential such that (Vs) is less than (Vc) in absolute value, does not prevent the formation of the image which presents a potential (Vi) less than (Vc) in absolute value.

When the inverter (222) is in position X and an anterior electrostatic image of negative potential (Vi) partially falls under the hole (214), there would be partial erasure in the absence of a "screen" electrode (219). However, the screen potential (Vs) is chosen so that (Vs) is greater than (Vi) in absolute value and the presence of the electrode (219) prevents the passage of positive ions from the hole (214) to the dielectric layer (221) (see example IV-5).

The insertion of a "screen" electrode (219) into the ion generator has other advantages than avoiding the discharge of the image under the conditions described above. The "screen" electrode can be used alone or in conjunction with the "control" electrode to control the formation of the matrix-shaped image. With Vs = O, no latent image is produced due to the discharge phenomenon. Thus, a three-level control of the matrix-shaped image is possible in an electrographic printer.

The "screen" electrode (219) determines unexpected control of the size of the image. If using the configuration of a dot matrix shown in Figure 4 with elongated electrodes, the size of the image can be controlled s

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by varying the size of the screen hole (218) (see example 6). In addition, by using such a configuration, all the variables other than the screen potential (226) being constant, a higher screen potential has been found to lead to points of smaller diameter ( see example 7). This technique can be used for the formation of fine or contrasting images. It has also been found that appropriate choices of (Vs) and of (Vc) allow an increase in the distance between the generator (210) and the dielectric layer (221) while maintaining a constant diameter for the image points. This is obtained by increasing the absolute value of (Vs) while keeping the difference (Vs-Vc) constant (see Example 8).

The shape of the image can be controlled by the use of a "screen" electrode given in an electrographic matrix printer (see Example 9). The screen holes (218) can, for example, take the form of fully formed characters which are no longer the simple round or square shapes of the control holes (214).

The electronic diagram used to control the electrographic printer of figure 14 can be modified to open the possibility of raising to a potential (Vb) the device, as shown in figure 15. The element (231) is a generator of 'impulses. The size of the control pulse can be varied to produce potentials (Vc) and (Vs) by choosing an appropriate base potential. For example, the following combinations all determine Vs = -700 V and Vc = -800 V:

no print pulse is applied.

Example 6

The electrographic printer of Example 5 was tested with different diameters for the holes (218) in the screen and the dimension of the resulting electrostatic image measured. The following results are representative:

screen hole diameter (mm)

image diameter (mm)

0.38

0.38

0.25

0.30

0.20

0.25

15

It has generally been found that a reduction in the size of the screen holes determines a reduction in the latent image without compromising on the image load.

20

Example 7

The electrographic printer of example 5 was tested with screen potentials, Vs, different and the dimension of the resulting electrostatic image measured. The following results are representative:

screen potential (volts) image diameter (mm)

30 300 -400 -500 -600

0.55 0.43 0.30 0.20

(D

Vb =

-600 V;

SVs =

—100 V;

ÔVc =

-200 V

(2)

Vb =

-500 V;

8Vs =

-200 V;

8Vc =

-300 V

(3)

Vb =

-400 V;

8Vs =

-300 V;

8Vc =

-400 V

(4)

Vb =

-300 V;

SVs =

-400 V;

8Vc =

-500 V

(5)

Vb =

-200 V;

8Vs =

-500 V;

8Vc =

-600 V

The advantages mentioned above are illustrated by the following examples:

Example 5

A 0.025 mm thick sheet of stainless steel is laminated on both sides of a 0.025 mm thick sheet of mica (muscovite). The sheet is coated with resin and photo-etched with a configuration similar to that shown in Figure 4 with holes 0.15 mm in diameter. A second sheet of 0.15 mm thick mica is applied to the sheet according to the teaching in FIG. 13. A “screen” electrode with holes of 0.375 mm in diameter respecting the same configuration as that of the elongated electrodes is photoetched in a 0.025 mm thick stainless steel sheet and applied to the second sheet of mica, the holes of the elongated electrodes and the screen being concentric. This construction determines a charge head which is used for obtaining a latent electrostatic image on dielectric paper as shown in FIG. 14, with Vc = -500V, Vs = -400 V, and an alternating potential (212) 1 kV peak at a frequency of 500 kHz. A spacing of 0.15 mm is maintained between the print head and the dielectric layer (221). Vc takes the form of a 20 msec printing pulse. Under these conditions, a latent image in the form of a point of approximately —300 V is produced on the dielectric surface. This image is then colored and inlaid to constitute a matrix with dense points. The ion current extracted from the discharge head as collected by an electrode 0.15 mm from the head is 0.5 mA / cm2. When the "screen" electrode (219) is omitted, any electrostatic image placed under the control hole is erased when

It has generally been found that an increase in the potential of the screen determines a reduction in the size of the latent image without compromising on the charge of the image.

40 Example 8

The electrographic printer of Example 5 was tested with different spacings between the print head and the dielectric layer (221). A variation of the screen potential, Vs, and the constant maintenance of the difference 45 between the potentials Vs and Vs does not cause any variation in the dimension of the resulting electrostatic image. The following results are representative:

spacing (mm) Vs (volts) Vc (volts) image diameter

50 (mm)

-400 -500 0.38

-500 -600 0.38

-600 -700 0.38

It has generally been found that in addition to an increase in the spacing between the printing head and the dielectric surface, an increase in the potential Vs determines an image 6o of constant diameter without compromising on the charge of the image.

Example 9

The electrographic printer of Example 1 was modified so that the screen had holes (48) in the form of a slot and no longer simple holes. The resulting colored latent electrostatic image was oval in shape.

0.15 0.25 0.33

B

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Claims (8)

632856 2 1. Method for forming an electrostatic image by means of ion production, characterized in that it comprises - the application of an alternating potential (114-A) of sinusoidal, square or triangular shape, between a first (102-1, î S 2-1) and a second electrode (102-2,112-2) respectively emitting and controlling separated by a solid dielectric element (101,111) having a surface not covered by the first electrode, from which causes an aerial breakdown with production of ions between the first electrode (102-1,112-1) and the surface not covered with the dielectric element (101, 111), - the extraction of the ions, coming from the aerial breakdown, by a direct voltage, determining an electric field, applied to the second electrode (112-2), - depositing ions on a dielectric element (115) carried by a conductor (112-3) to form the electrostatic image with the ions extracted and deposited. 2. Method according to claim 1, characterized in that the aerial breakdown is caused in a hole (112-h) of the second control electrode (112-2) coming either from a cut if it is full, either of its manufacture if it is materialized by a woven screen with open meshes. 3. Method according to claim 2, characterized in that there are control electrodes (132-1,2,3,4; 133-1,2,3,4) forming networks whose intersections which determine a matrix correspond on one side of the solid dielectric element, each to a hole (125), ions are extracted from the holes by the simultaneous application of an electric discharge and an external field, and the latent electrostatic image on a dielectric layer (115) with the ions extracted before coloring and expanding for posterior transfer. 4. Method according to claim 1, in which the second control electrode is subjected to a direct voltage (Vc) for the extraction of ions, characterized in that the extraction of the ions is controlled by a screen electrode (219) having holes (218) whose dimension adjustment allows the size of the electrostatic image to be determined, separated from the second control electrode (215) by a solid dielectric element (217) presenting itself holes (216), and by applying a direct voltage (Vs) between the screen electrode (219) and the conductor (223) carrying a dielectric layer (221) such that (Vs) is in value absolute smaller than (Vc) and that (Vs) does not prohibit the extraction of ions while determining the dimension of the electrostatic image, and which is formed on the dielectric layer (221) with the extracted ions, the electrostatic image, using relative displacement between the first emitting electrode (211) and the layer dielectric (221), and by means of a regulation by the selective application of a direct voltage (Vc) to the dielectric layer (221) on which the electrostatic image has a potential (Vi) relative to the conductor (225) supporting the dielectric layer, and such that (Vi) = (Vs) in order to prevent erasure of the image, and by choosing an appropriate distance between the screen electrode (219) and the dielectric layer ( 221). 5. Method according to claim 4, characterized in that one uses a multiplicity of screen electrodes (219) and control electrodes (215) whose intersections determine a matrix, and that one modulates the extraction of ions from the holes (214) of the control electrodes (215) by means of the screen electrodes (219). 6. Apparatus for implementing the procedure according to claim 1, characterized in that it comprises - a solid dielectric element (111,213), - a first emitting electrode (112-1,211) on one of the sides of the solid dielectric element (111,213) with an aerial breakdown zone between itself and a surface not covered with the solid dielectric element, - a second control electrode (112-2,215) on the other side of the solid dielectric element (111-213), - means (114,212) for applying an alternating potential having a sinusoidal shape, square or triangular, of frequency varying between 60 Hz and 4 MHz, between the electrodes (112-1,112-2; 211,215) leading to a breakdown aerial, and - means (114-3,224) for creating an electric field applied to the second electrode (112-2,215) for the extraction of the ions originating from the aerial breakdown and for transferring them to another element consisting of a dielectric layer (115,221) carried by a driver (112-3,225). 7. Apparatus according to claim 6, characterized in that the second control electrode, possibly multiple (132-1,2,3,4), has holes (135) arranged to form characters or dots and by the the fact that the first possibly multiple emitting electrode is constituted by a selection bar (133-1,2,3,4), a solid dielectric element (131) made of a material selected from a plastic film, glass , ceramic, mica and, in particular, muscovite, being disposed between the electrodes. 8. Apparatus according to claim 7, characterized in that it comprises an additional element which comprises a dielectric recording element (115,221) applied to a conductor (112-3,225) and which can enter in relative displacement relative to the first electrode (112-1,211), a third screen electrode (219) consisting of a woven screen with open mesh or a mask pierced with holes (218), a solid dielectric layer (217) separating the screen electrode (219 ) of the control electrode (215) and of the solid dielectric element (213), which like the control electrode (215) and the screen electrode (219) contains holes (216) for the extraction of ions which are wider than those corresponding to them in the control electrode (215), a direct voltage source (Vc) between the control electrode (112-2, 215) and the complementary element (112-3,225 ) can be modulated so as to form an electrostatic configuration on the dielectric recording element rement (115,221) maintained at the voltage (Vi) relative to its conductive support (112-3,225), and a direct voltage source (Vs) of the same polarity but of lower absolute value than (Ve) and (Vi) between the screen electrode (219) and the complementary element (225).