DE102021106328A1 - Process for cleaning a print head for ink printing - Google Patents

Abstract

A method according to the invention for cleaning a print head for ink printing with at least one print nozzle (5) and a piezo actuator (6) assigned to the print nozzle, with the steps: providing a cleaning device (10) with an ultrasonic actuator (14), and activation of the ultrasonic actuator during cleaning, is characterized by the following step: activation (15) of the piezo actuator (6) during cleaning, the piezo actuator being matched to the activation (16) of the ultrasonic actuator (14). is controlled. The invention advantageously makes it possible to improve the cleaning of printheads using ultrasound while preventing damage to them.

Description

invention

The invention relates to a method for cleaning a print head for ink printing with the features of the preamble of claim 1.

field of technology

The invention is in the technical field of the graphics industry and there in particular in the field of industrial, i.e. highly productive ink printing (inkjet) on flat substrates, i.e. the image-wise application of the finest drops of liquid ink on sheet, web, foil or label-shaped printing materials , preferably made of paper, cardboard, plastic, metal or composite material, using print heads. The invention relates to the cleaning of the print heads.

State of the art

Each print head of a printing machine generally has a large number of individually controllable printing nozzles. The nozzles are generally arranged in a row of nozzles or in a plurality of rows of nozzles parallel or at an angle to the printing material transport direction. Each nozzle produces an ink droplet when activated. If a nozzle is permanently activated, it prints a visible line in the color of the ink on the substrate.

Printheads and especially their nozzle plates can get dirty, which is why the printheads have to be cleaned from time to time. It is known to use automated cleaning devices for this purpose. It is also known to use cleaning liquid. It is also known to improve the cleaning process through the use of ultrasound.

In practice, however, it has been shown that the use of ultrasound is not unproblematic and, in extreme cases, can lead to the destruction of the print heads and in particular their sensitive piezo actuators.

the EP997290A1 discloses cleaning using ultrasound in the field of inkjet printing. Likewise the EP2955027A1 .

the DE102020105736A1 discloses ultrasonic inkjet printhead cleaning. The document also reveals that the meniscus pressure in the print head can be varied during cleaning - similar to what is known as purging: from sucking in the cleaning liquid to rinsing it out again. However, the document does not give any further details on this procedure and does not describe what should be taken into account.

Technical task

It is therefore an object of the present invention to provide an improvement over the prior art, which in particular makes it possible to improve the cleaning of printheads using ultrasound while preventing damage to them.

Solution of the problem according to the invention

According to the invention, this object is achieved by a method according to claim 1 .

Advantageous and therefore preferred developments of the invention result from the dependent claims as well as from the description and the drawings.

A method according to the invention for cleaning a print head for ink printing with at least one print nozzle and a piezo actuator assigned to the print nozzle, with the steps: providing a cleaning device with an ultrasonic actuator, and controlling the ultrasonic actuator during cleaning, is characterized by the following Step out: Activation of the piezo actuator during cleaning, the piezo actuator being activated in a way that is coordinated with the activation of the ultrasonic actuator.

Advantageous Forms and Effects of the Invention

The invention advantageously makes it possible to improve the cleaning of printheads using ultrasound while preventing damage to them.

As part of investigations into solving the problem, it was worked out that the solution window for the control parameters for the ultrasonic actuator in ultrasonic cleaning, in which the print heads are not destroyed, is very small and thus the efficiency of the cleaning is severely restricted: Since each print head has different resonance frequencies or sensitive frequencies, different parameters would be required for each printhead (or for each ink used); The passive piezos in the print head would be stimulated to oscillate by the active ultrasonic actuator, possibly resonate and be destroyed as a result; The undesired movement of the piezos in the print head that is generated in this way would also generate an undesired voltage, which could destroy the control electronics of the piezos.

According to the invention, the piezo actuators in the print head do not remain passive or inactive during cleaning, but are also activated, ie they are active (as part of their activation)—at least temporarily overlapping with the activity of the ultrasonic actuator. Damage and corresponding costs can be prevented by the coordination according to the invention between the activation of the ultrasonic actuator (the cleaning device) and the piezo actuator (the print head). Accordingly, the tuning takes place in such a way that damage to the print head and in particular to a piezo actuator of the print head is avoided. The tuning takes place in particular in such a way that the piezo actuator does not perform any movements and/or vibrations that damage itself and/or other piezo actuators during the ultrasonic cleaning. In this case, the tuning can take place synchronously (or synchronously with one another; or synchronized).

In the prior art, for example, "purging" occurs during ultrasonic cleaning. However, this is not a coordinated control, because the process of "purging" only takes place at some point during the cleaning with ultrasound and it can very well lead to damage.

Developments of the invention

Preferred developments of the invention (in short: developments) are described below.

A development can be characterized in that the piezoelectric actuator is controlled in phase opposition to the control of the ultrasonic actuator.

The anti-phase control effectively prevents undesired damage to the print head or its piezo actuators. Damaging overvoltages in the print head can be effectively prevented. The anti-phase tuning effectively prevents the piezo actuator from oscillating passively, as it actively performs a counter-movement. In this sense, "opposite phase" means: the piezo actuator is controlled in time in such a way that it actively executes a movement or an oscillation which is opposed to a passive movement or oscillation (mediated by the active ultrasonic actuator). In the optimal case with optimal tuning, the piezo actuator then performs no movement or vibration, otherwise at least a greatly reduced movement or vibration.

A further development can thus be characterized in that the piezo actuator is controlled in phase opposition to the control of the ultrasonic actuator in such a way that passive oscillation of the piezo actuator is avoided.

A further development can be characterized in that the piezoelectric actuator is controlled in phase opposition to the control of the ultrasonic actuator and is controlled with a phase shift.

This phase shift advantageously leads to an intensive fluid flow (ink, cleaning agent) being set up directly at the inlet of the printing nozzle for a short time, which specifically removes the dirt or corresponding particles in this extremely sensitive area and thus improves the cleaning effect. In order to achieve a particularly good cleaning effect in the area of the entrance to the print nozzle, the piezo actuators in the print head can even be activated in phase for a short time in order to achieve a particularly large fluid flow in this area. However, only so briefly that the piezo actuators in the print head or their control electronics are not destroyed.

A further development can thus be characterized in that the piezoelectric actuator is initially controlled in phase with the control of the ultrasonic actuator and is then controlled in phase opposition.

If necessary, the pulse length of the activation of the piezo actuators can also be varied in order to achieve an ideal result of the cleaning effect and protection against damage or destruction.

The features and combinations of features disclosed in the above sections Technical field, invention and developments as well as in the following section Working examples represent further advantageous developments of the invention - in any combination with one another.

character list

• 1 zeigt eine Vorrichtung beim Durchführen eines bevorzugten Ausführungsbeispiels eines erfindungsgemäßen Verfahrens.
• Die 2 und 3 zeigen Diagramme zu bevorzugten Ausführungsbeispielen eines erfindungsgemäßen Verfahrens.

The figures show preferred exemplary embodiments of the invention and the developments. Features corresponding to one another are provided with the same reference symbols in the figures. Reference symbols that are repeated in the figures have been partially omitted for the sake of clarity.

• 1 shows a device when carrying out a preferred embodiment of a method according to the invention.
• the 2 and 3 show diagrams of preferred exemplary embodiments of a method according to the invention.

1 shows a printing machine 1, in particular an inkjet printing machine, for printing a printing material 2, in particular paper, cardboard or film, which comprises at least one print head 3, preferably a large number of print heads. Each print head of the printing press has what is known as a nozzle plate 4, in which a large number of printing nozzles 5—as respective openings in the nozzle plate—are formed. Each pressure nozzle is assigned an actuator 6, which has a piezo that can be controlled to move and/or vibrate and is therefore referred to as a piezo actuator. During printing, the actuators are controlled by a digital computer 7 in such a way that the printing nozzles eject ink 8 as ink drops and a printed image is thereby generated on the conveyed printing material. 1 also shows that dirt 9 can adhere to the nozzle plate or that dirt 9 can get into the printing nozzles.

1 12 shows the print head 3 not in the printing state, but during cleaning. In this case, a cleaning device 10 is positioned adjacent to the nozzle plate 4, for example below the nozzle plate. The positioning can be done by the cleaning device performing a movement 11 towards the print head, or vice versa.

The cleaning device 10 shown comprises a chamber 12, preferably formed in a housing, in which a cleaning liquid 13 is accommodated or stored. The cleaning device also includes an actuator 14, which generates ultrasound and is therefore referred to as an ultrasonic actuator. The ultrasonic actuator can also be a piezo actuator. The ultrasonic actuator can be arranged, for example, on or in the bottom of the chamber and is preferably also controlled by the digital computer 7 for cleaning. During cleaning, ultrasound is generated and transmitted to the cleaning liquid. The chamber is partially open, i.e. has an opening 12a, preferably at its top.

During cleaning, the cleaning liquid 13 contacts the nozzle plate 4 in the area of the opening 12a and cleans it, supported by ultrasound. In addition, the printing nozzles 5 and preferably also their interior can be cleaned, for which purpose cleaning liquid gets into the printing nozzles, e.g. is actively pressed or sucked. Soiling 6 is removed in this way.

The activation of the piezo actuator 6 - or the several piezo actuators - during cleaning is carried out according to the invention in a manner coordinated with the activation of the ultrasonic actuator 14.

2 shows an example of tuned driving. In the diagram shown, the amplitude A of the activation is plotted over time t. The upper curve 15 shows the activation of the piezo actuator 6 and the lower curve 16 the activation of the ultrasonic actuator 14. Both curves are shown in a simplified manner and preferably form a respective sequence of square-wave signals (each with a high and a low level). The zero line can preferably be assumed separately for each curve at the level of the low level. It can be seen that the two actuators 6 and 14 are preferably controlled in phase opposition: Whenever the ultrasonic actuator 14 is controlled (or strongly controlled), the piezo actuator is not (or weakly) controlled. In this advantageous way it can be achieved that the piezo actuator 6 supports the cleaning process and immediately prevents the ultrasonic actuator 14 from damaging the piezo actuator 6 . The latter means that the piezoelectric actuator is preferably controlled in phase opposition to the control of the ultrasonic actuator in such a way that passive oscillation of the piezoelectric actuator is avoided.

3 shows another example of tuned driving in a similar diagram as in 2 . It can be seen that the piezo actuator 6 is preferably controlled in phase opposition to the control of the ultrasonic actuator 14 and coordinated with a phase shift 17 .

Reference List

1

printing press

2

substrate

3

printhead

4

nozzle plate

5

pressure nozzles

6

Piezo actuators (of the pressure nozzles)

7

calculator

8th

ink

9

Contamination, e.g. particles

10

cleaning device

11

movement

12

chamber

12a

opening

13

cleaning fluid

14

ultrasonic actuator (of the cleaning device)

15

Curve: activation of piezo actuator

16

Curve: control of ultrasonic actuator

17

phase shift

QUOTES INCLUDED IN DESCRIPTION

This list of documents cited by the applicant was generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Patent Literature Cited

• EP 997290 A1 [0006]
• EP 2955027 A1 [0006]
• DE 102020105736 A1 [0007]

Claims (5)

Method for cleaning a print head for ink printing with at least one print nozzle (5) and a piezo actuator (6) assigned to the print nozzle, with the steps: providing a cleaning device (10) with an ultrasonic actuator (14), and controlling the ultrasound -Actuator during cleaning, characterized by the step: activating (15) the piezoelectric actuator (6) during cleaning, the piezoelectric actuator being controlled in a manner coordinated with the activation (16) of the ultrasonic actuator (14). procedure after claim 1 , characterized in that the piezoelectric actuator (6) is controlled in phase opposition to the control (16) of the ultrasonic actuator (14). procedure after claim 2 , characterized in that the piezoelectric actuator (6) is controlled in phase opposition to the activation (16) of the ultrasonic actuator (14) in such a way that passive oscillation of the piezoelectric actuator is avoided. Method according to any of the preceding claims 2 or 3 , characterized in that the piezoelectric actuator (6) is controlled in phase opposition to the control (16) of the ultrasonic actuator (14) and with a phase shift (17). Method according to any of the preceding claims 2 until 4 , characterized in that the piezoelectric actuator (6) on the activation (16) of the ultrasonic actuator (14) is initially controlled in phase and then in phase opposition.

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