EP3129232A1 - Sturdy drop generator - Google Patents

Abstract

The invention relates to a device for forming and ejecting drops of an inkjet of a CIJ printing machine, said device comprising: a) a cavity for containing an ink and comprising an end provided with a nozzle (10) for ejecting drops of ink, and b) actuator means (21, 22, 32, 41, 42) in contact with the cavity. The invention relates to a device wherein the jet velocity modulation from the nozzle (10) has a value of ΔVj(f_t) at the operating frequency, and does not vary, at a temperature equal to 15 °C and to 35 °C, in a frequency range of +5 kHz around the operating frequency f_t, outside the range between 0.25ΔVj(f_t) and 4 ΔVj(f_t). In order to achieve this, the acoustic impedance of the cavity is suitable for limiting the influence thereof on the jet velocity modulation.

Description

 ROBUST DROP GENERATOR

DESCRIPTION

TECHNICAL FIELD AND STATE OF THE PRIOR ART The invention relates to improving the operation of a print head of an ICJ printer to make it more robust with respect to the environmental variations (in particular temperature) encountered. during industrial use of this type of printer.

 This improvement passes by an increase of the robustness of the function of stimulation of the generator of drops with respect to the temperature.

 Continuous Inkjet (CIJ) printers are well known in the field of coding and industrial marking of various products, for example to mark barcodes or the expiry date on food products directly on the production line and at high speed. This type of printer also finds application in the field of decoration where the graphic printing possibilities of the technology are exploited.

 CIJ printers continuously generate jets of drops, some of which are selected and oriented towards the printing medium while the others are recovered for recycling. These printers have several typical subsets as shown in Figure 1:

 First, a print head 1, generally offset from the body of the printer 3; it is connected to the latter by a flexible umbilical 2 gathering the hydraulic and electrical connections necessary for the operation of the head by giving it a flexibility that facilitates integration on the production line.

 The body of the printer 3 (also called desk or cabinet) usually contains three subsets:

an ink circuit 4 in the lower part of the console (zone 4 '), the main purpose of which is, on the one hand, to supply ink to the head at a stable pressure and of adequate quality, and on the other hand to support ink jets not used for printing.

 a controller located at the top of the console (zone 5 '), capable of managing the sequencing of actions and of performing the processes enabling activation of the various functions of the ink circuit and the head.

 - An interface 6 which gives the operator the means to implement the printer and to be informed about its operation.

 This description can be applied to continuous jet printers (Cl J) called binary or continuous jet multi-deflected.

 The binary CIJ printers are equipped with a head whose drop generator has a multitude of jets whose drops can only be oriented towards 2 paths: print trajectory or recovery trajectory.

 In multi-deflected continuous jet printers, each drop of a single jet (or a few spaced jets) can be deflected on different paths corresponding to different commands. A succession of drops under different commands can thus scan the area to be printed in a direction that is the direction of deflection, the other scanning direction of the area to be printed is covered by relative displacement of the print head. and the printing medium 8. Generally the elements are arranged such that these two directions are substantially perpendicular.

 The deviated continuous ink jet printer heads have various functional subassemblies. Figure 2 schematizes in particular a print head of a multi-deflected CIJ printer. It consists of:

 means 10, 63 for generating a jet of drops called drop generator or stimulation body;

 means 62 for recovering the ink not used for printing;

means 65 for deflecting the drops intended for printing; means for controlling and controlling the process of deflection of the drops (synchronization of the formation of the drops with the deflection controls).

 Referring to Figure 2 which schematizes a multi-deflected ICJ print head, there is a drop generator 60 in which a cavity is fed with electrically conductive ink. This ink, maintained under pressure, by the ink circuit 4, generally outside the head, escapes from the cavity by at least one calibrated nozzle 10 thus forming at least one ink jet 7.

 A periodic stimulation device 63 is associated with the cavity in contact with the ink upstream of the nozzle 10; it transmits, in ink, a periodic modulation (pressure) which causes a modulation of speed and radius of the jet at the outlet of the nozzle. When the dimensioning of the elements is adequate, this modulation amplifies in the jet under the effect of the surface tension forces responsible for the capillary instability of the jet, until the rupture of the jet. This break is periodic and occurs at a precise distance from the nozzle at a point called "breaking" 13 of the jet, which distance depends on the stimulation energy.

 In the case where a stimulation device, called an actuator, whose driving member is a piezoelectric ceramic, is in contact with the ink of the cavity upstream of the nozzle, the stimulation energy is directly related to the amplitude of the electrical control signal of the ceramic. The prior art teaches other means of stimulating the jet (thermal, electro-hydrodynamic, acoustic, ...) but stimulation using piezoelectric ceramics remains the most widespread for its efficiency and its relative ease of use. Implementation.

 At its breaking point 13, the jet, which was continuous leaving the nozzle, is transformed into a train 11 of identical and regularly spaced ink drops.

The drops are formed at a temporal frequency identical to the frequency of the stimulation signal and for a given stimulation energy, while any other parameter is stabilized elsewhere (in particular the viscosity of the ink), there is a precise phase relationship ( constant) between the periodic stimulation signal and the breaking instant, itself periodic and of the same frequency as the stimulation signal. That is to say that at a precise moment of the period of the stimulation signal corresponds a precise moment in the dynamics of separation of the drop of the jet.

 Without additional action (this is the case where the drops are not used for printing), the drop train travels along a path 7 collinear with the ejection axis of the drops (nominal trajectory of the jet) which joins, by geometric construction of the print head, the gutter 62 recovery. This gutter 62 for recovering the unprinted drops captures the unused ink which returns to the ink circuit 4 in order to be recycled.

 To print, the drops are deflected and spaced from the nominal trajectory 7 of the jet. They then escape the gutter and follow oblique trajectories 9 which meet the support to be printed 8 at the various desired points of impact. All these trajectories are in the same plane. The placement of the drops on the matrix of impacts of drops to be printed on the support, to form characters, for example, is obtained by the combination of an individual deflection of the drops in the deflection plane of the head with the relative displacement. between the head and the print medium (usually perpendicular to the plane of deflection). In deflected continuous jet printing technology, the deflection is achieved by electrically charging the drops and passing them into an electric field. Practically, the deflection means of the drops comprise an individual charge electrode 64 for each jet, located in the vicinity of the breaking point 13 of the jet. It is intended to selectively charge each of the drops formed at a predetermined electric charge value which is in general different from one drop to another. For this purpose, the ink being maintained at a fixed potential in the drop generator 60, a voltage slot of a determined value, conveyed by the control signal, is applied to the charging electrode 64, this value being different with each drop period.

In the control signal of the charging electrode, the instant of application of the voltage is done a little before splitting the jet to take advantage of the electrical continuity of the jet and attract a given amount of charge, depending on the value the tension at the end of the jet. This variable load voltage allowing the deflection is typically between 0 and 300 volts. The voltage is then maintained during the fractionation to stabilize the charge until the electrical isolation of the loose drop. The voltage remains applied a little later to take into account the hazards of breaking moment.

 It is therefore sought to synchronize the instant of application of the voltage with the splitting process of the jet. In case of desynchronization, the drop concerned is not loaded correctly, its load is lower or even zero.

 The deflection means of the drops also comprise a set of 2 deflection plates 65 placed on either side of the trajectory of the drops downstream of the charging electrode. These 2 plates are brought to a high fixed relative potential producing an electric field Ed substantially perpendicular to the trajectory of the drops, capable of deflecting the electrically charged drops which engage between the plates. The amplitude of the deflection is a function of the charge, the mass and the speed of these drops.

 In order to control the deflection of the drops for printing, it seeks to produce quality break in the range of environmental conditions provided in the specifications.

 We therefore seek to ensure:

 on the one hand, that the break is in the field of the charging electrode and therefore at a determined distance from the nozzle (breaking position);

 - And, on the other hand, that the breaking of the jet is done in a stable and reliable manner (breaking quality: which will be specified below). This is done by an optimal adjustment of the stimulation which is achieved practically by acting on the stimulation energy. In most cases of the prior art, the stimulation energy is controlled by the level Vs of the periodic voltage signal applied to the stimulator (piezoelectric component).

A break is considered stable and reliable (of good quality), when it makes it possible to guarantee an optimal charge of the drops in a field of operation of the printer characterized in particular by a temperature range (conditioning the viscosity of the ink ) for a given ink. Specifically, just before breaking, the drop is connected by a tail to the next drop during training (see Figure 3A). The shape of this tail determines the quality of the break. The most characteristic forms of a problem break are the following (but many more or less stable intermediate situations may exist):

 - very thin tail (see fig.3B) which is liable to break unstably (superficial tension cohesion forces become weak compared to electrostatic forces). When there is a very large electric field between two successive drops charged at very different values (in the case of a heavy charge followed by a low charge), a peak effect phenomenon at the tail creates electrostatic forces such as that particles of charged material are torn off the very fine tail of the heavily loaded drop and join the weakly charged drop by transferring charges. As a result, the drops no longer have their nominal load, the deflection is disturbed and the print quality deteriorates.

 - tail having a lobe between 2 narrowing (see fig.3C), which can break in 2 places and create a satellite isolated from the drop, which embeds part of the charges for the concerned drop:

 * If its speed is faster than the jet (fast satellite), the satellite and its charges will join the concerned drop and rebuild a nominal situation without noticeable impact on the print quality;

 * If the speed of the satellite is identical to that of the jet (infinite satellite) or does not reach the concerned drop before its deflection, it will be badly charged and the satellites will be violently deflected at the risk of fouling the print head;

 * If it joins the next drop (slow satellite) it will transfer loads from the concerned drop to the next and disrupt the deflection.

The shape of the break, in addition to the rheological characteristics of the ink, is related to the level of stimulation (excitation intensity). The shape of the break determines the quality of the breaking, that is to say its ability to guarantee the correct charge of the drops. In general, it changes, when the excitation increases, to pass from a break to satellites, then to a break without satellite. The satellite is defined as a secondary drop resulting from the breakage of the main drop.

 By further increasing the level of stimulation, the breakup returns to a satellite regime. At the same time, the position of the break relative to the nozzle changes according to the curve of FIG.

 The latter represents the profile of the characteristic f giving the breaking distance (Lb) between the nozzle 10 and the breaking point 13, as a function of the stimulation voltage VS (Lb = f (VS)). This curve will be called thereafter: stimulation curve. This is established by scanning the values of the stimulation excitation voltage VS and determining Lb for each VS value.

 When the stimulation excitation increases (from a low value), the nozzle / breakoff distance (Lb), which starts from a high value (natural breaking of the jet), decreases and goes through a minimum called "point of crawl, "and then lie down again. The shape and actual position of this curve depends on many parameters, in particular the nature of the ink and the temperature. The print head is designed so that the functional part of this curve is, at least in part, in the field of the charging electrode despite the variability of the parameters mentioned. On the other hand, there is a functional area related to the quality of breakage in which the impression is satisfactory (the load of the drops is correct). The intersection of the zone correctly positioned in the electrodes and the functional zone of breaking quality corresponds to the operational stimulation range. This stimulation range is characterized by an entry point (Pe) on the left and an exit point (Ps) on the right as shown in figure 4. The stimulation system will be satisfactory if the operational stimulation range is sufficiently well defined regardless of the conditions of use of the printer.

At least two distinct modes of piezoelectric pacing operation are used in state of the art inkjet printers: these are resonant and non-resonant pacing modes. Non-resonant pacing is relatively difficult to implement and requires substantial energy because the actuator must provide all the energy needed to create the displacement of the portion of the actuator in contact with the ink to generate the pressure modulation upstream of the nozzle. In return, this mode is relatively tolerant to the variability of the excitation conditions.

 Comparatively, resonant pacing has a much more advantageous yield in the context of a periodic stimulation which leads to the periodic breaking of a jet into drops at a fixed frequency, as is often the case in the printing processes of continuous jet type. Indeed, in this case it is very effective to design an actuator as an oscillating or vibratory system, tuned substantially on the frequency of emission of the drops; a weak periodic excitation can then maintain an amplified stationary wave which will generate the amplitude of displacement necessary for the modulation of pressure upstream of the nozzle.

 Under reasonable conditions of implementation, a simple piezoelectric ceramic (used in D33 mode, the electric field created between 2 electrodes deposited on the ceramic then producing a stretching or a longitudinal contraction thereof according to the direction polarization and the polarity of the electrical signal) can not be used alone as an actuator because it would not have a sufficient amplitude of deformation (of the order of a nanometer only) to create the modulation of ejection speed of expected ink; it is therefore attached to a room, called resonator, used for amplification of movement. The ceramic / resonator assembly is called an actuator.

 It has been observed that, for certain inks and sizing of the drop generator, the efficiency of the stimulation is not stable as a function of temperature.

This can go as far as the impossibility to operate the printer at certain distinct temperatures of at least 15 ° C or 20 ° C, and / or in certain temperature ranges, in particular at 5 ° C or 15 ° C, and at 35 ° C and / or 45 ° C (and / or 50 ° C) and / or between these different values taken in pairs, especially between 15 ° C and 35 ° C or between 5 ° C and 45 ° C (or even 50 ° C).

 Indeed, under certain conditions, the stimulation becomes completely ineffective and the operational range of stimulation moves and / or atrophies up to, in some cases, disappear, which makes it impossible to adjust the machine.

 In some cases, it is possible to attempt to adjust the stimulation setting according to the predictable range of temperature evolution during the production session in which the printer is used. But this is not always possible.

 Finally, if one wishes to compensate for this instability, it is necessary to implement additional means (temperature control at the head, for example), which imposes an additional cost too.

 There is therefore the problem of finding a device and a method which allow satisfactory operation at at least 2 different temperatures of at least 15 ° C or 20 ° C, in particular at 5 ° C (and / or or at 15 °), and secondly at 35 ° C, and / or at 45 ° C and / or at 50 ° C, preferably between any two of these values, in particular between 15 ° C and 35 ° C or between 5 ° C and 45 ° C (or even 50 ° C).

 Another problem, in a system implementing a resonant mechanical actuator, the resonance of the actuator is coupled to the resonance of the fluid, in particular by the fact that the ratio of acoustic celestialities, on the one hand in the material used for the resonator (for example stainless steel) and secondly in the fluid (about 5000 m / s in the resonator, about 1250 m / s in the fluid) is of the order of 4, a quarter of a length d 'wave. The consequences of this report are the coupling mentioned above.

STATEMENT OF THE INVENTION

 The invention aims to solve these problems.

 According to the invention, a device for forming and ejecting drops of an ink jet from an ICJ printing machine comprises:

a) a cavity for containing an ink and having an end provided with an ink drop ejection nozzle, b) means, forming an actuator, in contact with the cavity.

 In such a device, the acoustic impedance of the cavity, close to the nozzle, has a value Z-r (ft) at the working frequency of the cavity and the actuator.

Preferably, this acoustic impedance does not vary, or varies little, in a frequency range of + 5 kHz around the working frequency f _t , so that the variation of the speed modulation in the nozzle remains between on the one hand, 0.25 (or 0.5), and, on the other hand, 2 (or 4), times the modulation of speed at the reference temperature (for 25 ° C for example), and this at at least 2 temperatures positive at least 10 ° C or 20 ° C, especially at 15 ° C and 35 ° C, preferably also at 5 ° C, and / or at 10 ° C and / or 20 ° C, preferably still at 45 ° C or even 50 ° C, more preferably at any temperature within a temperature range which contains at least the range [15 ° C - 35 ° C], or even at least the range [5 ° C-35 ° C] ° C - 50 ° C].

 Such a device according to the invention makes it possible to move the resonant and antiresonance frequencies, due to the ink cavity, so that their drift as a function of the temperature does not cause them to cross the frequency of stimulation of the jet, to at least 15 ° C and 30 ° C (or at 35 ° C), preferably also at 5 ° C, and / or at 10 ° C and / or at 20 ° C, more preferably at 45 ° C or even at 50 ° C, more preferably at any temperature in a range of between 15 ° and 35 ° and more generally between 5 ° and 50 ° C. These temperatures and / or these temperature ranges are in fact those of the operating specifications of many printers.

 Preferably, said cavity is such that the ratio between the length of the actuator (or actuator) mechanical and the length of the, or a portion of the cavity for accommodating a column of fluid, is strictly greater than 4; this ratio can be for example between 4 and 6 or 4 and 10 or 100.

 According to a first embodiment, the internal shape of the cavity may comprise:

a first cylindrical zone, having a first diameter, and a first length, measured along a longitudinal axis of said cavity, a second cylindrical zone having a second diameter, different from the first diameter, and a second length, measured along a longitudinal axis of said cavity.

 A cavity having at least two cylindrical sections of different diameters is thus created so as to shift the eigenmodes of the ink cavity for sound velocities in the usual inks. Cylindrical sections of different diameters make it possible to vary the length of the fluid.

 The actuator means, for example a piezoelectric ceramic, may be directly in contact with the internal volume of the cavity.

 The means forming the actuator may comprise a resonator element. The actuator is then resonant.

 According to one embodiment, this resonator element comprises a resonator body disposed in the cavities.

 In another embodiment, the walls of the cavity form at least a portion of the resonator.

 The resonator may be metallic or mineral in nature, for example stainless steel, or aluminum, or beryllium, or brass, or copper, or diamond, or glass, or gold, or iron, or lead , or TMMA, or silver, or titanium.

 The resonator body may include a first portion having a first diameter and a second portion having a second diameter, different from the first.

 The invention also relates to a device for forming and ejecting drops of an ink jet from an ICJ printing machine, this device comprising:

 a) a cavity for containing an ink and having an end provided with an ink drop ejection nozzle,

b) means, forming a resonator, in contact with the cavity, made of a material chosen from aluminum, beryllium, brass, copper, diamond, glass, gold, iron, lead, TMMA, silver, or titanium. The length of the ink cavity is generally comparable to the length of the flange resonator, the latter being chosen to allow the mechanical resonance of the actuator.

 The physical properties of the resonator are adjusted to allow the resonance of the device at a given frequency.

 The choice of a material other than stainless steel, and possibly the length of the bar and therefore of the ink cavity, makes it possible to move the unwanted resonant and antiresonance frequencies in the ink outside the range. useful (resonance of the actuator).

 The choice of such a material for the means forming the resonator thus makes it possible to eliminate, again, parasitic resonances due to a liquid contained in the cavity.

 The means forming the resonator may comprise a piezoelectric element.

 The resonator can be inserted in a resonator body of constant section or variable in the longitudinal direction.

 This resonator body may comprise a first portion having a first diameter and a second portion having a second diameter, different from the first.

 Both embodiments can be combined to optimize the final implementation.

 In one or other of the two embodiments, a device for forming and ejecting drops according to the invention may contain an ink, for example an ink in which the speed of sound is between 800 and 2000 m. / s.

 The invention also relates to a printing machine of the continuous ink jet (CIJ) type, this machine comprising:

 a print head provided with a device for forming and ejecting drops of an ink jet according to one of the embodiments described above,

an ink circuit, - Means for controlling the circulation of the ink and the print head.

 The invention also relates to a method for forming ink drops, in which a device as described above or a machine as described above is used.

 The invention makes it possible to preserve the principle of resonant stimulation with its advantages (efficiency, cost).

 It can be applied to different types of drop generator implementation.

 The combination of the two embodiments presented (cavity having several acoustic impedances, and specific material chosen for the resonator) makes it possible to limit certain drawbacks peculiar to each mode; it makes it possible to obtain a compromise between:

 - a satisfactory size, since it is related to the length of the bar (depending inter alia on the speed of sound);

 - ease of cleaning the cavity, in relation to the complexity and dead volume of ink in the cavity.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of the structure of a deviated continuous jet printer,

 FIG. 2 is a diagram of a print head of a deviated continuous jet printer,

 FIGS. 3A-3C show various breaking configurations, FIG. 3A showing a good quality break, FIG. 3B a fine-tipped break (with the risk of tearing of the material) and FIG. 3C a broken lobe (with risk of satellites),

FIG. 4 is a curve indicating the evolution of the breaking distance as a function of the excitation of the stimulation, FIGS. 5A-5E show stimulation body structures 20, 30, 40, 50 and 60 to which the invention can be applied,

 FIG. 6 is a stimulation efficiency curve, giving the breaking length as a function of the excitation frequency of the jet,

 FIGS. 7A-7B represent results obtained with a stimulation body of the type of FIG. 5D,

 FIG. 8 illustrates a schematic model of a stimulation body,

FIG. 9 is an electrical analogy of the equivalent diagram of a stimulation device,

 FIGS. 10A-10B show the frequency response of a stimulation body for two different ink temperatures,

 FIG. 11 represents other complementary results;

 FIGS. 12A-C represent test results obtained with another type of stimulation body,

 FIG. 13A represents the evolution of the acoustic impedance as a function of the frequency and FIG. 13B represents the evolution of the modulation of the speed of the jet as a function of the frequency,

 FIGS. 14A-E represent stimulation body structures embodying the invention,

 FIGS. 15A-15C show test results obtained with a stimulation body with the invention,

 FIG. 16 gathers ultrasonic speed data for different inks, as a function of temperature,

 FIG. 17 is a schematic representation of the control means of an inkjet printer

DETAILED PRESENTATION OF PARTICULAR EMBODIMENTS

FIGS. 5A, 5B, 5C, 5D and 5E show five embodiments of stimulation actuator in a stimulation body 20, 30, 40, to which applied the invention. Some (FIGS. 5A, 5D) comprise a resonator which is intended to dive into the ink when it is present in the cavity.

 The stimulation body 20 of FIG. 5A comprises an envelope 25 whose internal volume preferably has a cylindrical shape and extends along an axis XX '.

 The body 20 further comprises an actuator comprising a ceramic 21, made of a piezoelectric material, of cylindrical shape along the axis XX '. The actuator is mounted in the casing 25 of the modulation body 20.

 This ceramic is metallized on its two faces 210, 212, perpendicular to the axis XX '. It is secured, coaxially, to a cylindrical metal bar 22. For example the bonding is achieved by bonding with an adhesive, which may advantageously be a conductive adhesive.

 According to the illustrated embodiment, this bar comprises a circular flange 23 on which is fixed the face 212 of the ceramic.

 The casing 25 may be provided with a bearing surface or an internal bearing surface 250, which is perpendicular to the axis XX 'of the cylinder and which is provided with an orifice 252 through which the cylindrical metal bar 22 can to be introduced. A bearing surface 230 of the circular collar 23 can thus bear against the internal bearing surface 250.

 Mechanical means, not shown, to center and clamp the flange 23 (thus the actuator) on the surface 250.

 The internal volume of the envelope 25, located under the surface 250 and the flange, defines an isolated cavity 24.

 In operation, the cavity is fed with ink under pressure through a conduit 26.

A nozzle 10 from which the jet exits is placed at the bottom of the cavity 24, and the assembly is calculated so that the active face 222 at the end of the bar 22 is found above and close to the nozzle 10, preferably at a distance of a few tenths of mm, for example between 2/10 ^th and 5/10 ^th mm mm. Each of the internal elements (actuator, envelope 25, nozzle 10) of the modulation body is of circular section and these different elements are placed coaxially with each other on the axis XX '.

 For practical reasons, the bar 22 is preferably:

 - of significant hardness (machinable by machining);

 in a conductive or metallized material, for transferring the zero voltage applied to the ink to one of the electrodes of the ceramic 21;

 - insensitive to corrosion if in contact with the ink.

 A material that can be used is stainless steel, which has all the features mentioned above.

 By construction, the bearing surface 27 of the flange 23 corresponds to a vibration node of the actuator, which avoids losses of efficiency by transmission of energy in the structure of the modulation body.

 On the other hand, it is preferable that the end 220 of the bar 22, which is above the nozzle 10, has a maximum amplitude of movement which corresponds to a vibration belly.

 In practice, the actuator can be tuned so that the resonance is in the vicinity of the working frequency (the so-called "drops" frequency, or the frequency with which the drops are to be generated), but not exactly the same for not make the system too sensitive to variations in the conditions of implementation of the actuator (mechanical tolerances of an actuator to another for example). The tuning is generally in the air, at a frequency shifted from the working frequency, to take account of the frequency slip related to the difference in impedance existing when the bar is in different environments (ink for example ).

 In this example, the part of the bar 22 under the flange 23 is installed in the cavity 24 (body of the drop generator) whose length is substantially identical to that of the bar 22.

In operation, the electrode 210 of the ceramic 21 is connected to means 27 for energizing. The body 25 may be connected to a mass 29 which will be transferred to the electrode 212 via the collar 230. FIG. 5B describes a second embodiment of resonant modulation body 30.

 Its operation is close to that described above in connection with FIG. 5A.

There is a cavity 34 of cylindrical internal shape delimited by two end surfaces 320, 322, perpendicular to the axis XX '. Pressurized ink is supplied into this cavity through a conduit 36. An end of the ^era this tubular cavity is closed by the partition wall 322 perpendicular to the axis XX '. A nozzle 10 is formed in the partition 320 2 ^nd end, to let out a jet along the axis XX '.

 It is the envelope 32, which delimits the cavity 34, which performs the function provided by the bar 22 of the first embodiment. It is excited by a piezoelectric ceramic 31 secured by mechanical means or by bonding to the partition 322. The ceramic-envelope assembly forms a resonator, the partition 322 being at a vibration node, the maximum amplitude of displacement being at the of the plate 320, provided with the nozzle 10. The length L of the envelope is thus chosen to create a standing wave in the vicinity of the working frequency, in the length of the envelope 32. In this case, the influence the impedance brought by the ink present in the cavity must be taken into account in order to tune the set to the correct frequency.

 In operation, an electrode of the actuator (for actuating the ceramic 31) is connected to means 37 for energizing. The envelope 32 can be connected to a mass 39.

 FIG. 5C describes a third embodiment, in which a piezoelectric ceramic 41 is annular and is placed in a groove 48 of a circular envelope 42 having a tubular cavity 44. The cavity is closed at the top by a partition 422 and, in bottom, is a plate 420 provided with a nozzle 10 ejection drops. The ink supply is via a duct 46.

On assembly, the ceramic 41 is clamped between the blanks 48a and 48b of the groove. Under the effect of a periodic electric field created between electrodes, arranged in a ring on the faces of the ceramic element 41, perpendicular to its axis, the latter deforms longitudinally and transmits this vibration to the envelope 42 to which she is solidarisée. This excitation is transmitted to the nozzle 10 and then to the jet. As in the embodiment of Figure 5B, it is the envelope that plays the role of the resonator.

 In operation, the actuator 41 is connected to means 47 for energizing, this electrode is electrically isolated from the envelope 42. The envelope 42 can be connected to a mass 49.

 Figure 5D describes a fourth embodiment, which is in fact a variation of the first mode described above. Numerical references identical to those of FIG. 5A denote identical or corresponding elements. The references 51 and 52 respectively denote the piezoelectric ceramic and the resonator.

In contrast to the structure of FIG. 5A, the resonator 52 comprises, from the flange 53, 3 sections 52 ₁ , 52 ₂ and 52 ₃ of different diameters: a first 52i of diameter slightly smaller than the diameter of the orifice in which the actuator is inserted, a second 52 ₂ of smaller diameter and which will allow to delimit a volume 54 in which the ink will be stored, a third 523 of still smaller diameter and it ends the conduit which will bring the ink to the nozzle. In fact, the difference between the first diameter and the diameter of the wall of the casing 25 in which the actuator is inserted makes it possible to circulate the ink, injected by the lateral duct 26. This type of actuator is generally used to generate drops of so-called "average" size and its shape is optimized for the operating conditions (in particular the working frequency) in a given space imposed by the mechanics implemented on the print head. In this figure, we have identified areas A, B, C which are used in the following description.

 The part of the bar under the flange 53 23 is installed in the cavity (body of the drop generator) whose length is, again, substantially identical to that of the resonator 52 of the cavity 54.

 The explanations already given above in connection with FIG. 5A and in particular those relating to the connection of the energizing means and the working frequency of the actuator are applicable here.

The print head can have a mechanical configuration that is common for many types of drop generators that produce drops of different size (to simplify: big, medium and possibly, small) so that operate at different frequencies. The size and the inputs / outputs can therefore be identical for all types of generator; the cavity length can also be very close for these different types. So that the different types of resonator can operate at different frequencies while maintaining a length between flange and substantially identical nozzle, one can act on the shape of the bar. Consequently, the bar for a head G (the lowest frequency) is a single cylinder whose length is the largest (FIG. 5A for example), and that of a head M (higher frequency) has a more complex shape (2 diameters, Figure 5D for example) which keeps a substantially identical length to the head G operating at a higher frequency.

 But the problem to be solved, presented in the present application, and in particular below, which is that parasitic resonances generated in the liquid column interfere with the stimulation as a function of temperature, remains the same. The parasitic nature of these resonances has not been demonstrated in the prior art, in particular in JP 2006-076039 or JP-2005-081643, or US 5063393 or JP-S58-3874.

 Figure 5E shows another type of device to which the invention can be applied. Numerical references identical to those of FIG. 5B designate the same elements.

 There is a cavity 34, of cylindrical inner shape, delimited, on the side of the nozzle 10, by an end surface 320 perpendicular to the axis XX '. Pressurized ink is fed into this cavity via a conduit 36.

 The other end of this tubular cavity is in direct contact with an actuator, here a piezoelectric ceramic 31 (itself held by a peripheral flange on the wall of the cavity)

 In this figure the cavity is elongated, along the axis XX '. But it can also be curved.

In operation, an electrode of the actuator 31 is connected to means 37 for energizing. The envelope 32 can be connected to a mass 39. In this device, the envelope 32, which delimits the cavity 34, does not perform a function such as that provided by the bar 22 of the first embodiment. The ceramic - shell assembly does not form a resonator. The ink is directly vibrated by the actuator 31 and resonances are formed in the cavity at the working frequency.

 This type of device presents the same problems as those presented above, in particular for other devices such as those of FIGS. 5A-5D.

 In general, the optimum working frequency of a jet is determined by the different parameters defining the jet. These parameters include:

 the diameter of the nozzle (which can be between 40 μιη and 80 μιη),

- the jet speed (which can be between 18 and 24 m / s),

the physico-chemical parameters of the ink: surface tension (for example between 20 and 60 mN / m), dynamic viscosity (for example between 2 and 10 cps) and density (for example between 800 and 1400 kg) / m ³ ).

 The operating frequency can be adjusted using means 27, 37, 47 for applying a voltage to the piezoelectric element.

 The stimulation efficiency is represented by the break length Lb as a function of the excitation frequency of the jet.

 Lb can be measured by observing the jet with a camera and a stroboscopic lighting synchronized on the drop period (this makes it possible to freeze the image of the drops in formation). The distance between the nozzle and the micrometer displacement breaking of the camera is then measured.

 Another technique is described in WO 2012/2107560

(See in particular the description in connection with FIGS. 5A-5C of this document), or in WO 2011/012641, when the drops are loaded (with a constant drop formation frequency).

In general, it is considered that the shorter the breaking length, the greater the stimulation efficiency. The curve of FIG. the evolution of U as a function of the excitation frequency of the jet. The resonance frequency of the jet is the frequency for which the amplification of the velocity or radius modulation is the most important. In general, the frequency of the actuator is adjusted close to this frequency. Indeed, the jet being defined by its diameter, its exit velocity of the nozzle and the fluid that constitutes it (responsible for the capillary instability of the jet by means of the surface tension of this fluid), the jet behaves as a system resonating at a given privileged frequency. When it is periodically excited by a speed modulation, the capillary instability reflects it into a periodic variation of the diameter of the jet which will be amplified until the rupture of the jet. The length Lb where this rupture is located as a function of the excitation frequency is representative of the resonance of the jet for a given stimulation voltage.

 According to the above, the optimum excitation frequency vo is that which corresponds to the absolute minimum of the length Lb.

 However, it has been found that the real curves the evolution of Lb as a function of the excitation frequency of the jet, examples of which are shown in FIGS. 12A-12C (which will be discussed in more detail below), do not show These real curves show that the actual frequency response is disturbed by additional frequency events.

 More precisely, it has been demonstrated that, during the operation of any one of the stimulation bodies, three resonance systems are put into play: the resonance of the jet, the resonance of the actuator or resonator and the resonance of the fluid cavity of the drop generator. In other words, certain frequency behaviors have been observed, which correspond neither to the resonance of the actuator nor to the resonance of the jet.

 The instability of the jet is excited by the actuator, which thus ensures its stimulating function. The actuator is preferably designed so that the two resonant frequencies, that of the jet and that of the actuator, are close.

With respect to these two resonances, the resonance of the fluid cavity is a parasitic resonance. It causes the formation, in the ink, of a standing wave which is very sensitive to temperature. This standing wave is superimposed on the excitation of the actuator.

 For the family of so-called "resonant" actuators, the resonance frequency of the actuator depends on the speed of the acoustic waves in the material of the resonator bar and the dimensioning thereof. In the case of the structure of FIG. 5A, the length of the resonator is such that, at the resonant frequency, there is a vibration node at the retaining flange and a belly at the end.

 The resonator (or the envelope in the embodiments of FIGS. 5B and 5C) is generally made of stainless steel, material in which the speed of sound is of the order of Cinox = 5790 m / s.

The properties of some inks are such that the velocity of the waves in the ink is approximately 4 times less than in stainless steel (C _e ncre = 1200 m / s). As a result, the ink cavity also constitutes a resonator in which a standing wave can develop whose resonance or anti-resonance frequency will be close to the resonant frequency of the actuator.

 The velocity of the waves in stainless steel (or, more generally, in the constituent material of the bar) is very insensitive to temperature whereas that of the waves in the ink is very sensitive to temperature (variation between -3 and - 4 m / s per ° C). Data relating to the evolution of this celerity as a function of temperature are collated in FIG. 16 for inks based on solvent MEK (methyl ethyl ketone), alcohol or water. In this figure, the data of the speed of sound in an ink # 1 (whose solvent is MEK) and # 2 (whose solvent is alcohol) show a fairly high variability. The variability is lower for an # 3 ink, with a "water" base.

The resonance modes in the resonator and in the cavity are very similar and evolve in different ways depending on the temperature. The resonance and antiresonance modes of the fluid cavity can therefore move as a function of temperature, by crossing the mode of the resonator, which itself varies only slightly as a function of temperature. This results in disturbances of the stimulation in certain temperature ranges. A first study conducted on this problem concerns the case of a drop generator provided with a stimulation body of the type of FIG. 5D.

 FIG. 7A shows curve I, which represents the evolution of Ve, that is to say of the input voltage of the stimulation range, as a function of temperature. As seen on this curve, at the input of range, the stimulation voltage remains stable, in other words it is a reflection of the effectiveness of the stimulation. On the other hand, this tension tends to increase significantly for a sweep from low to high temperature from 25 ° C.

 In this same figure, the curve II represents the evolution of Vs, that is to say of the output voltage of the stimulation range, as a function of the temperature. There is a peak on this curve II, at about 25 ° C.

 Curve III represents the evolution of Vs / Ve, that is to say the ratio of input voltage / output voltage of the stimulation range, as a function of temperature. This report is representative of the robustness of the stimulation: the larger it is, the easier the printer will be to adjust because a single stimulation voltage makes it possible to form drops of quality over the entire temperature range. It is found here that from about 25 ° C, the drift is very important.

 Curve IV represents the evolution of the tension at the cusp Vr. This is initially stable, then, as the input voltage increases with temperature, from about 25 ° C.

 It was possible to establish curves which represent the evolution of the breaking length Lb as a function of the temperature (from 5 ° C. to 45 ° C., in steps of 5 ° C.) and the stimulation voltage. These curves are represented in FIG. 7B.

From these curves, we sought to determine how the efficiency of the stimulation changes as a function of temperature. For this, at a given voltage, it appears that the breaking length Lb can vary by a factor of 2 as a function of the temperature. Based on the theory of capillary instability, we obtain the expression: With:

 Lb: breaking length

 a: jet radius at the nozzle outlet

 Vj: average jet speed

 AVj: jet speed modulation (result of the stimulation process)

^: dimensionless growth rate of the modulations substantially constant over the working range (in particular the temperature range)

 We: Weber number The velocity modulation varies exponentially with the breaking length and therefore the stimulation varies in proportions much larger than a factor 2.

 The purpose being to compare modulation levels at different temperatures, it is shown that the stimulation efficiency falls between 20 ° C and 40 ° C. The influence of the temperature can vary by a few% the input parameters (typically by the surface tension, ...), which is unrelated to the orders of magnitude on the stimulation efficiency.

 To explain this sudden variation in efficiency, we can consider:

- a non-linearity, unidentified to date (unlikely);

 - or a phenomenon of resonance.

 One can thus be interested in the body of stimulation, by looking for the resonances in the solid and the liquid:

 As a first approximation it is reasonable to consider that the materials of the resonator for example the ceramic and stainless steel for the bar are stable over a range of a few tens of degrees. The charge brought back by the ink, on the actuator, does not make it possible to explain the drastic evolution on the efficiency of the stimulation.

In the liquid (everywhere the ink is present), there may be an acoustic resonance phenomenon, since the largest dimension is of the order of the wavelength. At 83 kHz and for a celerity of the order of 1200 m / s (in a MEK base ink), the wavelength is typically 15 mm, which is shorter but nevertheless comparable in order of magnitude to the height the stimulation body (here about 21 mm, in an example of geometry of Figure 5D).

 One can establish a relationship that expresses the dependence between the modulation generated by the piezoelectric actuator and AVj, the jet velocity modulation, including the phenomenon of propagation in the ink. The complete transfer function can be determined and the existence of resonance frequency related to the ink and in the vicinity of the working frequency. These frequencies (resonance or transmission zero (anti-resonance)) will then be subjected to a sensitivity study as a function of temperature. It is interesting to check whether these frequencies are drifting, and / or crossing the working frequency (imposed by the actuator).

 The drop generator can be interpreted schematically to list the main functional elements. Figure 8 (and its equivalent, in terms of electrical circuit, shown in Figure 9) shows the simplified version of the drop generator by highlighting 4 elements:

 - the source term: the piezoelectric actuator that modulates the ink flow (which is the inflow);

 - the terms of loss: these are the outgoing flows that balance the incoming flow. There are 3 terms: the ink block 520 under the actuator 52, the nozzle 50 and the upper part 550 of the stimulation body in which an acoustic wave can be propagated.

 The resonator body, for example stainless steel, is considered as non-deformable: the walls admit a zero speed condition whether in flow or propagation.

 We will re-specify the physical behavior of the functional elements of the drop generator and the equations associated with them. For this purpose, the impedances of each of the elements are determined.

The pressure drop across the nozzle 50 is described by the Navier Stokes equations. In sinusoidal mode, the movement of the mass of ink trapped in the nozzle is limited by the terms of inertia. The impedance of the nozzle will be noted Zb:

P L

 Z, = i (û

With:

 use: length of the nozzle

 Sb: section of the nozzle

 P: density of the ink

ω ^' · pulse at working frequency

The ink block 520 under the actuator concerns the column at the nozzle inlet (this column is located in the removable nozzle plate but before the zone 521 which connects it to the nozzle 50) and the "disk" of ink located under the active face of the actuator. For the column, the diameter is for example 500 μιη, compared to the nozzle diameter, again taken as an example, of 50 μιη. The speed of the ink in the hold is therefore very low (factor 100) compared to the nozzle. The fluid can therefore be considered as immobile (no inertia effect). The impedance of the hold is therefore reduced to its compressibility term noted Z _c :

i ω Ve

Where Ke is the compressibility and Ve the volume of ink of the zone 521. The waveguide 550 is an acoustic element delimited by the active face of the resonator; it goes up to the level of the shoulder 53 on which rests the resonator. This zone is flooded with liquid, so we consider the liquid ring between the resonator and the sheath of the stimulation body.

It is recalled that the column of liquid admits variations of section, the impedance of this column, by section, is given by the formula of the theory of the lines (in electrical analogy): Z _AB cos (& L _B ) + i Z _B sin (& L _B )

cos (k L _B ) + i - ^ - sin (& L _B )

Where ZBC is the input impedance of the AB acoustic impedance section Zb terminated a load impedance ZAB.

 The piezoelectric actuator admits, for its part, a resonant behavior that can be modeled by the localized constant approximation (mass-spring analogy). With regard to the relative impedances of the actuator with respect to the fluid, the actuator is dominant: at the first order, the resonant frequency of the stimulation assembly is calibrated on the resonance of ½ Langevin (the resonator) in the air .

 Since the working frequency is fixed (83.3 kHz), this mechanical resonance will not be considered for more legibility of the model. The resonant assembly is thus assimilated to a flow source, it is the volume of ink stirred at the end of the resonator: Q ..

 The elementary impedance terms being defined for the output flow, it is possible to determine the pressure P at the end of the bar. The pressure drop in the nozzle is related to its Zbuse impedance and gives the flow rate as a function of the frequency or the jet speed modulation for a given nozzle section.

 The above formulas made it possible to draw the frequency response curve (FIG. 10A) at a temperature of 5 ° C., namely the modulus of the jet speed modulation as a function of frequency. The unit of speed is normalized, which allows, in relative terms, to locate the frequencies for which the stimulation is reinforced.

(resonance phenomenon) or weakened (transmission zero, anti-resonance).

It can be seen from this figure that in the frequency range of interest, namely 80 - 90 kHz, there are two remarkable frequencies Fl and F2 which will affect the level of efficiency of the stimulation at 83.3 kHz. This frequency congestion is not a problem if these frequencies are stable in the operating environment of the printer; at most the level of stimulation may be different from one printer to another. But these frequencies Fl, F2, evolve according to the temperature which seems to be the disruptive parameter of robustness for stimulation. Simulations with "MathCad" software make it possible to identify the celerity of the ink as a highly influential parameter. At room temperature (see Physics Handbooks of 1990 to 1991 - 71 ^th edition - pages 14-32 and velocity measurements in real inks of the curve in Figure 16), the speed of the ink typically ranges from -3 to - 4 m / s per ° C.

 The same simulation was carried out over a temperature range of 45 ° C., as experimentally explored, which made it possible to show a frequency offset of F1, F2 of about 10 kHz (FIG. 10B). The sign of dependence of the celerity according to the temperature is important because the sliding in temperature makes the frequency F2 troublesome, while Fl leaves the zone of working frequency.

 This frequency shift may seem rather weak; nevertheless, combined with the proximity of Fl and F2 around 83.3 kHz, it is understood that it is possible to have significant variations in pacing levels when F2 crosses the working frequency.

 The tests reported above have made it possible to demonstrate an acoustic resonance phenomenon within the fluid cavity. This phenomenon is dependent on the speed of propagation of the acoustic waves within the ink; a dependence, depending on temperature, appears, which positions the events, in frequency, more or less close to the working frequency.

 Complementary results (real measurements) have been made, with the same type of stimulation agreements. These measurements implement a stimulation body identical to the previous simulated situation, with equivalent settings; the results are presented in Figure 11.

For these measurements, it was realized, with a low voltage (weak stimulation), the measurement of the breaking length Lb during a frequency sweep, this at different temperatures (5 ° C - 45 ° C), in order to visualize the events on the beach 70 - 100 kHz. The breaking length Lb is measured. These measurements are carried out over the temperature range of 5 ° C to 45 ° C, with a pitch of 10 ° C, using the following parameters: - M EK white pigment base ink,

 - jet speed: 20m / s

 stimulation signal (50% duty cycle time) generated by a laboratory apparatus,

 standard stimulation body (with the structure of FIG. 5D) equipped with a piezoelectric actuator whose resonant frequency is close to the operating frequency (which is the frequency of generation of the drops).

 The results shown in Figure 11 show many events around the 83.3khz working frequency. The curves intersect as a function of temperature and the absolute minimum of break length drifts significantly as a function of temperature. This operation degrades the robustness of the stimulation.

 These complementary results confirm the disturbances observed and already reported above. Furthermore, they illustrate the difficulty of - or even the impossibility of - maintaining a stable operation of a device generating drops at at least 2 positive temperatures at least 15 ° C apart or 20 ° C, for example d on the one hand at 5 ° C and / or 15 ° C and, on the other hand, at 30 ° C and / or 35 ° C and / or 45 ° C, more generally in a temperature range from from 5 ° C or 15 ° C to 35 ° C or 45 ° C or even 50 ° C, on the other hand.

 Other works have confirmed the hypothesis of the influence of disturbances related to the resonances present in the fluid cavity. Real measurements have been made on a generator of head drops G whose mechanical simplicity (cavity and resonator bar are cylindrical, the type of that of Figure 5A) makes it easier to calculate the resonant behavior of the fluid cavity.

 Complementary tests were thus performed for a stimulation body of the type of that of FIG. 5A.

More precisely, the breaking length, as a function of frequency, was studied in low stimulation for 3 different temperatures. Since the stimulation voltage is 7 volts, it is always possible to be in the presence of a "slow" satellite and thus, according to the linear theory of capillary instability, to directly link the break length to the stimulation efficiency. . The temperatures tested were 5 ° C, 25 ° C, and 45 ° C.

 The ink used is a pressurized white pigmented M EK base ink to achieve a constant jet speed of 20 m / sec. The tests were not performed at constant wavelength; as a result, the speed of the jet is not readjusted as a function of frequency, and we obtain a pa ra bolique-type envelope, which reflects the physical phenomenon of capillary instability which will be taken into account in the exploitation of the results.

In FIGS. 12A-C, the points resulting from the measurement of Lb, and the resonant and anti-resonance frequencies in the cavity, calculated numerically from the mechanical configuration of the generator and the celerities of the sound in FIG. the ink at different temperatures. Transmission (anti-resonance) zeros are identified by vertical bars. Peaks Pc (Figure 12 A and B), or Pci, Pc ₂ (Figure 12C) represent the resonance peaks in the liquid.

 For 5 ° C (Figure 12A):

 The theoretical model has been adjusted with a celerity in the ink of c =

1170 m / s. The resonant frequency of the actuator is approximately 64 kHz. The model also gives 2 trailing zeros corresponding to 46 kHz and 74 kHz. For 46kHz, we find the associated efficiency decline; but, for 74 kHz, it was not possible to record the values, the break being in the "noise" of the natural break.

 The model also predicts a resonance peak around 57 kHz clearly observed on the break length curve. The resonance phenomenon at 64 kHz is also highlighted, it is preponderant in terms of amplitude because imposed by the actuator.

 For 25 ° C (Figure 12B):

 The theoretical model has been adjusted with c = 1100 m / s, a slope of -

3.5m / s / ° C. The 2 transmission zeros are located at about 42 kHz and 69 kHz. This is well confirmed by the experimental data which translate, at these frequencies, by a sub-efficiency of stimulation. An acoustic resonance in the ink cavity is also well evidenced at around 53 kHz. The resonance of the actuator is also clearly visible, but the resolution is not sufficient to locate precisely this minimum breaking length which is probably between 63 kHz and 64 kHz.

 For 50 ° C (Figure 12C):

 The theoretical model was adjusted with c = 1030 m / s, a slope of - 3.5m / s / ° C. The first zero is slightly before 40kHz and the second at 65kHz. The latter is very close to the working frequency and is therefore superimposed on the resonance peak of the actuator located at 64kHz.

 To solve the anomalies observed above, it is proposed to adjust the acoustic impedance of the system, more particularly that of the fluid cavity, near the nozzle 10.

 This acoustic impedance varies according to the frequency, especially when it varies around the working frequency.

FIG. 13A shows the typical evolution of this acoustic impedance (near or at the level of the nozzle 10) as a function of the frequency and for a given temperature. The working frequency of the system (ie: the cavity and the actuator) is identified by f _t and Zr (ft) denotes the value of said acoustic impedance at this working frequency. This working frequency is defined by the cavity and by the resonator in the case of FIGS. 5A-5D. In the case of FIG. 5E, it is defined by the geometry of the stainless steel cylinder 32.

As can be seen in FIG. 13A, the acoustic impedance varies smoothly or smoothly around f _t . But, when disturbances of the type explained above appear, one or more peaks Pi, P ₂ , resonance or anti-resonance, appear on this graph, in particular in the vicinity of the working frequency, for example within a range of + 10 kHz or + 5 kHz around the latter.

 This impedance variation has the effect of varying the amplitude of the jet velocity modulation (or else stimulation efficiency) in the nozzle and therefore the breaking length.

In addition, the graph of Figure 13A changes with temperature. Peaks such as peaks p1, p2, not present in the target frequency range, at a certain temperature, for example at 5 ° C or 15 ° C, may appear in this same frequency range, at another temperature, for example at 30 ° C or 35 ° C.

According to the invention, a frequency range [f ₁ , f ₂ ] of + 10 kHz or + 5 kHz is defined around the working frequency f _t . The system is such that, when the frequency varies in this range, the value of the speed modulation in the nozzle at a temperature T, relative to the speed modulation in the nozzle at 25 ° C, does not vary outside of an interval between, on the one hand, 0.25 (or 0.5) and, on the other hand, 2 (or even 4), and that on the one hand, 15 ° C and, on the other hand, at 35 ° C, preferably also at 5 ° C, and / or at 10 ° C and / or 20 ° C, more preferably at 45 ° C or even 50 ° C, more preferably at temperature in a temperature range of at least 15 ° C (or 10 ° C or 5 ° C) to at least 35 ° C (or 40 ° C or 45 ° C or 50 ° C). An example of this velocity modulation interval is represented by horizontal bold lines in FIG. 13B. This avoids:

on the one hand, the presence, in an interval close to f _t , of peaks (such as P'1 and P'2 in FIG. 13B) reflecting disturbances;

- On the other hand a drift of such peaks, to f _t , depending on the temperature.

 It is noted that the impedance can be calculated according to the formula already mentioned above. From this calculation we can deduce the modulation of jet velocity and its variations under the effect of temperature.

 This velocity modulation can also be estimated or deduced from the measurement of the variations of U (of which, moreover, the formula has been given above) as a function of the frequency, at constant excitation voltage. Indeed, a variation of U reflects an impedance variation.

Alternatively, it is possible to measure or estimate pressure variations as a function of frequency. At the nozzle 10, these pressure variations represent or reflect the variations of U as well as the variations of acoustic impedance (ie of jet velocity modulation). The solution proposed above can be obtained by modifying the configuration of the internal volume of the stimulation body, intended to receive the ink, giving it a shape making it possible to achieve an acoustic impedance variation.

 In other words, the internal volume comprises at least a first portion, having a first acoustic impedance, and at least a second portion, having a second acoustic impedance, different from the first acoustic impedance.

 For example, it is possible to introduce into the cavities an element, or means, making it possible to carry out this impedance variation. Embodiments of this solution are shown in FIGS. 14A-14E.

 The device of FIG. 14A (respectively 14B, 14C, 14D, 14E) corresponds to that of FIG. 5A (respectively 5B, 5C, 5D, 5E) with the same reference numerals to designate the same elements. In each of these figures 14, a ring 27, 37, 47, 57, of annular shape, has been introduced into the internal volume of the cavity. The outer diameter of this ring is substantially equal to the inner diameter of the envelopes 25, 32, 42, while its inner diameter does not impede the flow of fluid. The material for this ring is preferably the same as that of the resonator, for example stainless steel.

 In these figures, the ring is shown in the lower part of the cavity. Alternatively, it could be arranged in another part, for example according to the arrangement shown in phantom in each of these figures. It would then have the same role of modifying the acoustic impedance of the cavity.

 More generally, it can also be seen in these figures that the internal shape of the cavity comprises:

 a first cylindrical zone 25i, 32i, 42i, 52i, 62i of a first diameter, and of a first length, measured along a longitudinal axis of said cavity,

a second cylindrical zone ₂ , 32 ₂ , 42 ₂ , 52 ₂ , 62 ₂ , of a second diameter, different from the first diameter, and a second length, measured along a longitudinal axis of said cavity. In the case where the ring of each of FIGS. 14 is positioned according to the position indicated in broken lines, the first cylindrical zone and the second cylindrical zone are different from those mentioned above.

 As will be shown below, the differences, or variations, in acoustic impedance induced in the examples of FIGS. 14 by the various diameters in the cavity make it possible to eliminate, from the working frequency zone. , the parasitic frequencies which result from the resonances specific to the cavity containing the liquid and thus to stabilize the modulation of velocity.

The different diameters make it possible to vary the length of the fluid. In the case of structures of Figures 14A and 14D, in which the resonator is immersed in the cavity for receiving the fluid, we created a ratio between the length _L of the actuator (or actuator) mechanical (comprising the piezoelectric element 21, 51, the flange 23, 53 and the part 22, 52, which is in contact with the fluid) and the length Lf of the, or of a portion of the cavity intended to receive a column of fluid preferably greater than 4; this ratio can be for example between 4 and 6 or 4 and 10 or 100. In the case of Figure 14D, the length Lf corresponds to the length of the portion of the zone B not occupied by the ring 57. Even if a fluid column remains, the length of which is not modified by the presence of the ring, the modification of the length of a portion of the cavity, intended to receive the fluid, eliminates, from the working frequency zone, parasitic frequencies.

 Tests were performed with a stimulation body structure according to FIG. 14D, with a ring whose length at the end of the study was 3.6 mm. The results are illustrated in FIGS. 15A-15C:

 FIG. 15A represents the evolution of the voltages Ve, Vs, Vr and of the ratio Vs / Ve, as a function of the temperature; this figure 15A shows that there is an almost linear variation of the piezoelectric instructions. It therefore compares very advantageously with the results which have been commented on above in connection with FIG. 7A;

FIG. 15B shows the breaking length Lb, as a function of the activation voltage, at various temperatures (5 ° C. to 45 ° C., with a pitch of 10 ° C., at 5 ° C., 15 ° C, 25 ° C, 35 ° C, 45 ° C); we find that the curves are stacked correctly, in a good order; again, the comparison with the curves of FIG. 7B is very advantageous,

 FIG. 15C shows the break length Lb, as a function of frequency, at various temperatures (5 ° C - 45 ° C, with a pitch of 10 ° C, at 5 ° C, 15 ° C, 25 ° C, 35 ° C, 45 ° C); the curves are stacked correctly, in a good order according to the temperature, and do not intersect. This result is much greater than that observed in Figure 11 where the order is incorrect and where the curves intersect.

 Additional tests were carried out with a standard "standard MEK base" ink and then with an "alcohol base" ink. The results obtained are similar with the previous 2 inks and confirms the optimum nature of the 3.6mm ring.

 The presence of the ring makes it possible to reduce the volume of the cavity in ink, which facilitates the rinsing of the drop generator during maintenance operations.

 The above tests show that the invention makes it possible to obtain a robust operation throughout the temperature range envisaged and ink (through celerity). The invention makes it possible to eliminate any disturbing event on the stimulation efficiency. There is a marked improvement in most of the curves obtained, that is to say that we move from a random operation to a well-controlled operation.

 The embodiment of the invention with the insertion of a ring into the cavity of the modulation body can be replaced by the direct machining of the ring function in the modulation body which then becomes monobloc and which presents variations of section, thereby having a profile identical or similar to that shown in Figures 14A-14E.

According to another embodiment, the differences in velocity of sound waves in various materials other than stainless steel are exploited. The stainless material used for the resonator is therefore replaced by one of these other materials. This solution makes it possible to satisfy the conditions stated above in connection with FIG. 13B.

 This solution also makes it possible to modify the length of the resonator while keeping the same operating frequency. The choice of another material is accompanied by a modification of the length of the resonator, which in first approach is proportional to the ratio of the celestialities;

 If the celerity is greater than in stainless steel, the bar (case of FIGS. 5A and 5D or 14A and 14D) under flange of the resonator will be lengthened; conversely, if the celerity is lower, the bar under collar will be shortened. The length of the resonant cavity containing the fluid may therefore be modified, for example according to the previous teaching according to the present invention:

the jet velocity modulation, at the exit of the nozzle, having a value AVj (ft) at the working frequency of the cavity and the actuator, and this modulation of the velocity of the jet, at the temperature of 15 ° C. and at a temperature of 35 ° C, not varying, within a frequency range of + 5 kHz around the working frequency f _t , outside the range of between 0.25 AVj (f _t ) and 4 AVj ( f _t ). ;

 - And / or the ratio between the length of the actuator (or actuator) and the mechanical length of the or a portion of the cavity for accommodating a fluid column being strictly greater than 4; this ratio can be for example between 4 and 6 or 4 and 10 or 100.

 In this case, the resonant and resonant frequencies of the fluid cavity will be displaced and rejected outside the operating area of the stimulation.

Table I collects data relating to the speed of sound waves in these other materials. Speed

 Material

 (m / s) (ft / s)

 Aluminum 6420 21063

Beryllium 12890 42530

Brass 3475 11400

Copper 4600 15180

Diamond 12000 39400

Glass 3962 13000

Pyrex glass 5640 18500

3240 10630 Gold

Iron 5130 16830

Lead 1158 3800

Lucite 2680 8790

Money 3650 12045

Steel 6100 20000

Stainless steel 5790 19107

Titanium 6070 20031

 Table I

If one of these other materials is retained for the resonator bar, then the effects of disturbance of the sound waves in the ink will not manifest.

 More generally, all metallic materials - other than stainless steel - or minerals may be suitable.

This choice also makes it possible, in addition, to reduce the length of the resonator, and thus the length of the cavity, which makes it possible, even more, to avoid the parasitic resonances as explained above. Whether the structure of the stimulating body is that of one of FIGS. 5A-5D or 14A-14D, the disturbance effects due to resonance in the cavity containing the ink will not occur.

 An ink jet device or printer for implementing a method of forming ink drops, with a device according to one of the embodiments detailed above, is of the type which has already been described hereinafter. above in conjunction with Figures 1 and 2.

 Such a device therefore comprises:

 a drop generator 60 containing electrically conductive ink, maintained under pressure, by an ink circuit, and emitting at least one ink jet,

 a charge electrode 64 for each ink jet, the electrode having a slot through which the jet passes

 an assembly consisting of two deflection plates 65 placed on either side of the path of the jet and downstream of the charging electrode,

 a gutter 62 for recovering the jet ink not used for printing in order to be returned to the ink circuit and thus be recycled.

 The operation of this type of jet has already been described above in connection with FIGS. 1 and 2. It will be recalled here simply that the ink contained in the drop generator escapes from at least one calibrated nozzle 10 thus forming at least one ink jet. Under the action of a periodic stimulation device placed upstream of the nozzle (not shown), consisting for example of a piezoelectric ceramic placed in the ink, the ink jet breaks at regular time intervals, corresponding to the period of the stimulation signal, in a specific location of the jet downstream of the nozzle. This forced fragmentation of the ink jet is usually induced at a point called "break"

13 of the jet by the periodic vibrations of the stimulation device.

In addition to the means above, such a device may further comprise means 5 for controlling and regulating the operation of each of these means taken individually, and the applied voltages. These means 5 are described below in more detail with reference to FIG. 17. In this figure, a set of controller means 5 comprises circuits, which make it possible to send to the print head the voltages making it possible to drive the latter and in particular the voltages to be applied to the electrodes as well as the piezoelectric excitation voltage. electric.

 This assembly 5 can additionally receive downstream signals coming from the head, in particular the signals measured by means of a position and / or drop speed sensor, and can process them and use them for the control of the head and the ink circuit. In particular, to process the signals from such a sensor, it may comprise means for analog amplification of a signal of this sensor, means for digitizing this signal (A / D conversion transforming the signal into a list of digital samples), means for denoising it (for example one or more digital filters of the samples), means for searching for the maximum (the maximum of the list of samples)

 This controller assembly 5 may communicate with means 500 for sending and / or receiving fluids to and from the print head.

 This controller assembly 5 may communicate with the user interface 6 to inform a user about the status of the printer and the measurements taken, in particular, of the type described above. It comprises storage means for memorizing the instructions relating to data processing, for example to carry out a method or to implement an algorithm of the type described above.

 According to an exemplary embodiment, the controller 5 comprises an onboard central unit, which itself comprises a microprocessor, a set of non-volatile memories and RAM, peripheral circuits, all of these elements being coupled to a bus. Data may be stored in the memory areas, including data for implementing a method according to the present invention or for controlling a device according to the present invention.

The means 6 allow a user to interact with a printer according to the invention, for example by performing the configuration of the printer to adapt its operation to the constraints of the production line (cadence, printing speed, etc.) and more generally its environment, and / or the preparation of a production session to determine, in particular the content of the printing to be carried out on the products of the production line, and / or by presenting the real-time information of the production monitoring (condition of the consumables, number of marked products, ...). These means 6 may comprise display means.

 Means may also be provided to supply or carry the different electrodes to the desired voltages. These means comprise in particular voltage sources.

 A stimulation body according to the invention, and a method of operating a stimulation body according to the invention, as described above, applied to a printer of the type described in connection with FIGS. 1 and 2, the operation has been recalled above, provides a robust stimulation, which does not present the problems presented in the introduction to the present application in connection with the known devices. In particular, the stimulation is much more stable, at least 2 temperatures at least 15 ° C or more apart, especially at 15 ° C and 30 ° C (or 35 ° C), preferably also at 5 ° C C, and / or at 10 ° C and / or at 20 ° C, more preferably at 40 ° C or 45 ° C or even at 50 ° C, more preferably at any temperature in a range of between 15 ° and 35 ° and more generally between 5 ° and 50 ° C.

 With a device and a method according to the invention, the frequencies

"Parasites" are found, regardless of the temperature in one of the ranges mentioned above, of the operating frequency range used. For example, this operating range is between 50 kHz and 150 kHz depending on the chosen diameter and speed of the jet.

Claims

Apparatus for forming and ejecting drops of an ink jet from an ICJ printing machine, said device comprising:

 a) a cavity (25, 32, 42) for containing an ink and having an end provided with a nozzle (10) for ejecting drops of ink,

 b) actuator means (21, 22, 32, 41, 42) in contact with the cavity,

device in which the jet velocity modulation, at the outlet of the nozzle (10), has a value AVj (f _t ) at the working frequency of the cavity and the actuator, and this modulation of jet velocity, at the temperature of 15 ° C and the temperature of 35 ° C, does not vary, within a frequency range of + 5 kHz around the working frequency f _t , outside the range of 0.25AVj (f _t ) and 4AVj (f _t ).

2. Device according to claim 1, wherein the modulation of jet velocity, at the outlet of the nozzle (10), does not vary, also at temperatures of 5 ° C and 45 ° C and / or 50 ° C, in a frequency range of + 5 kHz around the working frequency f _t , outside the range 0.25 AVj (f _t ) and 4 AVj (f _t ).

3. Device according to claim 1 or 2, wherein the internal volume of the ink cavity comprises at least a first part, having a first acoustic impedance, and at least a second part, having a second acoustic impedance, different from the first one. acoustic impedance.

4. Device according to one of claims 1 to 3, the internal shape of the cavity comprising:

a first cylindrical zone (25i, 32i, 42i, 52i, 62i) having a first diameter, and a first length, measured along a longitudinal axis of said cavity, a second cylindrical zone (25 ₂ , 32 ₂ , 42 ₂ , 52 ₂ , 62 ₂ ) having a second diameter, different from the first diameter, and a second length, measured along a longitudinal axis of said cavity.

5. Device according to claim 4, the cavity having a cylindrical internal shape, of diameter equal to said first diameter, and being provided with a cylindrical ring (27, 37, 47) whose inner diameter is equal to said second diameter.

6. Device according to claim 4, the cavity being delimited by a pa ri having a first cylindrical portion, of internal diameter equal to said first diameter, and having a second cylindrical portion, whose inner diameter is equal to said second diameter.

7. Device according to one of claims 1 to 6, the resonator means comprising a member (21, 31, 41) piezoelectric.

8. Device according to one of claims 1 to 7, the actuator being directly in contact with the internal volume of said cavity.

9. Device according to one of claims 1 to 7, the actuator comprising resonator means.

10. Device according to claim 9, the resonator means comprising a body (22, 52) resonator disposed in said cavity.

11. Device according to claim 10, said body (22, 52) of resonator being made of stainless steel, or aluminum, or beryllium, or brass, or copper, or diamond, or glass, or gold, or iron, or plom b, or TM MA, or silver, or titanium.

The device of claim 10 or 11, said resonator body (52) having a first portion (52i) having a first diameter and a second portion (52 ₂ ) having a second diameter, different from the first.

13. Device according to claim 9, the internal volume of the cavity being delimited by a wall (32, 42) forming a resonator.

14. Apparatus for forming and ejecting drops of an ink jet from an ICJ printing machine, said device comprising:

 a) a cavity for containing an ink and having an end provided with an ink drop ejection nozzle,

 b) means (21, 22, 32, 41, 42) forming a resonator, in contact with the cavity, made of a material chosen from aluminum, beryllium, brass, copper, diamond, glass, gold, iron, lead, TMMA, silver, or titanium.

15. Device according to claim 14, the resonator means comprising a piezoelectric element (21, 31, 41).

16. Device according to one of claims 14 or 15, the resonator means comprising a body (22, 52) resonator disposed in said cavity.

The device of claim 16, said resonator body (52) having a first portion (52i) having a first diameter and a second portion (52 ₂ ) having a second diameter, different from the first.

18. Device according to one of claims 14 or 15, the internal volume of the cavity being delimited by a wall (32, 42) forming a resonator.

19. Continuous inkjet type printing machine, this machine comprising:

 - a print head (1), provided with a device for forming and ejecting drops of an ink jet according to one of claims 1 to 18,

 an ink circuit (4),

 means (5) for controlling the circulation of the ink and the print head.

20. A process for forming ink drops, in which a device according to one of claims 1 to 18 or a machine according to claim 19 is implemented.