FR2477072A1 - Printing head for matrix dot electrostatic printer - has metal covers on a polyester support panel used esp. for light-weight heads for bidirectional operation - Google Patents

Abstract

A printing head for a matrix dot electrostatic printer is made by laminating two comb-shaped metal sheets on to opposite faces of a panel of insulating material so that the ends of the two combs are laterally staggered so that the prongs of one metal plate are out of phase with the prongs on the other plate. Pref. the insulating panel is of 'Mylar' (RTM - polyethylene terephthalate) and the metal plates are of beryllium bronze with tips hardened by a coating of titanium carbide or nitride. Pref. the thicknesses of the metal plates and the insulating panel are similar. Opt. the metal prongs are cut in opposite ends of a common sheet, the middle of which is wrapped over the top of the insulating panel.

Description

Electric discharge writing head for dot-matrix printer machine and its manufacturing process
The present invention relates to a writing head comprising a plurality of electrodes for printing dots, by electrical discharge between at least one of the above electrodes and a writing medium having an electrically conductive surface layer, and it also relates to a process for manufacturing such a writing head.

 The writing heads that the invention aims to improve are intended to be used in association with electro-sensitive papers having an electrically conductive surface layer, in particular metallic (thin layer of aluminum for example). A determined potential difference created between an electrode and the conductive layer of the paper causes an electrical discharge which causes the conductive layer to snap and causes a black underlayer to appear there, for example made of graphite.

 Up to now, writing heads of this kind comprise several electrodes consisting respectively of rods or styli aligned and separated from each other by a predetermined interval which depends, on the one hand, on the definition (or number of points to be printed per unit of length) and, on the other hand, of the electric voltage which is applied to them. Taking into account the risk of electric breakdown between neighboring electrodes if, for a given operating voltage, the electrodes are brought too close together, it is not possible, with these known writing heads, to make appear on the paper juxtaposed (or even overlapping) dots so that the signs (for example letters and / or numbers) finally constituted by a set of dots have a drawing continuity close to that of signs printed in a conventional manner (for example with an office typewriter).

 Furthermore, in known machines, the writing head is designed and arranged in such a way that the flexible styli are inclined (for example at 450) relative to the paper; this arrangement makes it possible to keep them easily applied to the paper during the movement of the writing head during the writing phases.

 However, this inclination of the styli authorizes writing only in one direction of movement of the writing head. When the writing head is moved in the opposite direction, the styli could catch the paper and tear it if they were kept applied to it. During the return movement of the head, it is therefore necessary to lift the latter, which obviously excludes any possibility of writing during this return.

Such unidirectional writing considerably limits the performance of known machines and is unacceptable in the case of machines with very high writing speed which it is sought to develop at present.

 Furthermore, the friction of the electrodes on the paper, on the one hand, and the repeated action of electrical discharges, on the other hand, cause rapid wear of the ends of the styli.

Up to now, this difficulty has been overcome by increasing both the length and the diameter of the styli, but such a solution is no longer possible for very fast machines designed to write signs consisting of dots juxtaposed or partially overlapping.

In addition, in view of these wear problems, the styli were made of tungsten wires, a relatively expensive material. However, in view of the above-mentioned elongation used, the styli had become too flexible.

 An attempt had been made to reinforce the wires, by coating them in an abrasive material. But this solution has proven to be too expensive and too difficult to implement.

 The object of the invention is essentially to remedy these drawbacks by producing a writing head which is arranged to form juxtaposed or even overlapping points, which is bidirectional - that is to say capable of writing whatever or the direction in which it is moved relative to the paper - and which is provided with electrodes made of a less expensive material and more resistant to wear by friction and to electro-erosion than tungs tene.

The invention also aims a writing head which can be manufactured simply, quickly and at shortest cost.

 According to a first aspect of the invention which relates to the writing head itself, such a head is characterized in that the electrodes are distributed in at least two parallel rows spaced from each other in each row, these two rows being arranged with respect to each other so that an electrode of a row is situated opposite an interelectrode interval of a neighboring row, and in that at least one support is provided, electrically insulator, in the form of a flat plate with two main parallel or substantially parallel faces, on which the two rows of electrodes are respectively fixed.

 Thanks to this arrangement, the electrodes are spaced apart from each other by a sufficient distance to avoid any electrical breakdown between them. The formation of a column of dots on the paper is carried out by activation of the electrodes of a first row, then , after incremental movement of the writing head to bring the second row of electrodes into the plane previously occupied by the first row of electrodes1 the electrodes of the second row are activated. Given the relative offset of the electrodes of the two rows, the points secondarily obtained are inserted between the points obtained previously and the inter valle between two successive points can be as reduced as desired without prejudice to the electrical insulation of the electrodes between them.

Of course, this arrangement can be envisaged for any number of rows of electrodes separated from each other by an appropriate number of insulating supports; the points which can thus be formed can then be as close together and as numerous per unit of length as desired.

 Such an approximation certainly requires a much greater fineness of the active ends of the electrodes, which is not however a handicap since the electrodes are secured to an insulating support and therefore no longer have to have a mechanical resistance, which is provided by the insulating support.

On the other hand, this constructive solution makes it possible to ensure very good alignment of the electrodes in each row, and therefore of the points formed on the writing medium, which the solutions retained so far have not permitted.
Finally, the arrangement of the writing head of the invention allows it to be placed in the machine in such a way that it is arranged perpendicular to the writing support. As a result, the writing head of the invention is functional regardless of the direction in which it is moved: printing can then be bidirectional, which considerably increases the printing capacities of the machine which is equipped with 'such a head.

 Preferably, the interval between two consecutive electrodes of the same row is at most equal to the transverse dimension of an electrode parallel to said row.

Thus, the points obtained on the writing medium are juxtaposed or else overlap each other, which makes it possible to obtain a more attractive visual aspect and greater readability for the signs thus composed.

 More preferably, the electrodes have, at least at their active end, a quadrangular cross section, in particular rectangular or square.

 Such an arrangement further increases the continuity of the design of the signs made up of points since there then remains no discontinuity between successive points, nor any contour undulation as may occur when the overlapping points are round. The visual aspect of sten signs is thus greatly improved.

Advantageously, to give them greater stiffness and greater hardness, the electrodes, at least at their active end, consist of a metal core coated with a layer of a material chosen from titanium nitride and titanium carbide, deposited by ion bombardment.

 As a result, the wear by friction and electro-erosion of the ends of the electrodes is greatly reduced.

By the same token, the metal core no longer needs to possess any particular mechanical qualities and, in replacement of tungsten, which is too expensive, one can use a very inexpensive material such as beryllium bronze or chrysocale.

The insulating support can advantageously be made of "Mylar".

In a particular embodiment, it is provided that the thicknesses of the electrodes are equal to that of the insulating support.

According to another aspect of the invention which relates to a method of manufacturing a writing head in accordance with the invention, the following stages are planned to be followed - metal strips are subjected to a selective chemical attack to form electrode combs, - a metal strip is cut to form electrode combs, - an electrically insulating support having two parallel or approximately parallel faces is cut, - two electrode combs are fixed respectively on the two faces of the insulating support, so that the fingers of a comb extend opposite the intervals between the fingers of the other comb, - the assembly thus formed is cut so as to eliminate the portion comprising the skeletons of the combs, and to obtaining a set of electrodes electrically isolated from each other, - and the edge of the set thus obtained is corrected corresponding to the active ends of the electrodes.

Preferably, the cutting of the metal strip is a chemical cutting.

 Because the precision of a chemical cut is very high, it is possible to be assured of high precision on the spacing of the electrodes between them, and therefore on the relative position of the points obtained on the writing medium. .

The invention will be better understood on reading the following description in which reference is made to the accompanying drawings in which - FIG. 1 is a perspective view on a very large scale of a writing head according to the invention, - Figure 2 is a bottom view of the writing head of Figure 1, - Figures 3 to 5 are schematic views illustrating different stages of the manufacturing process according to the invention, - and Figures 6 and 7 illustrate respectively two other methods of manufacturing writing heads according to the invention.

Referring first to Figures 1 and 2, the writing head 1 is constituted by a support plate 2 of an electrically insulating material and of rectangular shape, and by two rows of metal electrodes 3 and 3 'arranged respectively on the two large faces of the plate 2.

 The electrodes 3, 3 'are in the form of metal strips arranged relative to each other so that the ends 4 of the electrodes 3 are located respectively opposite the intervals 5' extending between two electrodes 3 ', and that conversely, the ends 4 ′ of the electrodes 3 ′ are situated respectively opposite the intervals 5 extending between two electrodes 3.

As shown in FIGS. 1 and 2, all the electrodes have the same width 1 and all the intervals between electrodes are equal and have the value 1. In addition, all the electrodes have the same thickness e and the support plate 2 also has a thickness equal to e. However, it is possible to envisage other embodiments in which the thickness of the plate 2 is an integer multiple of e (for example 2 e or 3 e)
Under these conditions, the active surfaces of the electrodes (that is to say the surfaces of the cross sections 6, 6 'of the ends 4, 4') are all equal and the ends 4 and 4 'are offset to correspond, dimensionally, so exact at intervals 5 'and 5, respectively.

At their opposite ends 7, 7 ′, the electrodes may have any suitable shape and size with a view to their electrical connection with members of the machine which will be equipped with the writing head 1. For example, while maintaining a constant thickness, the electrodes 3, 3 'have a sinuous longitudinal shape such that they deviate from each other, that is to say that the ends 7 or 7' are further apart from each other than are the 4 or 4 'ends. In addition, the width of the ends 7, 7 'is significantly greater than that of the ends 4, 4'. Thanks to these provisions, the writing head 1 can easily be inserted into an appropriate female-type electrical connector which will provide the connections provided with the machine. The removability thus provided greatly facilitates the replacement of a used writing head or damaged.

The writing head 1 which has just been described can be manufactured in the following manner (with reference to FIGS. 3 to 5 in which the same reference numerals have been kept to designate the elements identical to those of FIGS. 1 and 2) .

 In a sheet of the metal constituting the electrodes 3, 3 ′, a comb 8 is cut comprising the electrodes 3 (for example) and a skeleton 9, that is to say a metallic part bonding the electrodes 3 together (FIG. 3 ).

 Although various methods can be used to make this cut, it is however advantageous to carry out a chemical cut using an agent suitable for the metal considered, in a manner known to those skilled in the art. 'art.

 The advantage provided by chemical cutting is that it is possible to obtain very high dimensional accuracy on the electrodes; in particular, it is possible to obtain very high precision on the spacing pitch of the active ends of the electrodes.

Then assembled by bonding two combs 8, 8 'respectively on the two oppo-ed sides of a support plate 2 previously cut from a sheet of appropriate thickness (Figure 4).

Then cut the assembly 9-2-9 'along the cutting line 10 so as to separate the skeletons 9, 9' of the electrodes 3, 3 '(Figure 5), and rectify the opposite side so that the edge of the plate 2 and the surfaces 6, 6 'are perfectly coplanar. There is thus obtained, easily and quickly, a read head such as the head 1 of FIG. 1, with all the electrodes 3, 3 ′ electrically isolated from each other.

 To avoid having to make two identical combs, it is also possible to envisage making, according to the same process, a double li comb (see FIG. 6) having two opposite parts 12, 12 ′ each corresponding to the comb 8 of FIG. 6, but offset with respect to each other so that the electrodes 3, 3t of one of the parts are respectively located in the extension of the interelectrode intervals 5 ', 5 of the other part.

 The double comb is then placed there straddling the edge of the support plate 2 and, by folding along the lines 13, 13 ′, the two parts 12, 12 ′ are folded down along the corresponding large faces of the plate 2; after bonding, the assembly is sectioned along the cutting line 10, as above, to separate the skeleton of the electrodes 3, 3 'which are then electrically independent from each other.

 The electrodes 3, 3 'can be obtained from a tungsten sheet of suitable thickness.

However, to reduce wear by friction and electro-erosion, and thus increase the lifetime of the electrodes, is deposited, at least on the ends 4, 4 ', by ion implantation of titanium nitride or carbide of titanium
Because it is this very resistant surface layer which now rubs on the reproduction medium and is subjected to electro-erosion, the metal constituting the electrode is no longer subjected to wear. It is no longer necessary to have recourse to tungsten, which is an expensive metal, and the metallic "core" can be made of a very cheap metal such as beryllium bronze or chrysocale, which is covered then by ion bombardment of a layer of several microns of nitride or titanium carbide.

 As for the plate 2, it can be made of "Mylar".

According to yet another manufacturing process, one starts from a flexible printed circuit, that is to say comprising at least one thin layer of a metal such as copper secured to a flexible insulating support, such as " Mylar ".

 As shown in FIG. 7, on a strip 14 of such a material, selective conductive stripping is carried out in a conventional manner, parallel conductive strips which, at one end of the strip 14, terminate respectively in portions 3 thinned in width.

Preferably, to simplify manufacture, a double-sided printed circuit is produced, as shown in FIG. 7, the selective pickling of the copper layers being carried out in such a way that the ends 3, 3 ′ constituting the electrodes are mutually offset.
However, for example when it is not possible to have strips for printed circuits having sufficient thicknesses of insulation and / or copper, several strips are produced having the appropriate printed circuits, which are then assembled by bonding face to face against face. insulating and / or copper face against copper face.

In all cases, such an arrangement is particularly advantageous because, in addition to a significant simplification of the manufacturing process, it also makes it possible to simplify the connections between the electrodes 3, 3 ′ and the apparatus to which they must be connected by removing the connector. electrical connection and connecting wires.

The integration of the electrodes in the connecting wires makes it possible to reduce the causes of bad contacts and therefore to improve the reliability of the device.

 Of course, still for reasons of mechanical strength, the copper strips 15, 15 ', in the area of the electrodes 3, 3', are covered with a hard metallic layer as mentioned previously.

 The writing head according to the invention has many advantages linked to its original design.

It makes it possible to write points of very small dimensions and juxtaposed, or even partially overlapping, on an appropriate support, without this resulting in damage to the electrical insulation between electrodes.

 The writing head according to the invention has many advantages linked to its original design.

It makes it possible to write very small and juxtaposed, or neutral, partially overlapping points on an appropriate support, without causing damage to the electrical insulation between electrodes.

For example, the active section 6 of the electrodes is 125 x 260 microns for the standard definition of 3.85 points per millimeter, or 125 x 130 microns for the standard definition of 7.7 points per millimeter, while , in machines known at present, the tungsten wire styli had a diameter of 0.3 to 0.4 millimeter.

 Printing takes place in two stages: first, all or part of the electrodes 3 are excited, which write corresponding points on the writing medium; the writing head is then moved in an incremental pitch corresponding to the thickness of the support plate 2, the electrodes 3 ′ then being in the plane previously occupied by the electrodes 3; all or part of the electrodes 3 ′ are then excited, which write corresponding points in all or part of the intervals not previously covered by the electrodes 3.

 The above-mentioned manufacturing process makes it possible to produce electrodes with a square or rectangular active surface, leading to the writing of points which are perfectly juxtaposed and which are aligned with each other with very high precision. This eliminates the wavy contours obtained so far by overlapping circular points obtained using round section styli.

Finally, the abandonment of tungsten and its replacement by a very cheap metal to constitute the "cores" of the electrodes, combined with a simple and rapid manufacturing process lead to an inexpensive pruning head.

 As is obvious and as follows from elsewhere-s from the above, the invention is in no way limited to those of its modes of application and embodiments which have been more especially envisaged; on the contrary, it embraces all its variants.

 In particular, it is possible to envisage increasing the number of rows of electrodes, by combining several rows of electrodes and several support plates, to increase the definition.

Claims (15)

 1 - Writing pad comprising a plurality of electrodes for printing signs, by electrical discharge between at least one of the above electrodes and a writing medium having an electrically conductive surface layer, characterized in that the electrodes are distributed in at least two parallel rows spaced from each other in each row, these two rows being arranged with respect to each other so that an electrode of a row is situated opposite an interelectrode interval of a neighboring row, and in that there is provided at least one support, electrically insulating, in the form of a flat plate with two main faces which are parallel or substantially parallel, on the top of which the two rows of electrodes are respectively fixed.  2 - writing head according to claim 1, characterized in that the interval between two consecutive electrodes of the same row is at most equal to the transverse dimension of an electrode parallel to said range.  3 - writing head according to claim 1 or 2, characterized in that the electrodes have, at least at their active end, a quadrangular cross section, in particular rectangular or square. 4 - writing head according to any one of claims 1 to 3, characterized in that the electrodes, at least at their active end, consist of a metal core coated with a layer of a material chosen from titanium nitride and titanium carbide, deposited by ion bombardment.  5 - writing head according to claim 4, characterized in that the metal core is beryllium bronze or chrysocale. 6 - writing head according to any one of claims 1 to 5, characterized in that the electrically insulating support is made of "Mylarr '.  7 - writing head according to any one of the preceding claims, characterized in that the thicknesses of the electrodes and of the electrically insulating support are oval.  8 - writing head according to any one of the preceding claims, intended to equip a document production machine capable of reproducing on a writing medium signs constituted by a selection of suitable dots, the dots being printed on the writing support column by column distant from each other <9'in a determined step, characterized in that the thickness of the insulating support of the writing head is equal to the above step.  9 - Writing head according to claim 8, characterized in that, in the functional position in the machine, it is arranged perpendicular to the writing medium.  10 - writing head according to any one of claims 1 to 9, characterized in that it is constituted by a flexible printed circuit and in that it is in one piece with a cable connecting to the machine also constituted by a flexible printed circuit. 11 - Method for manufacturing a writing head according to any one of claims 1 to 9, characterized in that it comprises the following stages - a metal strip is cut to form electrode combs, - an electrically cut support insulator having two parallel or approximately parallel faces, - two electrode combs are fixed respectively on the two faces of the insulating support, in such a way that the fingers of one comb extend opposite the intervals between fingers of the other comb, - the assembly thus cut is cut so as to eliminate the portion comprising the skeletons of the combs, and to obtain a double set of electrodes electrically isolated from each other, - and the edge of the assembly thus obtained is rectified corresponding to the active ends of the electrodes.  12 - A method for manufacturing a writing head according to any one of claims 1 to 9, characterized in that it comprises the following steps - a metal strip is cut to form a double comb comprising two comb portions arranged back to back with the fingers of the comb portions extending respectively in opposite directions, - an electrically insulating support is cut having two large parallel or approximately parallel faces, - the double comb is placed astride the edge of the insulating support so that the fingers extend on either side of said edge and the double comb is folded over said edge to fold the two comb portions respectively on the two large parallel faces of the insulating support so that the free ends of the fingers of a comb portion are located respectively opposite to the inter-electrode intervals of the other comb portion, - the two comb portions are secured not thus arranged on the insulating support, - the assembly thus cut is cut so as to eliminate the part comprising the skeleton of the double comb and to obtain a double set of electrodes electrically isolated from each other, - and the edge is rectified of the assembly thus obtained corresponding to the active ends of the electrodes.  13 - Method according to claim 12, characterized in that, in the double comb, the fingers are arranged so that the fingers of a comb portion are arranged respectively in the extensions of the interdigital intervals of the other comb portion. 14 - Method according to any one of claims 11 to 13, characterized in that the cutting of the metal strip is a chemical cutting.  15 - Process for manufacturing a writing head according to claim 10, characterized in that the techniques of manufacturing printed circuits are used to form, on an insulating plate having two metallized faces, the desired electrode configuration.