EP1007363B1 - Control system for spraying electrically conductive liquid - Google Patents

Description

The present invention relates to a system for electrically sprayed liquid control driver. Such a system is in particular usable in an inkjet printhead using the continuous jet process.

In a projection control system liquid having one or more continuous jets used in inkjet printers, each jet of electrically conductive liquid is split into drops. The drops are electrically charged and their path is then deflected by a field electric which, depending on information to reproduce, deviate each drop either to a gutter ink recovery, either to the support where ink must be deposited.

In continuous jet printers, the ink is pressurization upstream of an ejection nozzle. To the output of the nozzle, there is emission of a continuous jet. This continuous jet is processed by the control system projection of liquid through several organs performing various functions. The jet is first divided in drops by an organ controlled by a signal splitting. Simultaneously, the drops separating of the continuous jet are electrically charged under the effect of the electric field established between the charging electrode and the liquid. They then pass in a field deflection electric generated between two electrodes or deflection plates to be deflected according to the value of this electric field. On leaving the system liquid spray control, drops are either recovered to return to the circuit ink supply, either deposited on the support.

In practice, control systems for projection of liquid put into service on printers exhibit a number disadvantages. They require many parts produced and positioned with precision. These parts are complex and must be separated by so-called safety distances and / or by shields and by empty or insulating spaces ensuring the separation of functions, which unnecessarily lengthens the path of the drops. The pieces making each function create discontinuous surfaces which cause local electric field elevations in the interior space, suitable for discharges electric. These surfaces are also difficult to clean when removing material residue to inside the print head. Rooms performing each function being supported by insulators, their surfaces can charge electrically variably and the liquid undergoes then parasitic electric fields. The result random deflections of the drops. With such control systems, the electrical voltages set work can reach 10 kV.

In the current state of the art, it is often done call for deflections of drops in space to atmospheric pressure. The drops being charged electrically beforehand, they undergo a force proportional to their charge and to the electric field. This electric field is obtained by two plates conductive close to the trajectory of the drops and subject to a potential difference between them. He has was proposed, in document GB-A-2 249 995, to coating one of the deflection plates with a coating dielectric to avoid discharges accidental electrostatic and / or to adapt the potential of the free space where the drops pass. According to document US-A-4 845 512, this coating dielectric could have an electrical bias permanent (electret) to generate potential electric or part of it. This would be done without electrical connection. In this environment, the projection of liquid and the presence of gas subjected to fairly intense electric fields create particles of matter and charges moving in free space. Projection and electrical forces entrain these free elements (particles and charges) on the walls around the jet. In particular these elements clump together on the surface free of the insulation subjected to the electric field and are attracted by opposite charges. So, they come compensate for the charges of the electrode or electrode previously coated with insulation. Therefore, the useful field in free space is reduced gradually in favor of growth in the field electric in the dielectric. The effectiveness of the deflection is reduced in relation to the reduction in electric field in free space.

Document WO 94/16896 recommends the use of electrically conductive plastic for realize a projection control system of electrically conductive liquid. This allows to reduce the cost, the number of additional parts such as shields and simplify wiring. Matter electrically conductive plastic also picks up electrical charges. This plastic material can be polyacetylene which is a conductive polymer intrinsic. Preferably, it is a resin plastic such as Nylon®, polyester, acetal containing conductive fibers (carbon, steel stainless) coated with nickel. The heterogeneity of a fibrous resin grows on the surface, particularly with molded manufacturing. The insulating part of the fibrous plastic being rather on the surface, the static charges can accumulate there. The effect expected conductivity therefore decreases in surface area and deflection drifts appear as it is announced in documents US-A-4,845,512 and GB-A-2 249 995. To improve the homogeneity of surface, it is possible to machine surfaces functional parts involved, but this increases their manufacturing cost.

In addition, the use of volatile liquid in the constitution of the ink causes condensation. Gradually, according to the ventilation internal printer, partial voltages various surrounding gases and gradients of temperature, the parts close to the inkjet get line with liquid. This causes phenomena of conduction on the walls of the deflection electrodes and reducing the air gap between jet and electrodes. We then note a drift in the deflection of the drops at during use of the control system projection.

To remedy this problem it has been proposed, in US-A-5,001,497, to heat the electrode of deflection affected by resistance electric to vaporize the liquid deposited. employment of such resistance was criticized in the document GB-A-2 249 995 because of the heat given off by this resistance and because of the current value necessary for its proper functioning.

WO-A-88/01572 (corresponding to the patent US-A-5,070,341) discloses a control system for projection of an electrically conductive liquid and emitted in the form of a jet under pressure, corresponding in the preamble of claim 1.

A harmful phenomenon in these printheads inkjet comes from the possible interaction between drops in flight. A good control system for projection must provide a drop path weak to fight against this phenomenon.

Some manufacturers have chosen not to coat the conductive deflection plates with a dielectric material. In order to avoid discharges accidental electrostatic, they place resistors in the power supply circuit deflection plates to limit the flow of discharge in the circuit. Several types of discharge may occur during operation a printer.

The first type of discharge is given in the case of a voltage applied between two well-polished plates. The electric field is identical everywhere and the shock ionization conditions are fulfilled uniformly on average. Thermal agitation causes, in a place, at a given time, an elevation brutal current which then passes from a quasi value null to a gigantic value if there is no resistors in the circuit. The stored energy is used almost completely in a brief instant related to the shape of the storage capacitor, and this shape defining the electromagnetic regime of the transient of discharge. The power density dissipated is gigantic and concentrated very locally. In the when using connected metal plates by about three meters of cable to the power supply high voltage, the stored energy can exceed 1 mJ.

In other cases, electrical leaks are particular sources in space (spikes conductive, insulation faults, foreign bodies) where the field strong enough locally generates a source of ions or electrons. The flow from this source is regulates more or less using the space charge created. We first obtain a stable current following presumably Langmuir's law, and then fluctuations in the current appear, remaining finished. This second discharge case causes variations of the deflector field, and also variations of the burden of gout. This reduces the accuracy of inkjet printers.

To solve problems related to the first type of discharge, it is known to place resistors of protection also for the safety of people and material (coating the electrodes may EDM), the risk of fire. The location of these resistors must be done to break up the stored electrical energy. Above all, it is necessary reduce the energy stored in hazardous locations like the deflection space. There are commonly, the order of 20 µJ stored in this space.

A first object of the present invention is to reduce the number of mechanical components and projection control system liquid.

A second object of the present invention is to remove discontinuities from the internal surfaces of the liquid spray control system.

A third object of the present invention is to reduce the path of the drops subjected to interactions between them in the control system of projection of liquid.

A fourth object of the present invention is integrate the electrical circuits necessary for liquid spray control system in a same component.

• un premier et un deuxième élément monolithique présentant chacun une surface continue, disposés de façon que les surfaces continues soient en vis-à-vis et définissent entre elles un espace dans lequel le jet sous pression est émis par ladite buse, lesdits éléments incluant des moyens conducteurs possédant des moyens de liaison électrique à des potentiels électriques permettant d'établir continûment des potentiels sur lesdites surfaces continues pour obtenir ladite charge électrique des gouttes et ledit champ électrique de déflexion,
• des moyens électroniques de commande desdits potentiels et de contrôle de l'intensité des courants électriques pouvant circuler sur lesdites surfaces continues,

caractérisé en ce que la surface continue du premier élément est conductrice et possède des moyens de liaison électrique à l'un desdits potentiels, la surface continue du deuxième élément est constituée par une face d'un support isolant, cette face étant équipée de pistes conductrices possédant des moyens de liaison électriques à des potentiels choisis parmi lesdits potentiels, un revêtement résistif, ayant une valeur de résistance carrée comprise entre 5 MΩ et 100 MΩ, s'étendant continûment sur ladite face.These objects are achieved by the present invention which relates to a system for controlling the spraying of an electrically conductive liquid and emitted in the form of a jet under pressure by at least one nozzle, comprising means for splitting the jet of liquid into drops, means for electrically charging said drops and means for applying an electric deflection field to said charged drops, comprising:

• first and second monolithic elements each having a continuous surface, arranged so that the continuous surfaces face each other and define between them a space in which the pressurized jet is emitted by said nozzle, said elements including means conductors having means of electrical connection to electrical potentials making it possible to continuously establish potentials on said continuous surfaces in order to obtain said electrical charge of the drops and said electrical deflection field,
• electronic means for controlling said potentials and for controlling the intensity of the electric currents which can flow on said continuous surfaces,

characterized in that the continuous surface of the first element is conductive and has means of electrical connection to one of said potentials, the continuous surface of the second element is constituted by one face of an insulating support, this face being equipped with conductive tracks having means for electrical connection to potentials chosen from said potentials, a resistive coating, having a square resistance value of between 5 MΩ and 100 MΩ, extending continuously on said face.

The continuous surface of the first element can also be covered with a resistive coating continued.

The first and second elements can also present ways to split the jet of liquid into drops and tilt the jet. These means make it possible to apply a field electric on the jet and may include means resistive and capacitive means. In this case, the resistive means are advantageously constituted by part of the resistive coating which has, preferably discontinuities in certain portions in order to increase the efficiency of the jet fractionation. The capacitive means may include said coating supported by an insulating layer, this insulating layer serving as a dielectric and being supported by conductive means supported by said insulating support.

• la figure 1 représente, en vue longitudinale, la partie mécanique d'un système de commande de projection d'encre selon la présente invention,
• la figure 2 est une vue selon le plan II-II de la figure 1,
• la figure 3 est une vue de détail agrandie de la partie mécanique représentée à la figure 1,
• la figure 4 est un diagramme montrant l'évolution d'un potentiel électrique le long d'une surface d'un dispositif de projection d'encre selon l'art antérieur,
• la figure 5 est un diagramme montrant l'évolution d'un potentiel électrique le long d'une surface d'un dispositif de projection d'encre selon l'invention,
• les figures 6 et 7 sont des schémas explicatifs de deux modes de commande de la stimulation dynamique du jet d'encre.

The invention will be better understood and other details and particularities will appear on reading the description which follows, given by way of example, accompanied by the appended drawings among which:

• FIG. 1 represents, in longitudinal view, the mechanical part of an ink spraying control system according to the present invention,
• FIG. 2 is a view along the plane II-II of FIG. 1,
• FIG. 3 is an enlarged detail view of the mechanical part shown in FIG. 1,
• FIG. 4 is a diagram showing the evolution of an electrical potential along a surface of an ink spraying device according to the prior art,
• FIG. 5 is a diagram showing the evolution of an electrical potential along a surface of an ink spraying device according to the invention,
• Figures 6 and 7 are explanatory diagrams of two modes of control of the dynamic stimulation of the ink jet.

By way of example, the following description will relate to an ink spray control system for continuous jet print head. The ink can be issued according to one or more jets which are split in drops. The ink drops, charged electrically, are then deflected by a field electric to either lead to a circuit of ink recovery and recycling, either to a support on which the ink must be deposited.

As shown in Figure 1, ink 3, contained in a cavity 1, is emitted under pressure by a nozzle 2. The ink jet 4, emitted by the nozzle 2, is projected into space 5 defined by the surfaces continuous presented by two elements 6 and 8, these surfaces facing each other. Multiple jets ink, such as jet 4, can be emitted from several nozzles between these continuous surfaces, such as the shows figure 2.

Element 6 includes a flat insulating support 60, for example made of alumina, the face of which towards the space 5 supports conductive tracks 62 to 66 and a resistive coating 67. The conductive tracks 62 to 66 electrically connect the resistive coating 67 to voltage generators, respectively 32 to 36, intended to supply control potentials, respectively U ₂ to U ₆ . The other face of the support 60 can also support conductive tracks, for example 71, 72, 73, resistive coatings, for example the resistive coating 74, the dielectric coating 76 and electrical or electronic components, for example the component 75 ( see figure 2).

Electrical or electronic components deposited on the support 60 can be circuits integrated analog or logic, transistors diodes, capacitors, a transformer. They allow for voltage increases, current and voltage measurements, the generations of signals needed to split the inkjet (if applicable) and at the expense of the drops, generations of supply voltages.

The electrical connections between the two faces support 60 can be done by metallized holes, such as metallized hole 77. This is the description of a monolithic element made up in the mass of electrical insulation, which is usually used as component support e. These components perform the functions control interface upstream, to the electrodes of liquid projection.

The element 8, in the example described, comprises a continuous support 81, for example of alumina, or of another insulating material, covered with a continuous resistive coating 82. A voltage generator 31 makes it possible to provide a potential U ₁ to the continuous resistive coating 82. The element 8 can also consist simply of metal or of another conductive material offering a continuous surface. The voltage generator 31 is then directly connected to the material of this element.

The ink jet 4, which enters the projection control system according to the invention, has an electric potential U _jet which will be taken as the reference potential to simplify the explanations. This ink jet can be provided beforehand with a kinematic disturbance depending on the time and leading to the separation of the jet into drops after a period of time, for example by means of a resonator included in the cavity 1. It can on the contrary be devoid kinematic disturbance when leaving the nozzle, in which case splitting into drops is carried out by the projection control system according to the invention.

We will now describe, in relation to FIG. 3, the way in which the jet splitting into drops can be carried out using the projection control system according to the invention. The insulating support 60 supports, in its part located near the ink ejection nozzle, three electrodes 11, 12 and 13 arranged successively in the direction of the ink jet and covered with an insulating layer 15. The tracks conductive 62 (see also Figure 1) are deposited on the insulating layer 15, so as to frame the electrodes 11 and 12. The resistive coating 67 covers both the conductive tracks 62 and the insulating layer 15. This resistive coating 67 has discontinuities (that is to say interruptions) on three small regularly spaced portions 16, corresponding to the upstream part (in the direction of advancement of the jet) of the conductive tracks 62, this to avoid propagation of the signal U ₂ origin on the coating 67 against the jet.

The electrodes 11, 12 and 13 are brought to the potential U _jet , the electrode 81 to the potential U ₁ and the conductive tracks 62 to the potential U ₂ . At the entrance, the ink jet 4 undergoes two attractive forces coming from the potentials U ₂ and U ₁ . These two forces are in opposition. Their difference produces the inclination of the incident jet constantly and / or dynamically if the potential U ₂ is variable. This provides the jet with a kinematic disturbance over time leading to the subsequent separation of the jet into drops. The jet then progressing in the liquid projection control system has its inclination and its disturbance which are amplified. Due to the establishment of the potential U ₂ on the resistive coating 67, the force coming from the potential U ₂ evolves progressively and is replaced by a force coming from the signals U ₃ then U ₄ supplied to the conductive tracks 63 and 64. The dynamic disturbance of the potential U ₂ depending on the time rapidly decreases by the presence of the insulating layer 15 as will be seen below. The static inclination given first by the potential U ₂ then progressively evolves according to the potential U ₃ . The force from the potential U ₁ essentially changes by modifying the distance between the jet and the position of the potential U ₁ .

The kinematic disturbance given by the fractionation signal reduces the diameter of the jet in certain places under the action of surface forces. This progresses until the cancellation of this diameter. It is the splitting of the jet, or the breaking. It is the moment of sampling of the electric charge of the drop formed according to the potentials U ₃ , U ₄ and U ₁ associated with the distances between the liquid and these potentials. In the example described here, the potentials U ₃ and U ₄ are equal and represent the charge control signal. This gives a certain independence of the electric charge of the drop with respect to the place of fractionation.

Since entering the system, the jet or the drops are constantly deflected by the action of forces from the surrounding potentials and the charges of the drops and the jet. The charged drops then evolve into a space where the deflection field remains large and becomes constant over time. We move away from the influence of the charge control provided by the potentials U ₃ and U ₄ . The free space between the potentials of the resistive coating 67 and U ₁ is defined increasing according to the needs of the printer to be defined. In practice, this prevents the drops from approaching the internal surfaces of the system in an unstable manner. The definition of the potentials brought to the resistive coating 67 is predefined to guarantee an operation without electric shocks, and without risk for the cohesion of the drops. Thus, the drops obtained by the fractionation signal have, at the output of the system according to the invention, trajectories controlled by the charge signal supplying the potentials U ₃ , U ₄ and by the tilt signal supplying the potential U ₂ .

The different static potentials used in the ink spraying control system according to the invention are obtained by electrical circuits within the reach of those skilled in the art. As an indication, one can use a chopping transistor defining a low voltage potential at the terminals of the primary of a step-up transformer and having several secondary. Diodes connected to the transformer secondary provide positive and negative rectified voltages of the same amplitude. This makes it possible to obtain the supply voltages of two amplifiers supplying the potentials U ₂ , U ₃ and U ₄ . The potential U ₁ is obtained in an analogous manner. The potentials U ₅ and U ₆ can be obtained using multiplier cells formed by diodes and capacitors and which make it possible to obtain multiples of the peak-to-peak voltage appearing at a secondary of the transformer.

To control the precision and check the operation of the system, a control device is provided, this device receiving the voltage measurements representing the result of the voltage behavior in the deflection X. Thus, the measurements are used to modify either the low voltage supplying the 'together or the pace of the chopper, i.e. the information sent to obtain the potentials U ₃ , U ₄ or U ₂ . This makes it possible to obtain the constancy of the deflection X with respect to the variations of the circuits for obtaining the electrical voltages.

Due to the use of a coating resistive in the part of the system according to the invention corresponding to the deflection plates of known art, there is no electrostatic energy storage likely to suddenly discharge into this part. The protective resistors used in certain devices of the known art are eliminated. The use, in the system according to the invention, of resistive coatings and the existence of currents parasites does not modify the deflection of the drops taking into account the means implemented above to master precision.

In the present invention, a variable air thickness between the jet inlet and the out of the drops. We use the increased field electric possible at short distance. This is good known and is illustrated by the Paschen curve defining the voltage giving an ionization not controllable, in a pressurized gas, between two conductive plates separated by a given distance. This, combined with the actual deflection of the drops loaded, provides the particular curvature of the surface to be generated. Reducing free space gives a substantial decrease in the amplitude of control voltages with increased deflection efficiency. The present invention makes it possible to drop the voltages used at 2300 V compared to a classic design using 8000 V.

q(n) :

succession de charges de gouttes,

Ce :

capacité entre la goutte et l'électrode de charge,

v(n) :

succession des tensions de charges de gouttes.

Another advantage linked to short distances comes from the reduction of the "historical load". This comes from the influence of the drop already loaded on the charge acquired by the drop leaving the jet. The value of the historical charge can be given by the coefficient a in the charge transfer formula: q (n) = -Ce [v (n) - av (n-1) -...]

q (n):

succession of charges of drops,

This:

capacity between the drop and the charging electrode,

v (n):

succession of drop charge voltages.

As expressed in the formula, one can also state the charge according to the current voltage of the charging electrode and according to the voltage at the formation of the previous drop. The value of a is given essentially by the ratio between the capacity between two drops in flight and Ce. Here the distance between the drop and the electrode is reduced. It rises and thus reduces. The development of the charge of the drops becomes less sensitive to this phenomenon.

When the printer starts, even satisfactory, small ink deposits are inevitable inside the projection system. We also risk larger malfunctions if accidentally liquid is deposited on the coating resistive or on the rare places where the insulation appears between two surface conductors.

In the prior art, as illustrated in FIG. 4, an ink deposit causes the existence of a disturbing current which passes through it. The diagram of the potentials U is compared with a set of electrodes 22, 23 and 24 carried respectively to potentials U ₂₂ , U ₂₃ and U ₂₄ and separated by insulating parts 25 and 26. The surface 27 of the part insulator 25 is easily polluted by parasitic electrical charges. If an ink deposit 28 occurs between the electrodes 23 and 24, a disturbing current i will flow in the ink deposit above the insulating part 26. The potential diagram shown is obtained with corresponding potential variations to intense electric fields, in particular for the insulating part 25. The potentials and the currents are then modified and the measurements make it possible to alert the control member. According to predefined criteria, the system can decide to modify orders or to stop periodic closings of the chopper. Depending on the type of ink, it is possible to wait for the ink to dry and the resistance of the parasitic deposit to change, and to return the chopper to operation for a new measurement of the disturbance.

According to the present invention, a very small proportion of free insulation surface can be reached by the ink. This is illustrated by FIG. 5 which takes up the principle of the invention: presence of an insulating support 60 supporting conductive tracks 62, 63 and 64 and a continuous resistive coating 67. The conductive tracks 62, 63 and 64 are brought respectively to the potentials U ₂ , U ₃ and U ₄ . The presence of an ink deposit 18 between the conductive tracks 63 and 64 causes the circulation of a small disturbing current i between the tracks 63 and 64. The resistive coating 67 makes it possible to define and reduce the electric field on the insulating. Thus, the potential drop between the electrodes is organized. The associated potential diagram clearly shows that the surface electric field is weak between the conductive tracks.

The insulation is no longer accessible to the static field of free space, the charges flow on the surface, without taking the time to disturb the deflection of the liquid.

This principle makes it possible to define the potentials continuous intermediates on the surfaces between potentials imposed by the conductive tracks, such as we can see it in figure 5. A minor deposit gives a lower disturbance current and if it is enough, it does not cause a degradation of the printer accuracy, or major alert to the supervisory body.

The resistive coating deposited on the support insulator 60, and possibly on electrode 81, can have a square resistance of 5 MΩ to 100 MΩ.

The ink used by inkjet printers contains a volatile liquid which creates condensation, especially on nearby surfaces inkjet. Gradually, in printers known art, according to internal ventilation, partial voltages of the various gases, gradients of temperature, surfaces close to the inkjet will line with liquid, which causes conduction on the walls. We then notice a drift of the deflection of the drops.

In the case of the present invention this requires a value domain of the square resistance of the resistive coating. The use of such a coating advantageously provides the potential for desired surface and local heating of this same area. So the surfaces close to the inkjet can be moderately heated using potential differences used to control the ink movement. We can define a power of sufficient dissipation to raise by about 1 degree the surfaces with respect to the ink. We can predict the square resistance allowing the detection of dysfunctions linked to the disturbance of quantities electric during spurious ink deposits. Then, we have the heat dissipation paths from resistive coating and components near electrics.

The method according to the invention employs a surface continuous common to the functions, from the entry of the jet to the out of the drops. This reduces and even suppress local elevations of the electric field due to the use of small radii of curvature. So we can follow the discharge limits more precisely restricting operation and increasing the efficiency of deflection. We can thus delete landfills of the second type regulated by the law of Langmuir exposed above. In the system according to the invention, the potentials of the functions fractionation, load, deflection, are generated from continuously on a continuous surface to control the surface electric field of interface between functions.

The dimension along the axis of deflection begins at the entry of the jet with values of the order of several jet diameters. Field boundaries are increased by the smallness of the dimensions used. The electric fields of the present invention are greater than 1.5 MV / m used in conventional printers. They can reach 6 MV / m. Limiting factors come from imbalance of the liquid surface by electrical pressure counteracting the surface pressure. For the same desired deflection result, the effective distance from the necessary liquid path can be reduced.

As indicated above, we use voltages much weaker. So the potentials between three functions are reduced, and the distances necessary to arrange the interfaces or "distances "between functions are also reduced.

We obtain a significant reduction in the route overall drops. So the transit time of drops is reduced. The impulses given by the interaction forces are reduced in the same way.

• U_jet, U₁, U₂, U₃ et U₄ : plutôt des potentiels variables, de faibles valeurs commandant la trajectoire du jet et la charge de la goutte,
• U₅ et U₆ : plutôt des potentiels constants, de fortes valeurs, amplifiant la trajectoire initiale de la goutte.

The list of potentials around the inkjet is in order, from the inkjet nozzle (see Figures 1 and 3):

• U _jet , U ₁ , U ₂ , U ₃ and U ₄ : rather variable potentials, low values controlling the trajectory of the jet and the charge of the drop,
• U ₅ and U ₆ : rather constant potentials, high values, amplifying the initial trajectory of the drop.

From these potentials, we can summarize the various ways of controlling the potentials and give their consequences. It will be assumed that the ink jet emitted is closer to the potential electrode U ₁ than to the multipotential element U ₂ to U ₆ .

According to a first control mode, U _jet = 0, U ₁ = 0, U ₂ = 0, U ₃ = U ₄ , U ₅ = -400 V and U ₆ = -1200 V. U ₄ is the sampled control signal during the break. For a voltage U ₄ of +100 V, the drop is negatively charged and takes a trajectory giving positive X. The drop runs along the upper surface limit. For a voltage U ₄ of -350 V, the drop is positively charged and takes a trajectory giving X negative. The drop runs along the lower surface limit.

According to a second control mode, U _jet = 0, U ₂ = 0, U ₃ = -300 V, U ₄ = -350 V, U ₅ = -400 V and U ₆ = -1000 V. U ₁ is the signal of order sampled during the break. For a voltage U ₁ of +300 V, the drop is negatively charged, and takes a trajectory giving positive X. The drop runs along the upper surface limit. For a voltage U ₁ of -50 V, the drop is positively charged and takes a trajectory giving X negative. The drop runs along the lower surface limit.

According to a third control mode, U ₁ = 200 V, U ₂ = 0, U ₃ = -300 V, U ₄ = -350 V, U ₅ = -400 V and U ₆ = -1000 V. U _jet is the control signal sampled during breakage. For a voltage U _jet of -50 V, the drop is negatively charged and takes a trajectory giving positive X. The drop runs along the upper surface limit. For a voltage U _jet of +200 V, the drop is positively charged and takes a trajectory giving X negative. The drop runs along the lower surface limit.

Of course, other combinations are possible, particularly if all the tensions expressed above are multiplied by -1 and if the hypothesis of the proximity between the jet and the potential U ₁ is modified. The most characteristic combinations have been mentioned. This is guided by determining the charge sampled by the drop. The charged drop is then subjected to an almost constant field.

The first control mode gives the preferred combination, adaptable to the multijet. The jets and U ₁ , U ₂ , U ₅ , U ₆ potentials are common to the different jets. The control voltage is comparatively higher excursion.

The second control mode gives the preferred combination, adaptable to the monojet if one wishes to keep the simplicity of the potential U ₁ . It is possible to replace the equipotential U ₁ with a second monolithic circuit. This produces before each break a specific charge voltage in the manner of the potentials U ₃ , U ₄ . On the rest of the surface, a constant potential is defined, framing the charge controls.

The third control mode gives a variant, adaptable to the monojet. The drop charge control potential is that of the jet. The control voltage is comparatively lower excursion. The implementation is simple, the nozzle is at control potential. The ink supply to the nozzle is carried out under an insulating tube. For example, if the length of the insulating tube is 0.5 m, its internal section is 2 mm ² and if the resistivity of the ink is 8 Ω.m, the load resistance of the control is then equal 2 MΩ, which represents a slight disturbance for the charge control generator.

The potential U ₂ makes it possible to modify, by its static value, the inclination of the incident jet or / and to dynamically deflect the jet and / or to propagate a disturbance in lieu of liquid separation information. Already in patents US-A-5,001,497 and US-A-5,070,341, the principle of continuous jet deflection is realized. This makes it possible to deflect portions of liquid subsequently not electrically charged. In the method according to the invention, most of the deflection results from the force applied to the charged drop. The static U ₂ potential is used to adjust to compensate for jet alignment errors. The means of producing the control electrode and the electrical behavior are particularly advantageous in the present invention.

Returning to FIG. 3, the definition of resistive, conductive and insulating deposits defines a propagation of the potential U ₂ (t) in the direction of advance of the jet. Thus the dynamic potential of the resistive deposit, significantly close in amplitude and phase of U _2, is present over a variable extent according to the desired frequency of droplet formation.

• cd est la capacité répartie entre le revêtement résistif et le dépôt conducteur au potentiel U_jet en F/m, donnée par la couche isolante 15,
• rd est la résistance répartie du revêtement résistif en Ω/m,
• ω est la pulsation du fractionnement.

The signal penetration range is given by the formula: (cd.ω.rd) ^-½ for an amplitude greater than half of the dynamic signal and a phase less than 0.2 π * Radian. In this formula:

• cd is the capacity distributed between the resistive coating and the conductive deposit at the potential U _jet in F / m, given by the insulating layer 15,
• rd is the distributed resistance of the resistive coating in Ω / m,
• ω is the pulsation of the fractionation.

The value of cd is around 150 nF / m and that of rd is 2.5 GΩ / m. For a frequency of 100 kHz, we obtain a penetration range of 78 µm.

In static (at zero pulsation), the entire resistive coating is at the static potential U ₂ and makes it possible to obtain a significant static deflection to adjust the inclination of the jet.

At very high frequencies, the equivalent dynamic potential width of the electrode is that of the conductor at potential U ₂ . This width is defined to obtain the maximum of the fractionation for the highest frequency for the formation of the drops, at least for the smallest distance between two consecutive future drops.

To increase the efficiency of fractionation, it is advantageous to create discontinuities upstream of the conductive deposits at potential U ₂ . Thus, downstream of these deposits, the signal U ₂ (t) on the resistive coating is delayed over time, and thus the peaks of this signal accompany the liquid of the jet. This phenomenon cannot occur in the opposite direction to the jet, and quickly the signal on the upstream resistive deposit reduces the efficiency of the fractionation. Several sequences are provided associated with the conductors at potential U ₂ spaced by one or more distances between two drops. This principle of fractionation makes it possible to effectively stimulate for different ranges of frequency of formation of the drops.

Two modes of control of the dynamic stimulation of the jet can be specified. A first mode is similar to the process described in US-A-4,220,958. Its principle is to use a "pump electrode" adjacent to the column of fluid, connected to a source of electrical energy to establish an electric field. variable, developing a normal force on the fluid column, to cause the formation of drops at substantially constant spacing. As shown in FIG. 6, the length of an electrode for applying the potential U ₂ (t) is approximately half a spacing between drops. The period of the voltage U ₂ (t) is that of the formation of the drops.

According to the present invention, the effective length of the electrode establishing a variable electric field for developing a normal force on the jet is also of the order of half a spacing between drops. However, this effective length of the electrode is achieved by summing a fixed conductive electrode to which is added a diffuse length linked to the propagation of the variable signal U ₂ on the resistive deposit coupled to the capacitive deposit.

The method according to the invention allows a certain adaptation of the effective length of the electrode establishing a variable electric field on the jet. For the same construction of the electrode, the variation of the effective length of the electrode as a function of the frequency of the signal U ₂ (t) makes it possible to effectively stimulate over a larger frequency band.

In US-A-4,220,958, as in the US-A-4,658,269, only one aspect is developed symmetrical of the normal forces around the jet. A second control mode of dynamic jet stimulation is described in patent US-A-5,001,497. According to the latter patent, an asymmetrical aspect of electrical force on the jet helps deflect it. This one meets then the surface of the collector-section for select the liquid "sausages" to print vis-à-vis the liquid to be recovered.

According to the present invention, a deflection of the jet is created by the action of the electric field emitted by the resistive electrode to stretch the jet in the inflection points of its trajectory. The surface tension continues the flow of the liquid from these inflection points to then give the future breaks between drops. The advantage of this control mode is to define dimensions of the electrode twice as large in the direction of advancement of the jet. If the design proposed in US-A-4 220 958 was half a drop spacing for its electrode, the present mode of attack of the jet requires a drop spacing. As shown in Figure 7, the period of the voltage U ₂ (t) is then double that of the formation of the drops. Under the reference 50, the inflection points of the trajectory of the ink jet have been shown.

The lithography technique used for the production of conductive and resistive electrodes to then be coarser. This provides an advantage since the width dimension of the conductive track must be smaller than the spacing between drops. You can choose a half-spacing between drops for the width of the conductive track, the resistive track taking over to transmit the potential U ₂ .

For a broken 80 kHz inkjet, at a speed 20 m / s, the spacing between drops is then 250 µm. The width dimension of the track is then of 125 µm. This value is easily obtained by conductive ink deposition screen printing techniques thick layer type of the electronics industry.

Claims (11)

1. Spraying control system for an electrically conducting liquid (3) emitted in the form of a pressurized jet (4) through at least one nozzle (2), comprising:

   means of separating the liquid jet into drops,

   means of electrically charging the said drops,

   means of applying an electrical deflection field to the said charged drops,

   a first (8) and a second (6) monolithic element each with a continuous surface, arranged so that the continuous surfaces are facing each other and define a space (5) between them in which the pressurized jet (4) is emitted through said nozzle (2), said elements (6, 8) including conductive means having electrical connection means at electric potentials permitting the continuous establishment of potentials on said continuous surfaces so as to obtain said electrical charging of the drops and said electrical deflection field,

   electronic control means (31 to 36) for said potentials and means of checking the intensity of electrical currents that can circulate on said continuous surfaces,

   characterized in that the continuous surface of the first element (8) is conductive and has electrical connection means to one (U₁) of said potentials, the continuous surface of the second element (6) is composed of one face of an insulating support (60), said face having conducting tracks (62 to 66) with means of making electrical connections at potentials chosen among said potentials (U₂ to U₆), with a resistive coating (67) having a square resistance mm between 5 MΩ and 100 MΩ, extending continuously over the said face.
2. Spraying control system according to claim 1, characterized in that the continuous surface of the first element (8) is also covered with a continuous resistive coating (82).
3. Spraying control system according to either of the claims 1 and 2, characterized in that said elements (6, 8) are also provided with means of separating the liquid jet into drops and inclining the jet.
4. Spraying control system according to claim 3, characterized in that the means used to separate and incline the said jet are means that can apply an electrical field to the said jet.
5. Spraying control system according to claim 4, characterized in that the means used to apply an electrical field to said jet include resistive means and capacitive means.
6. Spraying control system according to claim 5, characterized in that the capacitive means comprise the said resistive coating (67) supported by an insulating layer (15), this insulating layer acting as a dielectric and being supported by conducting means (11, 12, 13) supported by the said insulating support (60).
7. Spraying control system according to either of claims 5 and 6, characterized in that the resistive means are composed of part of the resistive coating (67).
8. Spraying control system according to claim 7, characterized in that said part of the resistive coating (67) has discontinuities in some portions (16) in order to increase the jet separation efficiency.
9. Spraying control system according to any one of claims 1 to 8, characterized in that the first (8) or the second (6) monolithic element supports at least one of the electronic control and checking means (31 to 36).
10. Spraying control system according to claim 9, characterized in that the monolithic element supporting at least one of the electronic control and checking means (31 to 36) comprises an insulating support identical to the electronic components support.
11. Application of the spraying control system according to any one of the previous claims to an inkjet printer.

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