EP0949077A1 - Method for ejecting an electrically conductive liquid and continuous ink jet printing device using this method - Google Patents

Abstract

The method involves spraying a continuous jet of liquid (14) at a given speed (Yj), cause the jet to split at two distinct predetermined break points (C,L) so as to form drops (22,24) of liquid at a given emission frequency, and apply to these drops quantities of a different electrical charges, according to their break points. The same electrical field is applied onto the drops, so as to deviate only those drops (24) formed at a first (L) of the break points. The jet acted upon so that the two break points are separated by a distance DELTA D strictly less than the wavelength lambda of the jet, defined by the relation lambda = Vj divided by F. The same quantity of charge is applied on all the drops formed in a zone centered on the second break point (C) of length sensibly equal to lambda divided by 4. An Independent claim is also included for an ink jet printer using the method described.

Description

Technical area

The invention relates to a projection method of an electrically conductive liquid in the form at least one continuous stimulated jet.

The invention also relates to a device multi-nozzle printing using this process.

A printing device according to the invention can be used in all industrial fields related to marking, coding, addressing and industrial decoration.

State of the art

In the current state of the art, there are two major inkjet printing technologies continuously stimulated. These are respectively the technique deflected continuous inkjet and the technique of binary continuous inkjet.

Using the continuous inkjet technique deflected, electrically conductive ink, maintained under pressure, escapes from a calibrated nozzle. Under the action of a periodic stimulation device, the ink jet thus formed breaks at time intervals regular at a single point in space. This forced fragmentation of the inkjet is usually induced by periodic vibrations of a crystal piezoelectric placed upstream of the nozzle. From this breaking point, the continuous jet turns into a train of identical and regular ink drops spaced. In the vicinity of the breaking point is placed a first group of electrodes whose function is to transfer to each drop of the spray, selectively, a variable amount of electrical charge and predetermined. All the drops of the jet pass through then a second group of electrodes within which there is a constant electric field. Each drop undergoes then a deflection proportional to the electric charge previously assigned to it, and which points to a specific point on a print medium. The non-deflected drops are recovered by a gutter and recycled to an ink circuit.

In working inkjet printers according to this technique, a specific device is usually provided to ensure a constant synchronization between the moments of jet break and the application of the charge signals of the drops.

This technology is mainly characterized by the fact that a variable amount of electric charge is selectively transferred to each drop of jet, so that multiple levels of deflection are created. This feature allows a nozzle unique to print, by segments (lines of points of a given width), the entire pattern (character or graphic pattern). Moving from one segment to another is carried out by the continuous displacement, perpendicularly to the segments, of the printing medium opposite the printing device.

For applications requiring width slightly larger print, multiple devices monobuses printing (generally two to four) can be grouped within the same box.

When the print widths become important, the use of printing devices multi-nozzle becomes compulsory. Document EP-A-0 512 907 describes a multi-nozzle printing device (eight nozzles) using inkjet technology continuously deflected. By juxtaposing several devices multi-nozzle printing, more printing widths important can be obtained.

Inkjet printing devices continuous stimulated using the continuous jet technique stand out from printing devices using the technique of continuous deflected jet mainly by the fact that only an amount of electric charge predetermined can be transferred on demand, with every drop of the spray. A single level of deflection of drops is then created. Printing characters or of patterns therefore requires the use of devices multi-nozzle printing, in which the center distance the nozzles generally coincide with the spacing between impacts on the print medium. In general, drops intended for printing ("drops to print "in the rest of the text) are the drops not deflected. This technique is particularly suitable for high speed printing applications such as addressing, printing proofs in high resolution color, etc.

In jet printing devices binary continuous ink, certain components groups of charge and deflection electrodes can be made common to these two groups of electrodes. In all cases, the dedicated electrodes borne drops of each spray must be piloted individually, at the training frequency drops and at voltage levels that can reach 350 V.

Manufacturing and juxtaposition one step away very thin set of nozzles and electrodes of a multi-nozzle printing device operating according to binary continuous inkjet technique show up major cost and design issues.

The cost problems originate from the multiplication of charge electrodes and multiplication high voltage electronic circuits connected to these electrodes, which induce a connection important and complex.

Design issues are related to the very dense high-voltage connectors near the jets, which causes unwanted crosstalk. The effect of these crosstalk on print quality can be limited only by reducing the usage rate drops, and therefore speed printing.

In the article "Binary Continuous Thermal Ink Jet Break off Length Modulation "by Donald J. DRAKE, published in the Xerox Disclosure Journal, volume 14, no. 3 of May-June 1989, a device is proposed binary continuous jet multi-nozzle printing including the design has been modified in order to overcome the disadvantages mentioned above.

In accordance with conventional jet technology binary continuous, this article proposes to use two groups of electrodes, each of which is formed by a planar electrode. However, in this case, each electrode is common to all jets and subject to constant electrical voltage. The selection of drops to print and drops to recycle is done then by the individual control of the stimulation of each of the ink jets from the print head. In this effect, an individual stimulation device each of the jets is planned.

In this arrangement, the associated connectors to stimulation devices is located upstream nozzles and therefore distant from the jets. In addition, she carries lower voltage levels than those that are required to charge the drops. The effects of crosstalk are therefore reduced.

According to the article by Donald J. DRAKE, we apply on request to each jet a stimulation signal low or high level. When a signal from low level stimulation is applied, the place of breaking jet is fixed at a point farther from the nozzle only when the stimulation signal applied to the jet has a high standard.

In the first case, the jet breaking point is located opposite the first electrode, or charging electrode, brought to a constant voltage V _c . The drop which detaches at this instant then carries a charge Q1 and undergoes a deflection of angle δ1 in the field created by the second electrode, or deflection electrode, brought to a constant voltage V _d . This drop is collected by a gutter and recycled to the ink circuit of the printing device.

When the breaking distance is shorter, due to the application of a high level stimulation signal on the jet, the latter breaks at a point situated slightly before the charging electrode. The charge Q2 carried by the drop is then lower than in the previous case. The deflection δ2 induced by the deflection plane is therefore also less. The drop then avoids the gutter and reaches the print medium.

In this article, the difference between the two levels of stimulation of the jet is such that the distance d between the breaking points of the jet for each of these two levels is equal to the wavelength λ of the stimulated jet, that is to say ie the train of drops. The value of λ is provided by the ratio of the speed V _j of the jet to the frequency F of the stimulation signal: λ = V j / F .

Operating mode and design offered in this article however suffer from three major handicaps that limit the possibilities application of this process to a jet printer continuous ink.

The first handicap stems from the fact that the distance d between the two break points of the jet is equal to the wavelength λ of the drop train. This leads to a difficulty in operating the jet during the long break-short break transitions. In fact, it can be seen that when a drop to be printed is followed by a drop to be recycled, the condition d = λ theoretically leads to the simultaneous detachment of the two drops. The kinetics of charge transfers are then different from that associated with a short break-long break transition, which can induce different trajectories. In addition, any fluctuation of one or the other of the breaking distances, inevitable in an actual implementation of the method, leads to a modification of the operating conditions of the jet. For example, if d becomes slightly greater than λ, a transient cluster of two drops will be created during the long break-short break transitions. A redistribution of the induced charges, difficult to determine a priori, will take place and the trajectory of the drop to be printed will be altered.

The second handicap of the process described in Donald J. DRAKE's article stems from layouts proposed electrodes, which require a short distance between the surface of the jet and the charging electrode (from jet diameter order) to obtain a selection sufficient printing drops. The realization and the exploitation of such geometry within of a multi-nozzle continuous jet printing device raise several difficulties.

First, the start of such a device led printing, for ink jets escaping from the nozzles, during a transient phase during which aerodynamic braking predominates. In particular, a jet forms at the end of each jet ink volume larger than that drops formed during the steady state, and the jet trajectory is temporarily altered.

A consequence of starting the jets is therefore soiling of the nearby charging electrode immediately from the jet axis. This effect is rendered inevitable by the angular dispersion of each of the jets, itself derived from the levels of precision and repeatability achieved during the manufacture of the nozzles. It strongly disrupts the functioning of the device and limits its reliability. The cleaning of the charging electrode is then compulsory.

In addition, in steady state, any fluctuation of the trajectory of the jets around their axis (due, for example, to the passing presence of an impurity in the jet jet duct) can also deflect slightly spray and lead to dirt on the electrode load placed in the immediate vicinity of jets, which usually causes short circuits between the jet and the electrode.

Finally, the geometry of the charging electrode described in the aforementioned article, which induces the application quantities of electric charge both on the printing drops only on unprinted drops, constitutes a third handicap. Indeed, these quantities of charge and therefore the levels of deflection of the drops, vary strictly monotonous with the positions of the breaking points at within the electric field created by the electrode of load This means that the print quality of a multi-nozzle printing device comprising such a charging electrode directly depends on the accuracy with which the breaking point is positioned and regulated short for all jets of the device printing. At any breaking point different from this corresponds to a different point of impact on the print media. The management and control of a such a constraint is extremely difficult from a point from a technical point of view and would greatly increase the cost of a printing device operating in this manner.

In document US-A-4 638 328, it has been proposed to replace the piezoelectric stimulation elements by thermo-resistive elements generating thermal disturbances.

Furthermore, document US-A-4 220 958 describes a method of stimulating an ink jet, in which jet disturbance is accomplished by excitation electro-hydrodynamics (EHD). The stimulation device EHD proposed in this document is composed of one or several electrodes placed near the jet, in downstream of the nozzle, the length of each electrode being approximately equal to λ / 2.

Statement of the invention

The main object of the invention is a method of electrically conductive liquid using the binary continuous jet technique described in the above-mentioned article by Donald J. DRAKE, without presenting the disadvantages of this technique.

More specifically, the invention relates to a method projection of liquid by continuous jet, in which the process of charging the drops from jets are controlled regardless of the sequence of drops emitted, and the trajectory of the printable drops is not a strictly monotonic function of the position of the breaking point within the device charge.

• on émet au moins un jet continu de liquide à une vitesse V_j donnée ;
• on stimule le jet de façon à le fragmenter, à la demande, en deux points de brisure prédéterminés distincts, pour former des gouttes de liquide à une fréquence d'émission F donnée
• on applique sur les gouttes des quantités de charge électrique différentes, selon leurs points de brisure ; puis
• on applique un même champ électrique sur les gouttes, de façon à ne dévier que les gouttes formées en un premier desdits points de brisure, relativement éloigné ;
  caractérisé par le fait qu'on stimule le jet de façon telle que les deux points de brisure soient séparés par une distance ΔD strictement inférieure à la longueur d'onde λ du jet, définie par la relation λ = vj/F et on applique approximativement une même quantité de charge sur toutes les gouttes formées dans une zone centrée sur le deuxième point de brisure et de longueur sensiblement égale à λ/4.

According to the most general definition of the invention, this result is obtained by means of an electrically conductive liquid spraying process, in which:

• at least one continuous jet of liquid is emitted at a given speed V _j ;
• the jet is stimulated so as to fragment it, on demand, into two distinct predetermined breaking points, to form drops of liquid at a given emission frequency F
• different amounts of electric charge are applied to the drops, according to their breaking points; then
• the same electric field is applied to the drops, so as to deflect only the drops formed in a first of said breaking points, relatively distant;
  characterized by the fact that the jet is stimulated in such a way that the two breaking points are separated by a distance ΔD strictly less than the wavelength λ of the jet, defined by the relation λ = v j / F and approximately the same amount of charge is applied to all the drops formed in an area centered on the second breaking point and of length substantially equal to λ / 4 .

According to a preferred embodiment of the invention, said quantity is applied to the drops of different electrical charge by creating two contiguous areas located in the respective vicinity of the two breaking points and bringing these two areas to constant electrical potentials and opposite signs.

For this purpose, the jet can be passed successively between two pairs of oriented electrodes parallel to the jet and sized so that both break points are located between said electrodes, and applying on the two pairs of electrodes constant electrical voltages and signs opposites.

In this case, in order to avoid the inconvenience related to the immediate proximity between the surface of the jet and the load plan, we advantageously place each electrode at least equal distance from the jet axis twice the diameter of it.

Preferably, several transmissions are simultaneously transmitted continuous jets of liquid, parallel to each other, we stimulate separately each spray, we apply simultaneously on the drops of all the jets said quantities of different electrical charge, then apply simultaneously the same electric field on these drops.

• un réservoir pressurisé équipé de plusieurs buses aptes à émettre simultanément, à une vitesse V_j donnée, une pluralité de jets d'encre continus parallèles entre eux ;
• un moyen individuel de stimulation binaire de chacun des jets, apte à fragmenter ceux-ci, à la demande, en deux points de brisure prédéterminés distincts, pour former des gouttes d'encre à une fréquence d'émission F donnée ;
• un moyen de charge, commun à la pluralité de jets d'encre, pour appliquer sur les gouttes d'encre des quantités de charge électrique différentes, selon leurs points de brisure ;
• un moyen de déflexion, commun à la pluralité de jets d'encre, pour appliquer un même champ électrique sur les gouttes, de façon à ne dévier que les gouttes formées en un premier des points de brisure, relativement éloigné des buses ; et
• une gouttière de recyclage des gouttes déviées vers le réservoir pressurisé ;
  caractérisé par le fait que le moyen individuel de stimulation binaire de chacun des jets est piloté par des niveaux de tension prédéfinis tels que les deux points de brisure soient séparés par une distance strictement inférieure à la longueur d'onde λ du jet, définie par la relation λ = Vj/F , le moyen de charge étant apte à appliquer approximativement une même quantité de charge sur toutes les gouttes formées dans une zone centrée sur le deuxième point de brisure et de longueur sensiblement égale à λ/4.

The subject of the invention is also a device for printing by continuous ink jets, comprising:

• a pressurized tank equipped with several nozzles capable of simultaneously emitting, at a given speed V _j , a plurality of continuous ink jets parallel to each other;
• an individual means of binary stimulation of each of the jets, capable of fragmenting these, on demand, into two distinct predetermined breaking points, to form drops of ink at a given emission frequency F ;
• charging means, common to the plurality of ink jets, for applying different amounts of electric charge to the ink drops, according to their breaking points;
• deflection means, common to the plurality of ink jets, for applying the same electric field to the drops, so as to deflect only the drops formed at a first of the breaking points, relatively distant from the nozzles; and
• a recycling gutter for the drops diverted to the pressurized tank;
  characterized in that the individual means of binary stimulation of each of the jets is controlled by predefined voltage levels such that the two breaking points are separated by a distance strictly less than the wavelength λ of the jet, defined by the relationship λ = V j / F , the charging means being able to apply approximately the same amount of charge to all the drops formed in an area centered on the second breaking point and of length substantially equal to λ / 4 .

According to a first embodiment of the invention, the individual means of binary stimulation of each of the jets includes a piezoelectric element or thermo-resistive placed in the pressurized tank and individually controlled by an electronic circuit external.

According to a second embodiment of the invention, the individual means of binary stimulation of each of the jets includes two elements thermo-resistive placed in the pressurized tank, an external electrical circuit permanently delivering a periodic electrical signal for supplying a first of the thermo-resistive elements, corresponding to first breaking point and, on request, a signal complementary electric supply of the second thermo-resistive element, corresponding to the second breaking point.

Finally, according to a third embodiment of the invention, the individual means of stimulation binary of each of the jets includes a transducer individual placed in the pressurized tank and at minus a common electro-hydrodynamic excitation electrode placed near the jets, downstream of the nozzle, an external electrical circuit delivering permanently a periodic power signal electric excitation electrode electro-hydrodynamics, corresponding to the first point breakage and, on request, an electrical signal additional supply of the transducer individual, corresponding to the second breaking point.

Brief description of the drawings

• la figure 1 est une vue en perspective qui représente schématiquement un dispositif d'impression par jet d'encre continu conforme à l'invention ;
• les figures 2A et 2B sont des vues de côté qui illustrent très schématiquement les processus de charge et de déflexion dans le dispositif de la figure 1, respectivement pour les gouttes destinées au recyclage et pour les gouttes servant à l'impression ;
• la figure 3 est une vue en coupe comparable aux figures 2A et 2B illustrant une deuxième forme de réalisation de l'invention, dans laquelle chaque moyen individuel de stimulation binaire comprend deux éléments thermo-résistifs ; et
• la figure 4 est une vue en coupe schématique comparable aux figures 2A, 2B et 3, illustrant une troisième forme de réalisation de l'invention, dans laquelle chaque moyen individuel de stimulation binaire comprend un élément thermo-résistif et un dispositif commun de stimulation EHD.

We will now describe, by way of nonlimiting examples, various embodiments of the invention, with reference to the accompanying drawings, in which:

• Figure 1 is a perspective view which schematically shows a continuous ink jet printing device according to the invention;
• FIGS. 2A and 2B are side views which very schematically illustrate the loading and deflection processes in the device of FIG. 1, respectively for the drops intended for recycling and for the drops used for printing;
• FIG. 3 is a sectional view comparable to FIGS. 2A and 2B illustrating a second embodiment of the invention, in which each individual means of binary stimulation comprises two thermo-resistive elements; and
• FIG. 4 is a schematic sectional view comparable to FIGS. 2A, 2B and 3, illustrating a third embodiment of the invention, in which each individual binary stimulation means comprises a thermoresistive element and a common EHD stimulation device .

Detailed description of several embodiments preferential

Figure 1 schematically shows a continuous inkjet printing device putting implement the method of projecting a liquid electrically conductor according to the invention.

The device comprises a pressurized tank 10, equipped with a plurality of calibrated nozzles 12 (three in the figure) from which escape, at a given speed V _j , ink jets 14 parallel to each other and having between them a constant spacing.

Each ink jet 14 is associated with an individual means 16 of binary stimulation, placed in the reservoir 10 and controlled individually by an external electronic circuit 18. Each individual means 16 of binary stimulation fixes, on request, the place of breaking of each of the jets 14 at a short breaking point C , relatively close to the nozzle 12 or at a long breaking point L further from this nozzle. The drops formed at points C and L are designated respectively by the references 22 and 24. the drops 22 and 24 are all emitted at the same given emission frequency F.

A charging means 20, which will be described in more detail detail later, is placed in the vicinity of breaking points C and L. This charging means 20 is common to all ink jets 14. It applies different amounts of charge in drops 22 and 24, according to their breaking points.

Downstream of the charging means 20, the device print includes a sensor 26 designed to measure the speed of the ink jets 14. This sensor 26 is connected to an electronic circuit 28 which ensures the processing of data collected by the sensor. The circuit 28 is connected to a regulation loop (not shown) of the speed of the jets 14, according to an arrangement known to those skilled in the art. To simplify, the sensor 26 and its associated circuit have not been shown in Figures 2A to 4.

Downstream of sensor 26, the printing device includes deflection means 30 which applies a same constant electric field on ink drops 22 and 24 previously electrically charged in the load means 20. This deflection means 30 comprises two flat electrodes 32 and 34, common to all ink jets 14. These electrodes 32 and 34 are arranged on both sides of the ink drop trains 22 and 24 and a constant voltage is applied between them by a supply circuit 36. The deflection means 30 directs the charged drops 24 towards a gutter 38 which recycles them to a general ink circuit 40 of the device. The trajectory of the other drops 22, approximately uncharged, unaffected by the deflection means 30, so that these drops not loaded hit a print medium 42.

The charging means 20 comprises two groups of planar electrodes, respectively 42, 44 and 46, 48, the electrodes of each group being placed on either side of the jets 14. The two groups of electrodes are separated one on the other by a distance D (Figure 2A) parallel to the axes of the jets. The total length of the two groups of electrodes, parallel to the axes of the jets, is called S. As illustrated diagrammatically in FIG. 1, supply circuits 50 and 52 apply the same constant voltage V1 to the two electrodes 42 and 44 of the first group of electrodes and supply circuits 54 and 56 apply a same constant voltage V2 , of opposite sign to V1 , on the two electrodes 46 and 48 of the second group of electrodes. Two contiguous zones are thus created, in the vicinity of the breaking points C and L , respectively, brought to constant electrical potentials and of opposite signs.

As illustrated more precisely in FIGS. 2A and 2B, the electrodes 42 and 44 of the first group of electrodes are arranged symmetrically on either side of the jets 14 and each placed at a distance E from the axes of the jets. Preferably, this distance E is greater than or equal to twice the diameter d _j of the jets 14. This characteristic makes it possible to avoid soiling of the electrodes, both when the jets are started up and in steady state, in the presence of a impurity in the ejection duct. The reliability of the printing device is thereby increased.

The electrodes 46 and 48 of the second group of electrodes are also arranged symmetrically on either side of the jets 14 and at the same distance E from their axes.

When the printing device is in operation, the selection of a drop 24 not intended for the printing of the support 42 takes place by controlling the individual means 16 of binary stimulation of the corresponding jet 14 by an electrical signal whose level Vℓ is determined in order to induce the breaking of the jet at the predetermined long breaking point L , inside the charging means 20.

The selection of a drop 22 intended for the printing of the support 42 is carried out by controlling the individual means 16 of binary stimulation of the corresponding jet by an electrical signal whose level V _c will induce the breaking of the jet at the predetermined point of breaking short C also inside the charging means 20.

According to the invention, the distance ΔD between the two breaking points C and L is strictly less than the wavelength λ of the stimulated jets. The value of the wavelength λ is provided by the relation λ = V j / F . Any risk of creating a transient cluster of two drops, during the long break-short break transitions is thus avoided. Consequently, possible alterations in the trajectory of the drop to be printed are eliminated.

Any sequence of drops 24 not intended for printing or of drops 22 intended for printing is created by generating, on the individual means 16 for stimulating each of the jets and at the frequency F of emission of the drops chosen, a signal gathering the corresponding sequence of level V _c or Vℓ .

• la charge induite sur les gouttes à recycler 24, détachées du jet au point de brisure longue L, soit telle que le champ électrique constant engendré par le moyen de déflexion 30 incurve la trajectoire de ces gouttes vers la gouttière 38 (figure 2A) ;
• la charge induite sur les gouttes à imprimer 22, détachées du jet au point de brisure courte C, ainsi que dans une zone centrée autour de ce point et de longueur approximativement égale à λ/4, soit telle que le champ électrique constant produit par le moyen de déflexion 30 ne modifie pas la trajectoire de ces gouttes, qui peuvent ensuite atteindre le support d'impression 42 (figure 2B).

By placing the charging means 20 at a distance H (FIG. 2A) from the nozzles 12, for which the breaking points C and L are between H and H + S (that is to say placed inside the means of charging 20 of the drops) the values of H , S , D , E , V1 and V2 are fixed so that:

• the charge induced on the drops to be recycled 24, detached from the jet at the long breaking point L , or such that the constant electric field generated by the deflection means 30 curves the trajectory of these drops towards the gutter 38 (FIG. 2A);
• the charge induced on the drops to be printed 22, detached from the jet at the short breaking point C , as well as in an area centered around this point and of length approximately equal to λ / 4 , ie such that the constant electric field produced by the deflection means 30 does not modify the trajectory of these drops, which can then reach the printing medium 42 (FIG. 2B).

The trajectory of the drops to be printed 22 is therefore not a strictly monotonic function of the position of the breaking point within the charging device. On the contrary, the same point of impact is ensured on the printing medium, despite possible fluctuations in the short break point C. Print quality is thus ensured without any particular technical difficulty or increase in cost.

By way of nonlimiting example, the length S of the charging means 20 can be less than 2.5 mm, the voltage V1 applied to the electrodes 42 and 44 equal to 300 V, and the voltage V2 applied to the electrodes 46 and 48 equal to - 300 V. Each of the jets 14 has, for example, a diameter of 35 μm, a speed of 24 m / s and a stimulation frequency equal to 125 kHz.

In the first embodiment of the invention illustrated schematically in the figures 1, 2A and 2B, each of the individual stimulation means 16 binary consists of a piezoelectric element placed in the tank 10 and controlled individually by the external electronic circuit 18. The number of piezoelectric elements is equal to that nozzles 12 of the print head.

Alternatively, each of the piezoelectric elements constituting the individual stimulation means 16 binary can be replaced by an element thermo-resistive generating disturbances of nature thermal. For more details on such items thermo-resistive, their operation and manufacturing method, we will usefully refer to the document US-A-4,638,328.

When each individual means 16 of binary stimulation consists of a single thermo-resistive element associated with each nozzle 12 of the print head, this element is supplied by an electrical signal composed of a sequence of voltages V _c and Vℓ , corresponding the pattern to be printed.

According to a second embodiment of the invention, illustrated schematically in FIG. 3, each of the individual means 16 of binary stimulation includes two heat-resistant elements 16a and 16b associated with each nozzle 12 of the print head.

The first element 16a is supplied uninterruptedly by a periodic electrical signal of amplitude Vℓ . When it is the only one to be supplied, the jet is therefore broken at point L furthest from the nozzle.

The second element 16b, located as appropriate upstream or downstream of the first, is only activated when a drop 22 is to be printed. It then receives an electrical signal, preferably a voltage square wave, the amplitude and phase shift of which relative to the periodic signal applied to the first element 16a lead to displacement of the jet breaking point at the point C closest to the nozzle.

A third embodiment of the means individual binary stimulation of each of the jets 14 is illustrated diagrammatically in FIG. 4.

In this case, each individual means 16 of binary stimulation comprises an electrode 58, placed immediately downstream of the nozzles 12 and common to all of the jets. This electrode 58 constitutes a stimulation device by electrodynamic excitation (EHD). Such a device and its operation are described in document US-A-4 220 958. The electrode 58, the length of which is approximately equal to λ / 2 , fixes the jets breaking point at the point L furthest from the nozzles, when no other stimulation is applied to the jets.

Each individual means 16 of binary stimulation further comprises an individual transducer 60, preferably of the thermoresistive type, associated with each of the jets inside the reservoir 10. the transducers 60 are only activated to move the breaking points to the point C closest to the nozzle, when a drop 22 is to be printed. Compared to the embodiments described above, the embodiment of FIG. 4 makes it possible to extend the life of the thermo-resistive transducers by reducing their stress.

It should be noted that the process implemented by the printing device described can be applied to selective projection of any liquid electrically driver.

Compared to the liquid spraying process by continuous jet according to the prior art, this process allows you to control the charging process droplets from the jets whatever the sequence of drops emitted. In addition, the electrodes of the charging device drops are not located at close proximity to jets. In addition, the trajectory drops to print is not a strictly function monotonous of the position of the breaking point at within the charging device.

As we have already observed, a jet printer multi-nozzle ink produced according to the invention can be used in all applications related to industrial marking and coding. The domain of addressing, which requires speed and width printing, also represents an area of application of the invention. In addition, the absence individual electrodes facing the jet increases the number of nozzles per unit length on the printing device tank. This allows application of the invention to industrial decoration which requires increased resolution, in addition high print speed.

Claims (11)

Method for spraying an electrically conductive liquid, in which: at least one continuous jet of liquid (14) is emitted at a given speed V _j ; the jet is stimulated so as to fragment it, on demand, into two distinct predetermined breaking points ( C , L ), to form drops (22, 24) of liquid at a given emission frequency F ; applying different amounts of electric charge to the drops (22,24), according to their breaking points ( C , L ); then the same electric field is applied to the drops, so as to deflect only the drops (24) formed in a first ( L ) of said breaking points, relatively distant;
characterized by the fact that the jet (14) is stimulated so that the two breaking points ( C , L ) are separated by a distance ΔD strictly less than the wavelength λ of the jet, defined by the relation λ = V j / F , and approximately the same amount of charge is applied to all the drops (22) formed in an area centered on the second breaking point ( C ) and of length substantially equal to λ / 4 . Method according to claim 1, in which said different amounts of electric charge are applied to the drops (22,24) by creating two contiguous zones located in the respective vicinity of the two breaking points ( C , L ) and by bringing these two zones to constant electrical potentials and opposite signs. Method according to claim 2, in which the jet is passed successively between two pairs of electrodes (42,44; 46,48) oriented parallel to the jet (14) and dimensioned so that the two breaking points ( C , L ) are located between said electrodes, and by applying constant electric voltages ( V1 , V2 ) of opposite signs to the two pairs of electrodes. Method according to claim 3, in which each electrode (42,44; 46,48) is placed at a distance ( E ) from the axis of the jet (14) at least twice the diameter of the latter. Method according to any of the claims previous, in which we transmit simultaneously several continuous jets of liquid (14), parallel between them, we stimulate each spray separately, we apply simultaneously on the drops (22,24) of all throw said different quantities of electric charge, then we apply the same field simultaneously electric on the drops. Continuous inkjet printing device, comprising: a pressurized tank (10) equipped with several nozzles (12) capable of simultaneously emitting, at a given speed V _j , a plurality of continuous ink jets (14) parallel to each other; an individual means (16) of binary stimulation of each of the jets, capable of fragmenting them, on demand, into two distinct predetermined breaking points ( C , L ), to form drops of ink (22,24) at a given transmission frequency F ; charging means (20), common to the plurality of ink jets (14), for applying different amounts of electric charge to the ink drops (22,24) according to their breaking points; deflection means (30), common to the plurality of ink jets (14), for applying the same electric field to the drops, so as to deflect only the drops (24) formed in a first ( L ) of the break points, relatively distant from the nozzles; and a gutter (38) for recycling the deviated drops (24) to the pressurized tank (10);
characterized by the fact that the individual means (16) of binary stimulation of each of the jets (14) is controlled by predefined voltage levels ( V _c , V _l ) such that the two breaking points ( C , L ) are separated by a distance ( ΔD ) strictly less than the wavelength λ of the jet defined by the relation λ = V j / F , the charging means (20) being able to apply approximately the same amount of charge to all the drops (22) formed in an area centered on the second breaking point ( C ) and of length substantially equal to λ / 4 . Device according to claim 6, in which the charging means (20) comprises two pairs of electrodes (42,44; 46,48) oriented parallel to the jets and dimensioned so that the breaking points ( C , L ) are located between said electrodes, and means (50,52; 54,56) for applying constant electrical voltages and of opposite signs to the two pairs of electrodes. Device according to claim 7, in which the electrodes (42,44; 46,48) are planar and placed at a distance ( E ) from the axis of each of the jets (14), at least equal to twice the diameter of the jets. Device according to any one of the claims 6 to 8, in which the individual means of binary stimulation of each of the jets (14) includes a piezoelectric or thermoresistive element (16) placed in the pressurized tank (10) and controlled individually by an external electronic circuit (18). Device according to any one of Claims 6 to 8, in which the individual means (16) for binary stimulation of each of the jets (14) comprises two thermo-resistive elements (16a, 16b) placed in the pressurized reservoir (10), an external electrical circuit (18) permanently delivering a periodic electrical signal for supplying a first of the heat-resistive elements, corresponding to the first breaking point ( L ) and, on request, an additional electrical signal for supplying the second thermo-resistive element (16b), corresponding to the second breaking point ( C ). Device according to any one of Claims 6 to 8, in which the individual means (16) for binary stimulation of each of the jets (14) comprises an individual transducer (60) placed in the pressurized reservoir (10) and at least one electrode (58) common electro-hydrodynamic excitation placed near the jets, downstream of the nozzle (12), an external electrical circuit (18) permanently delivering a periodic signal of electrical supply to the electrode (58) d electro-hydrodynamic excitation, corresponding to the first breaking point ( L ) and, on request, an additional electrical signal supplying the individual transducer (60), corresponding to the second breaking point ( C ).

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