EP3322589A2

EP3322589A2 - Method for actuating an ink-jet print head

Info

Publication number: EP3322589A2
Application number: EP16758262.6A
Authority: EP
Inventors: Jan Franck
Original assignee: Individual
Current assignee: Individual
Priority date: 2015-07-13
Filing date: 2016-07-11
Publication date: 2018-05-23
Anticipated expiration: 2036-07-11
Also published as: CA2991393A1; US20190054741A1; JP6648251B2; CA2991393C; IL256571A; JP2018524213A; CN107835748B; EP3322589B1; CN107835748A; WO2017009705A3; AU2016291839A1; AU2016291839B2; IL256571B; WO2017009705A2; SG11201800283TA; US10556427B2; RU2692036C1

Abstract

The invention relates to a method for actuating an ink-jet print head, comprising at least one printing system that has a nozzle positioned on the side of an ink chamber that faces a substrate to be printed, said ink chamber being delimited at least in some sections, and preferably in the region thereof that faces away from the printing substrate, by a membrane which can be moved away from the ink chamber by electrically actuating a piezo-element that is mechanically coupled to said membrane, such that ink is sucked into the ink chamber from a storage chamber, and which membrane can be moved into the ink chamber such that an ink drop is pushed out through the nozzle, wherein the printing system, which consists of the ink chamber, membrane, piezo element and the electronic actuation element thereof, constitutes a vibrating structure that, upon a high-energy actuation, is excited to vibrate with a natural frequency fres exhibiting a resonance, i.e. the vibration with the cycle T_res = 1 / _fres is not or is only slightly attenuated, wherein the brightness of an image point being printed is varied by a sequence of a plurality of ink drops being pushed out of the same nozzle one after the other, for each image point, with a temporal spacing of T_tropf = 1 / _ftropf, and energy is only introduced into the printing system by said actuation signal precisely when an ink drop should also actually be pushed out.

Description

Method for controlling an inkjet print head

The invention is directed to a method for controlling an inkjet print head, comprising at least one printing system with a nozzle on the side of an ink chamber facing a substrate to be printed, which is bounded by a membrane at least regionally, preferably in its region facing away from the printing substrate electrical actuation of a mechanically coupled to the membrane piezoelectric element away from the ink chamber is movable to suck ink from a storage space in this, and is movable into that to squeeze out through the nozzle an ink drop, wherein the printing system consisting of ink chamber, membrane , Piezoelement and its control electronics, represents an oscillatory structure, which is excited at a high-energy activation to a vibration with a natural frequency f _res , which shows a resonance, ie, that the oscillation with the period T _res = 1 / f _res not or only is hardly damped, and wherein the brightness of a pixel to be printed is varied by successively _ejecting one sequence of one, one, two or more drops of ink from the same nozzle per pixel, with a temporal distance T _tr0 pf = 1 / dropping

A known ink printing system is shown in Figs. 1 to 4. 1, the outer, mechanical structure of an ink jet print head 1. Its central region forms an elongated plate 2, with adjustment and / or mounting holes 3, 4 and / or pins in the end-side end portions 5 of the plate. 2

Between these, the adjustment and fixing end portions 5 is a central, preferably thickened part 6 of the plate 2; There, in the flat side 7 of the plate 2 facing a substrate to be printed, a plurality of nozzles 8 for dispensing drops of ink in the direction open to the substrate to be printed. In the area of the flat side 7, the middle part 6 of the plate 2 provided with nozzles 8 can be raised in relation to the adjacent end areas 5, so that heads 9 of fastening screws projecting beyond the end areas 5 do not touch the substrate to be printed.

At the opposite flat side 10 of the plate 2 extends in the region of the central part 6 of the plate 2, a mechanism which is shown in Fig. 2 in section. It can be seen that behind each nozzle 8 is one of these associated printing system 11.

Each printing system 11 comprises its own ink chamber 12, from which only the single nozzle 8 opening into it is fed with ink. This ink chamber 12 is connected via a contrast cross section significantly reduced ink channel 13 with a feeding ink reservoir 14; through the ink channel 13, the ink chamber 12 can be refilled after printing an ink drop.

While the ink channel 13 opens laterally into the ink chamber 12, located on the nozzle 8 opposite side of the ink chamber 12, a membrane 15 which is fixed only along its peripheral edge, for example clamped. In its central region, the movable part of a piezoelectric element 16 is attached to the side facing away from the ink chamber 12 side of the diaphragm 15, which is already fixed to a solid, rear plate or on a solid, rear block 17.

By way of a drive circuit 18 connected directly to the piezoelectric element 16, this can be excited into contractions or expansions which are transmitted 1: 1 to the connected membrane 15 and therefore increase or decrease the volume within the ink chamber 12. When enlarging the ink chamber 12, ink is sucked from the ink reservoir 14 into the ink chamber 12, as the ink chamber 12 is reduced, an ink droplet is expelled from the nozzle 8 when the volume reduction is sufficiently strong and violent, that is, sufficiently large and fast, so that as a result, ink droplets bulging at the nozzle 8 will rupture.

In Fig. 3 it can be seen that a total of only a few ink reservoirs 14 are provided, preferably only one or two, to each of which a larger part of the ink chambers 12 is connected.

In Fig. 4 it can be seen that the piezoelectric elements 16 can be represented as capacitances in electrical terms, which charge when connected to a supply voltage 19 to cause a mechanical reaction, such as a contraction or expansion; they may also be discharged or reloaded to initiate the reverse mechanical response, for example by short-circuiting the two electrical terminals of one capacitor or by actively applying a different voltage.

The entirety of a printing system 11, that is, its ink chamber 12, membrane 15, piezoelectric element 16 and the drive circuit 18 form a vibratory system. Its natural frequency f _res depends on the geometric structure of the printing system 11 and on the properties of its components 12, 15, 16, 18. However, the natural frequency f _res does not need to be calculated but can be triggered by triggering a vibration by means of a sufficiently high-energy trigger signal the electrical connections of the piezoelectric element 16 are read. There, a largely undamped oscillation is recognizable whose period T _{res is} inversely proportional to the natural frequency f _res : f _res = 1 / T _res - A measuring circuit for the determination of this natural frequency f _res is conceivably simple: An active drive circuit 18 for a piezoelectric element generates an isolated drive pulse or a series of drive pulses spaced in time, for example at intervals of one or more seconds. If the signal curve is so strongly attenuated by the drive circuit 18 after delivery of the pulse or in each case a pulse that the natural frequency oscillation already decays within one oscillation period, the connection between the drive circuit 18 and the piezoelement 16 can be additionally interrupted after delivery of a pulse or are interrupted until the next pulse, so that the piezoelectric element 16 together with the connected mechanical components is left to itself and can oscillate freely without appreciable damping, so that a plurality of measurable oscillation waves follow each other. In this phase, an electrical voltage in the form of a gradually decaying wave is measurable at the electrical terminals of the piezoelectric element 16, with a frequency corresponding to the resonant frequency f _{res of} the entire system of electrical and mechanical components.

The voltage on the piezoelectric element is measured or recorded dynamically, for example with an oscilloscope or a storage oscilloscope, wherein the image representation or recording is triggered by the drive pulse. On the time scale of the oscilloscope screen or a stored signal recording, one can then read off the period T _{res of} a resonance oscillation and from this determine the resonance frequency f _res using the formula: f _res = 1 / T _res .

For the variable darkening of a pixel, up to k ink droplets per pixel can be delivered: n = 0, 1, 2, ... k. In this case, the time interval T _trop f between two successive drops of ink may be constant, for example T _seq / (k-1), that is to say a part of the entire image drop sequence T _seq , or a multiple thereof, in particular:

T _drip = T _seq * (kn) / (k-1) when printing n <k ink drops per pixel sequence.

Usually, the natural oscillation at the resonant frequency f _{res of} the printing system 11 is initially excited by a signal having a reduced amplitude, and then an ink drop per period T _{res of} the inherent shrinkage is emitted in the period grid prescribed thereby by means of a correspondingly set antineration, and although in each case at about the same phase of natural vibration. This requires a control to the effect that - at least in case of more than one drop of ink per pixel sequence:

Drop ^{- "} ^ res-

Thus, the printing system is subordinated to the natural frequency of the system; this natural frequency forms, so to speak, the clock in which printing takes place. However, it has been found that this clock frequency f _{res is} comparatively slow and therefore limits the printing speed. This in turn means that in practice the number of ink drops per image sequence is reduced as much as possible in order to still keep the printing speed to a tolerable level; However, this in turn results in a reduced printing accuracy, because then each ink droplet must have a comparatively large volume, and thus no fine gradations are possible.

From the disadvantages of the prior art described results in the invention initiating problem, a generic method such to further develop that the printing speed can be increased, and / or at the same or improved printing speed, the gradations in the color brightness can be refined.

The solution to this problem succeeds in that energy is introduced into the printing system via the activation signal only when an ink droplet is actually to be forced out.

By thus dispensing with the initiation of the natural oscillation before a pixel sequence, the printing system is initially in an idle state there. The first drop is printed with a large drive amplitude, but with a shortened pulse duration to minimize the energy introduced into the system, so that no or only minimal excitation occurs at the natural frequency and consequently the printing system does not settle after the first ink drop, but within less than a period T _{res of} the natural oscillation comes to rest again, preferably even within less than half the period T _{res of} the natural oscillation. This opens up the possibility of being able to deliver the drive pulse for the next following ink drop within a fraction of this period T _res . Consequently, no rhythmic suggestions take place in time with the natural frequency f _res so that no natural oscillation can build up in the course of several successive drive pulses. Rather, the printing system returns to its idle state after each ink drop.

It has proved to be favorable that the minimum time interval Tt _r0 pf between chronologically successive pressure control _{signals is} equal to the oscillation period T _res = 1 / f _{res of} the resonant natural frequency f _{res of} the printing system:

¹ drip res- The best is a certain safety distance Τ _ε = | T _res - T _drip | > 0 between the two period _values T _drip T _res , so that even in the case of directly successive control _signals no resonant oscillation is triggered or that a possibly triggered by successive control signals oscillation undergoes a sufficient damping. Such a safety distance can be defined by

^~ Γ _ε = | T _res - droplet I u * T _res , with 0 <μ <1, in particular with μ = 1/5, or μ = 1/4, or μ = 1/3.

The invention can be further developed such that the minimum time interval Τ _ίΓ0Ρ ί between temporally successive pressure control _{signals is} equal to or preferably smaller than three quarters of the oscillation period T _res = 1 / fres of the resonant natural frequency f _{res of} the pressure system:

Ttropf _S T _res / 1, 33.

This corresponds to a value for μ of μ = 1/4.

The invention can be further developed such that the minimum time interval T _trop f between temporally successive pressure control signals is equal to or preferably less than two thirds of the oscillation period T _res = 1 / fres of the resonant natural frequency f _{res of} the printing system:

Drop Tres / 1, 5.

This corresponds to a value for μ of μ = 1/3.

Another design rule states that the minimum time interval T _t r ₀ pf between temporally successive pressure drive signals is equal to or preferably less than half the oscillation period T _res = 1 / f _{res of} the resonant natural frequency f _{res of} the printing system:

Drop T £ _res / 1.66.

This corresponds to a value for μ of μ = 2/5.

Another design rule states that the minimum time interval T _tr0 pf between chronologically successive pressure control _{signals is} equal to or preferably smaller than half the oscillation period T _res = 1 / f _{res of} the resonant natural frequency f _{res of} the printing system:

Ttropf <Tres 1, 75.

This corresponds to a value for μ of μ = 3/8.

On the other hand, the minimum time interval T _tr opf between temporally successive pressure drive signals can not be made arbitrarily small, because otherwise either successive ink drops would unite during their flight or would not detach any ink drops from the nozzle at all. Therefore should also apply:

Tt _r0 pf> V * T _res , with 0 <v <1, in particular with v = 1/5, or v = 1/4, or v = 1/3.

The invention provides, for example, that the minimum time interval Ttropf between temporally successive pressure control signals is equal to or preferably greater than one third of the oscillation period T _res = 1 / fres of the resonant natural frequency f _{res of} the pressure system:

Drop> Tres / 4. This corresponds to a value for v of v = 1/4.

Therefore, the invention provides that the minimum time interval T _tr0 pf between temporally successive pressure control _{signals is} equal to or preferably greater than one third of the oscillation period T _res = 1 / f _{res of} the resonant natural frequency f _{res of} the printing system:

Drop. Tres / 3.

This corresponds to a value for v of v = 1/3.

Further advantages result from the fact that the minimum time interval Ttropf between chronologically successive pressure control signals is equal to or preferably greater than four tenths of the oscillation period T _res = 1 / f _{res of} the resonant natural frequency f _{res of} the pressure system:

Drop> Tres / 2.5.

This corresponds to a value for v of v = 2/5. Based on the above definition, for μ and v, μ + v <1.

Optimum results can be achieved if a following drive pulse occurs at a fairly precise time when a resonance oscillation initiated by the preceding drive pulse has just completed half a period, because then the renewed drive pulse is anticyclical to the preceding one and counteracts it, so to speak ideally clears. This can be optimally realized by the following design: Drop ^- T _res / 2.

In practice, it has been shown that even slightly different values still provide good results, ie:

0.4 ^* T _r e _S <T _trO p _f <0.6 * T _reS) or in particular:

0.45 ^* T _res <T _tr0 p _f <0.55 * T _res .

Such a control is thus completely different than the usual in the prior art, where the energies of successive drive pulses additively superimposed, so further amplify a resonant vibration. In the present invention, however, the energies of successive drive pulses superimpose subtractive, so extinguish each resonance oscillation. In other words, while in the prior art resonant vibrations are promoted, the invention aims to suppress resonant vibrations.

An advantageous side effect of the method according to the invention is that the droplet frequency can be at least doubled, possibly even increasing even further. As a result, several drops can be dispensed per pixel.

According to the invention, it is further provided that the size or the volume of an ink droplet does not depend on the duration or other properties of a preceding drive pulse. As a result not only several drops of ink per pixel can be generated, but even different large drops. The sizes or volumes of the ink droplets to be printed in a pixel sequence may thus be different and / or independent of each other.

The invention allows a development in that the row of different drop sizes is non-linear. It may, for example, be logarithmic, such as 8: 4: 2: 1. It is evident that with such a system, droplet sizes can be generated between a maximum volume V _max and a minimum volume V _min , for example V _max = 15 * V _min . For an ink quantity per pixel of 7 ^* V _min , one drop of the quantities 4 ^* V _min , 2 * V _min and 1 * V _min would then have to be generated; An ink quantity per pixel of 12 ^* V _min can be generated by means of one drop of the sizes 8 * V _min and 4 ^* V _min , etc.

The size or volume of an ink drop can be increased by increasing the amplitude of a drive pulse. As a result, the membrane 15 is deflected further and moved a larger volume of ink.

Another way to increase the size or volume of an ink drop is to increase the total duration of a drive pulse or the duration of the plateau phase of a drive pulse. This will enable a peeling drop to draw in a larger amount of ink.

The size or volume of an ink drop can also be increased by increasing the duration of the rising and / or falling edge of a drive pulse. This gives the mechanics more time to follow a drive signal, and also allows a larger amount of ink to be set in motion, which then eventually comes off in the form of a larger drop.

Finally, it is in accordance with the teaching of the invention that at most one pixel, to which the color intensity is assigned zero, corresponds to none Ink drop, a wildcard signal is output which, however, is too weak in intensity to cause the repulsion of a drop; However, if a pixel is not associated with zero color intensity, corresponding to one or more drops of ink, then there is no pre- or intermediate non-printing signal either before or between the drive pulses of that pixel sequence. On the one hand, the printing speed is not affected by superfluous intermediate signals; On the other hand, no superfluous energy is consumed, which in particular warms up the ink head and can therefore lead to inaccuracies, and finally, successive drive pulses are optimally adjusted to each other in such an optimal way that no additional signals are required for the desired suppression of resonant natural oscillations.

Further features, details, advantages and effects on the basis of the invention will become apparent from the Unteranspürchen and from the following description of a preferred embodiment of the invention and from the drawing. Hereby shows:

Figures 1a-c show various views of an ink jet print head.

Fig. 2 is a vertical section through a printing system of the ink jet print head of Fig. 1;

FIG. 3 shows the ink chamber system of the ink jet print head according to FIG. 1 in a schematic representation; FIG.

FIG. 4 shows the control electronics of the ink jet print head according to FIG. 1 in a schematic representation; FIG.

5 shows the time characteristic of a customary drive signal for a printing system according to FIG. 1 or the deflection signal of the piezoelectric element proportional thereto; FIG. 6 shows the time profile of the drive signal according to the invention for the printing system according to FIG. 1; FIG.

7 shows the possibilities of influencing the control signal for varying the

Drop size or drop volume, where "+" corresponds to an increasing influence on the drop size and a "-" a reducing influence;

8 shows an exemplary time profile of a sequence of on-drive pulses according to the invention, for the purpose of illustrating the possibility of varying the quantity of ink by overprinting several drops with different volumes; such as

9 shows an exemplary time course of a pixel sequence in the prior art, where all drive pulses are the same size and long, so that all ink droplets have the same volumes.

The structure of the ink jet print head 1 and one of its printing systems 11 has already been described in detail above with reference to FIGS. 1 to 4.

The usual operation of such a printing system is shown in Fig. 5. There, the deflection x or -x of the part of the piezoelectric element 16 coupled to the membrane 15 can be seen as the graph 20. The upper line of the signal corresponds to a kind of zero position of the piezoelectric element 16 or the membrane 15. In this case, the ink chamber 12 is just completely filled with ink. Applied downward, a deflection of the piezoelectric element 16 or the membrane 15 away from the ink chamber 12, resulting in an increase of the volume V in the ink chamber results. This increase in volume AV is approximately equal to the deflection Δχ of the membrane 16, multiplied by the base area F of the ink chamber 12 and that of the completely spanning membrane 16:

AV = Q ^* Δχ.

In Fig. 5, the displacement -x is shown along the ordinate, that is, an upward shift of the diaphragm 16 toward the ink chamber 12 is applied. Thus, when the graph 20 increases, the diaphragm 16 approaches the ink chamber 12 by the value | Δχ | and consequently, the volume V decreases by the amount | AV | = Q * | Δχ |, that is, Δν <0. In other words, upwards -Δχ is plotted, corresponding to -Δν.

Along the abscissa in Fig. 5, the time t is plotted.

A first pulse 21 starts at t = 0. In this case, the deflection 21 in Fig. 5 points downwards, i.e. Δχ becomes larger, so the ink chamber volume V increases, ink in an amount Δν is sucked into the ink chamber 12.

However, the deflection 21 does not take place far enough so that the sucked volume Δν is smaller than the volume of a drop V _drip :

AV <V _drip .

As a result, upon subsequent swinging back of the membrane 16, no ink droplet is pushed through the nozzle 8, but the ink surface bulges outwardly only through the nozzle 8, but without tearing.

By means of this high-energy measure, however, the natural oscillation with the resonance frequency f _{res is} triggered in the printing system 1, the period T _res = 1 / f _{res of} the resonant natural oscillation being approximately 15 ps in the illustrated example. Subsequently, the follow-on pulses 22 for expressing a plurality of ink droplets 24 are generated in a specific time frame. Each successive pulse 22 consists of a falling edge in FIG. 5, during which the volume of ink V in the ink chamber 12 increases by a volume AV, where:

AV = V _drip .

During a short plateau phase 23 of the sequence pulse 22, ink flows from the ink reservoir 14 through the ink channel 13 into the ink chamber 12 of the respective printing system.

Subsequently, the membrane 16 swings back to its original position, and the volume V within the ink reservoir chamber 12 decreases by ΔV. However, this amount of ink corresponds to the volume V _{dripping of} an ink drop, and this finally _breaks off beyond the nozzle 8.

Such a "shot" period, within which so ink is sucked into the ink chamber 12 and then pushed through the nozzle 8 until the beginning of the next suction movement of the membrane 16, corresponds to the period T _{res of} the natural vibration of the printing system 11, so that each Period begins at the same phase of the natural oscillation.

So that the period T _{seq of} an entire pixel sequence is at most about 50 MS, therefore, only a maximum of three ink droplets 24 can be delivered per pixel sequence. Further, since the volume of all the ink drops 24 is approximately the same, can therefore change the color intensity l _F a pixel only in a coarse grid, namely only in the stages

IF = 0 ^* I ₀ ;

IF = 1 ^* lo;

l _F = 2 ^* l ₀ ; l _F = 3 ^* l ₀ ; where l ₀ corresponds to the color intensity of a single drop of color of the respective size.

This means a maximum of 4 different values, which corresponds to an information that can be represented with 2 bits: 00 = 0; 01 = 1; 10 = 2; 11 = 3.

While this may be normal for an inkjet printer, it does not provide a good result, for example, the intensity values of the pixels of images captured by a camera are much finer, for example with 16 different color intensities per pixel and color (4 bits), or 64 color intensities, or with 128 or even 256 color intensities.

In order to solve the above problems in printing by ink printers in terms of a limited printing speed and a very limited resolution of the color intensity, the invention proposes - with otherwise identical printhead 1 and identical printing system 11, the driving method shown in FIG.

This method is based on the idea not to subordinate itself to the natural vibration of the printing system 11 with the resonance frequency f _res , but to avoid those so that the system is not excited to its natural vibration, so that each ink droplet 24 is not synchronous to a vibration of the printing system 11th but theoretically could be delivered at any time.

To avoid the natural vibration of the printing system 11 with the resonant frequency f _res , the invention provides several measures: On the one hand, in the method according to the invention, an excitation pulse 21 'preceding the actual pressure pulses 22' is completely missing.

In any case, therefore, the first pressure pulse 22 'has a stationary diaphragm 16 - defined conditions prevail in the pressure system 11, and the first ink droplet 24 is delivered with high precision.

Another measure to avoid self-oscillation is that the plateau phase 23 'is shortened. While in the prior art the time T _p i _at the plateau phase 23 is greater than the times T _ans t _> T _abf for the rising or falling ramp, the following now applies:

Tpiat ^< T _ans t

Tpiat ^< Tab

This is achieved by increasing the maximum deflection Ax _max , with approximately the same gradient of the rise and fall ramps. Characterized, can be used for suction of the same volume of ink AV time T _p i _a t required to be shortened as a result of a higher Differenzsaugdrucks between ink chamber 12 and Tinntenvorratsraum 14 the flow rate is increased.

Consequently, it can be realized that the following applies:

Tabf ⁺ T _p | at ⁺ T _a nst T _re s / 4, in particular

TABF ⁺ T _p i _a t ⁺ T _ans t ^<T _res /. 5

The consequence of this measure is that the spectrum of such a single wave is shifted towards higher frequencies, thus far more has greater frequency spacing to the resonant frequency f _res . As a result, the resonance frequency f _{res is} not triggered thereby.

Another measure for avoiding resonant natural oscillations in the printing system 11 is to further reduce the duration of a period T _trop f for the printing of an ink droplet, in particular such that the following applies:

T _trop f <T _res 1 »5.

As a result, there are no spectral _components in the spectrum at f _res , and as a result no natural oscillation is excited.

Even more, for example at a period of Τ _ίΓ0Ρ ί = T _res / 2, a possibly previously initiated natural oscillation of the frequency f _res is erased by an anticyclical phase position by means of a following pressure pulse.

On the other hand, the period duration should not be too short, so that successive ink droplets 24 remain separated from each other in the flight phase and not uncontrollably connect with each other in flight, otherwise the size of the sucked from the nozzle 8 drops 24 could deviate from the desired volume. To ensure this, the invention recommends the following inequality:

Drop> T _res / 3.

Such printheads 1 or printing systems 11 are particularly suitable for the process according to the invention, in which the movement of a membrane 15, which at least partially delimits the ink chamber 12, is effected by a piezoelement 16. Its activity direction is directed mostly perpendicular to the membrane 15. Nevertheless, there are various such piezo printheads, which in particular with regard to the arrangement of the Membrane 15 and the acting on that piezoelectric element 16 relative to the position and longitudinal direction of the nozzle 8 differ:

In the arrangement referred to in the art "Piston Shooter", the diaphragm 15 is located between the nozzle 8 and the piezoelectric element 16, and the action direction of the latter is in alignment with or parallel to the longitudinal direction of the nozzle 8.

In the so-called "Side Shooter", the membrane 15 is located laterally on the ink chamber 12, so to speak adjacent to the nozzle 8. While the membrane 15 may be parallel to the nozzle direction, the direction of action of the piezoelectric element 16 is perpendicular to the longitudinal direction of the nozzle 8, and consequently a squeezed drop 24 moves at an angle of 90 ° to the direction of action of the piezoelectric element 16.

Furthermore, there is the so-called "shared wall" arrangement, where by means of piezoelectric elements 16 is pressed from both sides against laterally arranged membranes, preferably in opposite directions, but the longitudinal direction of the nozzle 8 and the direction of a drop 24 leaving these are by 90 ° offset relative to the common line of the directions of action of the piezo elements 16 against each other.

The invention offers a number of advantages:

The timing of the fire signal is independent of an advance signal or a vibration, as is fired at frequency rest. At best, if a pixel is assigned the color intensity zero, corresponding to no ink drop, a dummy signal can be output, which is too weak in intensity, similar to the pre-signal generated in the prior art, to cause the ejection of a drop. Such a wildcard signal is intended only for the purpose of printing the ink within the ink comb 12 during non-use phases Condition with an optimal viscosity. In addition, a residual self-oscillation can be deleted from a preceding drive _signal if the drive takes place anticyclically, ie, at T _tr0 pf = T _res / 2.

With the method according to the invention, successive ink droplets 24 with different, independent droplet sizes can be realized which do not influence each other.

The drop size is only dependent on

the duration of the rising and / or falling edge of a pressure pulse (a faster edge results in a smaller drop size), and / or

the duration of a pressure pulse in total (a shorter pressure pulse results in a smaller drop size), and / or

the amplitude or height of a pressure pulse (at smaller amplitude the drop is smaller),

and, conversely, that is, larger drop sizes can be achieved by the opposite measure.

By varying the above parameters, it is possible to eject a plurality of drops 24 of different sizes or volumes in a single sequence for generating a pixel by means of a plurality of pressure pulses in order to achieve intermediate quantities of the color intensity.

The droplet size, however, is only conditionally dependent on the nozzle diameter, since the drop / meniscus does not vibrate during firing and is limited by the nozzle wheel, but depends purely on the energy of the pulse. In the current state of the art, the nozzle diameter is the determining element of the droplet size.

Furthermore, in the invention, the ejected drops 24 are very stable and precise. The maximum drop speed becomes lower; next to, in front of or behind the main drop there are no unwanted satellite drops.

With the method according to the invention, a much higher firing frequency is possible than in the prior art, with simultaneously increasing precision and variable droplet size. The frequency can be increased by about 100% to about 200%, at the same time finer shades of gray can be achieved.

When printing higher viscosity (ink) liquids, the saving is about 5 to 10%.

***

List of Reference Intaglio print head

plate

mounting hole

end

midsection

flat side

jet

screw heads

flat side

printing system

ink chamber

ink channel

Ink reservoir

membrane

piezo element

rear block

drive circuit

supply voltage

graph

first impulse

follow-up pulse

plateau phase

drops

Claims

claims

Method for controlling an inkjet print head (1), comprising at least one printing system (11) with a nozzle (8) on the side of an ink chamber (12) facing a substrate to be printed, which at least partially, preferably in its region facing away from the printing substrate, passes through a Membrane (15) is limited, which is movable by electrical actuation of a mechanically coupled to the membrane (15) piezo element (16) of the ink chamber (12) to suck ink from an ink reservoir (14) into this, as well as to that or can be moved into it, in order to push out an ink drop (24) through the nozzle (8), the printing system (11) consisting of ink chamber (12), membrane (15), piezo element (16) and its drive circuit (18 ), an oscillatory structure, which is excited in a high-energy activation to a vibration with a natural frequency f _res , which shows a resonance, ie, that the vibration m it period T _res = 1 / f _re s the color intensity of an image to be printed dot can only hardly attenuated or not, and wherein be varied by using per pixel, a sequence of a plurality of ink drops (24) from the same nozzle (8) in succession with a time interval Τ _ίΓ0Ρ ί = 1 / ftropf. characterized in that on the drive signal only then energy is introduced into the printing system (11), even if an ink droplet (24) is actually to be pushed out.

A method according to claim 1, characterized in that the minimum time interval T _tr0 pf between temporally successive pressure control _signals not equal to the oscillation period T _res = 1 / f _res the resonant natural frequency f _{res of} the printing system (11) is: T, drip res-

3. The method according to claim 1 or 2, characterized in that the minimum time interval T _drip between temporally successive pressure control _{signals is} equal to or preferably smaller than one and a half _times the oscillation period T _res = 1 / f _{res of} the resonant natural frequency f _{res of} the printing system ( 11):

T drop <T _res 1, 5.

4. The method according to any one of claims 1 to 3, characterized in that the minimum time interval T _tr0 pf between temporally successive pressure control _{signals is} equal to or preferably smaller than twice the oscillation period T _res = 1 / f _{res of} the resonant natural frequency f _res of the printing system (11):

T, drip _S T _r es 1 2.

5. The method according to any one of the preceding claims, characterized in that the minimum time interval T _tr0 pf between temporally successive pressure control _signals equal to or preferably greater than one-third of the oscillation period T _res = 1 / f _r it the resonant natural frequency f _res of the printing system (11):

6. The method according to any one of the preceding claims, characterized in that the minimum time interval T _tr0 pf between temporally successive pressure control _{signals is} equal to or preferably greater than two-fifths of the oscillation period T _res = 1 / f _re s the resonant natural frequency f _res of the printing system (11): Drop> T _res / 2.5.

A method according to any preceding claim, characterized in that the size or volume of an ink drop (24) is increased by increasing the amplitude of a drive pulse.

A method according to any one of the preceding claims, characterized in that the size or volume of an ink drop (24) is increased by increasing the total duration of a drive pulse or the duration of the plateau phase of a drive pulse.

9. The method according to any one of the preceding claims, characterized in that the size or the volume of an ink drop (24) is increased by the duration of the rising and / or falling edge of a AnSteuerimpulses is increased.

10. The method according to any one of the preceding claims, characterized in that the size or the volume of an ink drop (24) does not depend on the duration or other properties of a preceding AnSteuerimpulses.

11. The method according to any one of the preceding claims, characterized in that the sizes of the ink drops to be printed (24) of a pixel sequence are different and / or independently.

12. The method according to any one of the preceding claims, characterized in that the row of different drop sizes is non-linear.

13. The method according to any one of the preceding claims, characterized in that at most at a pixel to which the color intensity is assigned zero corresponding to no drop of ink, a placeholder signal is output, which is too weak in intensity, to the repulsion of a drop cause; However, if a pixel is not associated with zero color intensity, corresponding to one or more drops of ink, then there is no pre- or intermediate non-printing signal either before or between the drive pulses of that pixel sequence.