EP3436278B1 - Device and method for ink supply in digital printing - Google Patents

Description

The invention relates to a device and a method for supplying ink to a print head in digital printing with at least one print head, a supply tank for an ink medium, the supply tank being connected to the inlet of the at least one print head via a supply line, and a return tank for the ink medium, the The return tank is connected to the output of the at least one print head via a return line.

Digital inkjet printing using so-called drop-on-demand print heads is now a frequently used application in industry. A wide variety of inkjet printers for desktop and office printers have been known for many years. The print heads used in industry are, however, designed differently than those for desk and office printers and, due to the large areas to be printed, typically also consume significantly more ink. For example, when printing bottles, an annual consumption of several tons can occur. A suitable ink supply is therefore an essential component of such printing systems and has a decisive influence on the quality of the printing result and the profitability.

Drop-on-demand printheads have a small ink chamber behind each nozzle that is filled with the ink medium. The ink medium is ejected through the nozzles of the printhead by briefly generating an increased pressure in this ink chamber using various methods. The one from the EP 0 352 978 A2 known methods, a minimal part of the ink medium is heated by a heater in the ink chamber, so that part of the ink medium evaporates. The resulting expansion generates a pressure pulse.

In the case of the U.S. 5,124,716 A The method described above is used to generate a pressure pulse in the ink chamber of the print head via a piezo crystal. This pressure pulse forces a drop of the ink medium through the nozzle and therefore ejects it.

The print heads, which usually have around a thousand nozzles, require a negative pressure inside the ink chamber in front of the nozzles, since otherwise the low-viscosity inks would leak out of the nozzles despite the small, only micrometer-sized nozzle openings. This negative pressure is of the order of about -10 mbar.

There are basically two different types of printhead used in industry. Some work as "dead-end shooters", in which the ink medium is simply applied to the print head and the supply takes place via the geodetic height (cf. U.S. 4,638,337 A ). This means that the ink medium is transported to the printhead and this forms a dead end with its ink chambers for the ink medium until the ink is ejected through the nozzles. The pressure inside the ink chambers of the print head essentially corresponds to the pressure at the input of the print head. The negative pressure in the printhead can cause air to be sucked into the printhead through the nozzles. The resulting air bubbles in the ink chamber lead to the volume in the ink chamber becoming compressible, as a result of which a pressure build-up for sending out a drop is not possible in a reproducible manner.

The second option is what is known as the pumping over process, as it is, for example, from the U.S. 5,818,485 A is known. Here the ink is always pumped through the printhead again so that there is a steady flow of ink through the printhead. As a result, the air possibly sucked in through the nozzles can be removed from the ink chamber again.

These printheads were originally designed for planographic printing, in which the surface to be printed on lies in a horizontal plane and the ink is applied from top to bottom, as is the case with typical office printers. In the meantime, however, the print heads are also used in a vertical arrangement. This has the advantage that when printing three-dimensional objects, the side surfaces can also be printed without moving the side to be printed into a horizontal position. For example, when printing on bottles, they would otherwise have to be turned over in a very short time (around 1/10 of a second) and then erected again.

However, the vertical arrangement requires a completely different level of precision in the ink supply. The print heads themselves typically have a printing length over the nozzles, that is to say a distance from the lowest to the uppermost nozzle, of approximately 70 mm. Therefore, the pressure difference within the nozzles of a vertically arranged head due to gravity, assuming a density of 1000 kg / m ^ 3 alone, is 7 mbar. The lowest nozzle thus has an ink supply pressure 7 mbar higher than the uppermost nozzle. At a nominal pressure of -10 mbar, there is no longer any leeway for a tolerance. Every pressure fluctuation can therefore determine whether the quality is sufficient for a good print.

Known pumping systems have a first tank from which the ink is fed to the head with positive pressure by means of a pump, and a return pump that generates a negative pressure at the ink outlet of the printhead that is high enough to achieve the described negative pressure due to the flow resistance in the printhead Generate printhead.

Pumps refer to machines for conveying essentially incompressible liquids, such as an ink medium. Pumps are to be distinguished from compressors and compressors, which are machines for compressing and conveying gases and gaseous media.

The pressure prevailing inside the printhead in front of the nozzles is called meniscus pressure, which corresponds to the nominal pressure in the printhead and which, as described, should be a slight negative pressure. Another important factor for the function of the print head is the pressure difference between the flow and return, which is responsible for the delivery rate, i.e. the flow of the ink through the print head.

In order to obtain a uniform pressure, the known ink supplies work with pumps for the ink and pulsation dampers. These systems have a very complex structure and complex control. Despite the smoothing by the pulsation damper, pressure peaks can occur, which severely impair the print quality.

In an ink jet device according to US 2012/0062639 A1 Ink flows from an upstream sub-tank through a print head to a downstream sub-tank. An ink pressure at the printing nozzle is set in the secondary tank and in the secondary tank on the basis of the auxiliary variables "energy per volume". This variable takes into account the potential energy of the ink in the respective secondary tank due to its height position and, on the other hand, a pressure in the respective secondary tank. Temperature sensors are arranged on an upstream channel between the sub-tank and the print head and on a downstream channel between the print head and the sub-tank and are connected to a CPU. Furthermore, an air pressure source is provided on the sub-tank and an air pressure source is provided on the sub-tank and each connected to the CPU. The CPU controls at least one of the auxiliary variables “energy per volume” in the secondary tanks by controlling the air pressure sources as a function of the ink temperatures measured by means of the temperature sensors. This makes it possible to compensate for a temperature dependency of the flow resistance of the ink and to maintain a nozzle pressure.

US 2007/0252860 A1 discloses an apparatus similar to that of US 2012/0062639 A1 is similar. Here, too, the auxiliary variable “energy per volume” is controlled in a first tank and in a second tank by means of air pressure sources.

A complex printing system according to WO 2010/130397 A1 comprises a first auxiliary tank which is connected via a line to a connection of a print head serving as an inlet. A connection of the pressure head serving as an outlet is connected via a line with a pressure reducer to a multi-way valve, via which the line can optionally be connected to a second auxiliary tank. The pressures in the auxiliary tanks are set exactly so that the pressure in the first auxiliary tank is slightly greater than in the second auxiliary tank. The pressure in the first auxiliary tank can be, for example, slightly less than an atmospheric pressure.

A pressure regulator according to U.S. 5,555,005 A. comprises a collection chamber for the ink to be discharged to a printhead. Ink pressure is given by a volume of gas above the ink in the collection chamber. A pressure sensor is arranged at an inlet of the collection chamber. Ink flowing back from the printhead passes through a conduit to a return reservoir.

The object of the present invention is to ensure uniform pressures on and in the print head when supplying ink for print heads in digital printing.

This object is achieved by a device with the features of claim 1 and by a method with the features of claim 10.

Advantageous refinements of the invention are the subject matter of the subclaims.

The device according to the invention for supplying ink in digital printing has at least one print head which is to be supplied with an ink medium. This print head can be a drop-on-demand print head, which has an inlet and an outlet for the ink supply, so that the ink medium can flow through it continuously. The device also has a supply tank, which is connected to the inlet of the print head via a supply line, in order to supply the ink medium to it. Furthermore, the ink supply has a separate return tank, which is connected to the output of the printhead via a return line in order to discharge the unused, that is, unejected, ink medium. The flow and return can be designed as a pipeline, hose or the like. All line connections can be equipped with self-closing plug-in couplings. The ink medium can be any ink or other liquid that is ejected from the printhead in place of the ink. Functional materials that, for example, produce an electrically conductive structure or have other preferred properties are also conceivable.

A gas volume is formed in the supply tank and / or in the return tank. For this purpose, in the simplest case, the tanks cannot be completely filled with the ink medium, so that a volume filled with gas is arranged above the ink medium, that is to say above the ink liquid level. This volume can be filled, for example, with air, nitrogen or another gas, for example an inert gas.

An inert gas can prevent hardening, changing and / or contamination of the ink medium. The gas in the volumes in the supply tank and return tank is under pressure. This can be an overpressure or a negative pressure relative to the external pressure. As a further possibility, the supply tank and / or return tank can have a separate tank for the gas volume, which according to the invention can be regarded as a component of the supply tank or the return tank.

It is also possible for the supply tank and return tank to be designed as separate chambers of a large common tank.

The gas pressures in the supply tank and return tank create a pressure difference between the inlet and outlet of the print head for conveying the ink medium. This causes an ink flow from the supply tank through the pressure head into the return tank. The flow of ink is therefore not driven by fluid pumps in direct contact with the ink medium, but is effected by the gas pressures. The compressibility of the gas means that there are no strong pressure fluctuations in the ink medium. Using the gas volume as a buffer, pressure surges are filtered out naturally.

In a preferred embodiment, the device has a pressure control device in the supply tank and / or the return tank. This can be designed as a pressure build-up and / or pressure relief device. For example, a pressure build-up device and a pressure relief device can be provided in each of the two tanks, with which the pressure can be generated and kept constant. In this case, the pressure build-up device of the supply tank can be suitable for generating an overpressure in the supply tank, such as a compressor, for example. Another possibility is the regulated connection to a compressed air connection or a suitable reservoir. In the return tank, for example, a compressor operating in the opposite direction or a vacuum pump, which sucks gas from the return tank, can be provided in order to generate a negative pressure. Rotary vane pumps, diaphragm pumps and scroll pumps, among others, are particularly suitable. The latter two are oil-free pumps, which means that contamination of the ink medium can be avoided particularly efficiently. A regulated connection to an existing vacuum suction line or a vacuum reservoir is also possible. The pressure relief devices can be further active devices or passive devices such as ventilation valves and / or vent valves.

In a preferred embodiment, the device has a controller which is set up to regulate the pressures in the gas volumes in the supply tank and / or return tank so that the pressure difference between the inlet and outlet of the printhead and / or the meniscus pressure of the printhead correspond to a predetermined setpoint . The meniscus pressure is the pressure inside the printhead in front of the outlet nozzles of the printhead. The gas pressures can be regulated by controlling the pressure build-up and pressure relief devices of the supply tank and return tank. The setpoint values can be permanently specified in the control system or they can be dependent on further operating parameters or manual inputs. The control enables the pressures to be kept at the desired values, even if gases dissolved in the ink medium can degas and thus change the pressure, or if the ink medium has absorbed air bubbles which have thus entered the circuit.

In a further preferred embodiment, a pump is arranged as a circulation pump between the supply tank and the return tank, which pump can pump ink medium from the return tank into the supply tank. The ink circuit can be closed by such a pump so that the discharged ink medium in the supply tank can be replaced by ink medium from the return tank. The circulation pump can be regulated with a separate or a common control, which is also responsible for regulating the gas pressures. The pressure range of the circulation pump is not critical; it can, for example, only depend on the fill level of the supply tank.

In a further preferred embodiment, the device also has a level sensor in the supply tank and / or the return tank. The measured values of the sensors can be recorded by a controller and used to regulate the actuators, such as pumps, compressors and valves.

In a further preferred embodiment, the return tank has a refill pump. This can be in connection with an ink reservoir from which the pump can refill ink medium into the return tank. This means that the used ink medium, i.e. the ink medium ejected at the printhead, can be replaced.

In a further preferred embodiment, a shut-off valve is arranged in the flow line and / or in the return line. These shut-off valves are open during normal operation to allow flow of the ink to and from the printhead.

By shutting off the connection to the return tank, the meniscus pressure rises to the value of the pressure at the input of the printhead. This overpressure in the printhead ejects ink through all of the printhead's nozzle openings. In this way, any blocked nozzle openings can be flushed free, making it unnecessary to remove or change the print head.

By shutting off the connection to the supply tank, the meniscus pressure drops to the value of the pressure at the output of the print head. Due to the negative pressure in the printhead, air is sucked in through all nozzle openings of the printhead. In this way, possible impurities in the print head, which are too large to be flushed out through the nozzle openings, can be removed from the print head.

In a further preferred embodiment, a bypass line is arranged as a bypass around the print head. This allows the ink to flow past the print head from the supply tank into the return tank. At least one shut-off valve is provided in this bypass line in order to prevent the flow through the bypass line. This shut-off valve is closed during normal operation. The bypass line can be attached in such a way that the connection to the flow in the flow direction is upstream of a possible shut-off valve in the flow. In the return, the connection in the direction of flow can be in the return after a possible shut-off valve. This allows the ink to flow unhindered through the bypass when the shut-off valves are closed in the flow and return and when the shut-off valve in the bypass is open.

If, for example, a print head is to be changed, the shut-off valve in the flow can be closed in a first step, whereby the print head is sucked empty by the return flow. The shut-off valve in the return can then be closed and the shut-off valve in the bypass line opened. This means that the printhead can be removed and, if necessary, replaced by a new printhead without interrupting the flow of ink for a long time.

In a further preferred embodiment, a heating device is provided in or on the supply tank and / or return tank. This makes it easy to keep the ink medium at the temperature suitable for printing. Due to the direct heating of the tanks with their large surfaces, a heat exchanger arranged in the ink circuit can be dispensed with and the system in the circuit including all lines can still be heated.

In a further preferred embodiment, all parts can be accommodated in a single housing. In particular, this makes it easier to replace the ink supply device.

The invention also relates to a method for supplying ink. In this case, an ink medium is conveyed from a supply tank to a print head and from the print head to a return tank. The conveyance of the ink is brought about by a pressure in a gas volume of the supply tank and / or a pressure in a gas volume of the return tank. For example, the ink medium can flow from the supply tank through the print head to the return tank through a floor drain or a drain mounted close to the floor.

The ink medium can then be pumped from the return tank back into the supply tank to close the ink circuit. Because the movement of the ink medium is not effected by a direct drive of the ink medium by means of a pump, but rather by the gas pressures, pressure peaks can be successfully avoided. The circulating amounts are very small and very uniform at around 150 ml / h per print head. There is therefore only a very slight fluctuation in the fill level in the supply tank.

Particularly preferably, the pressure in the gas volume of the supply tank and / or return tank is set in such a way that the delivery rate, i.e. the flow through the print head, results from the differential pressure and a meniscus pressure is maintained in the print head. The setting of the pressures can be taken over by a control, which for this purpose can fall back on measured values from one or more sensors. For example, the gas pressures in the supply tank and return tank or the pressures of the ink medium at the inlet and outlet of the print head can be measured. The meniscus pressure can be determined from these values or also directly in the print head be measured. The meniscus pressure should typically be kept at a low negative pressure of around -10 mbar.

The level in the supply tank, that is to say the height of the ink liquid level in the supply tank, is preferably kept approximately constant during the supply of ink. For this purpose, measured values of a sensor can be compared with setpoint values and, if necessary, ink medium can be tracked from the return tank by the circulation pump. Due to the constant level of the ink liquid level, the gas pressure in the supply tank rarely needs to be readjusted.

The level in the return tank, that is to say the height of the ink liquid level, is preferably also kept in a predetermined range. Since the exact level in the return tank is not critical, refilling can only be initiated if the fill level falls below a minimum. For this purpose, measured values from a sensor can be compared with setpoint values and, if necessary, the ink medium can be replaced by refilling the return tank. This is necessary because the amount of ink returned to the return tank is less than the amount of ink medium used than the ink medium discharged from the return tank into the supply tank. The control can record all consumption values and report them to a higher-level control.

When the ink is refilled, the gas pressure in the return tank would rise and thus affect the printing process. Therefore, gas in the gas volume of the return tank is pumped out at the same time in order to keep the pressure more or less constant. However, since the ink consumption of a print head is very low at a few ml / h, the refill quantity also requires only a slight readjustment of the gas pressure.

Both tanks can therefore be kept small in order to keep the response time of the control short. A volume between 50 and 350 ml per tank is used.

The various control processes in the system are well decoupled from one another and can therefore be easily mastered.

At least one of the gas pressures can preferably be regulated as a function of the installation height of the print head. The difference in height between the ink liquid level in the supply tank and the print head can have an influence on the actual pressure at the input of the print head. The higher the print head is arranged above the supply tank, the greater the gas pressure in the supply tank must be selected in order to achieve the necessary pressure at the input of the print head. In the same way, the difference in height between the return tank and the print head can have an influence on the actual pressure at the output of the print head. The higher the print head is arranged above the return tank, the greater the gas pressure in the return tank must be selected in order to achieve the necessary pressure at the outlet of the pressure head. The absolute height above sea level must also be taken into account, as the external pressure already 100 m above sea level is 12 mbar lower than at sea level. The meniscus pressure must therefore be adapted in such a way that there is a slight negative pressure of -10 mbar with respect to the external pressure in order to prevent the ink medium from leaking through the nozzles.

At least one of the gas pressures is preferably adapted dynamically to different operating states or operating parameters. In the case of rotating digital printing machines, operating parameters include, for example, the rotational speed of the printing machine, the position of the print head on the printing machine and / or the alignment of the print head with respect to the axis of rotation and / or the direction of gravity in question. The dynamic adaptation of the gas pressures to these operating parameters allows new dynamic machine systems. Previous systems are static, stationary or move with uniform movement and thus with uniform forces on the ink medium in the system. However, there are new embodiments of digital printing machines that operate dynamically, in which, for example, the print heads rotate in a carousel. In such rotating systems, the ink medium in the print heads is subject to centrifugal force, depending on the distance to the rotation point and the speed. Up to now, such machines have been operated at a constant speed, since their ink supply has not permitted the ink pressure to be regulated. By adapting the gas pressures according to the invention to the speed or other operating conditions, these can now be varied dynamically. This can be carried out in a particularly simple manner, since the system has a certain inertia due to the gas volumes. Due to the compressible gas buffer in the gas volumes, there can be no strong fluctuations in the pressure regulation. This avoids that the negative pressure becomes too great and so the printheads suck in air or that the overpressure becomes too high and ink leaks out of the printhead. The control speed can therefore easily be adapted to a speed control of carousel machines. This allows the machine to be integrated into a bottle filling system, for example, and to operate it in accordance with the number of available bottles in such a way that a "stop-and-go" operation is avoided.

The invention is explained in more detail below using exemplary embodiments and with reference to the figures.

• Fig. 1 schematisch eine erfindungsgemäße Vorrichtung zur Tintenversorgung,
• Fig. 2 schematisch einen Ausschnitt aus einer erfindungsgemäßen Tintenversorgung.

Show it:

• Fig. 1 schematically an inventive device for supplying ink,
• Fig. 2 schematically a section from an ink supply according to the invention.

Fig. 1 shows an inventive device for supplying ink in digital printing. This consists of a preferably vertically arranged print head 1, which is connected at its inlet via a flow line 5 to a flow tank 2 and at its outlet is connected to a return tank 3 via a return line 6. A circulating pump 4 is arranged between the return tank 3 and the supply tank 2 and can convey ink from the return tank 3 into the supply tank 2. The return tank 3 is connected to a refill pump 10, which can pump ink from an ink reservoir 11 into the return tank 3. A heating element 7 for heating the ink medium is arranged in both the supply tank 2 and the return tank 3.

Flow tank 2 and return tank 3 are not completely filled with ink, but have a volume 20, 21 filled with gas above the ink liquid level 18, 19. This gas can be sucked off in the return tank 3 by a suction device 12, whereby the gas volume 20 of the return tank 3 a negative pressure is generated. A membrane pump can be provided as the suction device. To reduce the negative pressure, that is, to bring the pressure closer to the external pressure, the return tank 3 has a ventilation valve 13. The supply tank 2 has a compressor 14, with which gas can be conveyed into the gas volume 21 of the supply tank 2, creating an overpressure in it becomes. To reduce the overpressure, the supply tank 2 has a vent valve 15. The gas pressures 20, 21, together with other physical influences, such as the height of the ink liquid level, the installation height of the printhead and the flow resistance, an overpressure at the inlet of the printhead 1 and a negative pressure at the output of the printhead 1. The pressures can be measured by an overpressure sensor 16 in the flow and a negative pressure sensor 17 in the return directly at the input or output of the print head 1. To determine the fill level of the supply tank 2 and return tank 3, they each have a fill level sensor 8, 9.

A controller 22 measures the pressures of the overpressure sensor 16 and of the underpressure sensor 17 and compares them with setpoint values. The setpoint values for the absolute pressures at the inlet and outlet of the print head 1 can result from setpoint values for the pressure difference and the meniscus pressure. If the inlet pressure is too low, the gas pressure in the flow must be increased. For this purpose, the compressor 14 is activated by the controller 22 and gas is thus conveyed into the supply tank 2. If, on the other hand, the pressure at the inlet of the print head 1 is too high, the control 22 opens the ventilation valve 15 so that gas is released from the supply tank 2. In the same way, the pressure at the output of the print head 1 is regulated by the pressure regulating devices 12, 13 in the return tank 3. If the pressure at the output of the print head 1, that is to say at the vacuum sensor 17, is too high, the pressure in the return tank 3 must be reduced. For this purpose, the suction device is switched on by the controller 22 and gas is pumped out of the return tank 3 in this way. If, on the other hand, the pressure is too low, the controller 22 can open the ventilation valve 13 and thus let gas into the return tank 3. Instead of switching the compressor and the suction device on and off, it can be operated with adapted power as required. The controller 22 then regulates the power up or down as required.

If the pressures are correctly set by the controller 22, the overpressure in the supply tank 2 transports the ink through the supply line 5 to the inlet of the print head 1. The controller 22 also measures the ink liquid level 19 in the supply tank by the level sensor 9 and keeps it by regulating the circulation pump 4 between return tank 3 and supply tank 2 at an almost constant level.

Due to the pressure difference between the inlet and outlet of the printhead 1, the ink medium is conveyed through the printhead 1. The part of the ink which was not ejected by the print head 1 is transported back to the return tank 3 through the return line 6. Since less ink is returned to the return tank 3 as a result of the consumption of the ink than is pumped into the supply tank 2 by the circulation pump 4, the ink liquid level 18 in the return tank 3 drops, whereby the pressure of the gas volume 20 is also reduced. This is therefore readjusted by the ventilation valve 13 and the compressor 12. The controller 22 also determines the height of the ink fluid clearance 18 through the fill level sensor 8 and, if necessary, regulates the refill pump 10 in order to refill ink from the ink reservoir 11 into the return tank 3 in order to prevent the return tank 3 from emptying.

Fig. 2 shows a detail from an embodiment of a device according to the invention for supplying ink, in which a multi-way valve system is installed around the print head 1. This consists of a shut-off valve 22 in the flow line 5, a shut-off valve 23 in the return line 6 and a bypass line 24 with a bypass valve 25.

In normal operation, the shut-off valves 22, 23 are open and the bypass valve 25 is closed.

If the shut-off valve 23 is closed, the meniscus pressure in the printhead 1 rises, whereby an ink flow from all nozzle openings of the printhead 1 arises. This increased flow of ink loosens blockages in the nozzles and removes impurities from the print head 1.

If the shut-off valve 22 is closed and the shut-off valve 23 remains open, the meniscus pressure in the print head 1 drops to the pressure at the outlet of the print head 1. This leads to the printhead 1 being sucked empty and air from the environment being sucked in through the nozzles.

If the bypass valve 25 is opened, the ink flows past the print head 1 through the bypass line 24. When the shut-off valves 22, 23 are closed, the print head 1 can be removed for a print head change, for example, without interrupting the flow of ink from the supply tank 2 to the return tank 3.

Reference character table

1

Printhead

2

Feed tank

3

Return tank

4th

Circulation pump

5

Supply line

6th

Return line

7th

heater

8th

Fill level sensor return tank

9

Fill level sensor supply tank

10

Refill pump

11

Ink reservoir

12th

Suction device

13th

Ventilation valve

14th

compressor

15th

Vent valve

16

Overpressure sensor

17th

Vacuum sensor

18th

Ink liquid level return tank

19th

Ink liquid level supply tank

20th

Gas space return tank

21

Gas space supply tank

22nd

Shut-off valve flow

23

Return shut-off valve

24

Bypass line

25th

Stop valve bypass line

26th

control

Claims (15)

1. Device for ink supply in digital printing, having at least one print head (1), a supply tank (2) for an ink medium, the supply tank (2) being connected via a supply line (5) to the inlet of the at least one print head (1), and a return tank (3) for the ink medium, the return tank (3) being connected to the outlet of the at least one print head (1) via a return line (6), a gas volume (20, 21) being formed in the supply tank (2) and/or in the return tank (3), and in that the ink medium can be conveyed through the print head (1) by the pressure of the gas volume (20, 21) in the supply tank (2) and/or in the return tank (3)
   characterized in that a volume of the supply tank (2) and a volume of the return tank (3) are each between 50 ml and 350 ml,
   in that an overpressure sensor (16) for measuring an overpressure at the inlet of the print head (1) is provided in the feed line (5), and
   in that a vacuum sensor (17) for measuring a vacuum at the outlet of the print head (1) is provided in the return line (6).
2. Device for ink supply in digital printing according to claim 1, characterized in that the supply tank (2) and/or the return tank (3) has a pressure regulating device (12, 13, 14, 15) with which the gas pressure in the supply tank (2) and/or return tank (3) can be regulated.
3. Device for ink supply in digital printing according to claim 1 or 2, characterized by a control (22) which is configured to regulate the pressure in the gas volume (20, 21) of the supply tank (2) and/or return tank (3) in such a way that the pressure difference between the inlet and outlet of the print head (1) and/or the meniscus pressure of the print head (1) correspond to a target value.
4. Device for ink supply in digital printing according to one of the preceding claims, characterized in that a pump (4) is arranged between the supply tank (2) and the return tank (3).
5. Device for ink supply in digital printing according to one of the preceding claims, characterized in that the supply tank (2) and/or the return tank (3) have a filling level sensor (8, 9).
6. Device for ink supply in digital printing according to one of the preceding claims, characterized in that the return tank (3) is connected to a refill pump (10).
7. Device for ink supply in digital printing according to one of the preceding claims, characterized in that a shut-off valve (22, 23) is arranged in the supply line (5) and/or the return line (6).
8. Device for ink supply in digital printing according to one of the preceding claims, characterized in that a bypass line (24) with a shut-off valve (25) is arranged around the print head (1).
9. Device for ink supply in digital printing according to one of the preceding claims, characterized in that the supply tank (2) and/or the return tank (3) have a heating device (7).
10. Method for ink supply in digital printing, in which an ink medium is conveyed from a supply tank (2) to a print head (1) and at least partially from the print head (1) to a return tank (3), the supply tank (2) being connected via a supply line (5) to an inlet of the print head (1) and the return tank (3) being connected via a return line (6) to an outlet of the print head (1), the conveying of the ink medium being effected by a pressure in a gas volume (21) of the supply tank (2) and/or a pressure in a gas volume (20) of the return tank (3),
    characterized in that a volume of the supply tank (2) and a volume of the return tank (3) are each between 50 ml and 350 ml,
    in that an overpressure is measured at the inlet of the print head (1) by means of an overpressure sensor (16) in a feed line (5), and
    in that a negative pressure at the outlet of the print head (1) is measured by means of a negative pressure sensor (17) in the return line (6).
11. Method for ink supply in digital printing according to claim 10, characterized in that the pressure in the gas volume (20, 21) of the forward flow tank (2) and/or return flow tank (3) is set in such a way that a delivery rate results from the differential pressure between the inlet and outlet of the print head (1) and/or a meniscus pressure is maintained in the print head (1).
12. Method for ink supply in digital printing according to one of claims 10 or 11, characterized in that the level (19) of the ink medium in the supply tank (2) is detected and controlled by a sensor (9).
13. Method for supplying ink in digital printing according to one of claims 10 to 12, characterized in that the ejected ink medium is replaced by refilling the return tank (3).
14. Method for ink supply in digital printing according to one of claims 10 to 13, characterized in that the pressure in the gas volume (20, 21) of the supply tank (2) and/or of the return tank (3) is controlled as a function of the installation height of the print head (3).
15. Method for ink supply in digital printing according to one of claims 10 to 14, characterized in that the pressure in the gas volume (20, 21) of the supply tank (2) and/or of the return tank (3) is controlled as a function of an operating parameter.

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