EP0091073B1 - Electrostatic printing head - Google Patents

Description

The present invention relates to electrostatic image printing on a recording medium; it relates more particularly to an electrostatic printhead with multiple electrodes which, brought to high voltage, create by electrical discharge in air a latent electrostatic image on the recording medium moving past them.

This printing mode therefore requires that an air slot be made between the electrodes of the print head and the facing surface of the recording medium on which this latent image is printed. Indeed, to generate, by means of a discharge in a gas, an image of electrostatic charge, in addition to the sufficiently high voltage to which the selected electrodes are brought, it is common to adopt a discharge slot of approximately 10 _li m responding to the Pashen curves of discharge in gases, in this case in air at atmospheric pressure (cf. for example FR-A-2 205 850).

When the recording medium consists of a conductive paper substrate carrying on one of its faces a dielectric layer a few micrometers thick, the natural roughness of the paper or the use of spacers embedded in the dielectric layer makes it possible to obtain this discharge slot between the electrodes and the dielectric surface. However, when the recording medium has an almost smooth surface state, which is particularly the case when it is constituted by a dielectric film, or a metal drum covered with a dielectric layer, used as an intermediate medium on which is generated latent image which will later be transferred, after development, to a final support, it is necessary to provide, at the level of the print head itself, means creating this discharge slot.

Document FR-A-2138 789 describes a device for generating charge images, with geometrically limited space for the reproductive parts of an arrangement of electrodes, located opposite the highly ohmic dielectric layer of the information carrier. In an embodiment given in this document, an arrangement of electrodes is formed by distinct conductive styles and is embedded in an insulating material having, at this arrangement, a hollow defined at the diameter of the styles, or, as a variant, is formed by a conductive style in the form of a symbol and is embedded in an insulating material, the hollow of which is defined in the profile of the symbol, so that the lower surface of the electrode arrangement does not lie flush with the lower surface of the insulating material and that this hollow then constitutes the essential discharge slot. In another given embodiment, an arrangement of electrodes is constituted by a thin very conductive layer covering the walls of bores or openings made in an insulating material, or alternatively, by profiled recesses made in a conductive material ; also in these two cases, the discharge inside the bores, openings or recesses makes it possible to create the load images.

These electrostatic heads designed so that the necessary electrical discharge slot is defined therein do not lend themselves to easy industrial production. In addition, they do not seem adaptable to rapid and high resolution printing (up to 8 points per millimeter) obtained by multi-electrode electrostatic heads (1,728 electrodes), currently operating with a paper covered with a dielectric layer with surface irregularities, which use printed circuit technology to define, by the end portions of conductive tracks, the functions of electrodes.

In this type of electrostatic head using printed circuit technology, it is the front portions of the conductive tracks which are flush or even projecting over the edge of the substrate which constitute the electrodes. Usually, such an electrode printhead is produced in two lines by assembling back to back two substrates carrying printed tracks US-A-4180 824 or the two parts of a single folded and cut substrate (US-A -4,287,525). These heads, however, do not work with a recording medium with a too smooth surface state.

The object of the present invention is to produce a printhead in which an electrical discharge slot is provided at its electrodes to allow it to operate with a recording medium with a smooth surface state and in which the technology is used. printed circuits to define a linear arrangement of electrodes compatible with very high printing performance.

Another object of the present invention is to keep the distance separating each of the electrodes from the recording medium constant, regardless of defects which would normally cause variations in this distance.

The subject of the present invention is therefore an electrostatic printhead bearing on a recording medium with an almost smooth surface state, comprising a plurality of electrodes disposed recessed relative to the surface of said recording medium to define therebetween and said surface a discharge slot responding to the Pashen curves in the air, characterized in that said electrodes are constituted by the end portions of conductive tracks of a flexible printed circuit, and that said printed circuit is mounted, at the very less at the level of the electrodes and of the adjacent parts of the tracks, the back pressed against a convex support structure, and with its outer face coated, in outside the end portions of the conductive tracks, an insulating film of selected thickness so that by pressing on the recording medium it maintains said slot between said electrodes and said recording medium.

Such a head is therefore particularly suitable for an industrial production of heads with high printing performance on a recording medium with a smooth surface state such as a metal drum covered with a thin dielectric layer or dielectric strip running on a metal roller serving counter electrode.

Preferably, the supporting structure of the printed circuit or the metal roller on which the recording medium runs when the latter is a dielectric strip, is made of elastic material and is subjected to a pressure means holding the recording medium and the insulating film applied one against the other.

Under these conditions, by deformation of the elastic material, any machining imperfections that the metal drum of the recording medium or the roller in abutment against which the recording medium runs can be compensated for.

• les figures 1 et 2 représentent schématiquement une tête d'impression électrostatique selon l'invention, pour l'impression d'images électrostatiques latentes sur un tambour recouvert d'un matériau diélectrique et sur une bande en matériau diélectrique, respectivement,
• la figure 3 illustre schématiquement des éléments rentrant dans ladite tête d'impression,
• la figure 4 représente à échelle agrandie la zone notée IV de cette figure 3.

The invention will be better understood with the aid of the detailed description of an exemplary embodiment illustrated in the attached drawing. In this drawing:

• FIGS. 1 and 2 schematically represent an electrostatic printing head according to the invention, for printing latent electrostatic images on a drum covered with a dielectric material and on a strip of dielectric material, respectively,
• FIG. 3 schematically illustrates elements entering said print head,
• FIG. 4 represents on an enlarged scale the zone denoted IV of this FIG. 3.

With reference to FIGS. 1 and 2, it can be seen that the electrostatic print head comprises a flexible printed circuit 1, with conductive tracks such as 2 printed on one of the faces of the flexible substrate organized in two distinct networks of tracks, the parts of which facing terminals are aligned in two parallel rows and constitute the printing electrodes (not referenced in these two figures). It further comprises an insulating film 3, 10 to 20 fJ.m thick deposited on the printed face of circuit 1, at least outside the end portions of the tracks constituting the electrodes and the interval between the two rows of electrodes which are therefore not covered. This film 3 will advantageously be divided into two parts deposited identically on one and the other of these two networks of conductive tracks.

The printed circuit 1 is mounted with the back pressed against a supporting structure formed by a drum 4, so that the rows of electrodes defined on the external face of the printed circuit are arranged substantially along two generatrices of this drum. The drum 4 will advantageously be chosen of small diameter, of the order of 1 to a few centimeters, to obtain a head of small thickness but also so that it defines at the level of. electrodes a very convex convex profile which will be possibly deformable according to the choice which will be made as for the material constituting the drum.

The drum 4 is associated with an auxiliary support structure formed here by a U-shaped support 5. The lateral branches of this support receive the portions of the printed circuit 1 which are situated on either side of its median portion held on the drum and which define the connection ends of the conductive tracks to an electrode control circuit (this external control circuit of the problem treated by the present invention is therefore neither illustrated nor described). This U-shaped support can be secured to the axis 6 of the drum 4. The printed circuit against the drum 4 and against the support 5 can be maintained by means of an adhesive or an adhesive polyester film.

With reference to FIGS. 3 and 4 in which the arrangement of the conductive tracks 2 on the printed circuit substrate has been shown diagrammatically, it can be seen that these tracks are organized in two distinct networks and terminate in the longitudinal portion of the printed circuit, concerns the two networks, respectively in two parallel rows separated from each other by a distance x equal to one or a few printing lines. The terminal portions of the tracks 2, defining these two rows, not covered by the insulating film 3 deposited on the printed face of the circuit 1, constitute the electrodes 8 of the print head; the size of the inscribed points will therefore be, in one dimension, determined by these portions not covered by the insulating film. Regardless of the organization of the tracks 2, these electrodes 8 are of regular pitch along their row and are offset between them by a half-step from one row to another.

From the ends constituting the electrodes 8 the conductive tracks 2 of one and the other of the two networks extend respectively towards one and the other of the longitudinal edges of the printed circuit and end, advantageously in a stepped manner by group for obvious facilities for connection with the control circuit of the electrodes to be connected to these so-called connection ends. Thus with regard to FIG. 3, it can be seen that the conductive tracks are organized in groups of sixteen and that the connection ends for each group, noted 8-1, ... 8-8, 8-9 ... 8- 16 for one of the groups, are effectively tiered. These ends have moreover been advantageously brought into two parallel lines along which they are regularly distributed by being two by two facing each other, or, in a variant not illustrated, offset from one line to another. The printed circuit portions including these connection ends, or at least the connection ends themselves, are not covered by the insulating film 3.

Of course in practice, the conductive tracks can be arranged in different groups, taking into account the number of electrodes that the head comprises (for example 1728 electrodes), of the control circuit which will be associated with the print head and of facilities for connecting this control circuit to the ends of these tracks.

In the printhead according to the invention, by mounting the printed circuit on a convex surface, it is the terminal portions of the tracks, and not their front end faces which constitute the electrodes. The dimensions of these electrodes are then limited, depending on the desired size of the individual dots to be printed on the one hand by the width of the tracks, but also by the insulating film 3 appropriately positioned relative to the ends correspondan- ^- your tracks for leave only the desired lengths of end portions serving as printing electrodes uncovered.

In addition in the print head, the insulating film 3, by the extra thickness it creates on either side of the electrodes, constitutes the bearing face of the head on the recording medium, with respect to which the electrodes remain distant, and cleaning between the electrodes and the recording medium, whatever the even perfectly smooth surface condition that it may have, the ion discharge slot necessary for the generation of latent images.

The description of the printhead according to the invention will be completed with reference to Figures 1 and 2, depending on the type of recording medium used, such as a metal drum 10 covered with a dielectric layer, axis parallel to that of the head drum (Figure 1) or a dielectric film 11 (Figure 2), having a relatively smooth surface condition.

In FIG. 1, the periphery of the drum 10 covered with a dielectric layer, if necessary electrostatically charged by a corona device illustrated at 12, scrolls past the two rows of electrodes of the print head supported by the film insulator 3 on the dielectric layer. The writing of the latent image is then carried out by placing the individual electrodes and the metal drum at suitable potentials creating an ionic discharge in the air layer between the individual electrodes and the dielectric layer, normally defined by the thickness of the film. insulator 3 (from 10 to 20 fJ.m).

In the use of the print head with such a support, the drum 4 of the support structure of the head will advantageously be made of an elastic material and will receive a pressure P, for example of the order of 1 kg applied to its axis 6.

Under these conditions, the flexible print head comes to be applied strictly against the rigid drum 10 and adapts to its periphery. This arrangement makes it possible to get rid of any machining defects in this recording drum which would affect the thickness of the air layer along the rows of electrodes facing the dielectric layer, by compensating for them. by crushing the printhead which ensures positioning of each of the electrodes at a constant distance opposite the periphery of the printing drum.

In FIG. 2, the film of dielectric material 11, possibly electrostatically charged by an illustrated corona device 13 and mounted opposite a metal guide 15, is driven in front of the print head while being supported, at its level for a conductive roller 14 serving as a counter-electrode and then belonging to the print head.

In this case of use of the print head with a flexible recording medium, the drum 4 of the support structure of the head will also be made of flexible material and subjected to a pressure P applying it rigorously against the periphery of the fixed roller 14 or, in the illustrated variant, it is the roller 14 which will be deformable and applied against the drum 4 then rigid and fixed with respect to it.

Under these conditions there is the same advantage of positioning the electrodes at a constant distance from the flexible recording medium independently of any machining defects in the drum 4 or the roller 14.

Claims (5)

1. An electrostatic print head for printing on a recording medium having a substantially smooth surface, comprising a plurality of electrodes disposed at a distance of the surface of said recording medium in order to define between them and said surface a discharge slot corresponding to the Pashen curves of air discharge, characterized in that said electrodes are constituted by the end portions of conducting tracks of a flexible printed circuit (1) and that said printed circuit (1) is mounted, at least in the vicinity of the electrodes and the adjacing portions of the tracks, with its back pressed against a convex support structure (4) and having its outer face covered, except for the end portions of the conducting tracks (2), by an insulating film (3) of such a thickness to ensure that when it is pressed against the recording medium (1_'), the film maintains said slot between said electrodes (8) and said recording medium.

2. An electrostatic print head having two rows of electrodes separated from one another by a distance equal to n printing lines and staggered from each other by a half-step of electrodes, n being an integer equal to unity or to several unities according to claim 1, characterized in that said insulating film is divided into two portions which are distanced from each other by n + 2 printing lines, for maintaining uncovered the two rows of electrodes and the printing lines between them.

3. A print head according to one of claims 1 and 2, characterized in that said convex support structure is a drum (4) of resilient material on which said electrodes define a linear arrangement parallel to its axis.

4. A print head according to claim 3, characterized in that said drum is submitted to a pressure means (P) pressing said insulating film (3) onto said recording medium.

5. A print head according to one of claims 1 to 3, and comprising, facing said electrodes, a structure of electrically conductive material on which passes said recording medium in the form of a dielectric band, characterized in that at least one of the structures (4, 14) which carry said printed circuit and said recording medium respectively is made of resilient material and is submitted to a pressure means (P) pressing said insulating film and said recording medium against each another.

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