ROBUST DROP PRODUCER

Description

DOMAINE TECHNIQUE ET ÉTAT DE LA TECHNIQUE ANTÉRIEURE

[0001] The invention relates to improving the operation of a print head of a CIJ printer to make it more robust with respect to environmental variations (in particular temperature) encountered during industrial use of this type of printer.

[0002] This improvement involves increasing the robustness of the drop generator's stimulation function with respect to temperature.

[0003] Continuous inkjet (CIJ) printers are well known in the field of industrial coding and marking of various products, for example for marking barcodes or expiry dates 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.

[0004] CIJ printers continuously generate jets of drops, some of which are selected and directed towards the substrate to be printed, while the others are recovered for recycling. These printers have several typical sub-assemblies as shown in the figure. figure 1 : First of all, a print head 1, generally offset from the body of the printer 3; it is connected to it by a flexible umbilicus 2 bringing together the hydraulic and electrical connections necessary for the operation of the head, giving it a flexibility which facilitates integration on the production line.

[0005] The body of the printer 3 (also called the desk or cabinet) usually contains three subassemblies: 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 take charge of the ink from the jets not used for printing. a controller 5 located at the top of the console (zone 5'), capable of managing the sequencing of actions and carrying out the processing allowing the activation of the various functions of the ink circuit and the head. an interface 6 which gives the operator the means to operate the printer and to be informed about its operation.

[0006] This description can apply to continuous jet (CIJ) printers known as binary or multi-deflected continuous jet.

[0007] Binary CIJ printers are equipped with a head whose drop generator has a multitude of jets whose drops can only be directed towards 2 trajectories: printing trajectory or recovery trajectory.

[0008] In multi-deflected continuous jet printers, each drop of a single jet (or of a few spaced jets) can be deflected along various trajectories corresponding to different commands. A succession of drops subject to different commands can thus perform a scan of the area to be printed in one direction which is the deflection direction, the other scanning direction of the area to be printed is covered by relative movement of the print head and the support to be printed 8. Generally the elements are arranged in such a way that these 2 directions are substantially perpendicular.

[0009] Continuous inkjet 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 a drop generator or stimulation body, means 62 for recovering ink not used for printing, means 65 for deflecting the drops intended for printing, means for controlling and managing the drop deflection process (synchronization of the formation of the drops with the deflection commands).

[0010] Referring to the figure 2 which schematizes a multi-deflected CIJ print head, there is a drop generator 60 in which a cavity is supplied with electrically conductive ink. This ink, maintained under pressure, by the ink circuit 4, generally external to the head, escapes from the cavity by at least one calibrated nozzle 10 thus forming at least one ink jet 7.

[0011] A periodic stimulation device 63 is associated with the cavity in contact with the ink upstream of the nozzle 10; it transmits, to the ink, a periodic modulation (of 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 is amplified in the jet under the effect of the surface tension forces responsible for the capillary instability of the jet, until the jet breaks. This break is periodic and occurs at a precise distance from the nozzle at a point called the "break" 13 of the jet, a distance which depends on the stimulation energy.

[0012] In the case where a stimulation device, called an actuator, whose driving member is a piezoelectric ceramic, is in contact with the ink in the cavity upstream of the nozzle, the stimulation energy is directly linked to the amplitude of the electrical signal driving the ceramic. The prior art teaches other means of jet stimulation (thermal, electro-hydrodynamic, acoustic, etc.) but stimulation using piezoelectric ceramics remains the most widespread for its effectiveness and relative ease of implementation.

[0013] At its breaking point 13, the jet, which was continuous when leaving the nozzle, transforms into a train 11 of identical and regularly spaced ink drops. The drops form at a temporal frequency identical to the frequency of the stimulation signal and for a given stimulation energy, all other parameters being stabilized elsewhere (in particular the viscosity of the ink), there is a precise (constant) phase relationship 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 a precise instant in the period of the stimulation signal corresponds to a precise instant in the dynamics of separation of the drop from the jet.

[0014] Without additional action (this is the case where the drops are not used for printing), the train of drops travels along a trajectory 7 colinear with the drop ejection axis (nominal trajectory of the jet) which joins, by geometric construction of the print head, the recovery gutter 62. This gutter 62 for recovering unprinted drops captures the unused ink which returns to the ink circuit 4 in order to be recycled.

[0015] To print, the drops are deflected and moved away 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 impact points. All these trajectories are in the same plane. The placement of the drops on the drop impact matrix 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 support to be printed (generally perpendicular to the deflection plane). In deflected continuous jet printing technology, the deflection is obtained by electrically charging the drops and passing them through an electric field. In practice, the drop deflection means comprise an individual charging 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 to a predetermined electrical charge value which is generally different from one drop to another. To do this, the ink being maintained at a fixed potential in the drop generator 60, a voltage pulse of a determined value, carried by the control signal, is applied to the charging electrode 64, this value being different at each drop period.

[0016] In the charging electrode control signal, the voltage application time is a little before the jet splits to take advantage of the electrical continuity of the jet and attract a given quantity of charges, depending on the voltage value, at the end of the jet. This variable charging voltage allowing deflection is typically between 0 and 300 Volts. The voltage is then maintained during splitting to stabilize the charge until the detached drop is electrically isolated. The voltage remains applied for a little while after to take into account the hazards of the breaking time.

[0017] We therefore seek to synchronize the moment of application of the voltage with the process of splitting the jet. In the event of desynchronization, the droplet concerned is not charged correctly, its charge is weaker, or even zero.

[0018] The drop deflection means also comprise a set of 2 deflection plates 65 placed on either side of the drop trajectory 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 drop trajectory, capable of deflecting the electrically charged drops which engage between the plates. The amplitude of the deflection is a function of the charge, mass and speed of these drops.

[0019] In order to control the deflection of drops for printing, we seek to produce a quality break in the range of variation of environmental conditions provided for in the specifications.

[0020] We therefore seek to ensure: on the one hand, that the break is in the field of the charging electrode, therefore at a determined distance from the nozzle (break position), and, on the other hand, that the jet break is done in a stable and reliable manner (break quality: which will be specified below). This is done by an optimal adjustment of the stimulation which is practically achieved by acting on the stimulation energy. In a majority of 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).

[0021] A break is considered stable and reliable (of good quality), when it allows to guarantee an optimal charge of the drops in a printer operating range characterized in particular by a temperature range (conditioning the viscosity of the ink) for a given ink.

[0022] Concretely, just before breaking, the drop is connected by a tail to the next drop being formed (see figure 3A ). The shape of this tail determines the quality of the break. The most characteristic shapes of a problem break are the following (but many more or less stable intermediate situations can exist): very thin tail (see fig.3B ) which risks breaking in an unstable manner (the surface tension cohesion forces become weak compared to the electrostatic forces). When there is a very strong electric field between 2 successive drops charged at very different values ​​(case of a strong charge followed by a weak charge), a peak effect phenomenon at the tail creates electrostatic forces such that charged particles of matter are torn from the very thin tail of the highly charged drop and join the weakly charged drop by transferring charges. As a result, the drops no longer have their nominal charge, the deflection is disturbed and the printing quality deteriorates. tail having a lobe between 2 constrictions (see fig.3C ), which can break in 2 places and create a satellite isolated from the drop, which carries part of the charges intended for the drop concerned: *if its speed is faster than the jet (fast satellite), the satellite and its charges will join the drop concerned and reconstitute a nominal situation without any noticeable consequence on the print quality, *if the speed of the satellite is identical to that of the jet (infinite satellite) or does not join the drop concerned before its deflection, it will be poorly charged and the satellites will be violently deflected with the risk of clogging the print head, *if it joins the next drop (slow satellite) it will transfer charges from the drop concerned to the next and disrupt the deflection.

[0023] The shape of the break, in addition to the rheological characteristics of the ink, is linked to the level of stimulation (excitation intensity). The shape of the break determines the quality of the break, i.e. its ability to guarantee the correct charge of the drops.

[0024] Typically, it changes as excitation increases, from satellite breakup to satellite-free breakup. A satellite is defined as a secondary droplet resulting from the breakup of the main droplet.

[0025] By further increasing the stimulation level, the break returns to a satellite regime. At the same time, the position of the break relative to the nozzle evolves according to the curve of the figure 4 .

[0026] The latter represents the profile of the characteristic f giving the break distance (L b ) between the nozzle 10 and the break point 13, as a function of the stimulation voltage VS (L b = f (VS)). This curve will subsequently be called: stimulation curve. This is established by scanning the values ​​of the stimulation excitation voltage VS and determining Lb for each value of VS.

[0027] When the stimulation excitation increases (from a low value), the nozzle / break distance (L b ), which starts from a high value (natural jet break), decreases and passes through a minimum called the "cusp point", then lengthens again. The actual shape and 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 lies, at least partly, in the field of the charging electrode despite the variability of the mentioned parameters. On the other hand, there is a functional area related to the break quality in which the printing is satisfactory (the droplet charging is correct). The intersection of the correctly positioned area in the electrodes and the functional area of ​​break quality corresponds to the operational range of stimulation.This stimulation range is characterized by an entry point (Pe) on the left, and an exit point (Ps) on the right as shown in the . figure 4 The stimulation system will be satisfactory if the operational stimulation range is sufficiently well defined regardless of the printer operating conditions.

[0028] At least two distinct modes of operation of piezoelectric stimulation are used in state-of-the-art inkjet printers: resonant and non-resonant stimulation modes.

[0029] Non-resonant stimulation is relatively difficult to implement and requires significant energy because the actuator must provide all the energy required to create the displacement of the portion of the actuator in contact with the ink in order to generate the pressure modulation upstream of the nozzle. In return, this mode is relatively tolerant to variability in excitation conditions.

[0030] In comparison, resonant stimulation has a much more advantageous efficiency in the context of periodic stimulation which leads to the periodic breaking of a jet into drops at a fixed frequency, as is often the case in continuous jet printing processes. Indeed, in this case it is very efficient to design an actuator as an oscillating or vibratory system, tuned substantially to the drop emission frequency; a weak periodic excitation can then maintain an amplified standing wave which will generate the displacement amplitude necessary for pressure modulation upstream of the nozzle.

[0031] Under reasonable implementation conditions, a simple piezoelectric ceramic (used in D33 mode, the electric field created between 2 electrodes deposited on the ceramic then producing a longitudinal stretching or contraction of the latter depending on the direction of polarization and the polarity of the electrical signal) cannot be used alone as an actuator because it would not have a sufficient deformation amplitude (of the order of only a nanometer) to create the expected modulation of ink ejection speed; it is therefore attached to a part, called a resonator, used for motion amplification. The ceramic / resonator assembly is called an actuator.

[0032] It has been observed that, for certain inks and drop generator sizes, the stimulation efficiency is not stable as a function of temperature.

[0033] This may go as far as making it impossible 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 two by two, in particular between 15°C and 35°C or between 5°C and 45°C (or even 50°C).

[0034] In fact, under certain conditions, the stimulation becomes completely ineffective and the operational range of stimulation shifts and / or atrophies to the point, in some cases, of disappearing, which makes it impossible to adjust the machine.

[0035] In some cases, it is possible to try to adapt the stimulation setting according to the expected temperature evolution range during the production session in which the printer is used. However, this is not always possible.

[0036] Finally, if we want to compensate for this instability, we must implement additional means (temperature control at the head, for example), which also imposes an additional cost.

[0037] The problem therefore arises of finding a device and a method which allow satisfactory operation at at least two different temperatures of at least 15°C or 20°C, in particular, on the one hand at 5°C (and / or at 15°), and on the other hand 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).

[0038] Another problem, in a system implementing a resonant mechanical actuator, the resonance of the actuator couples to the resonance of the fluid, in particular by the fact that the ratio of the acoustic speeds, on the one hand in the material used for the resonator (for example stainless steel) and on the other hand in the fluid (approximately 5000 m / s in the resonator, approximately 1250 m / s in the fluid) is of the order of 4, or a quarter of a wavelength. The consequences of this ratio are the coupling mentioned above.

[0039] JP2006-76039 shows a device for forming and ejecting drops from an ink jet having a cavity containing an ink, the internal volume of this cavity comprising a first part, having a first acoustic impedance, and a second part, having a second acoustic impedance, different from the first acoustic impedance EXPOSÉ DE L'INVENTION

[0040] The invention aims to solve these problems.

[0041] Its first subject is a device for forming and ejecting drops from an ink jet of a CIJ printing machine according to claim 1

[0042] The acoustic impedance does not vary, or varies little, in a frequency range of ± 5 kHz around the working frequency ft, so that the variation of the speed modulation in the nozzle remains between, on the one hand, 0.25, and, on the other hand, 4 times the speed modulation at the temperature of 15°C at the temperature of 35°C.

[0043] Preferably the acoustic impedance does not vary, or varies little, in a frequency range of + 5 kHz around the working frequency ft, such that the variation of the speed modulation in the nozzle remains between, on the one hand, 0.5, and, on the other hand, 2 times the speed modulation at the reference temperature (for 25°C for example), and this at at least 2 positive temperatures at least 10°C or 20°C apart, in particular at 15°C and 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 included in a temperature interval which contains at least the interval [15°C - 35°C], or even at least the interval [5°C - 50°C].

[0044] Such a device according to the invention makes it possible to shift the resonance and antiresonance frequencies, due to the ink cavity, so that their drift as a function of temperature does not cause them to cross the jet stimulation frequency, at least at 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 within a range 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.

[0045] Preferably, said cavity is such that the ratio between the length of the mechanical actuator (or actuator) and the length of the, or a, portion of the cavity intended to accommodate a column of fluid, is strictly greater than 4; this ratio may be for example between 4 and 6 or 4 and 10 or 100.

[0046] 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.

[0047] This creates a cavity with at least two cylindrical sections of different diameters, so as to shift the frequency eigenmodes of the ink cavity for sound speeds in usual inks. Cylindrical sections of different diameters allow variation in the fluid length.

[0048] The actuator means, for example a piezoelectric ceramic, may be in direct contact with the internal volume of the cavity.

[0049] The means forming the actuator may comprise a resonator element. The actuator is then resonant.

[0050] According to one embodiment, this resonator element comprises a resonator body arranged in the cavities.

[0051] According to another embodiment, the walls of the cavity form at least part of the resonator.

[0052] The resonator can be of metallic or mineral 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.

[0053] The resonator body may comprise a first portion having a first diameter and a second portion having a second diameter, different from the first.

[0054] The length of the ink cavity is generally comparable to the length of the resonator under the collar, the latter being chosen to allow the mechanical resonance of the actuator.

[0055] The physical properties of the resonator are adjusted to allow the device to resonate at a given frequency.

[0056] 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 resonance and antiresonance frequencies, which are undesirable in the ink, outside the useful domain (actuator resonance).

[0057] The choice of such a material for the means forming the resonator therefore makes it possible to further eliminate parasitic resonances due to a liquid contained in the cavity.

[0058] The means forming the resonator may comprise a piezoelectric element.

[0059] The resonator can be inserted into a resonator body of constant or variable section in the longitudinal direction.

[0060] This resonator body may comprise a first part having a first diameter and a second part having a second diameter, different from the first.

[0061] A drop forming and ejecting device according to the invention contains an ink, for example an ink in which the speed of sound is between 800 and 2000 m / s.

[0062] The invention also relates to a continuous ink jet (CIJ) type printing machine, 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.

[0063] 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.

[0064] The invention allows the principle of resonant stimulation to be retained with its advantages (efficiency, cost).

[0065] It can be applied to different types of drop generator implementation.

[0066] The combination of the two embodiments presented (cavity presenting several acoustic impedances, and specific material chosen for the resonator) makes it possible to limit certain drawbacks specific to each mode; in particular, it makes it possible to obtain a compromise between: a satisfactory size, since it is linked to the length of the bar (depending among other things on the speed of the sound), an ease of cleaning of the cavity, in relation to the complexity and dead volume of ink in the cavity. BRÈVE DESCRIPTION DES DESSINS

[0067] There figure 1 is a diagram of the structure of a deflected continuous jet printer, the figure 2 is a diagram of a print head of a deflected continuous jet printer, the figures 3A - 3C represent various break configurations, the figure 3A representing a good quality break, the figure 3B a fine-tailed break (with risk of material tearing) and the figure 3C a lobe break (with risk of satellites), the figure 4 is a curve indicating the evolution of the breaking distance as a function of the stimulation excitation, the figures 5A - 5E represent structures of stimulation bodies 20, 30, 40, 50 and 60 to which the invention can be applied, the figure 6 is a stimulation efficiency curve, giving the break length as a function of the jet excitation frequency, the figures 7A - 7B represent results obtained with a stimulation body of the type of the figure 5D , there Figure 8 illustrates a schematic model of a stimulation body, the Figure 9 is an electrical analogy of the equivalent diagram of a stimulation device, the figures 10A - 10B represent the frequency response of a stimulation body for 2 different ink temperatures, the figure 11 represents other complementary results; the figures 12A-C represent test results obtained with another type of stimulus body, the figure 13A represents the evolution of the acoustic impedance as a function of the frequency and the figure 13B represents the evolution of the modulation of the jet speed as a function of the frequency, the figures 14A-E represent stimulation body structures implementing the invention, the figures 15A - 15C represent test results obtained with a stimulation body with the invention, the figure 16 collects ultrasonic velocity data for different inks, as a function of temperature, figure 17 is a schematic representation of the control means of an inkjet printer EXPOSÉ DÉTAILLÉ DE MODES DE RÉALISATION PARTICULIERS

[0068] We have represented in figures 5A, 5B, 5C , 5D and SE, five types of embodiment of stimulation actuator in a stimulation body 20, 30, 40, to which the invention can be applied. Some ( figures 5A , 5D ) include a resonator which is intended to plunge into the ink when it is present in the cavity.

[0069] The stimulation body 20 of the figure 5A comprises an envelope 25 whose internal volume preferably has a cylindrical shape and extends along an axis XX'.

[0070] 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.

[0071] This ceramic is metallized on its 2 faces 210, 212, perpendicular to the axis XX'. It is secured, coaxially, to a cylindrical metal bar 22. For example, the securing is achieved by bonding with an adhesive, which can advantageously be a conductive adhesive.

[0072] According to the illustrated embodiment, this bar comprises a circular collar 23 on which the face 212 of the ceramic is fixed.

[0073] 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 may be introduced. A bearing surface 230 of the circular collar 23 may thus come to bear on the internal bearing surface 250.

[0074] Mechanical means, not shown, make it possible to center and clamp the collar 23 (therefore the actuator) on the surface 250.

[0075] The internal volume of the envelope 25, located under the surface 250 and the collar, defines an isolated cavity 24.

[0076] In operation, the cavity is supplied with pressurized ink by a conduit 26.

[0077] A nozzle 10, from which the jet emerges, 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 above and close to the nozzle 10, preferably at a distance of a few tenths of a mm, for example between 2 / 10ths < mm and 5 / 10ths < mm.

[0078] Each of the internal elements (actuator, casing 25, nozzle 10) of the modulation body has a circular section and these different elements are placed coaxially with respect to each other, on the XX' axis.

[0079] For practical reasons, bar 22 is preferably: of significant hardness (formable by machining), in a conductive or metallized material, to transfer the zero of the electrical voltage applied to the ink to one of the electrodes of the ceramic 21, insensitive to corrosion if it is in contact with the ink.

[0080] One material that can be used is stainless steel, which has all the characteristics mentioned above.

[0081] By construction, the bearing surface 27 of the collar 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.

[0082] On the other hand, it is preferable that the end 220 of the bar 22, which is located above the nozzle 10, benefits from a maximum amplitude of movement which corresponds to a vibration antinode.

[0083] In practice, the actuator can be tuned so that the resonance is close to the working frequency (called the "drop" frequency, or the frequency with which the drops are to be generated), but not exactly identical so as not to make the system too sensitive to variations in the actuator's operating conditions (mechanical tolerances from one actuator to another, for example). Tuning is generally done in air, at a frequency offset from the working frequency, to take into account the frequency shift linked to the impedance difference existing when the bar is in different media (ink, for example).

[0084] In this example, the part of the bar 22 under the collar 23 is installed in the cavity 24 (body of the drop generator) whose length is substantially identical to that of the bar 22.

[0085] In operation, the electrode 210 of the ceramic 21 is connected to voltage means 27. The body 25 can be connected to a ground 29 which will be transferred to the electrode 212 via the collar 230.

[0086] There figure 5B describes a second embodiment of resonant modulation body 30.

[0087] Its operation is close to that described above in connection with the figure 5A .

[0088] There is a cavity 34, of cylindrical internal shape, delimited by two end surfaces 320, 322, perpendicular to the axis XX'. Pressurized ink is brought into this cavity by a conduit 36. A 1st end of this tubular cavity is closed by the partition 322 perpendicular to the axis XX'. A nozzle 10 is formed in the 2nd end partition 320, to let out a jet along the axis XX'.

[0089] It is the envelope 32, which delimits the cavity 34, which performs the function ensured by the bar 22 of the first embodiment. It is excited by a piezoelectric ceramic 31 secured by mechanical means or by gluing 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 level of the plate 320, provided with the nozzle 10. The length L of the envelope is therefore 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 of the impedance brought back by the ink present in the cavity must be taken into account to tune the assembly to the correct frequency.

[0090] In operation, an electrode of the actuator (to actuate the ceramic 31) is connected to voltage means 37. The casing 32 can be connected to a ground 39.

[0091] There figure 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, at the bottom, there is a plate 420 provided with a drop ejection nozzle 10. The ink supply is via a conduit 46.

[0092] During assembly, the ceramic 41 is clamped between the sides 48a and 48b of the groove. Under the effect of a periodic electric field created between electrodes, arranged in a crown on the faces of the ceramic element 41, perpendicular to its axis, the latter deforms longitudinally and transmits this vibration to the casing 42 to which it is secured. This excitation is transmitted to the nozzle 10 then to the jet. As in the embodiment of the figure 5B , it is the envelope which plays the role of the resonator.

[0093] In operation, the actuator 41 is connected to power-up means 47, this electrode is electrically insulated from the casing 42. The casing 42 can be connected to a ground 49.

[0094] There figure 5D describes a fourth embodiment, which is in fact a variant of the first embodiment described above. Reference numerals identical to those of the figure 5A y denote identical or corresponding elements. References 51 and 52 denote the piezoelectric ceramic and the resonator respectively.

[0095] Unlike the structure of the figure 5A , the resonator 52 comprises, from the collar 53, 3 sections 52 1 , 52 2 and 52 3 of different diameters: a first 52 1 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 make it possible to delimit a volume 54 in which the ink will be stored, a third 52 3 of even smaller diameter and it terminates 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 envelope 25 in which the actuator is inserted makes it possible to circulate the ink, injected by the lateral conduit 26. This type of actuator is generally used to generate drops of so-called "medium" size and its shape is optimized for the operating conditions (in particular the working frequency) in a given size imposed by the mechanics implemented on the print head.In this figure, we have identified zones A, B, C which are used in the rest of the description.

[0096] The part of the bar under the collar 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.

[0097] The explanations already given above in connection with the figure 5A and in particular those concerning the connection of the powering means and the working frequency of the actuator, are applicable here.

[0098] The print head can have a mechanical configuration that is common to several types of drop generators that produce drops of different sizes (to simplify: large, medium and possibly small) and therefore operate at different frequencies. The size and inputs / outputs can therefore be identical for all types of generator; the cavity length can also be very similar for these different types. So that the different types of resonator can operate at different frequencies while maintaining a length between collar and nozzle that is substantially identical, the shape of the bar can be adjusted. Consequently, the bar for a G head (lowest frequency) is a simple cylinder whose length is the greatest ( figure 5A for example), and that of an M head (higher frequency) has a more complex shape (2 diameters, figure 5D for example) which allows to keep a length substantially identical to the head G while operating at a higher frequency.

[0099] But the problem to be solved, set out 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 the temperature, remains the same. The parasitic nature of these resonances has not been highlighted in the prior art, in particular in documents JP 2006-076039 or JP-2005-081643, or even US 5063393 or JP-S58-3874.

[0100] There figure 5E represents another type of device to which the invention can be applied. Reference numerals identical to those of the figure 5B they designate the same elements.

[0101] There is a cavity 34, of cylindrical internal shape, delimited, on the side of the nozzle 10, by an end surface 320 perpendicular to the axis XX'. Pressurized ink is brought into this cavity by a conduit 36.

[0102] 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)

[0103] In this figure the cavity is elongated, along the XX' axis. But it can also be curved.

[0104] In operation, an electrode of the actuator 31 is connected to power-up means 37. The casing 32 can be connected to a ground 39.

[0105] In this device, the casing 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-casing assembly does not form a resonator. The ink is directly set into vibration by the actuator 31 and resonances are formed in the cavity at the working frequency.

[0106] This type of device presents the same problems as those presented above, especially for other devices such as those of figures 5A - 5D .

[0107] Generally speaking, the optimal working frequency of a jet is determined by the various parameters that define it. These parameters include: the nozzle diameter (which can be between 40 µm and 80 µm), the jet speed (which can be between 18 and 24 m / s), the physicochemical 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< ).

[0108] The operating frequency can be adjusted using means 27, 37, 47 for applying a voltage to the piezoelectric element.

[0109] The stimulation efficiency is represented by the break length L b as a function of the jet excitation frequency.

[0110] L b can be measured by observing the jet with a camera and strobe lighting synchronized to the drop period (this allows the image of the drops forming to be frozen). The distance between the nozzle and the break is then measured by micrometric movement of the camera.

[0111] Another technique is described in document WO 2012 / 2107560 (see in particular the description in connection with the figures 5A - 5C of this document), or in WO 2011 / 012641, when the drops are charged (at constant drop formation frequency).

[0112] Generally speaking, it is considered that the shorter the break length, the greater the stimulation efficiency. The curve of the figure 6 represents the evolution of L b as a function of the jet excitation frequency. The jet resonance frequency is the frequency for which the amplification of the velocity or radius modulation is the greatest. In general, the actuator frequency is adjusted close to this frequency. Indeed, since the jet is defined by its diameter, its nozzle exit velocity and the fluid that constitutes it (responsible for the capillary instability of the jet through the surface tension of this fluid), the jet behaves like a system resonating at a given preferred frequency. When it is excited periodically by a velocity modulation, the capillary instability reflects it in a periodic variation of the jet diameter which will be amplified until the jet breaks up. The length L b where this break is located as a function of the excitation frequency is representative of the jet resonance for a given stimulation voltage.

[0113] According to the above, the optimal excitation frequency v 0 is the one which corresponds to the absolute minimum of the length L b .

[0114] However, it was found that the actual curves of the evolution of Lb as a function of the jet excitation frequency, examples of which are shown in figures 12A-12C (which will be commented on in more detail below), do not present the ideal form of the figure 6 These real curves show that the real frequency response is disturbed by additional frequency events.

[0115] More precisely, it was possible to highlight that, during the operation of any of the stimulation bodies, 3 resonance systems are involved: the jet resonance, the actuator or resonator resonance and the resonance of the fluid cavity of the drop generator. In other words, certain frequency behaviors were observed, which correspond neither to the actuator resonance nor to the jet resonance.

[0116] The jet instability is excited by the actuator, which thus performs its stimulation function. The actuator is preferably designed so that the two resonant frequencies, that of the jet and that of the actuator, are close.

[0117] Compared to these two resonances, the fluid cavity resonance is a parasitic resonance. It causes the formation of a standing wave in the ink that is very sensitive to temperature. This standing wave is superimposed on the actuator excitation.

[0118] For the family of so-called "resonant" actuators, the resonant frequency of the actuator depends on the speed of the acoustic waves in the material of the resonator bar and on its sizing. In the case of the structure of the figure 5A , the length of the resonator is such that, at the resonance frequency, there is a vibration node at the level of the retaining collar and an antinode at the end.

[0119] The resonator (or the envelope in the embodiments of the figures 5B et 5C ) is generally made of stainless steel, a material in which the speed of sound is of the order of C stainless steel = 5790 m / s.

[0120] The properties of certain inks are such that the wave speed in the ink is approximately 4 times lower than in stainless steel (C ink ≈ 1200 m / s). As a result, the ink cavity also constitutes a resonator in which a standing wave can develop whose resonance or antiresonance frequency will be close to the resonance frequency of the actuator.

[0121] The speed of waves in stainless steel (or, more generally, in the material constituting the bar) is very insensitive to temperature, whereas that of waves in ink is very sensitive to temperature (variation between -3 and -4 m / s per °C). Data concerning the evolution of this speed as a function of temperature are gathered in figure 16 for MEK (MethylEthylKetone), Alcohol, or Water-based inks. In this figure, the sound velocity data for ink #1 (MEK-based) and #2 (alcohol-based) show quite a high variability. The variability is lower for ink #3, with a "water" base.

[0122] The resonance modes in the resonator and the cavity are very similar and evolve differently depending on the temperature. The resonance and antiresonance modes of the fluid cavity can therefore shift depending on the temperature, crossing the resonator mode, which varies only slightly depending on the temperature. This results in disturbances of the stimulation in certain temperature ranges.

[0123] A first study carried out on this problem concerns the case of a drop generator equipped with a stimulation body of the type of the figure 5D .

[0124] We have represented in figure 7A curve I, which represents the evolution of Ve, that is to say the input voltage of the stimulation range, as a function of the temperature. As can be seen on this curve, at the entry of the range, the stimulation voltage remains stable, in other words it reflects the effectiveness of the stimulation. On the other hand, this voltage tends to increase significantly for a scan from low to high temperature from 25°C.

[0125] In this same figure, curve II represents the evolution of Vs, that is to say the output voltage of the stimulation range, as a function of temperature. We see a peak on this curve II, at approximately 25°C.

[0126] Curve III represents the evolution of Vs / Ve, i.e. the input voltage / output voltage ratio of the stimulation range, as a function of temperature. This ratio is representative of the robustness of the stimulation: the higher it is, the easier the printer will be to adjust because a single stimulation voltage allows the formation of quality drops over the entire temperature range. We can see here that from around 25°C, the drift is very significant.

[0127] Curve IV represents the evolution of the voltage at the reversal point Vr. This is initially stable, then, like the input voltage, increases as a function of temperature, from around 25°C.

[0128] Curves were established 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 figure 7B .

[0129] From these curves, we sought to determine how the effectiveness of stimulation changes as a function of temperature. To do this, at a given voltage, it appears that the break length Lb can vary by a factor of 2 as a function of temperature. Based on the theory of capillary instability, we obtain the expression: Lb 2 a = We 2 γ Ln V j Δ V j With : Lb: break length a: jet radius at nozzle outlet Vj: average jet speed ΔVj: jet speed modulation (result of the stimulation process) γ : dimensionless growth rate of modulations substantially constant over the working range (in particular the temperature range) We: Weber number

[0130] The velocity modulation varies exponentially with the break length and therefore the stimulation varies in proportions much greater than a factor of 2.

[0131] The aim being to compare modulation levels at different temperatures, we show that the stimulation efficiency collapses between 20°C and 40°C. The influence of temperature can vary the input parameters by a few % (typically by surface tension, etc.), which is unrelated to the orders of magnitude on the stimulation efficiency.

[0132] To explain this sudden variation in efficiency, we can consider: a non-linearity, not identified to date (unlikely), or a resonance phenomenon.

[0133] We can therefore focus on the stimulation body, by looking for resonances in the solid and the liquid: As a first approximation, we can reasonably consider that the materials of the resonator, for example 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 allow us to explain the drastic change in the effectiveness of the stimulation.

[0134] In the liquid (wherever ink is present), there can be a phenomenon of acoustic resonance, since the largest dimension is of the order of the wavelength.

[0135] At 83 kHz and for a speed 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 of the stimulation body (here approximately 21 mm, in an example of geometry of the figure 5D ).

[0136] A relationship can be established that expresses the dependence between the modulation generated by the piezoelectric actuator and ΔVj, the jet velocity modulation, by including the propagation phenomenon in the ink. The complete transfer function can be determined and the existence of resonance frequencies linked to the ink and close to the working frequency can be investigated. 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 drift, or / and cross the working frequency (imposed by the actuator).

[0137] The drop generator can be interpreted schematically in order to list its main functional elements. Figure 8 (and its equivalent, in terms of electrical circuit, represented in figure 9 ) presents the simplified version of the drop generator by highlighting 4 elements: the source term: the piezoelectric actuator which modulates the ink flow (which is the incoming flow), the loss terms: these are the outgoing flows which balance the incoming flow. There are 3 terms here: the ink wedge 520 under the actuator 52, the nozzle 50 and the upper part 550 of the stimulation body in which an acoustic wave can propagate.

[0138] The resonator body, for example in stainless steel, is considered non-deformable: the walls admit a zero speed condition whether in flow or in propagation.

[0139] We will clarify the physical behavior of the functional elements of the drop generator and the equations associated with them. To this end, we determine the impedances of each of the elements.

[0140] The pressure drop across the nozzle 50 is described by the Navier Stokes equations. In sinusoidal flow, the motion of the ink mass trapped in the nozzle is limited by the inertia terms. The nozzle impedance will be denoted Z b : Z b = iω ρ L buse S b

[0141] With : L nozzle: nozzle length S b: nozzle section ρ : density of the ink ω : pulsation at working frequency

[0142] The ink shim 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 ink “disc” located under the active face of the actuator. For the column, the diameter is for example 500 µm, to be compared to the nozzle diameter, again taken as an example, of 50 µm. The speed of the ink in the shim 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 shim is therefore summed up by its compressibility term noted Z c : Z c = Ke i ω Ve

[0143] Where Ke is the compressibility and Ve is the ink volume of area 521.

[0144] The waveguide 550 is an acoustic element delimited by the active face of the resonator; it rises to the level of the shoulder 53 on which the resonator rests. This area is flooded with liquid, so we consider the ring of liquid between the resonator and the sheath of the stimulation body.

[0145] We recall that the liquid column admits variations in section, the impedance of this column, by section, is given by the formula of the theory of lines (in electrical analogy): Z BC = Z AB cos k L B + i Z B sin k L B cos k L B + i Z AB Z B sin k L B

[0146] Where Z BC is the impedance brought back to the input of the section AB of acoustic impedance Z b terminated by a load impedance Z AB .

[0147] The piezoelectric actuator, for its part, admits 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: to the first order, the resonant frequency of the stimulation assembly is set to the resonance of the ½ Langevin (the resonator) in air.

[0148] As the working frequency is fixed (83.3 kHz) we will not consider, for greater readability of the model, this mechanical resonance. The resonant assembly is therefore assimilated to a flow source, it is the volume of ink stirred at the end of the resonator: Q.

[0149] Since the elementary impedance terms are defined for the outlet flow rate, it is possible to determine the pressure P at the end of the bar. The pressure drop in the nozzle relative to its nozzle impedance Z gives the flow rate as a function of the frequency or the jet speed modulation for a given nozzle section.

[0150] The previous formulas allowed us to plot the curve ( figure 10A ) frequency response at a temperature of 5°C, namely the modulus of the jet velocity modulation as a function of frequency. The velocity unit is normalized, which allows, in relative terms, to locate the frequencies for which the stimulation is reinforced (resonance phenomenon) or weakened (zero transmission, anti-resonance).

[0151] This figure shows that, in the frequency range of interest, namely 80 - 90 kHz, there are two notable frequencies F1 and F2 which will influence the level of stimulation effectiveness at 83.3 kHz. This frequency congestion does not pose a problem if these frequencies are stable in the printer's operating environment; at most, the stimulation level can be different from one printer to another.

[0152] But these frequencies F1, F2, evolve according to the temperature which seems to be the parameter disturbing robustness for the stimulation. Simulations with the software "MathCad" make it possible to identify the speed of the ink as a strongly influential parameter. At room temperature (see the Handbooks of Physics 1990-1991 - 71st edition - pages 14-32 and the speed measurements in real inks of the curve figure 16 ), ink velocity typically varies from -3 to -4 m / s per °C.

[0153] The same simulation was carried out over a temperature range of 45°C, as explored experimentally, which made it possible to highlight a frequency shift of F1, F2 of approximately 10 kHz ( Figure 10B ). The sign of the temperature dependence of the speed is important because the temperature glide makes the frequency F2 troublesome, while F1 leaves the working frequency zone.

[0154] This frequency shift may seem quite small; nevertheless, combined with the proximity of F1 and F2 around 83.3 kHz, we understand that it is possible to have significant variations in stimulation levels when F2 crosses the working frequency.

[0155] The tests reported above have made it possible to highlight an acoustic resonance phenomenon within the fluid cavity. This phenomenon is dependent on the propagation speed of the acoustic waves within the ink; a dependence, as a function of temperature, therefore appears, which positions the events, in frequency, more or less close to the working frequency.

[0156] Additional results (actual measurements) were carried out, 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 .

[0157] For these measurements, the break length Lb was measured with a low voltage (weak stimulation) during a frequency sweep, at different temperatures (5°C - 45°C), in order to visualize the events over the 70 - 100 kHz range. The break length Lb is measured. These measurements are carried out over the temperature range from 5°C to 45°C, with a step of 10°C, using the following parameters: white pigment MEK base ink, jet speed: 20m / s stimulation signal (50% duty cycle slot) generated by a laboratory device, standard stimulation body (with the structure of the figure 5D ) equipped with a piezoelectric actuator whose resonance frequency is close to the operating frequency (which is the drop generation frequency).

[0158] The results illustrated in figure 11 show many events around the working frequency 83.3 kHz. The curves cross as a function of temperature and the absolute minimum break length drifts significantly as a function of temperature. This operation degrades the robustness of the stimulation.

[0159] These additional results confirm the disturbances observed and already reported above. Furthermore, they clearly illustrate the difficulty in - or even the impossibility of - maintaining stable operation of a droplet generating device at at least two positive temperatures at least 15°C or 20°C apart, for example 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, on the one hand, 5°C or 15°C to, on the other hand, 35°C or 45°C or even 50°C.

[0160] Other work has confirmed the hypothesis of the influence of disturbances linked to resonances present in the fluid cavity. Real measurements were carried out on a head drop generator G whose mechanical simplicity (cavity and resonator bar are then cylindrical, of the type of that of the figure 5A ) makes it easier to calculate the resonant behavior of the fluid cavity.

[0161] Additional tests were thus carried out for a stimulation body of the type of that of the figure 5A .

[0162] More precisely, we studied the break length, as a function of frequency, in low stimulation, for 3 different temperatures. The stimulation voltage being 7 Volt, it allows to always 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.

[0163] The temperatures tested were 5°C, 25°C, and 45°C.

[0164] The ink used is a white pigment MEK-based ink pressurized to achieve a constant jet speed of 20 m / s. The tests were not carried out at a constant wavelength; therefore, the jet speed is not readjusted according to the frequency, and a parabolic envelope is obtained, which reflects the physical phenomenon of capillary instability, which will be taken into account in the analysis of the results.

[0165] On the figures 12A-C , the points resulting from the measurement of Lb are represented, and the resonance and anti-resonance frequencies in the cavity, calculated numerically from the mechanical configuration of the generator and the sound speeds in the ink at different temperatures. The transmission zeros (anti-resonance) are identified by vertical bars. The peaks Pc ( Figure 12 A et B ), or Pc 1 , Pc 2 ( Figure 12C ) represent the resonance peaks in the liquid. Pour 5°C (figure 12A):

[0166] The theoretical model was fitted with an ink velocity of c = 1170 m / s. The actuator resonance frequency is approximately 64 kHz. The model also gives 2 transmission zeros, corresponding to 46 kHz and 74 kHz. For 46 kHz, the associated efficiency drop is clearly found; but for 74 kHz, it was not possible to record the values, the break being in the "noise" of the natural break.

[0167] 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 predominant in terms of amplitude because it is imposed by the actuator. Pour 25°C (figure 12B):

[0168] The theoretical model was adjusted with c = 1100 m / s, i.e. a slope of - 3.5 m / s / °C. The 2 transmission zeros are located at approximately 42 kHz and 69 kHz. This is well confirmed by the experimental data which result, at these frequencies, in an under-efficiency of stimulation. An acoustic resonance in the ink cavity is also clearly highlighted at around 53 kHz. The resonance of the actuator is also clearly visible, but the resolution is not sufficient to precisely locate this minimum break length which is probably between 63 kHz and 64 kHz. Pour 50°C (figure 12C):

[0169] The theoretical model was fitted with c = 1030 m / s, i.e. a slope of - 3.5 m / s / °C. The first zero is located slightly before 40 kHz and the second at 65 kHz. The latter is very close to the working frequency and therefore overlaps the actuator resonance peak located at 64 kHz.

[0170] To resolve 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.

[0171] This acoustic impedance varies according to the frequency, particularly when the latter varies around the working frequency.

[0172] We have represented, in figure 13A , 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 (in other words: of the cavity and of the actuator) is identified by ft and we designate by ZT (ft ), 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 figures 5A - 5D . In the case of the figure 5E , it is defined by the geometry of the 32 stainless steel cylinder.

[0173] As seen in figure 13A the acoustic impedance varies in a regular or smooth manner around ft. But, when disturbances, of the type explained above, appear, one or more peaks P 1 , P 2 , of resonance or anti-resonance, appear on this graph, in particular in the vicinity of the working frequency, for example in an interval of ± 10 kHz or ± 5 kHz around the latter.

[0174] This variation in impedance has the consequence of varying the amplitude of the jet speed modulation (or stimulation efficiency) in the nozzle and therefore the breaking length.

[0175] In addition, the graph of the figure 13A evolves as a function of temperature. Peaks such as peaks p1, p2, not present in the targeted frequency interval, at a certain temperature, for example at 5°C or 15°C, may appear, in this same frequency interval, at another temperature, for example at 30°C or 35°C.

[0176] According to the invention, a frequency range [f 1 , f 2 ] of ± 10 kHz or ± 5 kHz is defined around the working frequency ft. 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 an interval between, on the one hand, 0.25 (or 0.5) and, on the other hand, 2 (or even 4), and this at, 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 at 20°C, more preferably also at 45°C or even at 50°C, more preferably at any temperature within a temperature range from 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 speed modulation interval is represented by horizontal bold lines on the figure 13B . This avoids: on the one hand the presence, in an interval close to ft, of peaks (such as P'1 and P'2 on the figure 13B ) reflecting disturbances, on the other hand a drift of such peaks, towards ft, as a function of the temperature.

[0177] It is noted that the impedance can be calculated according to the formula already mentioned above. From this calculation we can deduce the jet velocity modulation and its variations under the effect of temperature.

[0178] This speed modulation can also be estimated or deduced from the measurement of the variations of L b (whose formula was given above) as a function of the frequency, at constant excitation voltage. In fact, a variation of L b reflects a variation of impedance.

[0179] Alternatively, it is possible to measure or estimate the pressure variations, as a function of the frequency. At the nozzle 10, these pressure variations represent or translate the variations in L b as well as the variations in acoustic impedance (i.e. jet speed modulation).

[0180] The solution proposed above can be obtained by modifying the configuration of the internal volume of the stimulation body, intended to receive the ink, by giving it a shape allowing a variation in acoustic impedance to be achieved.

[0181] In other words, the internal volume 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 acoustic impedance.

[0182] For example, an element, or means, can be introduced into the cavities to achieve this variation in impedance. The embodiments of this solution are shown in figures 14A-14E .

[0183] The device of the figure 14A (respectively 14B, 14C, 14D, 14E) corresponds to that of the figure 5A (respectively 5B, 5C, 5D, 5E) with the same numerical references to designate the same elements. On 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 external diameter of this ring is substantially equal to the internal diameter of the envelopes 25, 32, 42, while its internal diameter does not obstruct the flow of the fluid. The material for this ring is preferably the same as that of the resonator, for example stainless steel.

[0184] 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 broken lines in each of these figures. It would then have the same role of modifying the acoustic impedance of the cavity.

[0185] More generally, we also note, in these figures, that the internal shape of the cavity includes: a first cylindrical zone 25 1 , 32 1 , 42 1 , 52 1 , 62 1 of a first diameter, and of 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 , of a second diameter, different from the first diameter, and of a second length, measured along a longitudinal axis of said cavity.

[0186] In the event that the ring of each of the figures 14is positioned according to the position indicated in broken lines, the first cylindrical zone and the second cylindrical zone are different from those mentioned above.

[0187] As will be shown below, the differences, or variations, in acoustic impedance, induced, in the examples of the figures 14 , by the different 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 speed modulation.

[0188] The different diameters allow for variation in the fluid length. In the case of structures of Figures 14A And 14D, in which the resonator plunges into the cavity intended to receive the fluid, a ratio is created between the length L a of the mechanical actuator (or actuator) (comprising the piezoelectric element 21, 51, the collar 23, 53 and the part 22, 52, which is in contact with the fluid) and the length L f of the, or a, portion of the cavity intended to receive a column of fluid, preferably strictly greater than 4; this ratio may be for example between 4 and 6 or 4 and 10 or 100. In the case of the Figure 14D , the length L f corresponds to the length of the portion of zone B not occupied by the ring 57. Even if a column of fluid remains, the length of which is not modified by the presence of the ring, the modification of the length of a part of the cavity, intended to accommodate the fluid, makes it possible to eliminate, from the working frequency zone, the parasitic frequencies.

[0189] Tests were carried out, with a stimulation body structure according to the Figure 14D , with a ring whose length, at the end of the study, was 3.6 mm. The results are illustrated on the Figures 15A-15C : there Figure 15A represents the evolution of the voltages Ve, Vs, Vr and the ratio Vs / Ve, as a function of the temperature; this Figure 15A shows that there is an almost linear variation in the piezoelectric instructions. It therefore compares very favorably with the results which have been commented on above in connection with the Figure 7A , there Figure 15B represents the breaking length L b , as a function of the activation voltage, at different temperatures (5°C - 45° C, with a step of 10°C, at 5°C, 15°C, 25°C, 35°C, 45°C); we see that the curves are stacked correctly, in a good order; again, the comparison with the curves of the Figure 7B is very advantageous, the Figure 15Crepresents the breaking length L b , as a function of frequency, at different temperatures (5°C - 45° C, with a step of 10°C, at 5°C, 15°C, 25°C, 35°C, 45°C); the curves are stacked correctly, in a good order as a function of temperature, and do not cross. This result is much better than what was observed on the figure 11 where the order is incorrect and the curves intersect.

[0190] Additional tests were carried out with a “standard MEK base” type ink and then with an “Alcohol base” type ink. The results obtained are similar with the two previous inks and confirm the optimum nature of the 3.6mm ring.

[0191] The presence of the ring makes it possible to reduce the volume of the ink cavity, which makes it easier to rinse the drop generator during maintenance operations.

[0192] The above tests show that the invention makes it possible to obtain robust operation throughout the entire range of temperature and ink envisaged (through speed). The invention makes it possible to eliminate any disruptive event on the stimulation efficiency. There is a clear improvement on most of the curves obtained, that is to say that we move from random operation to well-controlled operation.

[0193] The embodiment of the invention with the insertion of a ring in 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 in section, thus having a profile identical or similar to that shown in Figures 14A-14E .

[0194] In another embodiment, the differences in sound wave speed in various materials other than stainless steel are exploited. The stainless steel material used for the resonator is therefore replaced by one of these other materials.

[0195] This solution makes it possible to satisfy the conditions set out above in conjunction with the Figure 13B .

[0196] This solution also allows the length of the resonator to be modified while maintaining the same operating frequency. The choice of another material is accompanied by a modification of the length of the resonator which at first glance is proportional to the ratio of the speeds;

[0197] If the speed is greater than in stainless steel, the bar (case of Figures 5A And 5D Or 14A and 14D) under the resonator collar will be lengthened; conversely, if the speed is lower, the bar under the collar will be shortened. The length of the resonant cavity containing the fluid may therefore be modified, for example according to the preceding teaching according to the present invention: the jet speed modulation, at the nozzle outlet, having a value ΔVj(ft ) at the working frequency of the cavity and the actuator, and this jet speed modulation, at the temperature of 15°C and at the temperature of 35°C, not varying, in a frequency range of + 5 kHz around the working frequency ft, outside the interval between 0.25ΔVj(ft ) and 4ΔVj(ft), and / or the ratio between the length of the mechanical actuator (or actuator) and the length of the, or a, portion of the cavity intended to accommodate a column of fluid, being strictly greater than 4; this ratio may be for example between 4 and 6 or 4 and 10 or 100.

[0198] In this case, the resonance and anti-resonance frequencies of the fluid cavity will be displaced and rejected outside the operating area of ​​the stimulation.

[0199] Table I collects data relating to the speed of sound waves in these other materials. Table I Material Speed (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 Or 3240 10630 Iron 5130 16830 Lead 1158 3800 Lucite 2680 8790 Money 3650 12045 Steel 6100 20000 Stainless steel 5790 19107 Titanium 6070 20031

[0200] If one of these other materials is chosen for the resonator bar, then the effects of sound wave disruption in the ink will not manifest.

[0201] More generally, all metallic materials - other than stainless steel - or minerals can be suitable.

[0202] This choice also makes it possible to reduce the length of the resonator, and therefore the length of the cavity, which further avoids parasitic resonances as described above.

[0203] Whether the structure of the stimulating body is that of one of the Figures 5A-5D Or 14A-14D , the disturbance effects due to resonance in the cavity containing the ink will not occur.

[0204] A device or inkjet 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 above in connection with the Figures 1 and 2 .

[0205] Such a device therefore includes: a drop generator 60 containing electrically conductive ink, maintained under pressure, by an ink circuit, and emitting at least one ink jet, a charging 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 trajectory of the jet and downstream of the charging electrode, a gutter 62 for recovering the ink from the jet not used for printing in order to be returned to the ink circuit and thus recycled.

[0206] The operation of this type of jet has already been described above in connection with the Figures 1 and 2. It will simply be recalled here 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, at a precise location in the jet downstream of the nozzle. This forced fragmentation of the ink jet is usually induced at a so-called “break” point 13 of the jet by the periodic vibrations of the stimulation device.

[0207] In addition to the above means, such a device may further comprise means 5 for controlling and regulating the operation of each of these means taken individually, and the voltages applied. These means 5 are described below in more detail in connection with the figure 17 .

[0208] In this figure, a set of means 5 forming a controller comprises circuits, which make it possible to send to the print head the voltages making it possible to control the latter and in particular the voltages to be applied to the electrodes as well as the piezoelectric excitation voltage.

[0209] This assembly 5 can further receive descending signals, coming from the head, in particular the signals measured using a drop position and / or speed sensor, and can process them and use them for controlling the head and the ink circuit. In particular, to process the signals coming from such a sensor, it can comprise analog amplification means for a signal from 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).

[0210] This controller assembly 5 can communicate with means 500 for sending and / or receiving fluids to and from the print head.

[0211] This controller assembly 5 can communicate with the user interface 6 to inform a user about the state of the printer and the measurements carried out, in particular, of the type described above. It comprises storage means for storing the instructions relating to the processing of the data, for example to carry out a method or implement an algorithm of the type described above.

[0212] According to an exemplary embodiment, the controller 5 comprises an on-board 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, in particular data for implementing a method according to the present invention or for controlling a device according to the present invention.

[0213] The means 6 allow a user to interact with a printer according to the invention, for example by configuring the printer to adapt its operation to the constraints of the production line (rate, printing speed, etc.) and more generally of 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 real-time information from the production monitoring (status of consumables, number of products marked, etc.). These means 6 may include display means.

[0214] Means may also be provided for supplying or bringing the various electrodes to the desired voltages. These means include, in particular, voltage sources.

[0215] 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 the Figures 1 and 2 , the operation of which has been recalled above, makes it possible to achieve 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 at least 2 temperatures separated by at least 15°C or more, in particular at 15°C and at 30°C (or at 35°C), preferably also at 5°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 within a range between 15° and 35° and more generally between 5° and 50°C.

[0216] With a device and a method according to the invention, the "parasitic" frequencies are removed, whatever the temperature in one of the ranges mentioned above, from the operating frequency domain used. For example, this operating domain is between 50kHz and 150kHz depending on the diameter and speed of the jet chosen.

Claims

1. Device for forming and ejecting drops of an ink jet of a CIJ printing machine, said device having a stimulating body comprising a) a cavity (25, 32, 42) containing an ink and comprising an end provided with a nozzle (10) for ejecting ink drops, the internal volume of said cavity comprising at least a first part having a first acoustic impedance, and at least a second part having a second acoustic impedance that is different from the first acoustic impedance, b) actuator means (21, 22, 32, 41, 42) in contact with the cavity, in which device the jet speed modulation, at the outlet of the nozzle (10), has a value ΔVj(ft) at the operating frequency of the cavity and of the actuator, and this jet speed modulation, at the temperature of 15°C and at the temperature of 35°C, does not vary, in a frequency range of +5 kHz around the operating frequencyft, outside the interval between 0.25ΔVj(ft) and 4ΔVj(ft).

2. Device according to claim 1, wherein the jet speed modulation, 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 operating frequencyft, outside the interval between 0.25ΔVj(ft) and 4ΔVj(ft).

3. Device according to any one of claims 1 or 2, the internal shape of the cavity comprising: - a first cylindrical zone (251, 321, 421, 521, 621) having a first diameter, and a first length, measured along a longitudinal axis of said cavity, - a second cylindrical zone (252, 322,, 422, 522622) having a second diameter that is different from the first diameter, and a second length, measured along a longitudinal axis of said cavity.

4. Device according to claim 3, 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 internal diameter is equal to said second diameter.

5. Device according to claim 3, the cavity being delimited by a wall having a first cylindrical portion, the internal diameter of which is equal to said first diameter, and having a second cylindrical portion, the internal diameter of which is equal to said second diameter.

6. Device according to any one of claims 1 to 5, the actuator being in direct contact with the internal volume of said cavity.

7. Device according to any one of claims 1 to 6, the actuator comprising resonator means.

8. Device according to claim 7, the resonator means comprising a piezoelectric element (21, 31, 41).

9. Device according to claim 7 or 8, the resonator means comprising a resonator body (22, 52) disposed in said cavity.

10. Device according to claim 9, said resonator body (22, 52) being made of stainless steel, or aluminium, or beryllium, or brass, or copper, or diamond, or glass, or gold, or iron, or lead, or TMMA, or silver, or titanium.

11. Device according to claim 9 or 10, said resonator body (52) comprising a first part (521) having a first diameter and a second part (522) having a second diameter that is different from the first.

12. Device according to claim 7, the internal volume of the cavity being delimited by a resonator wall (32, 42).

13. Continuous ink jet type printing machine, said machine comprising: - a print head (1) provided with a device for forming and ejecting drops of an ink jet according to any one of claims 1 to 13, - an ink circuit (4), - means (5) for controlling the flow of the ink and the print head.

14. Method for forming ink drops, wherein a device according to any one of claims 1 to 12 or a machine according to claim 13 is used.