EP3720719B1 - Printing method for a digital printing device - Google Patents

Description

The invention relates to a printing method for a digital printing device, comprising a print head with a plurality of printing systems and at least one control device in order to send control signals to the printing systems for generating ink droplets, each printing system having a nozzle, at least one ink chamber and one associated e.g. piezoelectric activator for ejecting ink drops from the relevant ink chamber through the relevant nozzle onto a substrate to be printed in response to a drive signal.

As an indirect or analogue printing process, offset printing is one of the most widespread printing techniques for book, newspaper, advertising and packaging printing. Before printing, an individual printing plate is first made, from which the color is then first transferred to a rubber cylinder during the printing process and from there to the substrate to be printed. A separate printing plate is required for each motif and each colour. The comparatively high quality of offset printing is offset by a comparatively low resolution, e.g. B, 150lpi or 120lpi (raster of 150 raster dots or 120 raster dots per square inch), but in which almost infinitely different dot sizes are available for each dot. With a given grid size, the color intensity can range from white to e.g. B. 100% magenta can be realized simply by increasing the size of the color dots.

With digital printing, on the other hand, there is no printing plate and the printed image is transferred directly from a computer to a printing device. Ink jet printers or laser printers are primarily considered as printing devices. By doing without a printing plate, digital printing - especially for smaller runs - easier, faster and cheaper. In common ink printers or inkjet printers, for example with piezo printheads, the print is either controlled by individual electrostatic charging of a continuous ink jet, which can then be deflected in a field depending on its electrical charge (continuous inkjet process, CIJ), or by the delivery of individual Drop on Demand (DOD) process.

However, such digital printers usually only manage 2 or 3 different droplet sizes per printing color, which correspond to the dot sizes of offset printing. While this is hardly noticeable when printing text or other black and white documents with a strong contrast between light and dark, inkjet printers are less suitable for printing color photos or black and white images with shades of gray. If, for this purpose, an attempt is made to break down each individual pixel into a smaller grid of, for example, 4 times 4 smaller dots and then to print 0, 1, 2 ... 15, 16 of these smaller grid dots, 16 different color intensities can already be achieved differentiate; However, these smaller brightness grid points of a pixel are still perceptible to the eye as points or at least as optical unrest, or even as a so-called moiree effect, i.e. microscopic structures repeat themselves regularly and thus produce a clearly perceptible or even unmistakable macroscopic pattern. In addition, the resolution of a pixel in, for example, 16 small raster dots would reduce the printing speed extremely, because 16 printing pulses are now required instead of a single printing pulse. Furthermore, complex algorithms are required in order to suppress the moiré effect mentioned above and similar disadvantageous effects.

From the state of the art are documents U.S. 2012/293577 A1 , U.S. 2013/083106 A1 and U.S. 2009/184995 A1 are known which disclose a printing method for a digital printing device.

The aim of the industry is a printing technique that makes use of the advantages of both printing techniques without their disadvantages. The requirements such a printing technique can be described as follows: The advantages of offset printing, namely high quality with low resolution, high speed, stable printing over a long period of time, but without the disadvantages of offset printing, i.e. the need to create printing plates, but rather a digital one Control, e.g. for the purpose of quick change of motif, etc.

Although many attempts have been made in this direction, problems have always been encountered, which are as follows in the current state of the art:
In order to achieve the optical quality of offset printing, digital printing requires a much higher resolution, as described above. A high printing speed with such a high physical resolution requires a very high number of printing nozzles with a correspondingly high density/resolution and high firing frequency (1200 dpi = 48 kHz / 2400dpi = 96 kHz) and at the same time very small drops (1 pl to 5 pl). In order to ensure high pressure stability over a long period of time, systems are partly built redundantly with "replacement nozzles" for nozzles that are not printing correctly and corresponding technical devices that recognize the same nozzles as defective and replace them accordingly.

For the machine builder, this means very high demands on mechanical components, especially with regard to the adjustment of the print heads, which have to be perfectly aligned with each other within a few micrometers. At the same time, these machines require complex monitoring of the nozzles, e.g. B. in the form of optical detection of defective nozzles during printing and appropriate replacement of these nozzles. The entire material transport has to be adapted to digital printing, since air turbulence strongly deflects the small droplets of 1 pl to 5 pl. Both the software and the hardware must be able to send enormous amounts of print data to the printing units, which must first be prepared for digital printing - similar to rip software, since print data usually has a resolution of 150 to 300 dpi the prepress stage, with each pixel being assigned an exact color value, this means that the print data is extrapolated or falsified to 1200 × 1200 or 1200 × 2400 dpi. The pure file grows uncompressed by a factor of 16 to 32. In addition, there are various algorithms that on the one hand conceal the mechanical tolerances of the machine and at the same time have to reproduce the perfect print image without any effects recognizable to the human eye plus the conventional color management for color fidelity, etc.

The production of printheads with ever higher physical resolution (nozzle density) and ever smaller drops also poses major problems for printhead manufacturers. The smallest mechanical deviations in production and/or nozzles that don't work lead to enormous rejects in production and to a correspondingly high end price for the print heads.

The work of the ink manufacturers and print head electronics developers is just as complex.

An alternative approach to developing a printing machine with the characteristics mentioned above - i.e. high quality with low resolution, high speed, stable printing over a long period of time without using printing plates - would be to control a print head in such a way that it is similar to the offset for each pixel prints a drop of the appropriate drop size.

Attempts have therefore already been made to change the number and/or size of drops while the nozzle size remains the same.

The usual way to do this is to multiply the drive pulses per image point. Depending on the time interval at which these control pulses follow one another, either several droplets are produced or—if the individual droplets combine in the area of the nozzle or in flight—larger droplets. This allows darker colors to be realized per pixel, e.g. a darker gray to black. However, it is not possible to reduce the droplet size in order to improve the shades of gray. Furthermore, such an increase in drop number or size requires multiple print pulses, which in turn reduces print speed. In addition, it has been found that larger ink drops normally have a higher speed than smaller ink drops, which can manifest itself in the form of an inconsistent print image.

The disadvantages of the prior art described result in the problem initiating the invention of further developing a generic printing method for an inkjet print head in such a way that the droplet size and/or speed can be influenced or adjusted with as little effort as possible. Of particular interest is the question of whether a uniform droplet speed can be ensured despite different droplet sizes.

This problem is solved by a printing process with the features of claim 1.

Within the scope of the control device for generating ink droplets of different sizes, a single waveform for the control signal with a predetermined time profile in the form of a single pressure pulse is preferably stored, comprising a first edge for increasing the volume of an ink chamber by means of the relevant activator, followed by a first hold time, while the ink is sucked into the relevant ink chamber, as well as a second flank to reduce the volume of the relevant ink chamber by means of the relevant activator, followed by a second hold time during which an ink droplet is shot out of the associated nozzle, the different droplet size being thereby achieved that at most with a single droplet size the entire waveform is transmitted as a control signal to the relevant activator, while with all other droplet sizes one or more sections of the stored, common waveform, esp consisting of the first edge, the first hold time, the second edge and the second hold time, are kept away from the relevant activator as part of the control signal.

The inventor has found that the droplet size can also be influenced significantly and reliably by suppressing one or more sections of a stored, common waveform, i.e. droplets with significantly different sizes can be generated, with the repeatability being very precise, i.e. the different droplet sizes are maintained with great precision. On the other hand, the individual suppression of individual sections of a saved, common waveform allows a more flexible influence on the droplet size. For example, the start and end times of portions of the common waveform to be transmitted can be influenced analogously, ie steplessly, while the number of different droplet sizes can be counted for different waveforms to be stored. This advantage also results in particular from the fact that within the scope of the present method, not entire pulses are faded in or faded out, as is the case, for example, when printing several drops in order to change the number of drops, but instead individual time segments of a single print pulse are faded in or faded out .

The invention recommends using a drive stage with a switched output or preferably a drive stage with a controlled or regulated output to drive an activator. In this context, a switched output is to be understood as meaning a circuit which can switch back and forth on the output side between two or more predetermined, preferably constant voltages, i.e. between 0 V and 20 V, for example not specified, but the output always follows the switched through constant voltage value with the highest possible speed. In the case of a controlled or regulated output, several or ideally any intermediate voltages are possible.

It is within the scope of the invention for the drive circuit to have a high impedance at its drive output, which is connected to an activator, during partial sections of the stored waveform that are not switched through. In this high-impedance state, the activator is left to its own devices. An activator "with a memory" that can save its last set state is therefore recommended.

It has proven advantageous that the control output of the control circuit, which is connected to an activator, is constructed in the manner of a push-pull circuit with two transistors connected in series on the output side. These transistors can be operated in different ways. On the one hand, one of two supply voltages can be switched through to the output, e.g. V _a,s = 0 V or V _a,s ± 20 V of both transistors can assume any intermediate values between the two supply voltages, e.g. 0 V ≤ V _3,r ≤ 20 V.

The two transistors connected in series on the output side of the control output of the control circuit connected to an activator should both have a high resistance on the output side when not switched through, in order to give the connected activator the possibility of not having to be influenced by the control circuit; Rather, the currently prevailing ink pressure or negative pressure could influence the output voltage, so that an immediate reaction to changed environmental conditions is possible.

As part of the activation of an activator according to the invention, the arrangement should be such that during the first or second flank the volume of an ink chamber is increased by means of the activator in question, and ink is sucked into the ink chamber in question during the first or second holding time that immediately follows .

On the other hand, the volume of the relevant ink chamber should be reduced by means of the relevant activator during the other, second or first edge, and an ink droplet should be shot out of the associated nozzle during the immediately following, second or first hold time.

Particular advantages result when a piezo element is used as an activator, which is in contact with an ink chamber in the area of an ink nozzle in order to influence the volume of the ink chamber. By applying different voltages, such a piezo element can be both contracted and expanded, so that a single, controllable element can be used to load or soak the ink chamber with ink and to push out or shoot an ink drop.

If the piezo element has conductive coatings on two opposite sides, these can be used as electrodes in order to control the piezo element.

Since the piezoelectric element between the two oppositely located coverings is not electrically conductive, an electrical charge applied as a result of an applied voltage remains constant given a high-impedance output of the drive circuit.

The electrical charge applied to the piezo element during a charging phase depends on the applied voltage on the one hand and on the duration of the charging phase on the other.

The invention can be further developed such that at least two intervals are selected from the stored waveform and transmitted to the activator.

At least two intervals selected and transmitted to the Activator should not be contiguous.

The invention provides that the intervals transmitted to the activator are selected from a first or second holding time.

The invention prefers that no edge is transferred to the activator in the case of one, several or all effective droplet sizes. Since no stored edge is transmitted, but rather only—in whole or in part—a few or all holding phases, a new setpoint value is specified instead of predetermined edges, which the output voltage can automatically approach.

An advantageous further development of the invention is that the stored waveform comprises two or more successive drive pulses, each of which has a first edge, followed by a first hold time, and, following the first hold time, a second, opposite edge, possibly followed by a second hold time. A first control pulse primarily serves to form an ink drop, and the second control pulse serves to shape it, in particular to reduce or supplement it, and/or to modify its movement, in particular to accelerate or decelerate.

Two or more drive pulses of the stored waveform can differ from one another, in particular with regard to the slope of the first edge and/or the second edge, and/or with regard to the first hold time and/or the second hold time, and/or with regard to the amplitude of the first and/or or second hold time, and/or in terms of the rise or fall time during the first edge and/or the second edge. In this way, the different requirements for different control pulses can be taken into account.

The amount of ink flowing into the ink chamber during a holding time when the volume of the ink chamber is increased depends on the degree of increase in volume and on the duration of the relevant increase in volume.

On the other hand, the volume of an ink drop shot out during a flank in which the volume of the ink chamber in question is reduced by means of the activator in question increases with the slope of the flank in question, in which the volume within the ink chamber is reduced as an exception, explicitly specified, for example, if necessary, with an intrinsic droplet size, with the entire stored waveform being passed on to the print head as a control signal; in most other cases, only hold phases are switched through, with the edge slope then being at its maximum in a transition phase, and thus higher than with the intrinsic droplet size. This effect can be so strong that the stored waveform is completely forwarded to the print head in the absence of a sufficient Slope is not even able to produce an ink drop, but this is only possible when a higher edge slope is achieved as a result of switching between constant voltage values according to the invention.

In addition, the volume of an ink droplet shot out during an edge of a drive pulse can increase with the duration of the effective holding time that immediately follows, because the droplet can form undisturbed in the meantime and can therefore grow to its full size.

On the other hand, the duration of the effective holding time immediately following an edge is determined by the time interval between a following edge of the drive signal.

A flank of a second drive pulse, which follows before the break-off of the ink drop emitted during a first drive pulse and increases the volume within the ink chamber (again) is able to reduce the volume of the ink drop, because as a result a larger part of the ink drop is held back, ie a flank which follows before the drop breaks off and increases the chamber volume pulls back a part of the drop, as it were.

It should also be noted that an edge of a second drive pulse, which decreases the volume within the ink chamber (again) after the ink drop formed during a first drive pulse, can increase the volume of the ink drop, namely if an additional amount of ink is emitted as a result.

After the formation of an ink drop during a first drive pulse, an additional amount of ink can be delivered if there is a sufficient amount of ink between the edge that increases the volume within the ink chamber (again) and a subsequent edge of a second drive pulse that decreases the volume within the ink chamber (again). has been sucked into the ink chamber, and if the slope reducing the volume inside the ink chamber (again) is sufficiently steep.

It is within the scope of the teaching of the invention that the speed of an ink drop shot out during an edge of a drive pulse increases with the amplitude or the stroke of the edge of the drive pulse. Because with increasing amplitude, increased pressure can be built up and thus a higher force, which results in a stronger acceleration of the ink to greater speeds.

Since the amplitude of the edge of a drive pulse increases with the duration of the actively impressed charging current for the activator, this duration of the charging current can be used to influence the speed of the ink droplets. An ink drop that is actually too fast can be prevented by shortening the duration of the charging current, i.e. the actively switched through holding phase, can be set slower, and an ink drop that is too slow will become faster by increasing the duration of the charging current.

The duration of the actively impressed charging current for the activator should be shorter for larger droplet sizes than for smaller droplet sizes, so that the speed of an ink droplet shot out during an edge of a drive pulse is approximately the same for all droplet sizes.

Fig. 1

eine Wellenform zur Ansteuerung einer Düse eines Druckkopfs gemäß dem Stand der Technik;

Fig. 2

eine erfindungsgemäß abgespeicherte, gemeinsame Wellenform für die Erzeugung von Ansteuersignalen für ein digitales Drucksystem;

Fig. 3

ein an ein Drucksystem übertragenes Ansteuersignal mit einer ausschnittweisen Vergrößerung im Bereich einer Flanke;

Fig. 4

eine der Fig. 3 entsprechend Ansicht einer anderen Ausführungsform der Erfindung;

Fig. 5

ein an ein Drucksystem übertragenes Ansteuersignal für die Erzeugung eines Tintentropfens mit einem Volumen von 5 pl (Pikoliter);

Fig. 5a

ein an ein Drucksystem übertragenes Ansteuersignal für die Erzeugung eines Tintentropfens mit einem Volumen von 7 pl;

Fig. 5b

ein an ein Drucksystem übertragenes Ansteuersignal für die Erzeugung eines Tintentropfens mit einem Volumen von 10 pl;

Fig. 5c

ein an ein Drucksystem übertragenes Ansteuersignal für die Erzeugung eines Tintentropfens mit einem Volumen von 12 pl;

Fig. 5d

ein an ein Drucksystem übertragenes Ansteuersignal für die Erzeugung eines Tintentropfens mit einem Volumen von 14 pl;

Fig. 5e

ein an ein Drucksystem übertragenes Ansteuersignal für die Erzeugung eines Tintentropfens mit einem Volumen von 17 pl;

Fig. 5f

ein an ein Drucksystem übertragenes Ansteuersignal für die Erzeugung eines Tintentropfens mit einem Volumen von 21 pl;

Fig. 6

eine andere Ausführungsform der Erfindung in einer der Fig. 2 entsprechenden Darstellung;

Fig. 7

eine wiederum abgewandelte Ausführungsform der Erfindung in einer Darstellung gemäß Fig. 6;

Fig. 8

eine nochmals veränderte Ausführungsform der Erfindung ähnlich der Darstellung von Fig. 6;

Fig. 9a

eine hinterlegte Wellenform als an ein Drucksystem vollständig übertragenes Ansteuersignal zur Erzeugung eines intrinsischen Tintentropfens mit einer Geschwindigkeit v₁;

Fig. 9b

ein anderes, an ein Drucksystem übertragenes Ansteuersignal in Form von zwei Teilabschnitten der abgespeicherten Wellenform, für die Erzeugung eines Tintentropfens mit einer Geschwindigkeit von 1,1 * v₁;

Fig. 9c

ein wiederum abgewandeltes, an ein Drucksystem übertragenes Ansteuersignal in Form von zwei anderen Teilabschnitten der abgespeicherten Wellenform, zwecks Erzeugung eines Tintentropfens mit einer Geschwindigkeit von 1,2 * v₁;

Fig. 9d

ein nochmals verändertes, an ein Drucksystem übertragenes Ansteuersignal, umfassend zwei abermals anders ausgewähte Teilabschnitte der abgespeicherten Wellenform, zwecks Erzeugung eines Tintentropfens mit einer Geschwindigkeit von 1,4 * v₁;

Fig. 9e

ein weiter modifiziertes, an ein Drucksystem übertragenes Ansteuersignal, umfassend zwei andere Teilabschnitte der abgespeicherten Wellenform, woraufhin der erzeugte Tintentropfen eine Geschwindigkeit von 0,9 * v₁ aufweist;

Fig. 10

eine beispielhafte Zuasmmenstellung von mehreren Zeitsignalen, welche die Auswahl und Zusammenstellung von jeweils unterschiedlichen Teilabschnitten der abgespeicherten Wellenform zu verschiedenen Ansteuersignalen für bestimmte Tintentropfengrößen veranschaulicht;

Fig, 11a

eine abgespeicherte Wellenform für einen Ansteuerimpuls mit einer Amplitude von 20 V;

Fig. 11b

einen teilweise an ein Drucksystem übertragenen Ansteuerimpuls, wobei jedoch die Haltephasen vollständig übertragen werden, so dass die Amplitude von 20 V erhalten bleibt;

Fig. 11c

einen wiederum teilweise an ein Drucksystem übertragenen Ansteuerimpuls, wobei allerdings die erste Haltephase nicht vollständig übertragen wird, so dass die Amplitude im Bereich beider Flanken von 20 V auf 16 V reduziert wird; sowie

Fig. 11d

einen wiederum teilweise an ein Drucksystem übertragenen Ansteuerimpuls, woei nunmehr die zweite Haltephase nicht vollständig übertragen wird, so dass die Amplitude im Bereich der zweiten Flanke von 20 V auf 16 V reduziert wird.

Further features, details, advantages and effects based on the invention result from the following description of a preferred embodiment of the invention and from the drawing. This shows:

1

a waveform for driving a nozzle of a print head according to the prior art;

2

a common waveform stored according to the invention for the generation of control signals for a digital printing system;

3

a control signal transmitted to a printing system with an enlarged section in the area of an edge;

4

one of the 3 according to view of another embodiment of the invention;

figure 5

a drive signal transmitted to a printing system for generating an ink droplet having a volume of 5 pl (picoliter);

Figure 5a

a drive signal transmitted to a printing system for producing an ink droplet having a volume of 7 pl;

Figure 5b

a drive signal transmitted to a printing system for generating an ink droplet having a volume of 10 pl;

Figure 5c

a drive signal transmitted to a printing system for producing an ink droplet having a volume of 12 pl;

Figure 5d

a drive signal transmitted to a printing system for producing an ink droplet having a volume of 14 pl;

Figure 5e

a drive signal transmitted to a printing system for producing an ink droplet having a volume of 17 pl;

Fig. 5f

a drive signal transmitted to a printing system for producing an ink droplet having a volume of 21 pl;

6

another embodiment of the invention in one of 2 corresponding representation;

7

a turn modified embodiment of the invention in a representation according to 6 ;

8

a further modified embodiment of the invention similar to that shown in FIG 6 ;

Figure 9a

a stored waveform as a drive signal completely transmitted to a printing system for generating an intrinsic ink droplet with a velocity v ₁ ;

Figure 9b

another drive signal transmitted to a printing system in the form of two subsections of the stored waveform, for the production of an ink drop with a velocity of 1.1 * v ₁ ;

9c

a drive signal modified in turn and transmitted to a printing system in the form of two other subsections of the stored waveform in order to produce an ink drop with a velocity of 1.2 * v ₁ ;

Figure 9d

a once again modified drive signal, transmitted to a printing system, comprising two again differently selected sections of the stored waveform, for the purpose of generating an ink droplet with a velocity of 1.4 * v ₁ ;

Fig. 9e

a further modified drive signal transmitted to a printing system comprising two other portions of the stored waveform whereupon the generated ink drop has a velocity of 0.9 * v ₁ ;

10

an exemplary compilation of a plurality of time signals, which illustrates the selection and compilation of different partial sections of the stored waveform for different control signals for specific ink droplet sizes;

Fig. 11a

a stored waveform for a drive pulse with an amplitude of 20 V;

Figure 11b

a drive pulse partially transmitted to a printing system, but with the sustain phases fully transmitted so that the 20 V amplitude is maintained;

11c

a drive pulse, which in turn is partially transmitted to a printing system, although the first holding phase is not is completely transmitted, so that the amplitude in the area of both edges is reduced from 20 V to 16 V; such as

Figure 11d

a control pulse that is in turn partially transmitted to a printing system, with the second holding phase now not being transmitted completely, so that the amplitude in the region of the second edge is reduced from 20 V to 16 V.

In 1 shows the procedure that is customary in the prior art for changing the gray level at a pixel, namely by a corresponding multiplication of the number of drive pulses in the area of the relevant pixel. If there instead of a single control pulse two or - as in 1 shown - three control pulses are generated and sent to the printing system in question, then the multiple amount of an ink jet drop reaches the relevant pixel to be printed. If a single ink drop is 7 pl (picoliters) in size, then two ink drops will be 14 pl and three ink drops will be 21 pl. Depending on how long the intervals between individual control pulses are, either individual drops are generated that only overlap or add up on the substrate to be printed, or the ink drops already combine in the area of the nozzle or during flight and then come as one single, large drops on the substrate. However, it is not possible to reduce the ink drop size - for example to less than 7 pl - in this way, and it is also impossible to create drops of intermediate sizes - ie for example drops with a volume of 10 pl.

In 2 Figure 1 shows what a waveform 1 used by the invention to generate all ink drops of different sizes looks like.

You can see the inactive starting position 2 at the voltage level 0V, at which the level is also at the end of the stored pulse sequence returns again. This is preferably an extreme value of the available voltage level. In the present example, the other extreme value is a voltage level of -20 V, which in 2 represents the lowest voltage level.

How to 2 can be inferred, the stored waveform 1 has a first edge 3 after the initial value 2, during which the volume within the ink chamber of the relevant printing system increases, followed by a first hold time 4, in which the ink has the opportunity due to the negative pressure in the enlarged ink chamber to flow into those. This is followed by a second flank 5, in which the volume of the ink chamber is reduced, followed by a second waiting time 6, in which a droplet is fired from the ink chamber through the nozzle in the direction of the substrate to be printed. The entire sequence, consisting of the initial waiting time 2, the first edge 3, the first holding time 4, the second edge 5 and the second holding time 6, consists of a sequence duration T of, for example, a total of 10 μs. For example, one fifth of the total sequence duration T can be allocated to the initial waiting time 2, the first edge 3, the first holding time 4, the second edge 5 and the second holding time 6, in the present example then 2 μs each.

In Figure 5a it can be seen by way of example that a droplet with a size of 7 pl is generated for a complete run-through of this stored waveform 2 as a control sequence.

In the Figures 3 and 4 two options are shown for how the stored waveform 2 can be converted into a control signal 7, 7' for a printing system.

The output or the output stage of the drive amplifier can be identical in both cases, e.g. with two transistors connected together in such a way that their collector-emitter paths are in lie in a row. Both transistors can then be switched on anti-cyclically, i.e. in such a way that one of the two is always conducting and the other is not. The potential at the connection node between the two transistors is then either at the upper potential (here 0 V) or at the lower potential (here -20 V) depending on the switching state.

Such an output can therefore simply be used to switch between two voltage levels, as is shown in 3 you can see. The slope of the flanks 8, 9 of the control signal is then not specified, but depends on the conditions of the printing system, in particular its capacity, and on the other hand on the available energy or the maximum current that can be supplied by the amplifier output Course of the flanks 8, 9 relatively smooth, as shown in Fig. 3a.

In the embodiment according to 4 exploits the fact that with suitable control of two series-connected transistors of an amplifier output, any intermediate value between the two input voltages (here 0 V on the one hand and -20 V on the other) can be generated. A very precise control of the amplifier output would be possible, for example, by feedback of the output voltage and a comparison with a specified target value, in which case the identity of the two voltages (if necessary multiplied by a factor, which is determined by using a voltage divider in the actual value acquisition) can be determined by means of a controller can be realized at the amplifier output) can be ensured. In such a case, a stored waveform 2 can be fed as a time variable to the setpoint input of such an amplifier, and this then provides exactly this voltage (or a multiple thereof) at its output as a control signal for a printing system. On the other hand, in order to be able to feed a stored waveform, i.e. a waveform stored in the form of digital values, as an analog desired value into such an amplifier stage, a digital-to-analog conversion of the stored waveform 1 necessary. Because the output value of a digital-to-analog converter used for this purpose can only be adjusted step by step - the stored digital values of waveform 1 are limited to a finite number of bits - you can see this in the edges 8', 9' of the output signal of an amplifier operated in this way or in the control signal 7' generated by it, a multiplicity of small stages 10, as in 4 to see.

According to the current state of the art, these steps 10 are irrelevant for the printing process, or they were ignored or not noticed. However, the following can be observed. The steps induce micro-vibrations in the piezo element, which are partially dampened by the ink, but ultimately still affect drop formation to the extent that there are assumed ideal values for the waveform of a single drop, unique to each different print head. These ideal values for the flanks and hold times were defined as the resonance of the print head because drop formation at these values was found to be ideal. Shorter times or longer times resulted in drop formation deterioration, most likely because the ink dampened the micro-vibrations too little or too much, thereby adversely affecting the drop. However, in the disclosure document WO 2017 / 009 705 A2 It has already been established that a significant reduction in the control period compared to the resonance period by a factor of more than 1.5 has a positive effect on droplet formation, presumably because after a certain reduction in edge time, the vibrations are reduced to such an extent that a favorable or Even better influence on the drop formation is obtained in relation to the assumed ideal value, in particular as a result of a reduction in the number of stages and therefore the amount and duration of the vibrations during the drop formation.

The invention makes use of this advantage below, as in FIGS Figures 5 to 5f is shown, which shows the drive signal 7 ', as it arrives effectively at a printing system. The time axis is always to the right, and the time-dependent amplitude of the control signal 7' is to the top. A solid line means that the voltage of the control signal 7' is explicitly specified at the relevant points in time and is transmitted to the relevant printing system, while a dashed line means that the control signal is not specified at these points in time. Either no control signal is generated at these times, or it is not transmitted to the activator of the printing system. This could be done, for example, by interrupting the supply line between the control stage and the activator, or by blocking both transistors of the output stage of the amplifier in question. It is also possible in this case to break the control loop or to disconnect or disconnect the setpoint of the control loop.

how to get out figure 5 can recognize, such a shortening of the control signal 7' to the second half of the stored waveform 1, in conjunction with a shortening of the second holding time 6, leads to the ink chamber filling less and consequently the ink drop produced becoming smaller than usual, e.g. only has a volume of 5 pl. This is thus achieved in that only the second half of the first hold time 4 and the first half of the second hold time 6 are passed through to the activator.

Be in accordance with Figure 5b For example, if only the first half of the first hold time 4 and the second half of the second hold time 6 are fed through to the activator, a drop of 10 pl is obtained. This is presumably because there is a long interval between the two predetermined periods of time in which the ink has a long time to flow into the ink chamber.

An even larger drop with a volume of about 12 pl is obtained according to Figure 5c e.g. by the fact that only the first half of the first holding time 4 is sent to the printing system, but the total second hold time is 6. This gives the drop a long time to form outside the nozzle because the pressure inside the ink chamber is maintained for a long time.

If both holding times 4, 6 of the stored wave function 1 are transferred completely to the activator of the printing system, but not the flanks 3, 5, then ink droplets with a size of approximately 14 pl are obtained. On the one hand, ink flows into the ink chamber at least during the entire first holding time 4; however, this ink flow is not interrupted by a second flank 5 because the second flank 5 is suppressed. It can therefore flow even more ink into the ink chamber than in the case of Figure 5a .

If, after triggering a first holding time 4 with the second holding time 6, one waits even longer than corresponds to the second edge 5, then an even larger quantity of ink can flow into the ink chamber, and one obtains according to Figure 5e a droplet size of 17 pl and according to Fig. 5f - here is opposite of Figure 5e waited even longer with the second control phase - a droplet size of even 21 pl.

the Figures 6 to 8 show further possibilities how the control signal 7' can be further optimized. The according to the Figures 5 to 5f modifiable part of the control signal, ie the initial waiting time 11, the first edge 8, the first holding time 12, the second edge 9 and the second holding time 13, can always be found in the middle of the illustrated control sequence.

For example, the initial waiting time 11 can be preceded by a small pre-firing pulse 14, which has an increasing effect on the droplet volume. In this way, for example, an oscillation of the system can be controlled, which can result in particularly steep flanks.

Alternatively or additionally, after the modifiable drive pulse, a counter-pulse 15 with a reduced amplitude can be generated the amount of drops has a reducing effect. The tearing off of the drop can thus be triggered prematurely.

7 shows a wave with a pre-pulse 14 and two waveforms 1 for generating one drop each, the two drops having the same size, so that in this way two or more ink drops of the same size can be assigned to one pixel. Preferably, the pre-pulse does not generate its own drop, but instead influences a drop that may immediately follow.

Furthermore, in 8 shown that the first, controllable pulse with an initial waiting time 11, first edge 8, first holding time 12, second edge 9 and second holding time 13 can also be followed by at least one further, preferably controllable, pressure pulse 17, so that one pixel can also have two or more ink drops can be assigned to different sizes.

If, for example, two or more sequences according to Figures 5a to 5f sent one after the other, an ink quantity of 42 pl can be applied to paper or to the substrate in a comparatively short time interval of only 20 µs. For this would be according to the usual procedure 1 double the time required.

If you turn it into the Figures 5 and 5a to 5f If, for example, three consecutive droplet impulses are assigned to one pixel in the options described above, in the ideal case (including the zero impulse) one can generate three different sized drops, each with eight possible droplet sizes, i.e. there are a total of 8 * 8 * 8 ± 512 different gray and brightness levels.

As a result of the invention, many different droplets are obtained from a small nozzle with, for example, a nominal droplet size of 7 pl Drop sizes, for example in a range from 5 pl or less to 50 pl or more.

In this way, even more different intermediate droplet sizes can be generated and thus a correspondingly large number of gray or brightness levels.

Furthermore, the size of pre-firing pulses or counter-pulses can also be influenced in a corresponding manner.

With the present invention, all the required droplet sizes are obtained from one and the same nozzle in order to print with a low physical resolution comparable to offset printing and, at the same time, to produce a high optical print quality. That means, for example, for an image to be printed with a resolution of 300 x 300 dpi despite a high printing speed of e.g. 1 m/sec. only a firing frequency of 12 kHz is required.

In addition, when using the method according to the invention, the demands on the software and/or hardware and/or on the mechanical engineering are also significantly lower than in the previously known prior art. The production of print heads with a low nozzle resolution or nozzle density as well as large nozzles is also made considerably easier by the invention.

How to Figures 9a to 9e can be inferred, the method according to the invention offers a further advantage, since the drop velocity can also be influenced by selecting suitable sections of the stored waveform.

First, the complete signal for the formation of an intrinsic drop should follow Figure 9a be considered which of the Figure 5a corresponds and therefore one Delivers 7 pl drop size. The speed of this "intrinsic droplet" of size 7 pl shall be denoted as v ₁ .

Will now only subsections A and B according to Figure 9b transferred to the printing unit, this corresponds to the procedure according to Figure 5c ; an ink droplet of size 12 pl is thus produced, with subsection A following subsection 4 in Figure 5c while subsection B corresponds to subsection 6 in Figure 5c is to be equated. It turns out that in this case the drop velocity is slightly increased, namely now at v = 1.1 * v ₁ .

Similarly, one can use the printing unit as in the figures 9c or 5d control. An ink drop of size 14 pl is then obtained at a drop velocity of v=1.1*v ₁ .

If you control the printing unit as in the Figures 9d or 5f, an ink drop of size 21 pl is obtained at a drop velocity of v=1.4*v ₁ .

On the other hand, a control according to Fig. 9e That the intrinsic drop velocity v ₁ can also be undercut, namely by a control that is about a mixture of Figures 5 and 5b is equivalent to.

The inventor has found that a combination of two or more drive pulses 18, 19 results in additional setting parameters that make it possible to vary the two variables, droplet size and droplet speed, in different ways.

Accordingly, one can choose from several different procedures for generating an ink droplet of a desired size in each case that mask for which to be forwarded to the printing unit Select portions of the common waveform that give identical or nearly identical drop velocity.

The result is in 10 shown:
There you can see a waveform 1", stored together for all ink drops of different sizes, with two sequential control pulses 18, 19.

As can be seen, the first drive pulse 18 takes a longer period of time than the following, second drive pulse 19. This is because the slope of the edge 3a" of the first drive pulse 18 has a lower slope than the corresponding edge 3b" of the second drive pulse 19, and flank 5a" is also flatter than flank 5b". Furthermore, the hold time 4a" of the first drive pulse 18 is longer than the hold time 4b" of the second drive pulse 19.

In addition, the levels of the holding times 4a" and 4b" could also be of different sizes, which, however, is not the case in the embodiment according to FIG 10 is not realized.

The parameters of the stored waveform 1" are selected in such a way that ink drops of different sizes but the same speed can be generated with this single waveform 1".

Various control signals 20a" to 20z" are shown below the stored waveform 1" which are more or less inverted, i.e. only the partial sections of waveform 1" are switched through to the printing unit during the low phases of the relevant control signal 20a" to 20z".

It turns out, however, that the complete switching through of both drive pulses 18, 19 with the control signal 20y" does not result in any ink drops supplies. This is due to the fact that the slope of the flanks 3a" and 5a" of the first drive pulse 18 is relatively small; the resulting ink drop is comparatively slow and moderate and is completely withdrawn again by the following second control pulse 19 .

With the control signal 20×", the second drive pulse 19 is not switched through, and an ink droplet with a size of 10 pl is obtained, which is however comparatively slow, precisely because of the small incline of the flanks 3a" and 5a".

In contrast, the control signal 20z" only forwards the second drive pulse 19 to the connected printing unit, while the first drive pulse 18 is suppressed. Although the edges 3b" and 5b" are somewhat steeper than the edges 3a" and 5a" of the first drive pulse 18 the holding phase 4b" is too short for an ink drop of sufficient size to form and also to detach from the printing unit with a sufficient pulse; again, no ink drop is formed at all.

However, it turns out that the special control signals 20a" to 20e" deliver ink drops of different sizes, which, however, have the same droplet speed:
The control signal 20c" comes closest to the single, "intrinsic" ink drop that is generated from the first control pulse 18 in the case of the control signal 20x"; this also delivers an ink droplet with a volume of 10 pl, which, however, is significantly faster than the ink droplet generated with the control signal 20×". The control signal 20×" only switches through the hold phase 4b" of the second drive pulse 19 and a part that is separated in time from it of the second holding phase 6b" of this second drive pulse 19. In contrast to the control signal 20z", however, the edges 3b" and 5b" are not predetermined, but are abruptly switched over to the respective voltage level 4b" or 6b" with high resistance, so that a maximum gradient of the flanks actually applied results, which enables the droplet to be detached. In addition, the immediately following section 6b" is not switched through, but a waiting time elapses in which the ink chamber can fill up. Because of the steep flanks, the ink drop in the control signal 20c" detaches itself from the print head under comparatively high pressure and therefore receives a comparatively high speed v.

Starting from the control signal 20c", which supplies an ink droplet with the volume 10 pl by selectively switching through parts of the second drive pulse 19, the control signal 20d" leads to a comparable procedure with regard to the first drive pulse 18; by selectively gating parts of it, an ink droplet with a volume of 12 pl is produced, but with the same velocity v.

The drive signal 20e" represents a slightly modified combination of the two control signals 20c" and 20d", a section of the first holding phase 4a" and the second holding phase 6a" of the first drive pulse 18 being switched through, as well as a section of the first holding phase 4b" in each case. and the second holding phase 6b" of the second drive pulse 19. Due to the long waiting time before the switched-through subsection 6b", a second ink droplet can be set in motion, presumably even before the droplet breaks off the ink droplet applied during the first drive pulse 18, i.e. these two ink droplets are either originally bonded together or fuse in flight and therefore impact the substrate to be printed as a single drop of ink. With this method a drop with a volume of 16 pl is obtained; the speed v is equal to the speed generated with the control signals 20c" and 20d".

Starting from the control signal 20x", which was originally discarded because the ink drop was too slow, but with a steeper second edge (at 5a"), a sufficiently fast ink drop is applied, which, however, is not increased by means of two switched-through partial sections 4b", 6b" of the second drive pulse 19 , but is reduced. Here the overall third edge (at 3b") again comes relatively shortly after the ink drop that was initially applied, but not as quickly as with the control signal 20y". In the manner of a counter-pulse, the components of the second control pulse 19 cannot therefore completely withdraw the ink drop that has been applied, but can reduce its volume. The resulting ink drop has a volume of 6 pl at the same velocity v,

The control signal 20a" results in a further reduction of the ink droplet volume. This is achieved by switching through only two mutually separate sections 4a", 6a" of the first drive pulse. The consequence of this is that the first two edges (at 3a" and 5a") become steeper and the ink drop thus obtains a comparable or even greater initial velocity; at the same time, however, as a result of the shortening of the ink loading phase to the plateau of the holding phase 4a", less ink is drawn into the ink chamber and therefore only a smaller ink drop forms . From this, even a partial volume is then withdrawn again by the partially switched-through second drive pulse 19, in the manner of a counter-pulse. Finally, an ink drop of only 4 pl is obtained, but again with the speed v due to the high initial speed.

As can be seen, it is possible to generate ink drops of sizes 4 pl, 6 pl, 10 pl, 12 pl and 16 pl from the single stored waveform 1" by masking and switching through certain sections using the control signals 20a" to 20e". which, however, all have the same drop velocity v.

In the Figures 11a to 11d Another possibility for influencing the droplet size and/or speed is shown, which can be used in particular when using piezo elements as activators:
In Figure 11a a stored waveform 1 is shown, which corresponds to waveform 1 2 is equivalent to. You can see the holding phases 2, 4, 6 and the two edges 3, 5. The amplitude is 20 V on each edge 3, 5.

In the Figures 11b to 11d shows how different sections 4, 6 are switched through to the connected printing unit from this uniform waveform 1 by means of different control signals.

According to Figure 11b the holding phases 4 and 6 are switched through completely. The result is that the output transistors of the control stage, which can naturally only drive a limited current, have enough time for the electrodes on the two opposite sides of the piezo element to fully adjust to the voltage of 0 V on the one hand and then to the voltage of 20 V on the other hand can charge. The course of the voltage 21 between the two electrodes of the piezoelectric element follows the predetermined waveform 1 fairly exactly, as Figure 11b can be clearly seen.

In the 11c On the other hand, the hold phase 4 is not switched through completely, but only to a very small extent, e.g. for a duration of about a quarter or fifth of the entire hold phase 4. This is so short that the switch-through duration between the two high-impedance phases of the output stage of the Control circuit is not sufficient to bring enough charge carriers to the electrodes of the piezoelectric element, which are required to lower the voltage 21 between those of initially 20 V to 0 V. Rather, the stroke or the amplitude in the area of the flank 3 is shortened, and the voltage 21 drops, for example, by only about 16 V, ie from 20 V to 4 V. Since the holding phase 6, which is then switched through, is long enough, the voltage 21 then again tends towards 20 V.

However, since the piezo element does not carry out the full stroke, but only a stroke of 4/5 of the maximum amplitude, for example, the chamber volume increases only to a correspondingly reduced extent. Less ink is sucked in, and therefore only an ink droplet with a smaller volume can be emitted on the second flank, for example with one corresponding to about 4/5 of the volume Figure 11b reduced value.

the Figure 11d shows a different procedure, but one that has a similar effect: It is not the switching through of holding phase 4 that is significantly shortened here, but the switching through of holding phase 6. The voltage 21 between the electrodes of the piezo element therefore initially follows the flank 3 of the stored waveform 1 almost exactly , i.e. also with an amplitude deviation of 20 V. The maximum possible amount of ink is thus sucked into the ink chamber.

However, the strong reduction in the switched-through holding phase 6 means that the voltage 21 in the area of the flank 5 does not have sufficient opportunity to rise again to the full value of 20 V. Because of the electrode capacitances to be recharged, only a voltage 21 of about 16 V can be achieved with the amount of charge applied. Again, only a smaller amount of ink is released from the printing unit, in the present example again only in an order of magnitude of 4/5 of the maximum value Figure 11b .

Another way of influencing the amplitude of a control signal is to provide not only a single supply voltage - e.g. 20 V - in addition to the ground potential 0 V, but several, i.e. 16 V and 20 V, for example, between which you can switch if necessary - for example between two control pulses 18, 19.

Reference List

1

waveform

2

initial waiting time

3

first flank

4

first holding time

5

second flank

6

second holding time

7

control signal

8th

flank

9

flank

10

Step

11

waiting period

12

first holding time

13

second holding time

14

pre-fire pulse

15

counter impulse

16

afterpulse

17

follow-up pressure pulse

18

first drive impulse

19

second drive pulse

20

control signal

21

tension

Claims (28)

1. Printing method for a digital printing device for the production of ink drops of different drop sizes, comprising a print head having a plurality of printing systems and comprising at least one control apparatus for feeding control signals to the printing systems for the production of the ink drops, wherein each printing system has a nozzle, at least one ink chamber and an activator, e.g. a piezoelectric activator, which is associated with the at least one ink chamber, for the discharge of ink drops from the ink chamber in question via the nozzle in question onto a substrate to be printed on, as a response to a control signal, wherein, in the context of the control apparatus, only a single waveform (1) for the control signal (7, 7') having a specified time curve is stored for ink drops of all sizes, comprising at least one printing pulse with, optionally, an initial waiting time (2), a first edge (3), followed by a first holding time (4), and, after the first holding time (4), a second, opposite edge (5), optionally followed by a second holding time (6), characterized in that the size and, optionally, the speed of ink drops is varied by virtue of the fact that, at most for a single, intrinsic drop size, the entire stored waveform (1) is transferred as a control signal (7, 7') to the activator in question, while for all other, effective drop sizes, only a portion of the common, stored waveform (1) is transferred to the activator in question, namely one or more selected portions of the initial watinig time (2), the first edge (3), the first holding time (4), the second, opposite edge (5), and the second holding time (6), while, in the control signal (7, 7'), one or more other portions of the stored, common waveform (1) are held off from the activator in question.
2. Printing method according to Claim 1, characterized in that a control stage with a switched output is used or preferably a control stage with a controlled or regulated output.
3. Printing method according to Claim 1 or 2, characterized in that, during portions of the stored waveform (1) that are not switched-through, the control circuit is in a high-impedance state at the control output thereof that is connected to an activator.
4. Printing method according to Claim 3, characterized in that the control circuit's control output that is connected to an activator is designed in the manner of a push-pull circuit with two transistors connected in series on the output side.
5. Printing method according to Claim 4, characterized in that the two transistors, which are connected in series on the output side, of the control circuit's control output, which is attached to a piezo element, are both high-impedance on the output side in a non-switched-through state.
6. Printing method according to one of the preceding claims, characterized in that during the first or second edge (3, 5), the volume of an ink chamber is increased by means of the activator in question, and during the thereupon immediately following first or second holding time (4, 6), ink is suctioned into the ink chamber in question.
7. Printing method according to Claim 6, characterized in that during the respective other, second or first edge (5, 3), the volume of the ink chamber in question is reduced by means of the activator in question, and during the thereupon immediately following second or first holding time (6, 4), an ink drop is fired out of the associated nozzle.
8. Printing method according to one of the preceding claims, characterized in that a piezo element is used as an activator, which piezo element is in contact with an ink chamber in the region of an ink nozzle in order to influence the volume of the ink chamber.
9. Printing method according to Claim 8, characterized in that the piezo element has conductive coatings on two opposite sides, which can be used as electrodes.
10. Printing method according to Claim 9, characterized in that the piezo element is not electrically conductive between the two opposite coatings, so that an electrical charge applied as the result of a voltage that is applied remains constant in the case of the high-impedance output of the control circuit.
11. Printing method according to Claim 9 or 10, characterized in that the electrical charge applied to the piezo element during a charging phase is dependent on the applied voltage as well as on the duration of the charging phase.
12. Printing method according to one of the preceding claims, characterized in that, in each case, at least two intervals are selected from the stored waveform (1) and are transferred to the activator.
13. Printing method according to Claim 12, characterized in that at least two intervals that are selected and transferred to the activator do not immediately follow each other.
14. Printing method according to one of the preceding claims, characterized in that the intervals transferred to the activator are selected from a first or a second holding time (4, 6).
15. Printing method according to one of the preceding claims, characterized in that, in the case of one, several or all effective drop sizes, no edge (3, 5) is transferred to the activator.
16. Printing method according to one of the preceding claims, characterized in that the stored waveform (1) comprises two or more sequential control pulses, each of which respectively has a first edge (3), followed by a first holding time (4), and, after the first holding time (4), a second, opposite edge (5), optionally followed by a second holding time (6).
17. Printing method according to Claim 16, characterized in that two or more control pulses of the stored waveform (1) differ from each other in terms of the slope of the first edge (3) and/or the second edge (5), and/or in terms of the first holding time (4) and/or the second holding time (6), and/or in terms of the amplitude of the first and/or second holding time (4,6), and/or in terms of the rise time or fall time during the first edge (3) and/or the second edge (5).
18. Printing method according to one of the preceding claims, characterized in that the quantity of ink flowing into the ink chamber during a holding time in the case of an increased volume of said ink chamber is dependent on the increase in volume, and/or on the duration of the regarding volume increase.
19. Printing method according to one of the preceding claims, characterized in that the volume of an ink drop fired out during an edge (5, 3), during which the volume of the ink chamber in question is reduced by means of the regarding activator, increases with the slope of the edge (5, 3) in question, in which the volume inside the ink chamber is reduced.
20. Printing method according to one of the preceding claims, characterized in that the volume of an ink drop fired out during an edge (5, 3) of a control pulse increases with the duration of the immediately following effective holding time.
21. Printing method according to one of the preceding claims, characterized in that the duration of the effective holding time immediately following an edge is determined by the time interval of a following edge of the control signal.
22. Printing method according to one of the preceding claims, characterized in that an edge of a second control pulse, which edge follows before the breaking off of the ink drop discharged during a first control pulse and is (once again) increasing the volume inside the ink chamber, reduces the volume of the ink drop, because consequently a larger part of the ink drop is withheld.
23. Printing method according to one of the preceding claims, characterized in that an edge of a second control pulse, which edge follows after the ink drop formed during a first control pulse and is (once again) decreasing the volume inside the ink chamber, increases the volume of the ink drop, when consequently an additional quantity of ink is discharged.
24. Printing method according to Claim 22 and 23, characterized in that, after the ink drop formed during a first control pulse, an additional quantity of ink is discharged, when a sufficient quantity of ink was suctioned into the ink chamber between the edge that is (once again) increasing the volume inside the ink chamber and a subsequent edge of a second control pulse, which edge is (once again) decreasing the volume inside the ink chamber, and when the edge that is (once again) decreasing the volume inside the ink chamber is sufficiently steep.
25. Printing method according to one of the preceding claims, characterized in that the speed of an ink drop fired out during an edge (5, 3) of a control pulse increases with the amplitude of the edge (5, 3) of the control pulse.
26. Printing method according to Claim 25, characterized in that the amplitude of the edge (5, 3) of a control pulse increases with the duration of the actively impressed charging current for the activator.
27. Printing method according to Claim 26, characterized in that the duration of the actively impressed charging current for the activator is lower in the case of larger drop sizes than in the case of smaller drop sizes.
28. Printing method according to one of the preceding claims, characterized in that the speed of an ink drop fired out during an edge (5, 3) of a control pulse is kept constant for all drop sizes.

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