Patent Application
20180065363
The invention relates to the improvement of the operation of a printing head of a CIJ printer to make it sturdier towards environmental variations (in particular temperature) found on industrial use of this type of printer.
This improvement involves an increase in the sturdiness of the stimulation function of the drop generator towards temperature.
Continuous ink jet (CIJ) printers are well known in the field of coding and industrial labelling for various products, for example to label bar codes or the expiration date on food products directly on the production line and at a high rate. This type of printer also founds application in the decorative field where graphic printing possibilities of the technology are exploited.
CIJ printers continuously generate drop jets some of which are selected and oriented to the support to be printed whereas the others are recovered to be recycled. These printers have some standard sub-assemblies as shown in
First, a printing head 1, generally offset from the body of the printer 3, is connected thereto by a flexible umbilical 2 joining the hydraulic and electrical connections required for operating the head by providing it with flexibility which facilitates integration on the production line.
The body of the printer 3 (also called a console or cabinet) usually contains three sub-assemblies:
This description can be applied to continuous jet (CIJ) printers called binary printers or multi-deflected continuous jet printers.
Binary CIJ printers are equipped with a head the drop generator of which has a multitude of jets the drops of which can only be oriented to 2 trajectories: printing trajectory or recover trajectory.
In multi-deflected continuous jet printers, each drop of a single jet (or spaced apart from a few jets) can be deflected on various trajectories corresponding to different commands. A succession of drops undergoing different commands can thus scan the zone to be printed along a direction which is the deflection direction, the other scanning direction of the zone to be printed is covered by a relative movement of the printing head and the support to be printed 8. Generally, the elements are arranged such that these 2 directions are substantially perpendicular.
The deviated continuous ink jet printing heads have different operating sub-assemblies.
Referring to
A periodical stimulation device 63 is associated with the cavity in contact with the ink upstream of the nozzle 10; it transmits, to the ink, a (pressure) periodical modulation which causes a modulation of velocity and jet radius from the nozzle. When the dimensioning of the elements is suitable, this modulation is amplified in the jet under the effect of surface tension forces responsible for the capillary instability of the jet, up to the jet rupture. This rupture is periodical and is produced at an accurate distance from the nozzle at a so-called break
point 13 from the jet, which distance depends on the stimulation energy. In the case where a stimulation device, called an actuator, the motive member of which is a piezoelectric ceramics, 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 signal for driving the ceramics. Prior art teaches other jet stimulation means (thermal, electro-hydrodynamic, acoustic, . . . ) but the stimulation using piezoelectric ceramics remains the most widespread thanks to its efficiency and relative workability.
At its breaking point 13, the jet, which was continuous from the nozzle, is transformed into a train 11 of identical and evenly spaced apart ink drops. The drops are formed at a time frequency identical to the frequency of the stimulation signal and for a giving stimulation energy, any other parameter being otherwise stabilised (in particular ink viscosity), there is an accurate (constant) phase relationship between the periodical stimulation signal and the breaking instant, itself periodical and with a same frequency as the stimulation signal. In other words, to an accurate instant of the period of the stimulation signal corresponds an accurate instant in the separation dynamic of the jet drop.
Without further action (this is the case where drops are not used for printing), the drop train travels along a trajectory 7 collinear to the drop ejection axis (nominal trajectory of the jet) which joins, by a geometric construction of the printing head, the recovery gutter 62. This gutter 62 for recovering non-printed drops uptakes the ink not used which comes back to the ink circuit 4 to be recycled.
For printing, the drops are deflected and deviated from the nominal trajectory 7 of the jet. Consequently, they escape from the gutter and follow oblique trajectories 9 which meet the support to be printed 8 at different desired impact points. All these trajectories are in a 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 achieved by combining an individual deflection of drops in the head deflection plane with the relative movement between the head and the support to be printed (generally perpendicular to the deflection plane). In the deviated continuous jet printing technology, the deflection is achieved by electrically charging drops and by passing them into an electric field. In practice, the means for deflecting drops comprise an individual charging electrode 64 for each jet, located in the vicinity of the break point 13 of the jet. It is intended to selectively charge each drop formed at a predetermined electrical charge value which is generally different from one drop to the other. To do this, the ink being held at a fixed potential in the drop generator 60, a voltage slot with a determined value, driven by the control signal, is applied to the charging electrode 64, this value being different at each gutter period.
In the control signal of the charging electrode, the voltage application instant is shortly before the jet fractionation to take advantage of the jet electrical continuity and attract a given charge amount, which is a function of the voltage value, at the jet tip. This variable charge voltage affording the deflection is typically between 0 and 300 Volts. The voltage is then held during the fractionation to stabilize the charge until the detached drop is electrically insulated. The voltage remains applied still a time after to take break instant issues into account.
Thus, it is attempted to synchronise the voltage application instant with the jet fractionation process. In case of desynchronisation, the drop in question is not properly charged, its charge is lower, or even zero.
The drop deflecting means also comprise a set of 2 deflection plates 65 placed on either side of the drop trajectory upstream of the charging electrode. Both these plates are put to a high fixed relative potential producing an electrical field Ed substantially perpendicular to the drop trajectory, capable of deflecting the electrically charged drops which are engaged between the plates. The deflection amplitude is a function of the charge, the masse and the velocity of these drops.
In order to control the deflection of the drops for printing, it is attempted to produce a quality breaking in the range of variation of the environmental conditions provided by specifications.
Thereby, it is attempted to make sure that:
A breaking is considered as stable and reliable (with a good quality), when it enables an optimum charging of the drops to be guaranteed in an operating range of the printer characterised in particular, by a temperature range (conditioning the ink viscosity) for a given ink.
Concretely, just before breaking, the drop is connected by a tail to the following drop being formed (see
The breaking shape, besides the rheological characteristics of the ink, is related to the stimulation level (excitation intensity). The breaking shape determines the breaking quality, that is its ability to ensure the proper charging of the drops.
Generally, it is modified, when the excitation increases, to switch from a satellite breaking, and then to a satellite-free breaking. The satellite is defined as a secondary drop from the breaking of the main drop.
By further increasing the stimulation level, the breaking goes back to a satellite regime. Meanwhile, the break position with respect to the nozzle changes by following the curve of
The latter represents the profile of the characteristic f giving the breaking distance (Lb) between the nozzle 10 and the break point 13, as a function of the stimulation voltage VS (Lb=f(VS)). This curve will be called in the following: a stimulation curve. This is set by scanning values of the stimulation excitation voltage VS and by determining Lb for each value of VS.
When the stimulation excitation increases (from a low value), the nozzle/break distance (Lb), which starts from a high value (natural jet breaking), decreases and passes through a minimum called a turn
, and then is extended again. The shape and the real position of this curve depend on many parameters, in particular the ink nature and temperature. The printing head is designed such that the functional part of this curve is found, at least partly, in the field of the charging electrode in spite of the variability in the parameters mentioned. On the other hand, there is a functional zone related to the breaking quality in which the printing is satisfactory (the charging of drops is proper). The intersection of the properly positioned zone in the electrodes and the functional zone of breaking quality corresponds to the stimulation operational range. This stimulation range is characterised by an input point (Pe) on the left, and an output point (Ps) on the right as indicated in
At least two distinct operating modes for the piezoelectric stimulation are used in ink jet printers of the state of the art: these are resonant and non-resonant stimulation modes.
The non-resonant stimulation is relatively difficult to implement and demands a significant energy because the actuator has to provide the entire energy necessary for creating the displacement of the actuator portion in contact with ink in order to generate the pressure modulation upstream of the nozzle. On the other hand, this mode is relatively tolerant to variabilities of the excitation conditions.
In comparison, the resonant stimulation has much more advantageous yield within the scope of a periodical stimulation which results in the periodic breaking of a drop jet at a fixed frequency, as is often the case in continuous jet type printing methods. Indeed, in this case, it is very efficient to design an actuator as an oscillating or vibratory system, substantially tuned to the drop emission frequency; a low periodical excitation can then maintain an amplified standing wave which will generate the displacement amplitude necessary for the pressure modulation upstream of the nozzle.
Under sensible conditions of implementation, a simple piezoelectric ceramics (used in mode D33, the electric field created between 2 electrodes deposited onto the ceramics thus producing a longitudinal stretching or contraction thereof as a function of the polarisation direction and the polarity of the electric signal) cannot be used on its own as an actuator because it would not have a sufficient deformation amplitude (in the order of one nanometre only) to create the expected ink ejection velocity modulation; thus, it is fixed to a piece, called a resonator, used for amplifying the movement. The ceramics/resonator assembly is called an actuator.
It could have been noticed that, for some inks and dimensionings of the drop generator, the stimulation efficiency is not stable as a function of temperature.
This can be up to the impossibility to operate the printer at some distinct temperatures of at least 15° C. or 20° C., and/or under some temperature ranges, in particular at 5° C. or at 15° C., and at 35° C. and/or at 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.).
Indeed, under some conditions, the stimulation becomes completely inefficient and the operational stimulation range is moved and/or is weakened up, in some cases, to disappear, which makes the machine setting impossible.
It can be tried, in some cases, to adapt the stimulation setting as a function of the predictable temperature change range during the production session during which the printer is used. But this is not always possible.
Finally, if this instability is desired to be compensated for, further means (temperature control of the head, for example) have to be implemented, which imposes an additional cost.
Consequently, there arises the problem of finding a device and a method, which allow for a satisfactory operation at at least 2 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.).
Another problem, in a system implementing a resonating mechanical actuator, is that the actuator resonance is coupled with the fluid resonance, in particular by the fact that the ratio of acoustic velocities, on the one hand in the material used for the resonator (for example stainless steel) and on the other hand in the fluid (about 5 000 m/s in the resonator, about 1 250 m/s in the fluid) in the order of 4, that is a quarter wavelength. The consequences of this ratio are the abovementioned coupling.
The invention aims at solving these problems.
According to the invention, a device for forming and ejecting drops of an ink jet of a CIJ printing machine includes:
a) a cavity for containing an ink and including an end provided with a nozzle for ejecting ink drops,
b) actuator means, in contact with the cavity.
In such a device, the acoustic impedance of the cavity, in the proximity of the nozzle, has a value ZT(ft), at the operating frequency of the cavity and of the actuator. Preferably, this acoustic impedance does not vary, or varies a little, in a frequency range of ±5 kHz about the operating frequency ft, such that the variation in the velocity 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 velocity modulation at the reference temperature (for 25° C. for example), and at at least 2 positive temperatures distant by at least 10° C. or 20° C., in particular at 15° C. and at 35° C., preferably also at 5° C., and/or at 10° C. and/or at 20° C., further preferably at 45° C. or even at 50° C., further preferably at any temperature in a temperature range which contains at least the interval [15° C.-35° C.], or even at least the interval [5° C.-50° C.].
Such a device according to the invention enables resonance and anti-resonance frequencies, due to the ink cavity, to be displaced such that their drift as a function of temperature does not cause them to intersect the jet stimulation frequency, at at least 15° C. and 30° C. (or at 35° C.), also preferably at 5° C., and/or at 10° C. and/or at 20° C., further preferably at 45° C. or even 50° C., further preferably at any temperature in a range between 15° and 35° and more generally between 5° and 50° C. These temperatures and/or temperature ranges are indeed those of operating specifications of many printers.
Preferably, said cavity is such that the ratio of the length of the mechanical actuator to the length of the or a portion of the cavity intended to accommodate a fluid column, is strictly higher than 4; this ratio can for example be between 4 and 6 or 4 and 10 or 100.
According to a first embodiment, the internal shape of the cavity can include:
Thus, a cavity having at least 2 cylindrical sections with different diameters is created, so as to displace their own frequency modes of the ink cavity for sound velocities in usual inks. Cylindrical sections of different diameters enable a variation in the fluid length to be made.
The actuator means, for example a piezoelectric ceramics, can be directly in contact with the internal volume of the cavity.
The actuator means can include a resonator element. The actuator is thereby resonating.
According to one embodiment, this resonator element includes a resonator body disposed in the cavities.
According to another embodiment, the walls of the cavity form at least one part of the resonator.
The resonator can be of a metal or mineral nature, for example of stainless steel, aluminium, beryllium, brass, copper, diamond, glass, gold, iron, lead, TMMA, silver, or titanium.
The resonator body can include a first part having a first diameter and a second part having a second diameter, different from the first one.
The invention also relates to a device for forming and ejecting drops of an ink jet of a CIJ printing machine, this device including:
a) a cavity for containing an ink and including an end provided with a nozzle for ejecting ink drops,
b) actuator means, in contact with the cavity, of a material chosen from aluminium, 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 resonator under a flange, the latter being chosen to allow for the mechanical resonance of the actuator.
The physical properties of the resonator are adjusted to enable the device to be resonated at a given frequency.
The choice of a material other than stainless steel, and possibly of the length of the bar and thus of the ink cavity, enables the resonance and anti-resonance frequencies, undesirable in ink, to be displaced off the useful range (actuator resonance).
The choice of such a material for the resonator means thus enables parasitic resonances due to a liquid contained in the cavity to be cancelled.
The resonator means can include a piezoelectric element.
The resonator can be inserted in a resonator body having a constant or variable cross-section in the longitudinal direction.
This resonator body can include a first part having a first diameter and a second part having a second diameter, different from the first one.
Both embodiments can be combined to optimise the final implementation.
In either or both embodiments, a device for forming and ejecting drops according to the invention can contain an ink, for example an ink in which the sound velocity is between 800 and 2 000 m/s.
The invention also relates to a continuous ink jet (CIJ) type printing machine, this machine including:
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 implemented.
The invention enables the resonant stimulation principle to be preserved with its advantages (efficiency, cost).
It can be applied to different implementation types of drop generator.
The combination of both embodiments introduced (cavity having several acoustic impedances, and specific material chosen for the resonator) enables some drawbacks unique to each mode to be limited; it makes it possible in particular to achieve a compromise between:
In
The stimulation body 20 of
The body 20 further includes an actuator comprising a ceramics 21, of a piezoelectric material, with a cylindrical shape along the axis XX′. The actuator is mounted in the envelope 25 of the modulation body 20.
This ceramics is metallized on its 2 faces 210, 212, perpendicular to the axis XX′. It is coaxially secured to a cylindrical metal bar 22. For example, the securement is made by gluing with a glue, which can advantageously be a conductive glue.
According to the embodiment illustrated, this bar includes a circular flange 23 on which the face 212 of the ceramics is attached.
The envelope 25 can be provided with a seat or an inner bearing surface 250, which is perpendicular to the axis XX′ of the cylinder and which is provided with a hole 252 through which the cylindrical metal bar 22 can be introduced. A bearing surface 230 of the circular flange 23 can thus bear against the inner bearing surface 250.
Mechanical means, not represented, enable the flange 23 (thus the actuator) to be centered and clamped to the surface 250.
The internal volume of the envelope 25, located under the surface 250 and the flange, defines an insulated cavity 24.
In use, the cavity is supplied with pressurised ink by 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 such that the active face 222 at the end of the bar 22 is located above and close to the nozzle 10, preferably at a distance of a few tenth mm, for example between 2/10th mm and 5/10th mm.
Each of the internal elements (actuator, envelope 25, nozzle 10) of the modulation body is of a circular cross-section and these different elements are coaxially placed with respect to each other, on the axis XX′.
For practical reasons, the bar 22 is, preferably:
One material that can be used is a stainless steel, which has all the characteristics mentioned above.
By construction, the bearing surface 27 of the flange 23 corresponds to a vibration node of the actuator, which avoids efficiency losses by energy transmission into the structure of the modulation body.
Besides, it is preferable that the end 220 of the bar 22, which is located above the nozzle 10, benefits from a maximum movement amplitude which corresponds to a vibration antinode.
In practice, the actuator can be tuned such that the resonance is located in the vicinity of the operating frequency (so-called “drop” frequency, or even frequency at which the drops are wanted to be generated), but not exactly identical not to make the system too sensitive to variations in conditions of implementation of the actuator (mechanical tolerances of an actuator to the other for example). The tuning is generally made in air, at a frequency offset from the operating frequency, for taking the frequency sliding, related to the impedance difference existing when the bar is located in different materials (ink for example), into account.
In this example, the part of the bar 22 under the flange 23 is placed in the cavity 24 (body of the drop generator) the length of which is substantially identical to that of the bar 22.
In use, the electrode 210 of the ceramics 21 is connected to powering means 27. The body 25 can be connected to a ground 29 which will be shifted to the electrode 212 through the flange 230.
Its operation is close to that described above in connection with
There is again a cavity 34, with a cylindrical internal shape, delimited by two end surfaces 320, 322, perpendicular to the axis XX′. Pressurised ink is brought into this cavity by a conduit 36. A 1st end of this tubular cavity is closed by the partition wall 322 perpendicular to the axis XX′. A nozzle 10 is formed in the 2nd end partition wall 320, to let a jet to out along the axis XX′.
It is the envelope 32, which delimits the cavity 34, which provides the function ensured by the bar 22 of the first embodiment. It is excited by a piezoelectric ceramics 31 secured by a mechanical means or by gluing onto the partition wall 322. The ceramics-envelope assembly forms a resonator, the partition wall 322 being at a vibration node, the maximum movement amplitude being located at 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 operating frequency, in the length of the envelope 32. In this case, the impedance influence brought about by the ink present in the cavity is to be taken into account to tune the assembly to the proper frequency.
In use, one electrode of the actuator (for actuating the ceramics 31) is connected to powering means 37. The envelope 32 can be connected to a ground 39.
Upon mounting, the ceramics 41 is clamped between the flanks 48a and 48b of the throat. Under the effect of a periodical electric field created between electrodes, disposed as a crown on the faces of the ceramics element 41, which are perpendicular to its axis, this is longitudinally deformed and transmits this vibration to the envelope 42 to which it is secured. This excitation is transmitted to the nozzle 10 and then to the jet. As in the embodiment of
In use, the actuator 41 is connected to powering means 47, this electrode is electrically insulated from the envelope 42. The envelope 42 can be connected to a ground 49.
Unlike the structure of
The part of the bar under the flange 53, 23 is placed in the cavity (body of the drop generator) the length of which is once again substantially identical to that of the resonator 52 of the cavity 54.
Explications already given above in connection with
The printing head can have a mechanical configuration which is common for several types of drop generators which produce drops with different sizes (to simplify: high, intermediate and possibly small), accordingly which operate at different frequencies. The overall space and the inputs/outputs can thus be identical for all types of generator; the cavity length can also be very close for these different types. For the different resonator types to be able to operate at different frequencies while preserving a length between flange and nozzle which is substantially identical, the bar shape can be acted on. Consequently, the bar for a head G (lowest frequency) is a simple cylinder the length of which is the highest (
But the problem to be solved, set out in the present application, and in particular herein 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 character of these resonances has not been emphasised in prior art, in particular in documents JP 2006-076039 or JP-2005-081643, or even U.S. Pat. No. 5,063,393 or JP-S58-3874.
Once again, there is a cavity 34, with a cylindrical internal shape, delimited, on the side of the nozzle 10, by an end surface 320 perpendicular to the axis XX′. Pressurised ink is brought into this cavity through a conduit 36.
The other end of this tubular cavity is in direct contact with an actuator, here a piezoelectric ceramics 31 (itself held by a peripheral flange to the wall of the cavity).
In this figure, the cavity is of an elongate shape, according to the axis XX′. But it can also be curved.
In use, an electrode of the actuator 31 is connected to powering means 37. The envelope 32 can be connected to a ground 39.
In this device, the envelope 32, which delimits the cavity 34, does not provide a function as ensured by the bar 22 of the first embodiment. The ceramics-envelope assembly does not form a resonator. The ink is directly vibrated by the actuator 31 and resonances are formed in the cavity at the operating frequency.
This type of device has the same problems as those introduced above, in particular for the other devices as those of
Generally, the optimum operating frequency of a jet is determined for the different parameters defining the same. Among these parameters, there are:
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 jet excitation frequency.
Lb can be measured by observing the jet with a camera and a stroboscopic lighting synchronised to the drop period (this enables the image of the drops being formed to be fixed). Then, the distance between the nozzle and the break is measured by micrometric displacement of the camera.
Another technique is described in document WO 2012/2107560 (see in particular the description in connection with
Generally, it is considered that the lower the break length, the higher the stimulation efficiency. The curve of
According to what is indicated above, the optimum excitation frequency v0 is that which corresponds to the absolute minimum of the length Lb.
However, it could have been noticed that the actual curves of the time change of Lb as a function of the jet excitation frequency, examples of which are represented in
More precisely, it could have been emphasised that, upon use 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, some frequency behaviours have been observed, which correspond neither to the actuator resonance nor to the jet resonance.
The jet instability is excited by the actuator, which thus ensures its stimulation function. The actuator is preferably designed such that both resonance frequencies, that of the jet and that of the actuator, are close to each other.
In comparison with these 2 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 comes to be superimposed to the actuator excitation.
For the so-called “resonating” actuator family, the resonance frequency of the actuator depends on the velocity of the acoustic waves in the material of the resonator bar and the dimensioning thereof. In the case of the structure of
The resonator (or the envelope in the embodiments of
The properties of some inks are such that the velocity of waves in the ink is around 4 times lesser than in stainless steel (Cink≈1 200 m/s). As a result, the ink cavity also makes up a resonator in which a standing wave can be developed, the resonance or anti-resonance frequency of which will be close to the resonance frequency of the actuator.
The velocity of the waves in stainless steel (or, more generally, in the material making up the bar) has a very low sensitivity to temperature whereas that of the waves in the ink is of a very high sensitivity to temperature (variation between −3 and −4 m/s per ° C.). Data regarding the time change of this velocity as a function of temperature are gathered in
The resonance modes in the resonator and in the cavity are very close to each other and change in differently as a function of temperature. The resonance and anti-resonance modes of the fluid cavity can thus be displaced as a function of temperature, by intersecting the mode of the resonator which in turn only varies very little as a function of temperature. As a result, there are disturbances in the stimulation in some temperature ranges.
A first study conducted on this problem relates to the case of a drop generator provided with a stimulation body of the type of
In
On the same Fig., the curve II represents the time change of Vs, that is the output voltage of the stimulation range, as a function of temperature. A peak is noticed on this curve II, at about 25° C.
Curve III represents the time change of Vs/Ve, that is the input voltage/output voltage ratio of the stimulation range, as a function of temperature. This ratio is representative of a sturdiness of the stimulation: the higher, the easier the printer to be set since a single stimulation voltage enables quality drops to be formed throughout the temperature range. Here, it is noticed that from about 25° C., the drift is very high.
Curve IV represents the time change of the voltage at the turn Vr. This is initially stable, and then, as the input voltage, increases as a function of temperature, from about 25° C.
Curves that represent the time change in the break length Lb as a function of temperature (from 5° C. to 45° C., by 5° C. pitch) and the stimulation voltage could be set. These curves are represented in
From these curves, it has been attempted to determine how the stimulation efficiency changes as a function of temperature. For this, at a given voltage, it appears that the break length Lb can vary by a factor 2 as a function of temperature. Based on the capillarity instability theory, the following expression is obtained:
with:
Lb: break length
a: jet radius from the nozzle
Vj: mean jet velocity
ΔVj: jet velocity modulation (result of the stimulation process)
γ: dimensionless growth rate of the modulations which is substantially constant on the operating range (in particular the temperature range)
We: Weber number.
The velocity modulation varies exponentially with the break length and thus the stimulation varies in proportions much higher than a factor 2.
Since the purpose is to compare modulation levels at different temperatures, it is shown that the stimulation efficiency dramatically drops between 20° C. and 40° C. The influence of temperature can vary by a few % the input parameters (typically by the surface tension, . . . ), which is irrelevant to the orders of magnitude on the stimulation efficiency.
To explain this abrupt efficiency variation, one can contemplate:
The stimulation body can thus be regarded, by searching for resonances in the solid and liquid.
As a first approximation, it can be reasonably considered that the materials of the resonator, for example ceramics and stainless steel for the bar are stable on a range of a few tens of degrees. The charge brought back by the ink, onto the actuator, does not enable the drastic change on the stimulation efficiency to be explained.
In the liquid (anywhere where the ink is present), an acoustic resonance phenomenon can exist as soon as its greatest dimension is in the order of the wavelength.
At 83 KHz and for a velocity in the order of 1 200 m/s (in a MEK-based ink), the wavelength is typically 15 mm, which is shorter but however comparable in order of magnitude to the height of the stimulation body (here about 21 mm, in an exemplary geometry of
A relationship which expresses the dependence between the modulation generated by the piezoelectric actuator and ΔVj, the jet velocity modulation, can be set by including the propagation phenomenon in the ink. The complete transfer function can be determined and the existence of resonance frequency related to the ink and in proximity of the operating frequency can be searched. 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 and/or intersect the operating frequency (imposed by the actuator).
The drop generators can be schematically construed in order to list the main functional elements thereof.
The resonator body, for example of stainless steel, is considered as being non deformable: the walls have a null velocity condition regardless of whether it is in flow or propagation.
The physical behaviour of the functional elements of the drop generator and the equations associated therewith will now be set out. For this, the impedances of each of the elements are determined.
The pressure drop through the nozzle 50 is described by the Navier Stokes equations. In the sinusoidal mode, the movement of the ink mass trapped in the nozzle is limited by the inertia terms. The nozzle impedance will be noted Zb:
with:
Lnozzle: nozzle length
Sb: nozzle cross-section area
ρ: ink density
ω: angular frequency at the operating frequency.
The ink wedge 520 under the actuator concerns the column at the input of the nozzle (this column is located in the removable nozzle plate but before the zone 521 which connects it to the nozzle 50), and the ink “disk” located under the active face of the actuator. For the column, the diameter is for example 500 μm, to be compared with the nozzle diameter, once again taken by way of example, of 50 μm. The ink velocity in the wedge is thus very low (factor 100) compared with the nozzle. The fluid can thus be considered as immobile (no inertia effect). The wedge impedance is thus only its compressibility term noted Zc:
where Ke is the compressibility and Ve the ink volume of the zone 521.
The waveguide 550 is an acoustic element delimited by the active face of the resonator; it rises up to the level of the shoulder 53 against which the resonator bears. This zone being flowed with liquid, the liquid ring is thus considered between the resonator and the sheath of the stimulation body.
It is reminded that the liquid column has section variations, the impedance of this column, per segment, is given by the formula of the line theory (in electrical analogy):
where ZBC is the equivalent impedance at an input of the segment AB with an acoustic impedance Zb terminated by a charge impedance ZAB.
The piezoelectric actuator has in turn a resonating behaviour that can be modelled by the localised constant approximation (mass-spring analogy). In view of impedances relating to the actuator with respect to the fluid, the actuator is dominating: in the first order, the resonance frequency of the stimulation assembly is set to the resonance of the ½ Langevin (the resonator) in air.
Since the operating frequency is fixed (83.3 KHz), this mechanical resonance will not be considered, for the model to be more legible. The resonating assembly is thus assimilated to a flow rate source, this is the ink volume agitated at the end of the resonator: Q.
The unit impedance terms are defined for the outflow rate, thereby it is possible to determine the pressure P at the end of the bar. The pressure drop in the nozzle equivalent to its impedance Znozzle gives the flow rate as a function of the frequency or even the jet velocity modulation for a given nozzle section.
The previous formulae have enabled the curve (
It is noticed in this Fig. that, in the frequency range of interest, that is 80-90 KHz, there are two noticeable frequencies F1 and F2 which will have an influence on the efficiency level of the stimulation at 83.3 KHz. This frequency overall space does not rise any problem if these frequencies are stable in the operating environment of the printer; at most, the stimulation level can be different from one printer to the other.
But these frequencies F1, F2 change as a function of temperature which seems to be the parameter disturbing the sturdiness for stimulation. Simulations with “MathCad” software enable the ink velocity as a strongly influencing parameter to be identified. At room temperature (see Handbooks of Physics 1990-1991—71th edition—pages 14-32 and the velocity measurements in actual inks of curve of
The same simulation has been made on a temperature range of 45° C., as experimentally explored, which enabled a frequency offset of F1, F2 of about 10 KHz to be emphasised (
This frequency offset can seem to be low enough; however, when combined to the proximity of F1 and F2 about 83.3 KHz, it is understood that it is possible to have high variations in the stimulation levels when F2 intersects the operating frequency.
The tests reported above have enabled an acoustic resonance phenomenon to be emphasised within the fluid cavity. This phenomenon is depending on the propagation velocity of the acoustic waves within the ink; a dependence, as a function of temperature, thus appears, which positions the events, in frequency, closer or less close to the operating frequency.
Complementary results (actual measurements) have been made, with the same type of stimulation tunings. These measurements implement a stimulation body identical to the previous simulated situation, with the following settings: the results are shown in
For these measurements, with a low voltage (low stimulation), the measurement of the break length Lb during a frequency scanning has been made, at different temperatures (5° C.-45° C.), in order to view the events on the 70-100 KHz range. The break length Lb is measured. These measurements are made on the temperature range from 5° C. to 45° C., with a 10° C. pitch, using the following parameters:
The results illustrated in
These complementary results confirm the disturbances observed and already reported above. On the other hand, they illustrate the difficulty, or even the impossibility, to maintain a stable operation of a drop generating device at at least 2 positive temperatures distant by about at least 15° C. or 20° C., for example on the one hand by 5° C. and/or 15° C. and, on the other hand, by 30° C. and/or 35° C. and/or 45° C., more generally in a temperature range ranging on the one hand from 5° C. or 15° C. to, on the other hand, 35° C. or 45° C. or even 50° C.
Other works have confirmed the hypothesis of the influence of the disturbances related to the resonances present in the fluid cavity. Actual measurements have been made on a drop generator with a head G the mechanical simplicity of which (cavity and resonator bar are thus cylindrical, of the type as in
Complementary tests have thus been conducted for a stimulation body of the type of that of
More precisely, the break length has been investigated, as a function of the frequency, in low stimulation, for 3 different temperatures. Since the stimulation voltage is 7 Volts, it enables always to have a “slow” satellite and thus, according to the linear theory of capillary instability, the break length to be directly related to the stimulation efficiency.
The temperatures tested were 5° C., 25° C., and 45° C.
The ink used is a pressurised white pigmented MEK-based ink to reach a constant jet velocity of 20 m/s. The tests have not been made at a constant wavelength; hence, the jet velocity is not readjusted as a function of frequency, and a parabolic type envelope is obtained, which reflects the physical capillary instability phenomenon which will be taken into account in exploiting the results.
In
For 5° C. (
The theoretical model has been adjusted with a velocity in the ink c=1 170 m/s. The resonance frequency of the actuator is about 64 kHz. The model further gives 2 transmission zeros, corresponding to 46 kHz and 74 kHz. For 46 kHz, the efficiency decrease associated is being found again; but, for 74 kHz, it has not been possible to read out the values, since the break is in the noise
of the natural break.
The model also predicts a resonance peak at approximately 57 kHz remarkably observed on the curve of break length. The resonance phenomenon at 64 kHz is also emphasised, it is prevailing in terms of amplitude because it is imposed by the actuator.
For 25° C.
For 50° C.
The theoretical model has been adjusted with c=1 030 m/s, that is a slope of −3.5 m/s/° C. The first zero is found slightly before 40 kHz and the second at 65 kHz. The latter is very close to the operating frequency and thus comes to be superimposed with the resonance peak of the actuator located at 64 kHz.
To solve the abnormalities observed above, it is suggested to adjust the acoustic impedance of the system, more particularly that of the fluid cavity, in the proximity of the nozzle 10.
This acoustic impedance varies as a function of frequency, in particular, when this varies about the operating frequency.
In
As seen in
This impedance variation results in varying the amplitude of the jet velocity modulation (or even the stimulation efficiency) in the nozzle and thus the break length.
Further, the graph of
According to the invention, a frequency range [f1, f2], of ±10 kHz or ±5 KHz, about the operating frequency ft is defined. The system is such that, when the frequency varies in this range, the value of the velocity modulation in the nozzle at a temperature T, with respect to the velocity 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 that at, on the one hand, 15° C. and, on the other hand, at 35° C., preferably also at 5° C., and/or 10° C. and/or 20° C., further preferably also at 45° C. or even 50° C., further preferably at any temperature included in a temperature range ranging from at least 15° C. (or 10° C. or 5° C.) to at least 35° C. (or to 40° C. or to 45° C. or to 50° C.). An example of this interval of velocity modulation is represented by horizontal bold lines in
It is noted that the impedance can be calculated according to the already above mentioned formula. From this calculation, the jet velocity modulation and its variations under the effect of temperature can be deduced.
This velocity modulation can thus be estimated or deduced from the measurement of the variations in Lb (the formula of which has moreover been given above) as a function of frequency, at a constant excitation voltage. Indeed, a variation in Lb reflects a variation in impedance.
Alternatively, it is possible to measure or estimate the variations in pressure, as a function of frequency. At the nozzle 10, these variations in pressure represent or reflect variations in Lb as well as variations in acoustic impedance (i.e. jet velocity modulation).
The solution provided above can be achieved by modifying the configuration of the internal volume of the stimulation body, intended to receive ink, giving it a shape enabling a variation in acoustic impedance to be made.
In other words, the internal volume includes at least one first part, having a first acoustic impedance, and at least one second part, having a second acoustic impedance, different from the first acoustic impedance.
For example, in the cavities, one element, or means, can be introduced, enabling this variation in impedance to be made. The embodiments of this solution are represented in
The device of
In these Fig, the ring is represented in the lower part of the cavity. Alternatively, it could be disposed in another part, for example according to the arrangement represented in dashed lines on each of these Fig. Thereby, it would have the same role of modifying the acoustic impedance of the cavity.
More generally, it is also noticed, on these Fig, that the internal shape of the cavity includes:
In the case where the ring of each of
As will be shown below, differences, or variations, in acoustic impedance, induced, in the examples of
The different diameters enable a variation in the fluid length to be made. In the case of the structures of
Tests have been made, with a structure of stimulation body according to
Complementary tests have been made with a “standard MEK based” type ink and then with an “alcohol-based” type ink. The results obtained are similar to the 2 previous inks and confirm the optimum character of the 3.6 mm ring.
The presence of the ring enables the volume of the ink cavity to be decreased which facilitates the rinsing of the drop generator during maintenance operations.
The tests above show that the invention enables a sturdy operation to be achieved throughout the temperature and ink range contemplated (through the velocity). The invention enables any disturbing event on stimulation efficiency to be removed. A sharp improvement is noted on most of the curves obtained, that is a random operation is switched 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 directly machining the ring function in the modulation body which therefore becomes a single piece and which has variations in cross-section area, thus having a profile identical or similar to what has been represented in
According to another embodiment, the differences in sound wave velocities in various materials other than stainless steel are exploited. The stainless steel material used is then replaced for the resonator with one of these other materials.
This solution enables conditions set forth above in connection with
This solution also enables the resonator length to be modified while keeping the same operation frequency. The choice of another material is accompanied with a modification in the resonator length which, in the first place is proportional to the velocity ratio.
If the velocity is greater than in stainless steel, the bar (case of
In this case, the resonance and anti-resonance frequencies of the fluid cavity will be displaced and rejected outside the stimulation operating zone.
Table I gathers data related to the sound wave velocity in these other materials.
If one of these other materials is retained for the resonator bar, then the disturbance effects of the sound waves in the ink will not be exhibited.
More generally, all the metal materials—other than stainless steel—or mineral materials can be suitable.
This choice further enables the length of the resonator, and thus the cavity length to be possibly reduced, which enables, furthermore, the parasitic resonances as set forth above to be avoided.
Regardless of whether the structure of the stimulation body is that of one of the
An ink jet device or printer for implementing a method for forming ink drops, with a device according to one of the embodiments detailed above, is of the type that has already been described in connection with
Such a device thus includes:
The operation of this jet type has already been described above in connection with
Besides the means above, such a device can further include means 5 for controlling and regulating the operation of each of these means taken alone, and the voltages applied. These means 5 are described below more precisely in connection with
In this Fig., an assembly of controller means 5 includes circuits, which enable the voltages for driving the printing head to be sent to the same and in particular the voltages to be applied to the electrodes as well as the piezoelectric excitation voltage.
This assembly 5 can further receive downlink signals, from the head, in particular the signals measured using a position and/or drop velocity sensor, and can process them and use them for controlling the head and the ink circuit. In particular, for processing the signals from such a sensor, it can include means for analogically amplifying this signal from this sensor, means for digitising this signal (A/D conversion transforming the signal into a list of digital samples), means for de-noising it (for example one or more digital filters for the samples), means for searching the maximum thereof (the maximum of the list of samples).
This controller assembly 5 can communicate with means 500 for sending and/for receiving fluids to and from the printing head.
This controller assembly 5 can communicate with the user interface 6 to inform a user about the printer state and the measurements performed, in particular of, the type of those described below. It includes storage means for storing instructions relating to data processing, for example for carrying out a method or carrying out an algorithm of the type described above.
According to an exemplary embodiment, the controller 5 includes an embedded central processing unit, which itself comprises a microprocessor, a set of non-volatile memories and RAM, peripheral circuits, all these elements being coupled to a bus. Data can be stored in the memory zones, in particular data for implementing a method according to the present invention or for controlling a device according to the present invention.
The means 6 enable 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 requirements of the production line (rate, printing velocity, . . . ) and more generally of its environment, and/or the preparation of a production session for determining, in particular, the printing content to make on the products of the production line, and/or by displaying information in real time for the follow-up of production (state of consumables, number of labelled products, . . . ). These means 6 can include viewing means.
Means can further be provided for supplying or bringing the different electrodes to the desired voltages. These means include in particular voltage sources.
A stimulation body according to the invention, and a method for operating a stimulation body according to the invention, as described above, applied to a printer of the type described in connection with
With a device and a method according to the invention, the “parasitic” frequencies are discarded, regardless of the temperature in any of the ranges discussed above, from the operating frequency range used. For example, this operating range is between 50 KHz and 150 KHz depending on the diameter and jet velocity chosen.
| Number | Date | Country | Kind |
|---|---|---|---|
| 14 53134 | Apr 2014 | FR | national |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 15302734 | Oct 2016 | US |
| Child | 15800403 | US |
