EP1628832B1 - Inkjet printer - Google Patents

Abstract

The invention relates to an inkjet printer (10) comprising a print head (1) which is equipped with an internal stimulation system (31). According to the invention, the stimulation system can be used to create: (i) an upstream break in a break position (11) upstream of a jet (30), said break forming drops (33) which are used for printing and jet segments (38) in a zero potential area; and (ii) a break in the jet (30) or jet segment (38) in a downstream break position (12), said break forming drops (43) which are recovered in a non-zero potential area. A sorting system (35) which is common to all of the jets (30) of the head can be used to simplify the head and to reduce the size thereof.

Description

TECHNICAL AREA

The invention is in the field of printheads and continuous ink jet printers. It also relates to a selective projection process parts of a conductive ink jet and in particular a continuous inkjet printing process. The method and printer according to the present invention can be used in all industrial fields related to writing, including marking, coding, addressing the personalization and decoration of industrial products.

STATE OF THE PRIOR ART

The typical operation of a continuous jet printer can be described as follows. Electrically conductive ink maintained under pressure escapes from a calibrated nozzle. Under the action of a periodic stimulation device, the ink jet thus formed is broken at regular time intervals into a single position of the space. Downstream of the breaking position of the jet, the continuous jet is transformed into a train of identical and regularly spaced ink drops. In the vicinity of the breaking position is placed a first group of electrodes whose function usually recognized is to transfer selectively and each drop of the jet a predetermined amount of electric charge.

The set of drops thus selectively loaded then passes through a second arrangement of electrodes in which a constant electric field prevails which will modify the trajectory of the charged drops.

In a first variant of deviated continuous jet printer, the amount of charge transferred to the drops of the jet is variable depending on the value of an electric potential applied to a charging electrode located in a drop formation zone. The potential applied to the charging electrode is determined according to the write command. This potential is in correspondence with the intended destination of the drop on the substrate or in a recovery gutter if the drop is not intended for printing, for the drop that will pass into the electric field determined by the potential of the electrodes charge. Another way to modify the value of the electric charge attributed to each drop is described in the patent application. FR 2471278 corresponding to the patent US 4346387 , consists in creating an electric charge field for example increasing in an axial direction of the jet and controlling the point of formation of the drops so that the potential of the breaking point is as in the previous case in correspondence with the intended destination drop on the substrate or in a recovery gutter if the drop is not intended for printing. Each drop records, as it passes through the second arrangement of constant-field electrodes, an increasing deflection with the electrical load previously assigned to it and is oriented towards a specific point of the print medium or towards the salvage gutter. This technology, thanks to its multiple levels of deflection, allows a single nozzle to print, by segment or frame, - point line of a given height -, the entirety of a pattern. The passage from one segment to another is effected by the continuous displacement, perpendicularly to said segment, of the substrate relative to the print head.

The second variant is that of the binary continuous stream. This technique differs mainly from the previous one in that the level of charge drops is binary. When passing through the deflection electrodes, drops are uniformly deflected or undirected depending on the load they have received. The printing of characters or patterns therefore generally requires the use of multi-nozzle print heads, the spacing of the orifices coinciding with that of the impacts on the print medium. It should be noted that in general the drops intended for printing are non-deflected drops, that is to say, whose binary level of charge is zero.

In both technologies, that of the deviated continuous jet and that of the binary continuous jet, the ink which is not used to mark the substrate is directed to an unused gutter or ink recuperator and is recycled in a circuit which is ink so that it returns to the print nozzles.

A method for breaking down the jet into drops is very well described, for example, in a patent bearing the number US Patent 4,220,958 whose inventor is Mr. CROWLEY. According to the method described by CROWLEY, the inkjet conductive passes through electrodes worn periodically at a relatively high potential. Under the action of these electrodes, the ink jet is charged. The charges are attracted to the electrodes so that a force transverse to the jet deforms the surface of the jet. The axial velocity of the jet and the transverse movement of the surface of the jet combine so that at a certain distance from the electrodes, the jet breaks in a succession of drops.

In the description of the prior art to his invention, CROWLEY cites a patent of Richard G. SWEET bearing the number US Patent 3,596,275 . According to this quote, an important point of an inkjet printer is the generation of drops. It is preferred that the drops are generated at a fixed frequency with constant mass and velocity. To achieve this goal, SWEET reveals three techniques that are represented at figures 1 , 2 and 10 of his patent.

According to a first technique, the ink emission nozzles are vibrated. According to a second technique, the liquid jet is excited electro-hydrodynamically with an electro-hydrodynamic exciter (EHD). A third technique is to impose a pressure variation on the liquid at the nozzle by means of a piezoelectric crystal introduced into a cavity for feeding the nozzle. This latter technique is dominant in the literature and is used for example in the IBM 6640 machine (registered trademark).

Compared with this state of the art, the invention of CROWLEY concerns an electro-hydrodynamic exciter wherein the length of the electrodes traversed by the ink jet is equal to half the distance between the drops.

Another method of stimulating the ink jet for its transformation into drops is described, for example in the patent US-A-4,638,328 DRAKE et al. . It is an activation by thermoresistive elements.

A second family of printing ink projection called drop on demand, is mainly implemented in office printers. This is to print text or graphic patterns in color on paper or plastic. In contrast to continuous jet printing, drop-on-demand technologies generate directly and only the ink drops actually needed to print the desired patterns. There is therefore no recirculating electrode or gutter ink between the outlet face of a nozzle and the print medium. These printers comprise a plurality of nozzles, each nozzle is associated with a stimulation device having the dual function of expelling a drop (kinetic energy) and controlling its formation (profile of the drop). This stimulation device, which is activated on demand by an electrical signal, has two main variants:

The "Bubble Jet" technology initially developed by Canon and Hewlett-Packard is mainly implanted in the field of office automation. A heating element placed in a duct produces the vaporization of the ink locally, the growth of the gas bubble produces the expulsion of a small drop of ink towards the print media.

The "piezoelectric" technology is based on the deformation of a piezoelectric ceramic to create an overpressure and thus project drops of ink. The fields of application of this technology concern office automation (Epson) or industrial printing (Trident, Xaar, Spectra).

The dot density offered by these printers of the order of 600 dots per inch results from the use of materials and manufacturing techniques developed for the microelectronics industry.

• une gamme d'encrë utilisable plus étendue et par conséquent une plus large variété de supports imprimables,
• une fréquence d'émission des gouttes plus élevée et donc une vitesse d'impression accrue (environ 100 kHz et quelques mètres par seconde contre environ 10 kHz et quelques centimètres par seconde),
• une distance d'impression de la face inférieure de la tête d'impression jusqu'au support supérieur (environ 10 à 30 mm contre 1 mm).

In the field of industrial printing, the performance of continuous ink jet printheads outperforms the capabilities of drop-on-demand models. The first offer:

• a wider range of usable ink and therefore a wider variety of printable media,
• a higher transmission frequency of the drops and therefore an increased printing speed (about 100 kHz and a few meters per second against about 10 kHz and a few centimeters per second),
• a printing distance from the underside of the print head to the upper support (approximately 10 to 30 mm against 1 mm).

• ■ de réalisation et de coût : la multiplication des circuits électroniques à haute tension reliés aux électrodes de charge et la multiplication de ces mêmes électrodes de charge induisent une commande électronique complexe et coûteuse,
• ■ d'utilisation et de performance : la connectique haute tension très dense à proximité du jet provoque des diaphonies indésirables dont l'effet sur la qualité d'impression ne peut être limité que par une réduction du taux d'utilisation des gouttes, et par conséquent, une réduction de la vitesse d'impression, et/ou une diminution de la résolution.

However, the simplicity of designing drop-on-demand printheads is not found in multi-binary continuous jet printers. Electrodes dedicated to the charge of the drops Each jet must be controlled individually, at the drop formation frequency and at voltage levels up to 350 volts. The fabrication and the juxtaposition at a very fine pitch of all the nozzles and the electrodes of a print head then reveal major problems:

• ■ realization and cost: the multiplication of high voltage electronic circuits connected to the charging electrodes and the multiplication of these same charging electrodes induce a complex and expensive electronic control,
• ■ of use and performance: the high-density high-voltage connection near the jet causes unwanted crosstalk, whose effect on the print quality can only be limited by a reduction in the drop utilization rate, and by therefore, a reduction in print speed, and / or a decrease in the resolution.

• Une première invention ayant pour inventeur Vago est décrite dans la demande de brevet EP 949077 ou US 6,273,559 prévoit un dispositif de stimulation fonctionnant à une fréquence F, et piloté par deux niveaux de tension. Selon la tension appliquée au dispositif de stimulation, le point de brisure du jet se produit en un point C ou en un point L. Avant d'aller plus loin, il convient de savoir ce qui suit.

Considérons un jet soumis à une stimulation périodique, ce dernier se brise en un train de gouttes avec une période spatiale appelée longueur d'onde. A l'intérieur d'une longueur d'onde peuvent se former plusieurs gouttelettes qui accompagnent la goutte principale (celle de plus fort volume). Dans le métier du jet d'encre ces gouttelettes secondaires sont appelées satellites. Cette notion se démarque sans ambiguïté du terme de tronçon lequel désigne des portions continues de jets comportant au moins deux longueurs d'ondes.
Pour cette première invention de Vago, la différence de niveau de tension appliquée aux moyens de stimulation est telle que les points de brisure du jet C et D sont séparés l'un de l'autre par une distance qui est strictement inférieure à la longueur d'onde du jet. Le point de brisure C est à une position où existe un potentiel égal à celui de l'encre, en sorte que les gouttes formées en C ne sont pas chargées. Ces gouttes non chargées ne sont pas déviées ultérieurement par des électrodes de déviation et vont imprimer le substrat d'impression. Le point de brisure L est à une position où existe un potentiel différent de celui de l'encre, en sorte que les gouttes formées en L sont chargées. Ces gouttes chargées sont déviées ultérieurement par les électrodes de déviation et sont dirigées vers une gouttière de récupération pour être recyclées dans le circuit d'encre. Le point C se trouve sensiblement à mi distance entre des jeux d'électrodes amont et aval portés à des potentiels égaux et de signe opposé. La distance C-L est trop courte pour créer des tronçons.In order to preserve the advantages of the binary continuous stream while overcoming the disadvantages, an alternative is to use a system of charging and deflecting drops common to all streams.

• A first invention invented by Vago is described in the patent application EP 949077 or US 6,273,559 provides a stimulation device operating at a frequency F, and controlled by two voltage levels. Depending on the voltage applied to the stimulation device, the jet breaking point occurs at a point C or at a point L. Before going any further, you should know the following.

Consider a jet subjected to periodic stimulation, the latter breaks into a train of drops with a spatial period called wavelength. Within a wavelength can form several droplets that accompany the main drop (the one of higher volume). In the ink jet business, these secondary droplets are called satellites. This concept stands out unambiguously from the section term which denotes continuous portions of jets having at least two wavelengths.
For this first invention of Vago, the difference in voltage level applied to the stimulation means is such that the break points of the jet C and D are separated from each other by a distance which is strictly less than the length of the current. wave of the jet. The breaking point C is at a position where there is a potential equal to that of the ink, so that the drops formed at C are not charged. These uncharged drops are not deflected later by deflection electrodes and will print the printing substrate. The breaking point L is at a position where there is a potential different from that of the ink, so that the drops formed in L are charged. These charged drops are later deflected by the deflection electrodes and are directed to a recovery gutter for recycling into the ink circuit. Point C is located approximately halfway between upstream and downstream electrode sets brought to equal potentials and opposite sign. The CL distance is too short to create chunks.

Patent Application No. FR 2 799 688 having in the USA the deposit number 09/685 064 of 10/10/00 object of a second invention of Vago, the publication in the newspaper Xerox disclosure ( Pincus - 1982, vol.7, p.23 ) describe a charging and sorting system based on a set of electrodes carried at constant potentials. The fragmentation of the jet is in the set of electrodes and preferably in front of a well identified electrode according to whether the jet portion is to be printed or collected by the gutter. During operation, the jet is in the form of a succession of drops electrically insulated, that is to say without embedded electrical load, physically separate, framed by electrically charged sections which are deflected to the gutter. The generation of isolated drops (of zero electrical charge) is triggered by an undescribed intermittent pacing system. In a manner known in itself, the intermittent stimulation of a jet can be provided by an ElectroHydroDynamic actuator (patent US 4,220,958 - Crowley ) or thermal ( US 3,878,519 - Eaton ). In both cases, it is called external stimulation techniques because they consist in acting on an already formed jet. An external stimulation technique easily makes it possible to form an isolated droplet in a jet inasmuch as the liquid flows in front of the stimulation device whose range of action is short-range, two configurations are presented.

In the absence of a stimulation signal, the jet is not disturbed and remains continuous until the natural breaking position.

The application of a stimulation signal selects a perfectly defined jet portion whose length depends only on the speed of advance of the jet and the duration of the excitation signal. Under the effect of surface tension, the appropriately selected length of stimulated jet will produce an isolated drop in the continuous stream.

In the second invention of Vago, the breaking position of the continuous jet, to form a drop on demand is placed in an area where an electrode common to all the nozzles of the print head maintains a potential equal to that of the ink in the print head. A charging electrode is placed downstream of this breaking position. As long as the jet is not broken, because the ink used is conductive, a jet portion placed downstream of the break position is in the area of influence of the charging electrodes. On the other hand, when the drops are formed before passing through the electric field of the charging electrodes, they are electrically insulated and do not charge.

These unloaded drops formed on demand are not deflected by deflection electrodes placed downstream of the charge electrodes. They will print the printing substrate. The sections that are loaded, are deflected by the deflection electrodes to a recovery gutter. In the second of Vago, the writing command of a drop does not as in continuous jet printers, at charging electrodes, placed in the ink flow downstream of the ink ejection nozzles but at the level of the stimulation means upstream of these ink jet nozzles. nozzles. Such a device in which the perturbation of droplet formation in the jet is performed upstream of the nozzle is said to internal stimulation. Vago's first and second inventions thus combine the advantages of drop-on-demand printing with those of the continuous stream.

STATEMENT OF THE INVENTION

• une suppression pour chaque jet du jeu d'électrodes individuelles de charge des gouttes et du circuit de commande associé à ce jeu d'électrodes individuelles.

The present invention aims, like the first and second inventions of Vago, to combine the advantages of drop-on-demand printing with those of the continuous jet. These benefits include:

• a suppression for each jet of the set of individual electrodes for charging the drops and the control circuit associated with this set of individual electrodes.

An application of the digital data defining the pattern to be printed no longer downstream of the nozzles, but upstream, at the level of the jet stimulating means. It is these data that will determine or not the formation of the drops used for printing.

This improves the quality of printing by eliminating the cross-talk by electrostatic coupling between the different jets of the same print head. In addition, the manufacturing is simplified and it decreases the overall size of the printheads.

The invention also aims at these advantages but with improvements which will be described below.

In the device described in the second invention of Vago, the charging electrodes must create a charging field in a separate area of the protection zone reserved for the drops intended for printing, at most the diameter of a drop. In this way the shortest sections whose length is about two drop diameters, have before break, a portion located in the charging zone and can be charged. In addition it is preferable that the charging electrodes have a zone of influence whose length in the direction of the axis of the jet, is large enough to ensure a charge of a section proportionally to the length of said section, and therefore to its mass. In this way the sections of different length and therefore of different masses, are all deviated in an identical manner and an inlet port of the recovery gutter can keep a reasonably small size, while ensuring the recovery of all sections regardless their length.

The present invention also aims at a better control of inkjet parts not intended for printing. It also aims to simplify the manufacture of the printheads by loosening the tolerances on the position of the electrodes common to all the nozzles of the head. It also aims at increased compaction of the overall dimensions of the print head, and a greater printing distance.

According to the invention, instead of breaking the jet, only to create the drops necessary for printing, the jet then being divided into drops and jet sections, it is also broken in a regular and controlled manner to create drops that will be for example, electrically charged and deflected by deflection electrodes. For this, the jet stimulating means, intended to break the jet, are capable of causing the jet to break in two positions of the jet axially separated from one another, an upstream breaking position and a downstream breaking position, the latter being further downstream in the direction of advancement of the jet than the upstream position. At the upstream breaking position the jet will be broken intermittently to create the drops of ink that will be used for printing. Thus after the upstream breaking position the jet can be continuously from the nozzle, if no intermittent drop has been formed, or on the contrary divided into drop (s) and section (s) if one or more intermittent drops have been formed . The upstream breaking position will be, for example, in an area in which electrodes maintain a potential equal to that of the ink in the print head, so that the intermittent drops will not be electrically charged. The downstream breaking position is, in the example discussed here, in an area where charging electrodes maintain a potential different from that of the ink in the print head so that the continuous drops will be electrically charged. At the downstream breaking position, the jet is broken if there is no break intermittent at the upstream position, on the other hand if there has been a break in the upstream position, the resulting jet section is continuously divided into drops. Thus after the downstream breaking position, the jet is entirely divided into drops. Deflection electrodes located downstream of the two breaking positions then make it possible to sort the charged drops and the uncharged drops to send the ones to a recovery gutter and the others to a printing medium.

• une tête d'impression à une ou plusieurs buses ayant un corps de tête logeant notamment pour chaque buse,
• un chemin hydraulique de l'encre comportant, une chambre de stimulation en communication hydraulique avec l'une des buses d'impression émettant un jet d'encre sous pression selon un axe de cette buse,
• des moyens internes de stimulation du jet d'encre émis par la buse couplés mécaniquement à l'encre logée dans la chambre de stimulation, ces moyens agissant sur le jet émis par la buse pour briser le jet de façon contrôlée, et
• des moyens de récupération de l'encre qui n'est pas reçue par un substrat d'impression,
• un générateur de signaux électriques de commande recevant un signal de commande et délivrant aux moyens de stimulation, des signaux de stimulation,
• un agencement d'électrodes de charge définissant autour de l'axe de la buse des zones amont et aval, la zone aval étant plus éloignée de la buse que la zone amont, des électrodes amont et aval de cet agencement étant reliées à des sources de potentiel électrique de façon à maintenir dans l'une des zones un potentiel égal à celui de l'encre se trouvant dans le corps de la tête d'impression, et dans l'autre de ces zones un potentiel différent de celui de l'encre se trouvant dans le corps de la tête d'impression,
• un agencement d'électrode de déflexion situé axialement en aval de l'agencement d'électrode de charge

caractérisée en ce que le générateur de signaux électriques de commande délivre aux moyens de stimulation des signaux provoquant la brisure contrôlée du jet de façon intermittente en une position de brisure amont située dans la zone amont pour former de façon intermittente une goutte, séparant ainsi le jet en une goutte et un tronçon et provoquant également la brisure contrôlée du jet ou de tronçons du jet de façon continue en une position de brisure aval, le jet continu émis par la buse étant ainsi transformé après la zone aval en un train continu de gouttes d'encre électriquement chargées et non chargées.Thus, the invention relates to an ink jet printer comprising:

• a print head with one or more nozzles having a head body housing in particular for each nozzle,
• a hydraulic ink path comprising a stimulation chamber in hydraulic communication with one of the printing nozzles emitting a jet of ink under pressure along an axis of this nozzle,
• internal means for stimulating the jet of ink emitted by the nozzle mechanically coupled to the ink housed in the stimulation chamber, these means acting on the jet emitted by the nozzle to break the jet in a controlled manner, and
• means for recovering the ink which is not received by a printing substrate,
• an electrical control signal generator receiving a control signal and delivering stimulation signals to the stimulation means,
• an arrangement of charge electrodes defining upstream zones around the axis of the nozzle and downstream, the downstream zone being further from the nozzle than the upstream zone, upstream and downstream electrodes of this arrangement being connected to sources of electrical potential so as to maintain in one of the zones a potential equal to that of the ink in the body of the print head, and in the other of these areas a potential different from that of the ink in the body of the print head,
• a deflection electrode arrangement located axially downstream of the charge electrode arrangement

characterized in that the electrical control signal generator supplies the stimulation means with signals causing the jet to be intermittently intermittently broken into an upstream break position in the upstream zone to intermittently form a drop, thereby separating the jet. in a drop and a section and also causing the controlled breaking of the jet or sections of the jet continuously to a downstream breaking position, the continuous jet emitted by the nozzle thus being transformed after the downstream zone into a continuous stream of drops of drip. electrically charged and uncharged ink.

The electrical control signal generator can be physically separated from the printer head. He can also be part of it physically. In the latter case, the invention also relates to the printer head.

In one embodiment, the printer or printer head according to the invention is characterized in that the upstream electrode of the charge electrode arrangement is connected to the same potential as the ink.

Thus, in this embodiment, the charged drops are those resulting from breaking the jet or sections of the jet in the downstream zone. They are deflected by the arrangement of deflection electrodes to the ink recovery means. Each period of the periodic signal creates a mechanical reaction of the stimulation means, this reaction causing the breaking of the jet or sections of the jet in the downstream zone. Each intermittent pulse of the pulse signal creates a mechanical reaction of the stimulation means causing the breaking of the jet in the upstream zone in a drop and a section. In a manner known per se, the charged drops could be directed towards the printing substrate and the uncharged drops to the ink recovery means. In this case, it suffices for the upstream breaking position, where the drops intended for printing are formed, to be in an area where an electrode arrangement maintains a potential different from that of the ink, whereas the potential maintained in FIG. downstream zone is at a value equal to that of the ink.

In one embodiment, the printer or the printer head according to the invention is characterized in that the stimulation means comprise a piezoelectric material, the generator of electrical control signals delivering to the stimulation means a signal of continuous printing formed by a periodic signal of period Tb, intermittently replaced by a pulse signal preceded and followed by transition signals.

In one embodiment, the printer or the printer head according to the invention is characterized in that the pulse signal delivered by the electrical control signal generator is constituted by a pulse comprising 3 consecutive voltage stages connected to the one to the next by a steep rising or descending front.

In one embodiment, the printer or the printer head according to the invention is characterized in that the pulse signal delivered by the electrical control signal generator is constituted by a succession of 3 rectangular pulses separated from each other by Lower level voltage levels at the lowest level pulse.

In one embodiment, the printer or the printer head according to the invention is characterized in that the periodic signal delivered by the electrical control signal generator is constituted by a signal whose spectrum is constituted by two lines of a first frequency and a line at a second double frequency of the first, other possible lines of the spectrum having coefficients much lower than the coefficients associated with the lines of the first or second frequency, for example a signal resulting from a combination of two signals sinusoidal. The periodic signal delivered by the electrical control signal generator may also consist of a combination of more than two sinusoidal signals.

In one embodiment, the printer or the printer head according to the invention is characterized in that the sum of the durations of the pulse signal and the transition signals delivered by the electrical control signal generator is equal to an integer number of periods of the periodic signal.

In one embodiment, the printer or the printer head according to the invention is characterized in that a Helmholtz frequency of a portion of a hydraulic path of the feed ink of a nozzle located in downstream of a restriction has a value located outside a bandwidth of the jet from this nozzle.

In one embodiment, the printer or the printer head according to the invention is characterized in that the hydraulic path of the ink has a restriction and in that the length of a hydraulic path between an inlet of the restriction and the nozzle is less than a quarter of the wavelength of the sound in the ink.

In an embodiment intended to avoid the creation of unwanted breaks, that is to say to avoid the formation of droplets between the drops that are actually wanted to form and the other parts of the jet or jet sections, the printer or the printer head according to the invention is characterized in that the system for stimulating a jet emitted by a nozzle is strictly non-resonant, ie the transfer function of the stimulation system is free of resonance peaks in the bandwidth of the jet. It is recalled that the transfer function of the stimulation system is defined as the relationship existing between the pressure induced by the action of the piezoelectric element and the speed modulation introduced into the jet ejection speed. The stimulation system includes therefore not only the stimulation means but also the hydraulic path of the ink in the body of the print head.

Explanations will be given later on how to obtain such a result.

In one embodiment, the printer or the printer head according to the invention is characterized in that the stimulation means comprise, in addition to the piezoelectric material, a membrane which is mechanically coupled to it, a resonance frequency of one vibrating element formed of the membrane and the piezoelectric material being greater than a cutoff frequency of the jet.

• on fractionne en outre le jet ou les tronçons résultant du fractionnement du jet en premières gouttes et tronçons, en secondes gouttes, les secondes gouttes résultant de ce dernier fractionnement étant dirigés vers la gouttière.

Finally, the invention also relates to a method of printing a support by means of a printer according to the invention in one of its embodiments in which an ink jet emitted by a nozzle of the printer is intermittently forming first drops that will strike the substrate to form points, and sections,
characterized in that

• the jet or the sections resulting from the splitting of the jet are further fractionated into first drops and sections, in second drops, the second drops resulting from the latter fraction being directed towards the gutter.

BRIEF DESCRIPTION OF THE DRAWINGS

• la figure 1 est un schéma en perspective destiné à expliquer le mode de fonctionnement d'une imprimante à jet d'encre selon l'invention ;
• la figure 2 comporte les parties a et b. La partie a est un schéma montrant le mode de brisure du jet en situation de non impression, la partie b est un schéma montrant le mode de brisure du jet en situation d'impression ;
• la figure 3 comporte les parties a à g. Chacune des parties montre une étape du mode habituel de brisure du jet ;
• la figure 4 comporte les parties a et b. Les parties a et b sont des graphiques portant en ordonnée des valeurs de tension et en abscisse des valeurs de durée, montrant chacun un exemple de signal impulsionnel pouvant être appliqué aux moyens de stimulation afin d'obtenir une brisure intermittente du jet ;
• la figure 5 comporte les parties a à d. Les parties a à d sont des graphiques portant en ordonnée des valeurs de tension et en abscisse des valeurs de durée, le graphique en partie a est un exemple de signal qui peut être appliqué aux moyens de stimulation afin d'obtenir une brisure sans défaut du jet en situation de non impression ; le graphique en partie c est un exemple de signal qui peut être appliqué aux moyens de stimulation afin d'obtenir une brisure sans défaut du jet en situation d'impression ; les graphiques des parties b et d représentent chacun un état logique d'un signal de commande d'impression ;
• la figure 6 est un exemple d'une coupe d'une tête d'impression faisant apparaître le chemin de l'encre dans un corps de cette tête ;
• la figure 7 est un graphique faisant apparaître la fonction de transfert d'un exemple de système de stimulation. Il comporte en abscisse la perturbation de vitesse apportée localement au jet en fonction de la fréquence d'une stimulation mécanique présente dans le circuit d'encre en amont de la buse.

Additional explanations and an example of how to make a printer or a head in accordance with the invention will now be given in conjunction with the accompanying drawings, in which

• the figure 1 is a perspective diagram for explaining the mode of operation of an ink jet printer according to the invention;
• the figure 2 includes parts a and b. The part a is a diagram showing the jet breaking mode in a non-printing situation, the part b is a diagram showing the jet breaking mode in a printing situation;
• the figure 3 includes parts a to g. Each of the parts shows a step of the usual jet breaking mode;
• the figure 4 includes parts a and b. Parts a and b are graphs plotted on the ordinate of the voltage values and on the abscissa of the duration values, each showing an example of a pulse signal that can be applied to the stimulation means to obtain intermittent breaking of the jet;
• the figure 5 includes parts a to d. The parts a to d are graphs plotting the voltage values on the ordinate and the duration values on the abscissa, the part-a graph is an example of a signal that can be applied to the stimulation means in order to obtain a faultless breaking of the duration values. jet in a non-printing situation; the graph in part c is an example of a signal that can be applied to the stimulation means in order to obtain a faultless break of the jet in a printing situation; the graphics of parts b and d each represent a logic state of a print control signal;
• the figure 6 is an example of a section of a print head showing the path of the ink in a body of this head;
• the figure 7 is a graph showing the transfer function of an example of a stimulation system. It comprises on the abscissa the speed perturbation provided locally to the jet as a function of the frequency of a mechanical stimulation present in the ink circuit upstream of the nozzle.

DETAILED PRESENTATION OF PARTICULAR EMBODIMENTS

The figure 1 shows schematically and in perspective the parts of a printer concerned by the invention. In this figure, the means of transport of the printing medium have not been represented in particular. This figure is essentially intended to explain the operation of a printer based on the present invention

In the embodiment shown, the printer 10 comprises one, as shown, or several print heads 1. On the figure 1 there is shown a head 1 having 3 nozzles 29 ejection of an ink jet 30. In reality the number of nozzles is much larger. For each of the nozzles, a body 23 of the print head comprises in particular a hydraulic path of the ink and a stimulation chamber 28 which will be described in more detail later in connection with the figure 6 . Each stimulation chamber 28 is, in itself known manner, constantly filled with an ink maintained at constant pressure by a supply of pressurized ink 27. Each stimulation chamber 28 comprises means Stimulator 31 each formed by a piezoelectric element 25 and a membrane 24. A generator 32 of control signals of the stimulation means 31 is connected to each of the piezoelectric elements 25. Control signals IMP for each of the means stimulation 31 are received by the circuit 32 preferably, as shown figure 1 , on a parallel bus comprising a path for each means 31. An ink supply circuit common to the chambers 28 is symbolized in this figure by arrows 14 showing that ink drops 43 formed at a downstream jet breaking position. 30 or sections 38 of this jet are recovered in a gutter 40 common to all of the nozzles of a head and directed to suction and pressurizing means symbolized by a block 13. Such an ink circuit feeding with pressurized ink 16 each of the chamber inlets 27 is itself known.

The pressure exerted on the ink is large enough to cause an ink jet 30 to be ejected through each ink ejection nozzle 29 at an average speed V i. A nozzle 29 has a section whose equivalent radius is equal to 'a', which is also approximately the radius of the jet 30. The stimulation device 31, controlled by the electrical signal generator 32, makes it possible to create a disturbance at the inside the chamber 28, causing the breaking of the jet 30 into drops 33, 43. In accordance with the invention, the electrical stimulation signals are such that they cause intermittently, intermittently, a breakage of the so-called intermittent jet in a first axial position 11, and secondly a second breaking of the jet at a second axial position 12 downstream of the first, said continuous breaking. The drops 33 are the drops resulting from the intermittent break and the drops 43 are the drops resulting from the continuous breaking. Examples of signals, capable of causing intermittent and continuous breaks, will be given later. A charge electrode 35 common to all the nozzles 29 is located downstream of the nozzles 29, in the direct vicinity of the axes of the nozzles 29. In the example described here, the charging electrode 35 is formed by a stack of two conductive materials. electricity 34, 37, separated by a layer 36 made of an electrically insulating material. The conductor 34 is the most upstream, the conductor 36 is the downstream of the charging electrode 35. The conductor 34 is connected to the same potential as the ink in a chamber 28, in general the zero potential of the mass electric. The conductor 36 is connected to a non-zero electrical potential Vc, different from that of the ink in a chamber 28. Downstream of the charging electrode is in the direct vicinity of the axes of the nozzles an electrode assembly 39 deflection. The set 39 of deflection electrodes is common to all the nozzles 29 of a head and is connected to a source of potential so that a uniform electric field E0, whose component perpendicular to a plane containing the axes of the nozzles 29 is preponderant. A recovery gutter 40 common to all the nozzles and located downstream of the set 39 of deflection electrodes and outside the axes of the nozzles 29 is used in a known manner to recover ink that is not used for printing. The ink used for printing is directed to a printing medium 41 on which each printing drop 33 forms a printing dot 58.

The operation of the print head is as follows.

In the example here commented the drops 33 are the drops that are used for printing. The drops 33 result from the intermittent breaking of the jet creating an isolated drop, called intermittent drop 33. The electric charge of the intermittent drops 33 is almost zero because they are formed at the first breaking position of the jet, facing the conductor 34 brought to the same potential that the ink in the chamber 28, in general the zero potential of the electrical mass. After intermittent breakage the jet 30 is split into the drop 33 and a jet section 38.

The drops 43 are those that are not used for printing. They are formed at the second breaking position, facing the conductor 37 of the charging electrode 35 brought to the non-zero electrical potential Vc, different from that of the ink in the chamber 28. The drops 43 embark electrostatically a greater electric charge in absolute value than the almost zero load embedded in the drops 33. The second breaking position 12 where the drops 43 are formed, is downstream of the first breaking position 11 where the intermittent drops 33 are formed. This break is called continuous downstream breaking of the jet sections 38, or jet 30 if intermittent breakage has not formed sections. All the drops which are detached from the jet then pass into the deflection zone defined by the deflection electrode 39. The ink drops 33, 43 passing through the deflection zone undergo an electrostatic force F = q.E0, where q is the electric charge of the considered drop. The intermittent drops 33, whose electric charge is almost zero, thus pursue an almost rectilinear trajectory along the axis of the nozzle 29, up to the printing medium 41. The trajectories of the drops 43 are deflected perpendicular to the axis of the jet according to their electric charge and terminate their trajectory in the recovery gutter 40, assuming that a judicious combination of electric potentials is applied to the charging and deflecting electrodes 35, 39. The ink collected in the gutter 40 is in known manner reinjected into the ink circuit to be reused.

The printing of a pattern results in a manner known per se from the selection of ink drops to be directed towards the printing medium 41 or to the trough 40 and a relative movement of the print medium 41 and the print head 1. In the above example commented, it is the uncharged drops, whose trajectory is not deflected, which are used for printing. This solution is generally preferred because the positioning accuracy of the drops contributing to the printing is greater, because the trajectory of these drops is shorter and less dependent on randomness relative to the exact mass drop, the value of the amount of electric charge on board and possible fluctuations of the deflection field. According to the invention, it is not excluded to use, as in some known embodiments deviated drops for printing, while the non-deflected drops are directed to the gutter.

One of the main advantages of the invention is that, as in the second invention of Vago, the set of charging electrodes 35, and of deflection 39 forming together a sorting system of the printing drops 33 and 43 of recovery, is common for all jets. However, since the sections 38 formed each time an intermittent drop 33 is formed are in a downstream position, also divided into drops 43, the groove 40 common to all the jets may be smaller in size because the guiding accuracy of the drops is improved.

The figure 2 is intended to illustrate the jet breaking modes to form the intermittent and continuous drops 43. On the figure 2 part a, one is in a phase where there is no printing, or in which there has been no intermittent break during the time taken by the jet to go from the upstream breaking position 11 to the downstream breaking position 12. In this case only a periodic signal breaks the jet continuously at the downstream position 12 to form the continuous drops 43. figure 2 , part b, there is shown the case where a drop 33 for example, is formed by a pulse of the break signal. In this case the jet 30 is split into a drop 33 and a jet section 38. This section carries the speed perturbation provided by the periodic signal. It is therefore broken at the downstream breaking position 12 to give continuous drops 43. Thus downstream of the downstream breaking position, the jet is entirely divided into drops 33 and 43.

Forms of electrical signals capable of causing, on the one hand, the intermittent breaking at the upstream breaking position 11, on the other hand the downstream continuous break at the downstream breaking position 12 and finally a combination of the break signals in the upstream and downstream positions. downstream will now be addressed.

It should be noted before that the intermittent break is a break designed to isolate a drop of a jet. This situation is different from the situation where a continuous train of drops is created, because in the case of the isolated drop, there is a tendency to the formation of satellite droplets and bulges which affect the quality of printing. To understand the interest of possible forms of the intermittent break signal it will be described below in connection with the figure 3 the breaking dynamics of an isolated drop corresponding to the invention in the case of the intermittent drop.

The figure 3 includes parts a to g. The sequence of the parts a to g shows a temporal succession of states of the intermittent breaking intended to make perceive the dynamics of the breaking. In a first stage represented in a, a speed perturbation brought by a temporary overpressure induced at the chamber 28 creates in the jet a belly 33a.

An intermittent drop 33 separates consecutively to two breaks: an upstream breaking 49 represented in part b by a space between the upstream part of the jet 30 and the downstream part, and a downstream breaking 50 represented in part c by a space between the drop 33 which at this stage is formed and the downstream portion of the jet 30 which thereby becomes a jet section 38. Upstream ligaments 51 and downstream 52 shown in parts b and c which respectively correspond to stretching of the upstream and downstream parts of the jet 30 with respect to the forming drop 33, may, if the stretch is large, give rise respectively to upstream and downstream satellite droplets 54 represented in part d. On part d we also see that the upstream and downstream parts of the jet on either side of the drop 33 in formation undergo swelling. As represented by the succession of states represented in part e, and f. These swellings of the ends of the jet and the jet section surrounding the forming drop 33, can also be detached to form ink drops 55, 56 shown in parts g. These ink drops 55, 56 upstream and downstream will be called subsequently upstream bead 55 and downstream bead 56. An upstream breaking length Lbam is defined as being the distance Lbam between the outlet face of the nozzle 29 and upstream breaking. 49, a downstream break length Lbav is defined as the distance Lbav between the outlet face of the nozzle 29 and the downstream break 50.

So that the beads 55, 56 are recovered in the channel 40, it is necessary that they embark a sufficient electrical charge, and therefore, they detach themselves sufficiently far downstream of the upstream and downstream breaks 49, 50 of the intermittent drop 33 to be at the moment of their detachment from the jet in the zone where there is a potential different from that of the potential of the ink in the room 29. That's why on the figure 3 Parts f and g show the beginning and the end of the detachment of the beads 55, 56 in the zone subjected to the influence of the electrode 37. Similarly, it is desirable for the upstream and downstream satellites 53, 54 to be absorbed. in other drops quickly, because they can cause significant soiling of the sorting system or even the print medium.

Any electrical signal applied to the stimulation device 31 and making it possible to obtain the characteristics of breaks such as the satellites and beads do not introduce printing defects as explained above, can be used to carry out the invention.

The figure 4 part a shows an example of electrical control signal that can be applied to the stimulation device 31 to control the shape of the intermittent breaks so as to ensure proper operation of the sorting between the printing drops 33 and the drops 43 to be recovered in the gutter 40.

The signal represented figure 4 part a consists of three consecutive levels of respective voltage levels U ₁ , U ₂ , and U ₃ , measured above a level U ₀ . The three levels have respective durations T ₁ , T ₂ , and T ₃ . Two consecutive levels are linked to each other by a steep rising or falling edge.

The durations T ₁ , T ₂ , and T ₃ of the three consecutive levels of voltage constituting the stimulation signal, are each close to a duration τopt. τopt is the duration of a rectangular pulse that would, if it were applied to the stimulation means 31, the shortest upstream intermittent breaking length, constant amplitude and for the same jet (same speed, same section, same ink) . τopt is a duration corresponding to a spatial disturbance of the jet of a length λopt / 2, where λopt is the optimal wavelength of the jet, ie the wavelength for which the coefficient of Amplification of capillary instability is maximal.

Like λopt ≅ 10.a for a viscous liquid, it follows that τopt = λopt / 2.Vj ≅ 5.a / Vj.

Recall that in the above formulas a is the equivalent diameter of the nozzle 29 which substantially corresponds to the diameter of the jet 30 and Vj is the ejection speed of the jet 30.

In the example commented in connection with the figure 4a the characteristic durations T ₁ , T ₂ , and T ₃ are chosen to be equal to each other, that is to say that we have: T1 = T2 = T3 ≅ τopt, thus the shape of the break obtained for the The formation of an intermittent drop 33 is stable, and therefore quite insensitive to small variations in jet velocity, viscosity, or other fluctuating jet properties.

In addition the principle of sorting the drops requires that the electrical charge on board by the drop intermittent 33 is, in this example, almost zero. However, the electrical charge actually carried by this drop depends on the geometrical configuration of the charging electrode 35, the electric potentials applied to the two conductors 34, 37 which constitute it, but also on the algebraic distance between the upstream and downstream intermittent fractures. (Lbav - Lbam).

The signal represented figure 4 part a allows to control this distance (Lbav - Lbam) between the two breaks forming an intermittent drop, so as to ensure a stable and well-defined trajectory of the drop to print.

The distance (Lbav - Lbam) between the upstream and downstream drop formation breaks can be adjusted by modifying certain parameters of the stimulation signal. In this embodiment, adjustment of the amplitudes U1, U2 and U3 of the bearings constituting the pulse signal makes it possible to adjust (Lbav-Lbam). More precisely, a decrease in the absolute value of the absolute difference | U1-U2 | between the voltage values of the first two stages has the effect of delaying the moment of the downstream breaking, and likewise a decrease of the absolute difference | U2-U3 | between the voltage values of the two last stages has the consequence of delaying the moment of upstream breaking. It is possible to choose T1 = 0 or T3 = 0, if one of the 3 levels of the signal is considered unnecessary by the skilled person, depending on the particular operation of the stimulation device considered. The signal presented makes it possible to correct the trajectory of the drop to be printed by empirically choosing the parameters of the signal which affect the distance (Lbav - Lbam) between the intermittent upstream break and the downstream intermittent break.

Another example of an impulse stimulation signal that can be used in one embodiment of the invention is described in FIG. figure 4 part b. This signal is composed of a succession of 3 rectangular pulses, a first of a duration D ₁ and of level U ₁ , a second of a duration T ₂ and of level U ₂ and a third of duration D ₂ and U level ₃ . The first and second pulses are separated from each other by a duration Tr ₁ , and the second and third pulses are separated from one another by a duration Tr ₂ . During the separation times between pulses the signal is at the base level U ₀ . If this signal is chosen in order to control the intermittent break, the durations are preferably T2 ≅ τopt; Tr1 = Tr2 ≅ τopt / 2; D ₁ and D ₂ close to τopt / 10 or τopt / 5 depending on the stimulation device to be controlled, τopt being defined as above. The distance between the upstream and downstream breaks of the intermittent drop 33 can then be adjusted by modifying U1 and / or U3: the instant of the downstream breaking is delayed when U1 / U2 increases, the moment of the upstream break is delayed when U3 / U2 increases.

It will now proceed to the description of a signal suitable for generating the breaking jet or jet sections in the second position called downstream position, producing the drops 43 which will be recovered by the channel 40.

The application of a simple sinusoidal signal would cause the creation of satellite droplets between the main drops 43 resulting from this break. In the embodiment described here, a continuous satellite-free break with a signal of sufficiently small amplitude to place the downstream continuous break in the vicinity of the charge conductor 37 is obtained by applying a two-mode signal, superposition of two sinusoidal signals of Fb and 2.Fb frequencies, amplitudes and phase shifts properly selected. The generated signal is of the form: $$\mathrm{Sb}\left(\mathrm{t}\right)\mathrm{=}\mathrm{ab}\mathrm{.}\left(\mathrm{<figure-callout\; id="2"\; label="sin"\; filenames="imgf0001.png"\; state="\{\{state\}\}">sin</figure-callout>}\left(\mathrm{2}\mathrm{\pi }\mathrm{.}\mathrm{Fb}\mathrm{.}\mathrm{t}\right)\mathrm{+}\mathrm{\alpha }\mathrm{.}\mathrm{<figure-callout\; id="4"\; label="sin"\; filenames="imgf0002.png"\; state="\{\{state\}\}">sin</figure-callout>}\left(\mathrm{4}\mathrm{\pi }\mathrm{.}\mathrm{Fb}\mathrm{.}\mathrm{t}\mathrm{+}\mathrm{\phi }\right)\right)$$

In formula (1) above Fb = 1 / Tb is the fundamental frequency of the continuous stimulation signal of drop formation 43. α> 0 is the relative amplitude of the second mode, and φ is its relative phase. Ab is a coefficient that determines the amplitude of the continuous stimulation signal for drop formation 43. The skilled person knows how to choose the values of the parameters α and φ to obtain a continuous break without satellite droplets. A signal as described above is represented figure 5 part a. It is a periodic signal of period Tb whose amplitude as a function of time is represented by the formula (1). If this signal is applied alone continuously, the jet is broken as shown figure 2 part a where only drops 43 are produced.

The combination of the generation signals of the drops 33 and 43 will now be explained. It will be examined successively the temporal combination of the two types of signals, their combination from the point of view of the relative amplitudes and finally a control mode for introducing a pulse signal into a succession of periodic signals.

From a temporal point of view, at least one period Tb of the periodic signal of continuous downstream stimulation is to obtain an intermittent drop replaced, for example, by the pulse control signal described in connection with the figure 4 part a. The combination of the pulse signal described in connection with the figure 4 part a and the periodic signal described in connection with the figure 5 part a is represented figure 5 part c. As the example of the figure 5 , part c, the total duration of the intermittent stimulation signal is equal to a value Ti. He is trained as shown figure 4 part a by a succession of three consecutive stages of respective duration T1, T2, and T3, T3 having in this example a zero duration, so that Ti = T1 + T2 + T3. As a rule, Ti ≠ n.Tb, n being an integer. In the embodiment chosen, the pulse stimulation signal is preceded by a downstream transition signal of duration tav, and followed by an upstream transition signal of duration tam. The durations of tav and tam are chosen so as to satisfy the condition tav + Ti + tam = n.Tb. In the example described in relation to the figure 5 in part c, the transition signals simply consist in maintaining the constant voltage between the interruption of the periodic signal of continuous stimulation and the beginning of the generation of the pulse signal. The times tav and tam are chosen so as to respect the integrity of the jet sections 38 on either side of the intermittent drop 33 to the zone of influence of the charge conductor 37 ( figure 1 ). The transition signals are also chosen so as to ensure the continuity of the electrical signal applied to the stimulation means 31 during the interruption and resumption of the generation of the periodic downstream continuous stimulation signal. Note that the transition signals can either or both have a zero duration.

The relative amplitudes of the periodic signal and the pulse signal, that is to say the relative values of Ab in the formula (1) defining the periodic signal and the value of U2 are chosen to correctly position the upstream and downstream broken positions. in the zones of influence of the charging electrode 35. The breaking lengths, that is to say the distance between the nozzle 29 and a breaking position, depend on the amplitude of the stimulation. To ensure effective separation of the drops 33 with respect to the drops 43, the distance between the intermittent breaking position 11 and the downstream continuous breaking position 12 must be sufficient, at least 20 times the radius of the jet. In the preferred embodiment, a distance between these two broken positions is close to 50 times the radius of the jet.

The generator 32 of electrical control signals adapted to generate on demand the pulse signal for creating an intermittent drop 33 and the periodic signal for continuous generation of drops 33 and connected for this purpose to the stimulation means 31, is in the mode described embodiment, controlled by means of a print command, for example a logic signal, for example a binary signal IMP represented on the Figures 5b and 5d . The signal IMP is a function of the data to be printed. When only the downstream continuous stimulation signal, of period Tb, is generated, the logic value of the boolean signal IMP remains at 0. It is this signal constantly at 0 which is represented figure 5b .

In the printing situation, the signal IMP goes to the value 1 for at least one period Tb, triggering the response of the electric control signal generator 32: thus, according to the preferred embodiment of the invention the signal generator 32 of control of the stimulation means 31 is able to combine a pulse-type signal and a periodic signal, by replacing an integer n of periods of the periodic signal by the pulse signal framed by transition signals.

Improvements that can be made to the print head according to the invention will now be discussed in connection with the Figures 6 and 7 which respectively represent an example of a section of a print head 1 showing the path of the ink in a body 23 of this head 1 and a graph showing on the abscissa the speed perturbation brought locally to the jet in function the frequency of a mechanical stimulation present in the ink circuit upstream of the nozzle.

The hydraulic path inside the body 23 of the print head 1 shown in section figure 6 along one or more xz planes, z being the direction of the jets 30 and x a direction perpendicular to z located in a plane perpendicular to the plane containing the axes nozzles 29 comprises, from upstream to downstream in the direction of flow of the ink, discrete functional elements. A reservoir 17 of pressurized ink 16 is in communication as represented by arrows 27 with an unrepresented ink supply line. The reservoir 17 is in communication with a narrow passage 18 called restriction. A first connecting tube 20 puts the restriction 18 in communication with the stimulation chamber 28. The stimulation chamber 28 is itself in communication with the nozzle 29 for forming the jet 30 by a second connecting tube 21. The nozzle 29 is pierced in a nozzle plate 22 which may comprise a plurality of nozzles aligned in a direction y perpendicular to the representation plane xz.

A wall portion of the chamber 28 is formed by a membrane 24 whose thickness along the Z axis is much smaller than its dimensions in the X, Y plane. On the outer face of the membrane 24, that is to say the one outside the chamber 28, is bonded a piezoelectric element 25.

When an electrical signal is applied to the piezoelectric element 25, the diaphragm pair 24 / piezoelectric element 25 which in this example forms the stimulation means 31 forms a vibrating element 31 which deforms in flexion, the effect of which is produce a modulation of the volume and the pressure in the chamber 28; this results in a modulation of the average ejection speed of the ink 16 at the nozzle 29. This type of actuator which is described in numerous patents was initially proposed by Silonics ( US-A-3,946,398 - Kyser & Sears ).

The need to form an isolated drop in a jet by the application of an intermittent signal as described figure 4 part a or b, and preferably to avoid the formation of satellite droplets such as 53, 54 described in connection with the figure 3 as well as the formation of a train of drops behind the isolated drop requires that the stimulation is strictly non-resonant. This means that the transfer function of the pacing system must be free of resonance peaks in the bandwidth of the jet 30. The transfer function of the pacing system is defined as the relationship between the pressure induced by the action of the pumping system. piezoelectric element 25 and the jet ejection velocity modulation 30.

The definition of the _jet jet bandwidth of jet 30 comes from the linear theory of capillary instability, the skilled person will find the following relation: $$\begin{array}{cc}{\mathrm{BP}}_{\mathrm{jet}}\in \left[0;{\mathrm{Fc}}_{\mathrm{jet}}\right]& {\mathrm{Fc}}_{\mathrm{jet}}=\frac{{\mathrm{<figure-callout\; id="2"\; label="V\; jet"\; filenames="imgf0001.png"\; state="\{\{state\}\}">V\; </figure-callout>}}_{\mathrm{jet}}}{2\pi R_{\mathrm{jet}}}\end{array}$$

• Vjet : vitesse du jet 30,par exemple 15 m/s
• Rjet : rayon du jet en sortie de buse 29, par exemple 15 µm.
• Fcjet = Fréquence de coupure du jet par exemple 160 kHz

For the digital application:

• Vjet: speed of the jet 30, for example 15 m / s
• Rjet: jet radius at nozzle outlet 29, for example 15 μm.
• Fcjet = Cutoff frequency of the jet eg 160 kHz

The stimulation system is capable of producing resonant frequencies F _R related to the mechanical and acoustic behavior of the device. To obtain a strictly non-resonant stimulation, it will be sought to place these resonant frequencies F _R at the outside of the jet bandwidth. Preferably, the following relation will be satisfied: $${\mathrm{F}}_{\mathrm{R}}\mathrm{>}\left(\mathrm{1}\mathrm{+}\mathrm{0.1}\right){\mathrm{Fc}}_{\mathrm{jet}}$$

For this we will seek to comply with one or more of the design rules below.

Mechanical and Acoustic Resonance (Design Rule # 1)

• L_M : terme d'inertie équivalent à une self en analogie électrique.
• C_M: terme d'élasticité équivalent à une capacité en analogie électrique.

The vibrating element 31 has a specific resonance frequency F _M which depends mainly on its geometry and the mechanical properties of the materials that compose it. $${\mathrm{F}}_{\mathrm{M}}\mathrm{=}\frac{\mathrm{1}}{\mathrm{2}\mathrm{\pi }\sqrt{\left({\mathrm{The}}_{\mathrm{M}}\mathrm{*}{\mathrm{VS}}_{\mathrm{M}}\right)}}$$

• L _M : term of inertia equivalent to a self in electrical analogy.
• C _M : elasticity term equivalent to a capacity in electrical analogy.

With the nominal values indicated in a dimension and material table in the appendix 1, the resonant frequency of the vibrating element 31 is typically of the order of 400 kHz.

In the absence of propagation phenomenon, we will focus on the frequency of Helmholtz F _H calculated from the terms of inertia and elasticity (electrical analogy) of each discrete element constituting the stimulation device namely the restrictor, the chamber and the nozzle as well as hydraulic connecting elements between these components if they exist.

• L_R : terme d'inertie (analogie électrique) associé à la restriction 18.
• L_B : terme d'inertie (analogie électrique) associé à la buse 29.
• C_M : terme d'élasticité en analogie électrique de l'élément vibrant 31.

With the nominal values indicated in the dimension and material table, the Helmholtz resonance frequency which is typically of the order of 200 kHz is located outside the jet bandwidth. In the particular case of the values proposed in the table given in Appendix 1, the frequency of Helmholtz F _H is calculated from the following simplified expression which retains only the terms whose weight is preponderant: $$F_H=\frac{1}{2\mathrm{\pi }\sqrt{\left(\frac{{\mathrm{The}}_B{\mathrm{The}}_R}{{\mathrm{The}}_B+{\mathrm{The}}_R}\right)*{\mathrm{VS}}_M}}$$

• L _R : term of inertia (electrical analogy) associated with the restriction 18.
• L _B : term of inertia (electrical analogy) associated with the nozzle 29.
• C _M : elasticity term in electrical analogy of the vibrating element 31.

Acoustic Resonance with Propagation (Design Rule 2)

Acoustic propagation phenomena can produce resonance peaks when one of the characteristic lengths of the stimulation system is not negligible compared to the length λ of the acoustic waves in the ink 16. By way of example, the wavelength λ is typically 7.5 mm in a water-based ink, MEK or alcohol for a cut-off frequency of jet Fc _jet 160 kHz and for an average speed of sound, for example in the MEK, of 1200 m / s. We mean by length feature any dimension of the restriction 18, the chamber 28, the first and second connecting tubes 20, 21, the nozzle 29 and the total path of the ink 16 in the stimulation system since the entry of the restriction 18 until the nozzle outlet 29. Ideally all the characteristic lengths of the stimulation system will be lower λ / 4 to overcome the propagation of acoustic waves. The constraint in λ / 4 sets the maximum characteristic length at 1.8 mm. It is generally easy to satisfy the λ / 4 stress for the nozzle 29, the restriction 18 and the connecting tubes 20, 21 as indicated in the dimension and attached material table. For the chamber 28, this rule may not be respected, because we are looking for a large chamber area to obtain a good stimulation efficiency, in this case, it is essential to proceed with the modeling of the transfer function for s' ensure that there is no resonance in the bandwidth of the jet.

For a stimulation system with the nominal dimensions indicated in the dimension and material table, it appears that its transfer function whose curve is presented in figure 7 does not have a resonance frequency in the jet bandwidth, resonance peak 26 at 200 kHz associated with the Helmholtz frequency.

For a jet cutoff frequency of 160 kHz and for a stimulation system having the ratings given in the table in Annex 1, the first resonance is around 200 kHz, which satisfies the criteria and precautions listed, it is easy to verify that the stimulation is non-resonant and advantageously allows to form a drop in a continuous stream ( Figures 6 and 7 ).

Optimization of Stationary and Unsteady Flow (Design Rule 4).

Under the effect of the piezoelectric element 25, a pressure pulse pushes ink 16 towards the nozzle 29 and pushes ink 16 towards the restriction 18, in fact these two elements constitute, for the chamber 28 the two output points of the ink 16. In order to maximize the effectiveness of the stimulation ie the speed modulation at the nozzle 29, it is desirable to adapt the impedance of the nozzle 29 to that of the restriction 18 which has a high acoustic impedance. The efficiency of the stimulation is defined by the ratio R _imp of the impedances L _B of the nozzle 29 and L _R of the restriction 18: $${\mathrm{R}}_{\mathrm{imp}}\mathrm{=}\frac{{\mathrm{The}}_{\mathrm{R}}}{{\mathrm{The}}_{\mathrm{B}}}\mathrm{=}\frac{{\mathrm{1}}_{\mathrm{R}}}{{\mathrm{S}}_{\mathrm{R}}}\frac{{\mathrm{S}}_{\mathrm{B}}}{{\mathrm{1}}_{\mathrm{B}}}$$

• l_R : longueur de la restriction 18
• l_B : longueur de la buse 29 dans la direction Z
• S_R : section droite de la restriction 18
• S_B : section droite de la buse 29

In the formula above:

• l _R : length of the restriction 18
• l _B : length of the nozzle 29 in the Z direction
• S _R : right section of restriction 18
• S _B : straight section of the nozzle 29

• ΔP : chute de pression statique entre l'entrée et la sortie de la buse 29 ou de la restriction 18
• Q : débit volumique
• R : rayon de la buse 29 ou de la restriction 18
• 1 : longueur de la buse 29 ou de la restriction 18,
• µ est la viscosité dynamique de l'encre.

In the idea of maximizing R _imp , the intuitive solution of choosing I _R >> I _B and S _R << S _B is uninteresting because it requires an ink pressure in the reservoir 17 too large. Indeed, the formation of the continuous jet 30 requires a static ink pressure upstream of the restriction 18 which strongly depends on the viscous charge losses in the stimulation system and in particular in the nozzle 29 and the restriction 18 which are the two areas of higher ink flow velocity. The hydraulic resistance of the nozzle 29 or the restriction 18 is described, as a first approximation, by the Poiseuille law according to the following generic expression: $$\frac{\mathrm{.DELTA.P}}{\mathrm{Q}}\mathrm{=}{\mathrm{R}}_{\mathrm{Hydro}}\mathrm{=}\frac{\mathrm{8}\mathrm{<figure-callout\; id="1"\; label="\mu "\; filenames="imgf0001.png"\; state="\{\{state\}\}">\mu </figure-callout>}\mathrm{1}}{\mathrm{<figure-callout\; id="4"\; label="\pi \; R"\; filenames="imgf0002.png"\; state="\{\{state\}\}">\pi \; </figure-callout>}{\mathrm{R}}^{}{\mathrm{}}^{\mathrm{4}}}$$

• ΔP: static pressure drop between inlet and outlet of nozzle 29 or restriction 18
• Q: volume flow
• R: radius of nozzle 29 or restriction 18
• 1: length of nozzle 29 or restriction 18,
• μ is the dynamic viscosity of the ink.

In order to reduce the hydraulic resistance of the restriction 18 compared with the nozzle 29 and while maintaining a good pacing efficiency, we will play on the equivalence [length ↔ section] by favoring a section and a length of the restriction 18 greater than those of the nozzle 29. The nominal dimensions given in the dimension and material table in annex 1, constitute a good compromise between the efficiency of the stimulation and the viscous pressure drop. For ratios of radius, respectively length, of the nozzle 29 and the restriction 18 being typically 1/3, respectively 1/10, we obtain: $$\{\begin{array}{l}{\mathrm{R}}_{\mathrm{Imp}}\cong 1\\ {\mathrm{R}}_{\mathrm{Hydro}}\left(\mathit{restrictor}\right)\cong \frac{1}{10}{\mathrm{R}}_{\mathrm{Hydro}}\left(\mathit{buzzard}\right)\end{array}$$

The volume contained in the parallelepiped-shaped chamber 28 is chosen such that the Helmholtz frequency of the system is not less than 200 kHz. The thickness of the chamber 28 (in the Z direction) must be as small as possible to provide a maximum surface area for the vibrating element 31 but nevertheless not less than the diameter of the nozzle 29 in order to minimize the viscous loss of pressure in the 28. This thickness resulting from a compromise will be chosen close to the diameter of the nozzle 29. The volume and the thickness being given, it fixes the surface of the chamber by ensuring good consistency with the design rule No. 1.

• un dispositif d'éjection de liquide permettant de former au moins un jet d'encre,
• un générateur de signaux électriques de commande,
• un dispositif de stimulation interne, c'est-à-dire en amont de la buse, permettant de fractionner le jet en créant des perturbations à sa surface en sortie de buse. Ce dispositif de stimulation est capable de créer une goutte isolée dans le jet lorsqu'on applique le signal impulsionnel approprié sur les moyens de stimulation,
• un système de tri consistant en un agencement d'électrodes portées à des potentiels électriques constants, et en une gouttière qui collecte les gouttes non imprimées.

Thus, a printer according to the invention includes:

• a liquid ejection device for forming at least one ink jet,
• a generator of electrical control signals,
• an internal stimulation device, that is to say upstream of the nozzle, for splitting the jet by creating disturbances at its surface at the nozzle outlet. This stimulation device is capable of creating an isolated drop in the jet when the appropriate pulse signal is applied to the stimulation means.
• a sorting system consisting of an arrangement of electrodes carried at electrical potentials constant, and in a gutter that collects the unprinted drops.

The invention makes it possible to use a common sorting system for a large number of jets, which eliminates the difficulties of producing the charging electrodes of a conventional binary printer, and makes it possible to take advantage of the advantages of the sorting system in intermittent stimulation. , especially its low cost of implementation. In addition, the stimulation being internal, congestion problems and difficulties related to external stimulation techniques are eliminated. The stimulation device controlled according to the principle of the invention also makes it possible to modify the behavior of the jet and the trajectory of the drops by the sole means of the stimulation signal, which simplifies the electronic part of the print head and gives a control very fine on jet stability and print quality. The combination of two stable breaks also helps to control the two trajectories of the two types of drops created by the simple adjustment of the stimulation signal parameters, which contributes to improving the reliability of the machine and the print quality.

Note that a print head using the invention may or may not include the circuit 32 for generating the break signals.

Annex 1

Size and material table Function Long (X) / Larg (Y) / Radius- Thickness (Z) Materials Restriction 18 250 μm / 130 μm / - 38 μm 316 stainless steel Connecting tube 20 - / - / 75 μm 38 μm 316 stainless steel Room 29 1000 μm / 410 μm / - 38 μm 316 stainless steel Vibrating element 31: - piezo ceramic 1000 μm / 410 μm / - 125 μm PZT - membrane 1000 μm / 410 μm / - 62.5 μm 316 stainless steel Connecting tube 21 - / - / 50 μm 475 μm 316 stainless steel Nozzle 29 - / - / 15 μm 50 μm 316 stainless steel

Annex 2 List of documents cited

• US-A-4,220,958 CROWLEY
• US-A-3,596,275 SWEET
• US-A-4,638,328 DRAKE AND AL
• FR 2 799 688 μ US 09/685 064
• Xerox Disclosure Journal ( Pincus - 1982, Vol. 7, p. 23

Claims (25)

1. An ink jet printer (10) comprising:

   - a generator (32) of electrical control signals receiving a control signal and delivering to stimulation means (31), stimulation signals,

   - a printing head (1) with one or more nozzles (29) having a head (1) body (23) notably accommodating for each nozzle (29),

   - a hydraulic path of the ink including, a stimulation chamber (28) in hydraulic communication with one of the printing nozzles (29) emitting a pressurized ink jet (30) along an axis of this nozzle (29),

   - internal means (31) for stimulating the ink jet (30) emitted by the nozzle (29), mechanically coupled with the ink (16) accommodated in the stimulation chamber (28), these means (31) acting on the jet (30) emitted by the nozzle (29) in order to break up the jet (30) in a controlled way, and

   - means (40) for recovering the ink which is not received by a printing substrate (41),

   - an arrangement (35) of charging electrodes defining around the axis of the nozzle (29), upstream and downstream areas, the downstream area being further away from the nozzle than the upstream area, upstream and downstream electrodes (34, 37) of this arrangement (35) being connected to sources of electric potential in order to maintain in one of the areas, a potential equal to that of the ink found in the body (23) of the printing head (1), and in the other one of the areas a potential different from that of the ink found in the body (23) of the printing head (1),

   - an arrangement (39) of deflection electrodes axially located downstream from the arrangement (35) of charging electrodes,

   characterized in that the generator (32) of electrical control signals delivers to the stimulation means (31), signals intermittently causing controlled breaking of the jet (30) in an upstream breaking position (11) located in the upstream area, to intermittently form a drop, thereby separating the jet into a drop and a jet section having a length larger than two wavelengths of the jet, and also causing controlled breaking of the jet (30) or of sections (38) of the jet (30) continuously in a downstream breaking position (12), the continuous jet (30) emitted by the nozzle (29) being thereby transformed after the downstream area into a continuous train of electrically charged and uncharged ink drops (33, 43).
2. The printer (10) according to claim 1, characterized in that the upstream electrode (34) of the arrangement of charging electrodes (35) is connected to the same potential as the ink (16).
3. The printer (10) according to any of claims 1 or 2, characterized in that the stimulation means (31) include a piezoelectric material (25), the generator (32) of electrical control signals delivering to the stimulation means (31), a continuous printing signal formed by a periodic signal of period T_b, intermittently replaced with a pulse signal preceded and followed by transition signals.
4. The printer (10) according to claim 3, characterized in that the pulse signal delivered by the generator (32) of electrical control signals is formed by a pulse including three consecutive voltage steps connected from one to the next by a steep rising or falling voltage edge.
5. The printer (10) according to claim 3, characterized in that the pulse signal delivered by the generator (32) of electrical control signals is formed by a succession of three rectangular pulses separated from each other by voltage steps with a level than the level of the pulse with the lowest level.
6. The printer (10) according to any of claims 3 to 5, characterized in that the periodic signal delivered by the generator (32) of electrical control signals is formed by a combination of two sinusoidal signals.
7. The printer (10) according to any of claims 3 to 5, characterized in that the periodic signal delivered by the generator (32) of electrical control signal is formed by a combination of more than two sinusoidal signals.
8. The printer (10) according to any of claims 3 to 5, characterized in that the sum of the durations of the pulse signal and of the transition signals delivered by the generator (32) of electrical control signals is equal to an integral number of periods of the periodic signal.
9. The printer (10) according to any of claims 1 to 8, characterized in that a Helmholtz frequency of a portion of a hydraulic path of the ink feeding a nozzle (29) comprising a restrictor (18) and the portion located downstream from this restrictor (18) has a value located outside a bandwidth of the jet (30) issued from this nozzle (29).
10. The printer (10) according to any of claims 1 to 8, characterized in that the hydraulic path of the ink includes a restrictor (18) and in that the length of a hydraulic path between an inlet of the restrictor and the nozzle (29) is less than the quarter of the wavelength of sound in the ink.
11. The printer (10) according to any of claims 1 to 8, characterized in that the system for stimulating a jet (30) emitted by a nozzle (29) is strictly non-resonant.
12. The printer (10) according to any of claims 3 to 8, characterized in that the stimulation means (31) include, in addition to the piezoelectric material (25), a membrane (24) which is mechanically coupled with it, a resonance frequency of a vibrating component formed by the membrane (24) and the piezoelectric material (25) is larger than a cut-off frequency of the jet (30).
13. A method for printing a medium by means of a printer (10) according to any of claims 1 to 12, wherein an ink jet (30) emitted by a nozzle (29) of the printer is fractionated in order to form in an upstream position, first drops (33) intended for printing, impinging a printing substrate in order to form points (58) and in a downstream position, second recovered drops directed towards a recovery trough (40),
    characterized in that,
    in the upstream position, the jet (30) is fractionated intermittently in order to generate the first intermittent drops (33) intended for printing, on the one hand, and jet sections with a length larger than two wavelengths of the jet on the other hand, and in that in the downstream position the jet or the sections (38) are further fractionated, resulting from the fractionation of the jet into first drops (33) and of sections (38) into said second drops (43) directed towards the trough (40).
14. An ink jet printer (10) head (1) comprising:

    - one or more printing nozzles (29) and a head (1) body (23) notably accommodating for each nozzle (29),

    - a hydraulic path of the ink including a stimulation chamber (28) in hydraulic communication with one of the printing nozzles (29) emitting a pressurized ink jet (30) along an axis of this nozzle (29),

    - internal means (31) for stimulating the ink jet (30) emitted by the nozzle (29) mechanically coupled with the ink (16) accommodated in the stimulation chamber (28), these means (31) acting on the jet (30) emitted by the nozzle (29) for breaking up the jet (30) in a controlled way, and

    - means (40) for recovering the ink which is not received by a printing substrate (41),

    - a generator (32) of electrical control signals receiving a control signal and delivering stimulation signals to the stimulation means (31),

    - an arrangement (35) of charging electrodes defining around the axis of the nozzle (29), upstream and downstream areas, the downstream area being further away from the nozzle than the upstream area, upstream and downstream electrodes (34, 37) of this arrangement (35) being connected to sources of electric potential so as to maintain in one of the areas a potential equal to that of the ink found in the body (23) of the printing head (1), and in the other one of these areas, a potential different from that of the ink found in the body (23) of the printing head (1),

    - an arrangement (39) of deflection electrodes axially located downstream from the arrangement (35) of charging electrodes

    characterized in that the generator (32) of electrical control signals delivers to the stimulation means (31), signals intermittently causing controlled breaking up of the jet (30) in an upstream breaking position (11) located in the upstream area, this intermittent breaking up generating first intermittent drops (33) intended for printing, and jet sections having a length larger than two wavelengths of the jet, and also causing controlled breaking up of the jet (30) or of the sections (38) of the jet (30) continuously in a downstream breaking position (12), the continuous jet (30) emitted by the nozzle (29) being also transformed after the downstream area into a continuous train of electrically charged and uncharged ink drops (33, 43).
15. The printer (10) head (1) according to claim 14, characterized in that the upstream electrode (34) of the arrangement of charging electrodes (35) is connected to the same potential as the ink (16).
16. The printer (10) head (1) according to any of claims 14 or 15, characterized in that the stimulation means (31) include a piezoelectric material (25), the generator (32) of electrical control signals delivering to the stimulation means (31), a continuous printing signal formed by a periodic signal with period T_b, intermittently replaced with a pulse signal preceded and followed by transition signals.
17. The printer (10) head (1) according to claim 16, characterized in that the pulse signal delivered by the generator (32) of electrical control signals is formed by a pulse including three consecutive voltage steps connected from one to the next by a steep rising or falling voltage edge.
18. The printer (10) head (1) according to claim 16, characterized in that the pulse signal delivered by the generator (32) of electrical control signals is formed by a succession of three rectangular pulses separated from each other by voltage steps with a level less than the level of the pulse with the lowest level.
19. The printer (10) head (1) according to any of claims 16 to 18, characterized in that the periodic signal delivered by generator (32) of electrical control signals is formed by a combination of two sinusoidal signals.
20. The printer (10) head (1) according to any of claims 16 to 18, characterized in that the periodic signal delivered by the generator (32) of electrical control signals, is formed by a combination of more than two sinusoidal signals.
21. The printer (10) head (1) according to any of claims 16 to 18, characterized in that the sum of the durations of the pulse signal and the transition signals delivered by the generator (32) of electrical control signals is equal to an integral number of periods of the periodic signal.
22. The printer (10) head (1) according to any of claims 14 to 21, characterized in that the Helmholtz frequency of a portion of a hydraulic path of the ink feeding a nozzle (29) comprising a restrictor (18) and the portion located downstream from this restrictor (18), has a value located outside a bandwidth of the jet (30) issued from this nozzle (29).
23. The printer (10) head (1) according to any of claims 14 to 21, characterized in that the hydraulic path of the ink includes a restrictor (18) and in that the length of a hydraulic path between an inlet of the restrictor and the nozzle (29) is less than the quarter of the wavelength of sound in the ink.
24. The printer (10) head (1) according to any of claims 14 to 21, characterized in that the system for stimulating a jet (30) emitted by a nozzle (29) is strictly non-resonant.
25. The printer (10) head (1) according to any of claims 16 to 21, characterized in that the stimulation means (31) include in addition to the piezoelectric material (25), a membrane (24) which is mechanically coupled with it, and in that a resonance frequency of a vibrating component formed by the membrane (24) and by the piezoelectric material (25), has a value located outside a bandwidth of the jet (30).

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