Croquelois, Jean-pierre (Lausanne, CH)
Erskine, William (Grand-Lancy/Ge, CH)
Gerber, Raymond (Moillesulaz, CH)

05/304004

09/10/1974

11/06/1972

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Battelle, Memorial Institute (Carouge, Geneva, CH)

347/55

101/170, 347/91, 101/122, 101/DIG.037

B41J2/005; G01D5/44

346/74ES 101/170,122,DIG.13,1,426 197/1R

3530441	METHOD AND APPARATUS FOR STORING AND RETRIEVING INFORMATION	September 1970	Ovshinsky
3545374	HIGH-SPEED PRINTER EMPLOYING A GAS DISCHARGE MATRIX	December 1970	Hendricks
3550153	HIGH SPEED NON-IMPACT PRINTING	December 1970	Haeberle et al.
3625604	APERTURE CONTROLLED ELECTROSTATIC PRINTING SYSTEM	December 1971	Pressman
3689935	ELECTROSTATIC LINE PRINTER	September 1972	Pressman
3715762	METHOD AND APPARATUS FOR GENERATING ELECTROSTATIC IMAGES USING IONIZED FLUID STREAM	February 1973	Magill et al.

Damm, E. P., "Nonimpact Printing Method", IMB Technical Disclosure Bulletin, Vol. 15, No. 9, Feb. 1973, page 2839. .
McCormack M. A. et al., "Acoustic Holographic Printer", IMB Technical Disclosure Bulletin, Vol. 13, No. 6, Nov. 1970, page 1621..

Pulfrey, Robert E.

Pieprz, William

Burns, Robert Lobato Emmanuel Adams Bruce E. J. L.

I claim

1. A method of continuous, high speed, non-impact printing with liquid electrically conductive ink on a sheet-like conventional print medium of ordinary paper or the like, by printing ink dots thereon in response to electrical control signals corresponding to a desired printing pattern, said method comprising the steps of:

2. In the method according to claim 1 wherein:

3. In the method according to claim 2 wherein the amplitude and duration of said voltage pulses are adjusted to control the size of said ink dots.

4. In the method according to claim 1 wherein said print medium is maintained at a short predetermined distance from said ink repelling layer during said attraction of the ink meniscus for contact thereof with said print medium.

5. A continuous, high speed, non-impact printer for producing dots with liquid electrically conductive ink on a sheet-like conventional print medium of ordinary paper or the like, in accordance with electrical control signals corresponding to a desired printing pattern, said printer comprising:

6. A printer according to claim 5 wherein said ink-carrying foil comprises a metallic ribbon having said ink repelling layer on one side thereof with said capillary perforations traversing said ribbon and said layer in a plurality of longitudinal rows, each row having at least one of said electrodes associated therewith for successive alignment of said one electrode with the perforations of one of said longitudinal rows.

7. A printer according to claim 5, wherein said ink-carrying foil includes spacing members projecting from said ink repelling layer for contact with one surface of said print medium.

8. A printer according to claim 5, wherein said capillary perforations have a cross-section which increases between said ink repelling layer and the opposite side of said ink-carrying foil.

FIELD OF THE INVENTION

This invention relates to non-impact printing with liquid ink and in particular to producing printing patterns by selectively attracting ink onto a conventional sheet-like printing medium of ordinary paper or the like, whereby to form ink dots thereon in accordance with these patterns.

The invention may be used for the transcription, in printed form, of information received in the form of electric signals from any external data source, whereby to provide, for example, printed texts or curves corresponding to the information output of computers, electronic table calculators, measuring instruments, telegraph or teletype receivers, and of apparatus for the transmission of images, for facsimile printing or the like.

BACKGROUND OF THE INVENTION

There are several known non-impact printers which are based on the common principle of selectively depositing stationary electric charges on the surface of a dielectric supporting medium in accordance with desired printing patterns, whereby to form a latent electrostatic image thereof, this image then being used to transfer ink by electrostatic attraction thereof from an ink carrying member onto a printing medium such as ordinary paper or the like. However, these two distinct steps of image formation and ink transfer present certain limitations with regard to high-speed printing with high resolution.

Thus, for example, the formation of a satisfactory latent image at high speed necessitates the use of intricate measures for ensuring precisely localized deposition of electric charges on the image supporting medium and for preventing any migration and hence dissipation of the charges in the medium. Moreover, subsequent transfer of ink to the printing medium, by means of the said image, requires precise positioning of the printing medium with respect to the latent image. Hence, while such two-step printing as described above, which involves image formation on the one hand and ink transfer on the other, may be suitable for certain applications, it has certain intrinsic limitations with regard to the nature and quality of the printing patterns which may be obtained thereby when printing at high speed.

In order to obviate some of these limitations, a printing process has been proposed, wherein said dielectric supporting medium for the latent image consists of a porous band on which the latent image is produced on one side and ink is deposited on the opposite side, whereby to cause attraction of the ink through the pores of the band by means of the electrostatic charges of the image. An ink pattern is thus formed for subsequent transfer onto a paper sheet or the like by contact thereof with said ink image. The dielectric supporting medium thus simultaneously constituts a latent image support and an inking member. However, printing involves three consecutive steps, namely image formation, ink attraction to the surface, and ink transfer to the paper by contact printing. Such a process has several limitations, in particular with regard to the control of selective ink transfer at high speed.

A printing device has also been proposed wherein an electrically insulating printing ink is retained in an electrically insulating, cylindrical screen. The device further comprises means for negatively charging the ink according to an electrostatic pattern and a positively charged drum for bringing a printing medium into contact with the screen to allow ink transfer onto the medium in accordance with the electrostatic pattern, by electrostatic force between the negatively charged ink and the positively charged drum. However, with such an arrangement, it is difficult to produce a very precise electrostatic pattern by selectively charging the ink and to ensure precise control of ink transfer only from individual selected points in the screen and also to ensure that no arbitrary transfer of ink can occur when the screen and the medium are in contact.

In a proposed variant of this device, the ink retained in the insulating screen is uniformly positively charged and the device comprises means for negatively charging the printing medium according to an electrostatic pattern, as well as means for moving the medium into contact with the screen to transfer the positively charged ink onto the medium in accordance with the electrostatic pattern, by the electrostatic force between the ink and the medium. However, the above-mentioned problems of ensuring precise control of ink transfer and of preventing arbitrary ink transfer nevertheless remain the same in this variant.

A principal object of the invention is to allow high-speed non-impact printing with liquid ink while largely obviating the above-mentioned drawbacks and limitations.

Another object of the invention is to allow controlled attraction of the ink so as to provide high speed ink transfer for printing ink dots of predetermined size.

A further object of the invention is to rapidly produce printed patterns having high-resolution by printing ink dots with high definition, the size whereof may be varied within certain limits, while using the same printing system.

Another object of the invention is to provide high-quality facsimile printing in such a manner as to print dots of different size, whereby to provide a progressive tonal range in the printed image.

Still another object of the invention is to provide means for completely eliminating any uncontrolled ink transfer during printing.

SUMMARY OF THE INVENTION

The present invention provides a method of non-impact printing with a liquid ink on a sheet-like dielectric printing medium consisting of ordinary paper or the like, by producing ink dots thereon in accordance with printing signals corresponding to a desired printing pattern, said method comprising the steps of:

placing said printing medium between an electrically conductive, perforated ink-carrying member and at least one electrode, said member having an electrically conductive ink retained therein in a plurality of capillary perforations of same size traversing said member, said member further being provided with a correspondingly perforated ink-repelling layer arranged to face one surface of said printing medium and said electrode being arranged adjacent to the opposite surface thereof and being selectively aligned with one of said capillary perforations;

maintaining said ink-carrying member at a constant potential; and

applying to said electrode a voltage pulse of given amplitude and short duration, corresponding to one of said printing signals, so as to momentarily create a localized electric field of predetermined intensity extending between said electrode and the ink-carrying member, whereby to provide selective attraction of ink from the said one perforation aligned said electrode and to thereby print an ink dot of predetermined size on said one surface of the printing medium.

The invention further provides a device for printing with liquid ink on a sheet-like dielectric printing medium, by producing ink dots thereon in accordance with printing signals corresponding to a desired printing pattern, said device comprising:

a perforated, electrically conductive ink-carrying member having a correspondingly perforated ink-repelling layer arranged on one side thereof, with a plurality of capillary perforations of same size traversing said member, said perforations having an electrically conductive ink retained therein by capillary action;

means for maintaining said ink-carrying member at a constant predetermined potential;

a printing head comprising at least one electrode having a size such as to allow selective arrangement thereof opposite no more than one of said capillary perforations at a time;

guide means adapted to allow relative arrangement of said ink-carrying member adjacent to said printing head, with said printing medium placed therebetween, so that one surface of the medium is arranged facing said ink-repelling layer and that said electrode is arranged adjacent to the opposite surface of said medium and in alignment with one of said perforations;

means for delivering said printing signals to the printing head, in the form of brief voltage pulses of predetermined amplitude and duration, so as to momentarily create a localized electric field of predetermined intensity extending between said electrode receiving said pulse and said ink-carrying member, to thereby provide selective ink transfer from the said selected perforation and to thereby print an ink dot of predetermined size due to said ink transfer onto said medium.

Various aspects of the invention and the particular significance of the combination of the features thereof appear in the following description, with reference to the accompanying drawings.

DESCRIPTION OF THE DRAWINGS

FIG. 1 shows in elevation the general lay-out of a printer according to an embodiment of the invention.

FIG. 2 is a sectional plan-view along II--II of FIG. 1, with partial removal of the inking member.

FIG. 3 is a sectional side-view along III--III of FIG. 1.

FIGS. 4a and 4b illustrate the printing zone of the printer shown in FIG. 1, respectively before and during ink attraction.

FIGS. 5 to 11 represent experimental curves showing the interrelationship of various parameters relevant to printing according to the invention.

SPECIFIC DESCRIPTION

In the general lay-out shown by way of example in FIG. 1, the printer according to this embodiment comprises an inking member in the form of a perforated inking ribbon 1 forming an endless loop mounted for movement thereof around three pairs of rollers 2a, 2b; 3a, 3b; and 4a, 4b past a stationary printing head 5, a printing medium in the form of a paper strip 6 being arranged between the said ribbon and printing head.

As may be seen in FIGS. 2 and 3, the said rollers are mounted within the closed loop formed by the inking ribbon 1 so as to guide the latter along its lateral edges, the rollers of each said pair being respectively mounted on a common shaft 7, 8 and 9, The rollers 3a, 3b constitute drive rollers having their common shaft 8 operatively connected to a drive system 10, while the rollers 2a, 2b and 4a, 4b constitute guide rollers which are mounted for free rotation on their respective common shafts 7 and 9. These guide and drive rollers consist of narrow drums, the roller pair 4a, 4b, at least, being made of metallic material to provide connection thereof to earth, via their shaft 9, as shown schematically in FIG. 1.

The inking ribbon 1 thus arranged for displacement in the direction of the arrows shown in FIG. 1, comprises a plurality of closely spaced capillary perforations 11 which are uniformly arranged in transversal and longitudinal rows and extend across the ribbon, in the zone thereof lying between the said rollers. The structure of the inking ribbon is shown in enlarged cross-section in FIG. 4 and will be further described in greater detail with reference to this figure.

The printer is further equipped with ink-feed means 12, shown schematically by a spray-head, whereby ink is distributed on the inner surface of the endless inking ribbon 1 and is drawn into the perforations 11 by capillary action, a doctor blade 13 further being provided adjacent to said surface to promote introduction of the ink into the inking ribbon 1 and to scrape off surplus ink from the inner surface thereof.

The strip of printing paper 6 is drawn off a supply roll 14 onto a receiving roll 15 having the shaft thereof also operatively connected to the drive system 10 so as to provide synchronized movement of the ribbon 1 and the paper strip 6 past the printing head 5.

The printing head 5 comprises a block 16 of insulating material having a curved upper surface which serves as a guide surface for exactly positioning the paper strip with respect to the inking ribbon 1 during passage thereof through the printing zone which is situated in the median plane of the printing head.

The printing head 5 moreover comprises a transverse linear array of seven electrodes 17 embedded in the insulating block 16 to extend vertically in said median plane thereof up to the said upper curved surface and to provide contact of the electrodes 17 with the paper strip 6 during passage thereof over the block 16.

The seven electrodes 17 are each associated with one of the longitudinal rows of perforations 11 and form a row extending transversely to the direction of movement of the ribbon 1, these electrodes being so arranged that the perforations of each longitudinal row of perforations are made to move past the electrode associated therewith.

A pulse generating means 18 is further connected to the electrodes of the printing head 5 and is adapted to deliver to each of the electrodes 17, printing signals S _p corresponding to a desired printing pattern, said signals being in the form of voltage pulses of given amplitude and short duration to provide controlled printing of ink dots as is explained further on.

The drive system 10 is operatively associated with the inking ribbon and the paper strip so that both are made to move past the printing head in synchronism with each other so that the transverse rows of seven perforations 11 are brought successively into alignment with the respective electrodes 17, as is shown in FIG. 3.

The drive system 10 may comprise any suitable drive means, such as an electric motor equipped with transmission means for example a toothed belt and gear system.

This motor may be either a constant-speed motor for providing continuous movement of the inking ribbon 1 and the paper strip 6, both at the same speed, or else a stepping motor for providing synchronized step-by-step movement wherein the inking ribbon 1 and the paper strip are stationary between consecutive steps thereof to allow ink transfer.

Since various drive systems for precise displacement of ribbons at high speed are well known in the printing field, among other fields, and the particular structure and arrangement thereof may thus be quite conventional, the drive system has merely been illustrated schematically by the block 10 and will not be described in further detail.

The pulse generating means 18 may likewise comprise any circuit of conventional design capable of selectively delivering to each of the electrodes 17 of the printing head a series of printing signals S _p in the form of voltage pulses of controlled amplitude and duration, said signals being delivered in accordance with data corresponding to a desired printing program. This data may be received from any outside programming means, for example a computer. Since the pulse generating means 18 may have any conventional design and need not constitute a part of the printing apparatus proper but may readily be a part of the printer programming means, it has been illustrated by the block 18 and will not be described in detail.

The structure of the inking ribbon 1 and its relative arrangement with respect to the paper strip 6 and to the printing head 5 are shown in enlarged cross-section in FIGS. 4a and 4b, which respectively show an enlarged cross-sectional view of the printing zone before and during the printing step. The inking ribbon 1 consists of a perforated metallic ink-carrying ribbon 1A provided with a correspondingly perforated ink-repelling layer 1B arranged thereon, on the outer side of the endless loop formed by the inking ribbon 1. The perforations 11 contain an electrically conductive printing ink which is maintained therein by capillary action as is indicated very schematically in FIG. 4a by the liquid column 20. The ink-carrying ribbon 1A must be electrically conductive and may be made, for example, of stainless steel, nickel, copper or aluminium, which is rendered more wettable (hydrophilic) by physical or chemical treatments such as are well known in the printing art. On the other hand, the layer 1B must be ink-repelling and any suitable known hydrophobic coating material such as tetrafluoroethylene (Teflon) or certain silicone compounds may be used for forming this layer, for example, by spraying onto the perforated metallic ribbon 1A. On the other hand, one may also envisage subjecting the conductive ribbon 1A to a surface treatment, such as electrolytic polishing, so as to provide its surface with hydrophobic properties. However, so long as this ribbon 1A is electrically conductive and wettable by the ink used for printing, while the layer 1B is relatively non-wettable by the ink, the choice of the particular materials and techniques used for producing the perforated inking ribbon are not critical as such.

As is shown in FIGS. 4a and 4b, the ink-repelling layer 1B lies between the conductive ink-carrying ribbon 1A and the strip 6 of printing paper and it has been found in practice that this provides a significant result with regard to the obtention of faultless printing at high speed. As is explained below, experimental results have consistently shown that ink transfer can be precisely controlled to provide printing of ink dots of given size at high speed. However, the exact action of the ink-repelling layer 1B, in this regard, cannot be fully explained.

Moreover, it will be readily seen that there are various parameters which may influence behaviour of the ink during transfer thereof at high speed, so that correlation of all these parameters is most difficult, if at all possible.

The following explanation, with reference to FIGS. 5 to 11 nevertheless shows the influence of various parameters, which has been determined experimentally and thus allows these parameters to be taken into account in order to achieve satisfactory control of ink transfer at high speed.

FIG. 4a shows various system parameters related to the dimensions of the composite inking ribbon 1 and to its relative arrangement with respect to the printing medium (paper). These parameters are:

diameter φ of the perforations 11;

thickness h _A of the conductive ribbon 1A;

thickness h _B of the ink-repelling layer 1B;

thickness h _m of the printing medium;

spacing x between the layer 1B and the printing medium (paper 6).

FIG. 4b shows three consecutive perforations of a longitudinal row, with the first and third perforations, containing ink at equilibrium under the effect of capillary action, while the ink is being attracted from the second perforation for transfer thereof onto the paper.

As may also be seen from FIGS. 4a and 4b, the inking ribbon 1 is further provided with spacers 21 which are fixed to the conductive ribbon 1A, and project through the layer 1B so as to come into contact with the paper 6 and thus determine the spacing x (see FIG. 4a) between the paper and the layer 1B.

The perforations 11 of the inking ribbon 1 all have the same size and may either be cylindrical (constant diameter) or, as is indicated on the left-hand side of FIGS. 4a, 4b, may have a cross-section which increases towards the free (inner) surface of the ribbon 1A, where ink is supplied by the feed means 12 (see FIG. 1). The use of such perforations with increasing cross-section has been found to be advantageous with regard to the introduction of the ink and the retention thereof in the perforations by capillary action, the meniscus of the ink being formed and held at that part of the perforations having minimum diameter. It is understood that FIGS. 4a and 4b merely show a conceptual model, but experimental results indicate that the ink meniscus is deformed and behaves in the general manner illustrated very schematically in these figures.

Thus, when liquid ink is spread on the surface of the conductive ribbon 1A, it enters the perforations 11 and is retained therein by capillary action, substantially as shown in FIG. 4a. The composite inking ribbon 1 thus ensures ink retention within the perforations thereof and thereby prevents uncontrolled ink transfer. Moreover, when a voltage pulse s _p of sufficient amplitude V _p and duration t _p is applied to the electrode aligned with one of the perforations 11 (see FIG. 4b), a localized electric field is momentarily produced therebetween and the meniscus of the ink is attracted and is thereby deformed axially so that it projects from the layer 1B for contact with the paper to provide ink transfer thereto, as is shown schematically in FIG. 4b.

The degree of deformation of the ink meniscus will evidently depend on various parameters such as: the attracting potentiel (V), the perforation diameter φ, the distance across which the meniscus must be displaced for contact with the paper (h _B and x possibly tending to zero), the duration of attraction, the surface tension (σ) and the viscosity (μ) of the ink.

The influence of these parameters is explained below with reference to the experimental results shown in FIGS. 5 to 11.

The curve of FIG. 5 shows (for φ = 0.2 mm) the variation of the maximum air gap x (see FIG. 4a) allowing ink transfer onto the paper, as a function of the ink-attracting potential V _p , i.e. the amplitude of the voltage pulse applied to the electrode, the duration t _p of the pulse being constant at 9.5 m/s. The ink-carrying ribbon 1A used is made of copper and has a thickness h _A of 0.1 mm, while the ink-repelling layer 1B is made of Teflon and has a thickness h _B of 0.05 mm. A water based printing ink of a commercially available type was used, having a surface tension σ of 39 dynes/cm and a viscosity μ of about 1 cp.

The areas lying under this experimental curve of FIG. 5 thus gives all values of V _p and x, in the considered ranges thereof, which provide ink transfer.

The three curves of FIG. 6 respectively show, for three ink-attracting potentials V _p , the variation of the gap size x as a function of the diameter φ of the perforations 11. The ink used, as well as the materials and thicknesses h _A and h _B of the inking ribbon are the same as indicated above with reference to FIG. 5.

The areas lying under these three experimental curves of FIG. 6 thus give, in the range considered, all allowable values of x and φ which provide ink transfer with the respective values of V _p .

The curves of FIGS. 7 and 8 respectively show the influence of the viscosity μ and of the surface tension σ of the ink on the minimum potential V _p necessary to provide ink transfer. In both cases, a composite copperteflon inking ribbon was used with φ = 0.2 mm, h _A = 0.1 mm and h _B = 0.05 mm. Moreover, the surface tension σ is constant at 68 dynes/cm in the ink used for the curve of FIG. 7, while the viscosity μ of the ink is constant at 1 cp in the ink used for the curve of FIG. 8. The duration t _p of the voltage pulses was also 9.5 ms, as before.

The curves of FIGS. 5 to 8 thus allow a suitable choice of the amplitude V _p of the voltage pulses used to provide printing, as a function of φ, x, σ and μ.

It may be noted that reduction of the gap size x is obviously favourable in that it allows V _p to be reduced, so that the gap may be eliminated altogether. Experiments have shown that no gap may be necessary, providing the surface tension of the ink is sufficiently high (e.g. 50 dynes/cm, or more) and the conductive ribbon 1A is highly wettable by the ink, as compared to the ink-repelling layer (1B). However, the use of a gap of small height of up to about 0.1 mm seems advisable for various practical reasons, in particular when the inking member and the printing medium are displaced at high speed during printing.

It may further be noted that the voltage pulse duration t _p = 9.5 ms which was used when establishing the experimental curves of FIGS. 5 to 8, was found to be much higher than the minimum "response time" necessary for ink transfer. However, the minimum pulse duration necessary for printing depends on various dimensional and operational parameters, as well as on the properties of the ink, and is thus difficult to determine for all cases. Experiments have nevertheless shown that the pulse duration t _p may be reduced to 4.5 ms without substantially modifying the relationships shown in FIGS. 5 to 8.

Moreover, satisfactory printing of ink dots has been achieved in practice with values of t _p as low as 0.1 - 0.2 ms.

FIGS. 9, 10 and 11 respectively show the variation of the size φ _d of the printed ink dots, as a function of V _p , t _p and x, when using the same ink with σ = 39 dynes/cm and μ = 1 cp.

FIG. 9 shows three curves giving dot size φ _d as a function of V _p and corresponding respectively to t _p = 9.5, 4.6 and 0.4 ms (for φ = 0.2 and x = 0.04 mm). The curves of FIGS. 9 and 10 (same ribbon and ink) thus show that the influence of pulse duration t _p on dot size φ _d is appreciable in the considered range.

FIG. 11 moreover shows the influence of the gap size x on the dot size φ _d for t _p = 9.5 and 0.4 ms respectively (for φ = 0.3 mm, h _A = 0.1 mm, h _B = 0.04 mm and V _p = 1,750 volts). This figure thus shows that the gap size is also a significant parameter with regard to dot size φ _d , which varies between 0.2 and 1.4 mm in the curve of FIG. 11.

The ink used for printing according to the present invention must be electrically conductive, although the conductivity thereof is not critical and may vary over a broad range. Thus any ink based on a polar solvent, for example a water-based ink, may be used for the purposes of the invention.

The above described structure of the inking ribbon, the arrangement of the conductive ink-carrying ribbon at a short predetermined distance from the surface of the printing paper, and the described association thereof with electrodes adapted to receive the printing signals from a data source, in the form of well determined high-voltage pulses of short duration, for attracting ink from the ribbon onto the paper, provide a particular result when combined together in the manner described above.

This result may be illustrated by the following advantages:

a. the conductivity of the ink-carrying ribbon, as well as that of the ink, render it possible to normally maintain all of the ink carried by the inking ribbon at exactly the same potential, for example at zero potential, as described;

b. the ink-repelling surface layer opposes lateral spread of the ink outside the ink-carrying perforations, on the underside of the inking ribbon which faces the print-receiving surface of the paper, and thus tends to keep the ink within the inking ribbon whereby to prevent uncontrolled ink transfer in the absence of an electric field acting thereon, while allowing instantaneous deformation of the meniscus for contact thereof with and ink transfer to the paper, when a voltage pulse is applied to an electrode situated on the opposite side of the paper, in alignment with the meniscus.

The fact that the ink dot size may be readily controlled by adjusting the amplitude or duration of the printing signals, allows the size of each individual dot to be varied considerably, without modifying the printing device.

Thus, alphanumerical characters of different size may be printed, while the line thickness may be readily adapted to each character.

Moreover, the advantages of the invention are by no means limited to the printing of alphanumerical characters. Indeed, the possibility of readily varying the dot size allows intricate printing patterns to be rapidly printed with contours of gradually varying thickness, while ensuring high definition and resolution.

It should be noted that the described embodiment has been given by way of an example and that the invention may be readily carried out by other embodiments wherein various components of the printer and the relative operative association thereof are substantially modified with respect to those described above and illustrated in the drawings.

Thus, for example, any perforated inking member with a similar structure may be used instead of an endless ribbon, while the printing medium need not be a continuous strip or band and may consist of a sheet of paper or the like.

Moreover, whereas one transversal row of electrodes has been shown and described, an array comprising any desired number of electrodes may obviously be used. Thus, for example, a linear array of 7 × 5 electrodes may be used for printing an alphanumerical character at one time, instead of in five successive steps, as illustrated. As a matter of fact, any number of arrays may be used for simultaneously printing several characters. Such multiplication of the electrodes would obviously allow a corresponding increase of the rate at which the characters are printed.

A linear electrode array extending over the width of a paper sheet may also be used, for example, while the paper is moved past the array, in a direction perpendicular thereto. In that case, the inking member may be a composite perforated band or sheet, which is either fixed or mobile, as the case may be.

The inking ribbon as well as the printing medium may moreover be stationary, while the printing head describes a scanning movement thereof during printing.