Method for actuating an ink-jet print head

Abstract

The invention relates to a method for actuating an inkjet print head, comprising at least one printing system having a nozzle on the side of an ink chamber which faces a substrate to be imprinted, and which is delimited at least in areas, preferably in its area facing away from the print substrate, by a diaphragm that is movable away from the ink chamber by electrically actuating a piezo element that is mechanically coupled to the diaphragm, so that ink is drawn into the ink chamber from a reservoir, and the diaphragm is movable into the ink chamber so that an ink drop is ejected from the ink chamber through the nozzle, wherein the printing system made up of the ink chamber, diaphragm, piezo element, and the electronic control system thereof represents an oscillatable structure which, when actuated at high energy, is excited to oscillate at a natural frequency f.sub.res that exhibits resonance; i.e. the oscillation with the period T.sub.res=1/f.sub.res undergoes little or no attenuation, and wherein the brightness of a pixel to be printed is varied in that, for each pixel, a sequence of multiple ink drops is ejectable in succession from the same nozzle at a time interval of T.sub.drop=1/f.sub.drop, and energy is only introduced into the printing system via the actuation signal precisely when an ink drop is actually to be ejected.

Claims

1. A method for actuating an inkjet print head (1), comprising at least one printing system (11) having a nozzle (8) on the side of an ink chamber (12) which faces a substrate to be imprinted, and which is delimited at least in areas, preferably in its area facing away from the print substrate, by a diaphragm (15) that is movable away from the ink chamber (12) by electrically actuating a piezo element (16) that is mechanically coupled to the diaphragm (15), so that ink is drawn into the ink chamber from an ink reservoir (14), and the diaphragm is movable toward or into the ink chamber so that an ink drop (24) is ejected from the ink chamber through the nozzle (8), wherein the printing system (11) made up of the ink chamber (12), diaphragm (15), piezo element (16), and the control circuit (18) thereof represents an oscillatable structure which, when actuated at high energy, is excited to oscillate at a natural frequency f.sub.res that exhibits resonance; i.e., the oscillation with the period T.sub.res=1/f.sub.res undergoes little or no attenuation, and wherein the color intensity of a pixel to be printed is varied in that, for each pixel, a sequence of multiple ink drops (24) is ejectable in succession from the same nozzle (8) at a period T.sub.drop=1/f.sub.drop for the ejection of an ink drop, characterized in that energy is only introduced into the printing system (11) via the actuation signal precisely when an ink drop (24) is actually to be ejected, wherein for the period T.sub.drop between (i) two subsequent printing control signals, and (ii) the oscillation period T.sub.res=1/f.sub.res of the resonant natural frequency f.sub.res of the printing system (11), it applies:
T.sub.dropT.sub.res/1.5.

2. The method according to claim 1, characterized in that the minimum period T.sub.drop between chronologically sequential print actuating signals is unequal to the oscillation period T.sub.res=1/f.sub.res of the resonant natural frequency f.sub.res of the printing system (11):
T.sub.dropT.sub.res.

3. The method according to claim 1, characterized in that the minimum period T.sub.drop between chronologically sequential print actuating signals is equal to or preferably greater than one-third of the oscillation period T.sub.res=1/f.sub.res of the resonant natural frequency f.sub.res of the printing system (11):
T.sub.dropT.sub.res/3.

4. The method according to claim 1, characterized in that the minimum period T.sub.drop between chronologically sequential print actuating signals is equal to or preferably greater than two-fifths of the oscillation period T.sub.res=1/f.sub.res of the resonant natural frequency f.sub.res of the printing system (11):
T.sub.dropT.sub.res/2.5.

5. The method according to claim 1, characterized in that the size or the volume of an ink drop (24) is increased by increasing the amplitude of a trigger pulse.

6. The method according to claim 1, characterized in that the size or the volume of an ink drop (24) is increased by increasing the overall duration of a trigger pulse or the duration of the plateau phase of a trigger pulse.

7. The method according to claim 1, characterized in that the size or the volume of an ink drop (24) is increased by increasing the duration of the rising and/or falling edge of a trigger pulse.

8. The method according to claim 1, characterized in that the size or the volume of an ink drop (24) is not a function of the duration or of other properties of a preceding trigger pulse.

9. The method according to claim 1, characterized in that the sizes of the ink drops (24) of a pixel sequence to be printed are different and/or independent from one another.

10. The method according to claim 1, characterized in that the series of different drop sizes is nonlinear.

11. The method according to claim 1, characterized in that in any event for a pixel having an associated color intensity of zero, corresponding to no ink drop, a placeholder signal is emitted which, however, is too weak in its intensity to bring about the triggering of a drop; however, if the color intensity of zero is not associated with a pixel, corresponding to one or multiple ink drops, a nonprinting preliminary signal or intermediate signal is not present either in front of or between the trigger pulses of this pixel sequence.

12. A method for actuating an inkjet print head (1), comprising at least one printing system (11) having a nozzle (8) on the side of an ink chamber (12) which faces a substrate to be imprinted, and which is delimited at least in areas, preferably in its area facing away from the print substrate, by a diaphragm (15) that is movable away from the ink chamber (12) by electrically actuating a piezo element (16) that is mechanically coupled to the diaphragm (15), so that ink is drawn into the ink chamber from an ink reservoir (14), and the diaphragm is movable toward or into the ink chamber so that an ink drop (24) is ejected from the ink chamber through the nozzle (8), wherein the printing system (11) made up of the ink chamber (12), diaphragm (15), piezo element (16), and the control circuit (18) thereof represents an oscillatable structure which, when actuated at high energy, is excited to oscillate at a natural frequency f.sub.res that exhibits resonance; i.e., the oscillation with the period T.sub.res=1/f.sub.res undergoes little or no attenuation, and wherein the color intensity of a pixel to be printed is varied in that, for each pixel, a sequence of multiple ink drops (24) is ejectable in succession from the same nozzle (8) at a period T.sub.drop=1/f.sub.drop for the ejection of an ink drop, characterized in that energy is only introduced into the printing system (11) via the actuation signal precisely when an ink drop (24) is actually to be ejected, wherein for the period T.sub.drop between (i) two subsequent printing control signals, and (ii) the oscillation period T.sub.res=1/f.sub.res of the resonant natural frequency f.sub.res of the printing system (11), it applies:
T.sub.dropT.sub.res/1.75.

13. A method for actuating an inkjet print head (1), comprising at least one printing system (11) having a nozzle (8) on the side of an ink chamber (12) which faces a substrate to be imprinted, and which is delimited at least in areas, preferably in its area facing away from the print substrate, by a diaphragm (15) that is movable away from the ink chamber (12) by electrically actuating a piezo element (16) that is mechanically coupled to the diaphragm (15), so that ink is drawn into the ink chamber from an ink reservoir (14), and the diaphragm is movable toward or into the ink chamber so that an ink drop (24) is ejected from the ink chamber through the nozzle (8), wherein the printing system (11) made up of the ink chamber (12), diaphragm (15), piezo element (16), and the control circuit (18) thereof represents an oscillatable structure which, when actuated at high energy, is excited to oscillate at a natural frequency f.sub.res that exhibits resonance; i.e., the oscillation with the period T.sub.res=1/f.sub.res undergoes little or no attenuation, and wherein the color intensity of a pixel to be printed is varied in that, for each pixel, a sequence of multiple ink drops (24) is ejectable in succession from the same nozzle (8) at a period T.sub.drop=1/f.sub.drop for the ejection of an ink drop, characterized in that energy is only introduced into the printing system (11) via the actuation signal precisely when an ink drop (24) is actually to be ejected, wherein for the period T.sub.drop between (i) two subsequent printing control signals, and (ii) the oscillation period T.sub.res=1/f.sub.res of the resonant natural frequency f.sub.res of the printing system (11) on the other hand, it applies:
T.sub.dropT.sub.res/2.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Further features, particulars, advantages, and effects based on the invention result from the subclaims and from the following description of one preferred embodiment of the invention, and with reference to the drawings, which show the following:

(2) FIGS. 1a, 1b and 1c show various views of an inkjet print head;

(3) FIG. 2 shows a vertical section of a printing system of the inkjet print head according to FIG. 1;

(4) FIG. 3 shows the ink chamber system of the inkjet print head according to FIG. 1, in a schematic illustration;

(5) FIG. 4 shows the electronic control system of the inkjet print head according to FIG. 1, in a schematic illustration;

(6) FIG. 5 shows the variation over time of a customary actuating signal for a printing system according to FIG. 1, and the deflection signal of the piezo element proportional thereto;

(7) FIG. 6 shows the variation over time of the actuating signal according to the invention for the printing system according to FIG. 1;

(8) FIG. 7 shows the options for influencing the control signal for varying the drop size or the drop volume, where + corresponds to an increasing influence on the drop size, and corresponds to a reducing influence;

(9) FIG. 8 shows a variation over time by way of example of a sequence of trigger pulses according to the invention, for illustrating the option of varying the ink quantity by overprinting multiple drops having different volumes; and

(10) FIG. 9 shows a variation over time by way of example of a pixel sequence in the prior art, where all trigger pulses have the same size and length, so that all ink drops have the same volume.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(11) The design of the inkjet print head 1 and one of its printing systems 11 has already been described in detail above with reference to FIGS. 1 through 4.

(12) The customary mode of operation of such a printing system is illustrated in FIG. 5. The deflection x or x of the portion of the piezo element 16 coupled to the diaphragm 15 is shown as a graph 20 in the figure. The upper line of the signal corresponds to a type of zero position of the piezo element 16 or of the diaphragm 15. The ink chamber 12 has just been completely filled with ink. Plotted at the bottom is a deflection of the piezo element 16 or of the diaphragm 15 away from the ink chamber 12, which results in an increase in the volume V in the ink chamber. This increase in volume V is approximately equal to the deflection x of the diaphragm 16 multiplied by the base surface area F of the ink chamber 12 or of the diaphragm 16 which completely spans the ink chamber:
V=Q*x.

(13) The displacement x is illustrated along the ordinate in FIG. 5; i.e., a displacement of the diaphragm 16 toward the ink chamber 12 is plotted at the top. Thus, as the graph 20 ascends, the diaphragm 16 approaches the ink chamber 12 by the value |x|, thus reducing the volume V by the quantity ||=Q*|x|; i.e., V<0. In other words, x, corresponding to V, is plotted at the top.

(14) The time t is plotted along the abscissa in FIG. 5.

(15) A first pulse 21 begins at t=0. The deflection 21 in FIG. 5 points downwardly; i.e., x becomes larger, and the ink chamber volume V thus increases, and ink in a quantity V is drawn into the ink chamber 12.

(16) However, the deflection 21 does not take place to the extent that the drawn-in volume V is less than the volume of a drop V.sub.drop:
V<V.sub.drop.

(17) Consequently, when the diaphragm 16 subsequently swings back, no ink drop is pressed through the nozzle 8, and instead the ink surface only bulges outwardly through the nozzle 8, but without falling off.

(18) Due to this high-energy measure, however, the natural oscillation at the resonance frequency f.sub.res is triggered in the printing system 11; in the illustrated example, the period T.sub.res=1/f.sub.res of the resonant natural oscillation is approximately 15 s.

(19) The following pulses 22 for ejecting multiple ink drops 24 are subsequently produced in a certain time grid. Each following pulse 22 in FIG. 5 is made up of a falling edge, during which the ink volume V in the ink chamber 12 increases by a volume V, where the following applies:
V=V.sub.drop.

(20) During a brief plateau phase 23 of the following pulse 22, ink flows from the ink reservoir 14 through the ink channel 13 into the ink chamber 12 of the printing system in question.

(21) The diaphragm 16 subsequently swings back into its starting position, and the volume V within the ink chamber 12 decreases by V. However, this quantity of ink corresponds to the volume V.sub.drop of an ink drop, and the ink drop ultimately falls off on the other side of the nozzle 8.

(22) Such a shot period, during which ink is thus drawn into the ink chamber 12 and subsequently ejected through the nozzle 8 until the beginning of the next intake movement of the diaphragm 16, corresponds to the period T.sub.res of the natural oscillation of the printing system 11, so that each period begins at the same phase position of the natural oscillation.

(23) In order for the period T.sub.seq of an overall pixel sequence to be at most approximately 50 s, only three ink drops 24 maximum per pixel sequence may therefore be delivered. In addition, since the volume is approximately the same for all ink drops 24, accordingly the color intensity I.sub.F of a pixel may be changed only in a coarse grid, namely, in the steps
I.sub.F=0*I.sub.0;
I.sub.F=1*I.sub.0;
I.sub.F=2*I.sub.0;
I.sub.F=3*I.sub.0;
where I.sub.0 corresponds to the color intensity of a single colored drop of the size in question.

(24) Thus, this involves a maximum of four different values, corresponding to a piece of information that is representable by 2 bits: 00=0; 01=1; 10=2; 11=3.

(25) Although this might be normal for an inkjet printer, it does not represent a good result, since, for example, the intensity values of the pixels of images that are recorded by means of a camera have a much finer resolution, for example with 16 different color intensities per pixel and color (4 bits), or with 64 color intensities, or with 128 or even 256 color intensities.

(26) In order to solve the above problems with regard to a limited printing speed and a very restricted resolution of the color intensity, with printing using inkjet printers, the invention, with an otherwise identical print head 1 and identical printing system 11, proposes the actuation method illustrated in FIG. 6.

(27) This method is based on the concept of not subordinating to the natural oscillation of the printing system 11 with the resonance frequency f.sub.res, but, rather, to avoid such, so that the system is not excited at all into its natural oscillation, and therefore each ink drop 24 does not have to be delivered synchronously with an oscillation of the printing system 11, and instead could theoretically be delivered at an arbitrary point in time.

(28) The invention provides several measures for avoiding the natural oscillation of the printing system 11 with the resonance frequency f.sub.res:

(29) Firstly, in the method according to the invention an excitation pulse 21 that precedes the actual print pulse 22 is completely absent.

(30) Therefore, in any case the first print pulse 22 encounters a diaphragm 16 at rest; defined conditions prevail in the printing system 11, and the first ink drop 24 is delivered with high precision.

(31) Another measure for avoiding the natural oscillation is to shorten the plateau phase 23. Whereas in the prior art, the time T.sub.plat of the plateau phase 23 is greater than the times T.sub.rising, T.sub.falling for the rising or falling ramp, the following now applies:
T.sub.plat<T.sub.rising
T.sub.plat<T.sub.falling

(32) This is achieved by increasing the maximum deflection x.sub.max, with the slope of the rising and falling ramps remaining approximately constant. The time T.sub.plat required for drawing in the same ink volume V may thus be shortened, since the flow velocity is increased due to a higher differential suction pressure between the ink chamber 12 and the ink reservoir 14.

(33) Consequently, the following relationship may be achieved:
T.sub.falling+T.sub.plat+T.sub.rising<T.sub.res/4,
in particular
T.sub.falling+T.sub.plat+T.sub.rising<T.sub.res/5.

(34) As a result of this measure, the spectrum of such an individual wave is shifted toward higher frequencies, and thus has a much larger frequency spacing from the resonance frequency f.sub.res. Consequently, the resonance frequency f.sub.res is not hereby triggered.

(35) Another measure for avoiding resonant natural oscillations in the printing system 11 is to further reduce the duration of a period T.sub.drop for the ejection of an ink drop, in particular in such a way that the following applies:
T.sub.dropT.sub.res/1.5.

(36) As a result, in the spectrum there are no spectral components at f.sub.res, and therefore there is also no excitation of natural oscillation.

(37) Moreover, for a period duration of T.sub.drop=T.sub.res/2, for example, due to a following print pulse a natural oscillation of frequency f.sub.res, possibly triggered beforehand, is once again quenched by an anticyclical phase position.

(38) On the other hand, the period duration also should not become too short, so that successive ink drops 24 in the flight phase remain separate from one another and do not uncontrollably combine with one another during flight, since otherwise the size of the drop 24 drawn in from the nozzle 8 could differ from the desired volume. To ensure this, the invention recommends that the following inequality be observed:
T.sub.dropT.sub.res/3.

(39) Particularly suitable for the method according to the invention are such print heads 1 or printing systems 11 in which the movement of a diaphragm 15 that at least partially delimits the ink chamber 12 is brought about by a piezo element 16. The activity direction of the piezo element is usually oriented perpendicularly with respect to the diaphragm 15. However, there are various piezo print heads of this type that differ in particular with regard to the configuration of the diaphragm 15 and the piezo element 16 acting on it, relative to the position and longitudinal direction of the nozzle 8:

(40) In the arrangement referred to in technical jargon as a piston shooter, the diaphragm 15 is situated between the nozzle 8 and the piezo element 16, and the direction of action of the latter is in alignment with or parallel to the longitudinal direction of the nozzle 8.

(41) In the so-called side shooter, the diaphragm 15 is situated to the side of the ink chamber 12, next to the nozzle 8, so to speak. Whereas the diaphragm 15 may be parallel to the nozzle direction, the direction of action of the piezo element 16 is perpendicular to the longitudinal direction of the nozzle 8, and therefore an ejected drop 24 moves at an angle of 90 relative to the direction of action of the piezo element 16.

(42) Furthermore, there is also the so-called shared wall arrangement, in which pressing by means of piezo elements 16 takes place from both sides against laterally situated diaphragms, preferably in opposite directions; however, the longitudinal direction of the nozzle 8 and the direction of a drop 24 exiting the nozzle are offset relative to one another by 90 with respect to the shared line of the directions of action of the piezo elements 16.

(43) The invention provides a number of advantages:

(44) The point in time of the firing signal is independent of a preliminary signal or an oscillation, since firing takes place when the frequency is quiescent. In any event, when the color intensity of zero is associated with a pixel, corresponding to no ink drop, a placeholder signal may be emitted which, similarly as for the preliminary signal generated in the prior art, is too weak in its intensity to bring about the triggering of a drop. Such a placeholder signal should be used only for the purpose of keeping the ink within the ink chamber 12 in a print-ready state, at an optimal viscosity, during phases of non-use. In addition, a residual natural oscillation from a preceding actuating signal may be quenched when the actuation takes place anticyclically, i.e., for T.sub.drop=T.sub.res/2.

(45) By use of the method according to the invention, successive ink drops 24 having different drop sizes that are independent of one another may be achieved which do not influence each other.

(46) The drop size in each case is a function only of the duration of the rising and/or falling edge of a print pulse (a more rapid edge results in a smaller drop size), and/or the overall duration of a print pulse (a shorter print pulse results in a smaller drop size), and/or the amplitude or magnitude of a print pulse (the drop is smaller at a smaller amplitude),
and in each case conversely; i.e., larger drop sizes may be achieved by the respective opposite measure.

(47) By varying the above parameters, it is possible, in a single sequence for creating a pixel by means of multiple print pulses, to eject multiple drops 24 having different sizes and volumes in order to achieve intermediate color intensity values.

(48) In contrast, the drop size is a function of the nozzle diameter only to a limited extent, since the drop/meniscus during firing does not oscillate and is delimited by the nozzle wheel, and instead is a function strictly of the energy of the pulse. In the current prior art, the nozzle diameter is the determining element for the drop size.

(49) In addition, in the invention the ejected drop 24 is very stable and precise.

(50) The maximum drop speed becomes smaller; there are no undesirable satellite drops next to, in front, of behind the main drop.

(51) With the method according to the invention, a much higher firing frequency is possible than in the prior art, at the same time with increasing precision and variable drop size. The frequency may be increased by approximately 100% to approximately 200%, and at the same time, finer gray graduations are achievable.

(52) The savings are approximately 5 to 10% for printing with high-viscosity (ink) fluids.

LIST OF REFERENCE NUMERALS

(53) 1 inkjet print head 2 plate 3 fastening hole 4 fastening hole 5 end area 6 middle portion 7 flat side 8 nozzle 9 screw heads 10 flat side 11 printing system 12 ink chamber 13 ink channel 14 ink reservoir 15 diaphragm 16 piezo element 17 rear block 18 control circuit 19 supply voltage 20 graph 21 first pulse 22 following pulse 23 plateau phase 24 drop