Method for adjusting ink temperature in ink printing process

Abstract

The invention relates to a method for regulating the temperature of ink in an ink printing process, in which ink (5) is pumped through an inlet line (21) and a return line (22) of an ink dispenser (20) and between them through a printing head (2) of the ink (5) supplied in parallel, in which an actual temperature value is compared with a predetermined target temperature value, the method is characterized in that a flow rate sensor (32) is operated between the input line (21) and the return line (22) and in parallel with the printing head (2) and measures the flow rate of the ink (5) or generates a flow rate-dependent measurement signal, and the flow rate sensor (32) is operated between the input line (21) and the return line (22) and in parallel with the printing head (2). Converting the flow rate or the measurement signal into a temperature auxiliary value by means of a respective predetermined characteristic curve; the auxiliary temperature value is used as an actual temperature value or as a target temperature value for adjustment.

Description

Technical Field

[0001] The present invention relates to a method for regulating the temperature of ink during ink printing.

[0002] The invention belongs to the field of graphic arts industry technology, in particular to the field of industrial, i.e. high-volume, inkjet printing on flat substrates, i.e. the image-based application of very fine liquid ink droplets on sheets, rolls, films or labels, preferably on substrates made of paper, cardboard, paperboard, plastic, metal or composite materials. The invention particularly relates to the subfield of circulating the liquid ink to the print head ejecting the ink droplets, thereby regulating the temperature of the liquid ink. Background Art

[0003] In industrial inkjet printing, it is known to provide a device for circulating ink supply to a plurality of inkjet print heads connected in parallel, i.e., an ink circuit. In addition to an ink storage container, such a device also includes an ink distributor (also called a "manifold") having an input line to the print head and a return line to the storage container and at least one pump for conveying ink through the line.

[0004] Known in industrial ink printing is to continuously measure the fluid dynamic pressure of the ink in the ink dispenser and keep it at a predetermined setpoint pressure by a regulating system. For this purpose, at least one pressure sensor is provided in the ink dispenser.

[0005] Furthermore, it is known in industrial inkjet printing to temperature-control, in particular to heat, the circulating ink during the printing process, i.e. to continuously measure the temperature and to keep it at a predetermined setpoint temperature by means of a control system. This is done primarily in order to set the ink viscosity required for trouble-free printing. For this purpose, at least one temperature sensor is provided in the ink dispenser.

[0006] DE102022109615A1 discloses a method for continuously determining and changing the viscosity of inkjet inks. Here, the ink temperature is used as a manipulated variable for regulating the pump speed in the ink circuit.

[0007] EP 3 741 571 A1 discloses a method for monitoring and adjusting the viscosity of ink during operation of a so-called “continuous inkjet” printer.

[0008] DE 69 213 542 T2 discloses a device for regulating the temperature of an inkjet print head or a thermal print head by means of a closed regulating loop.

[0009] WO2023285291A1 discloses a regulation system which takes the flow rate of the supply pump into account. However, this can cause problems if the rotation speed of the supply pump is too inaccurate.

[0010] In inkjet printers currently on the market, such as the “Gallus Labelfire” or “Gallus One” from Gallus Ferd. Rüesch AG, Switzerland, the temperature required for undisturbed droplet formation is regulated by directly measuring two ink temperatures at different locations in the ink dispenser.

[0011] The disadvantage of regulating the ink temperature using only the ink temperature measured at different locations as input variable is that it cannot detect changes in the ink rheological properties (e.g. due to production fluctuations between manufacturer batches or premature aging of the ink) and therefore cannot correct for them. However, the rheological properties of the ink play a decisive role in the interference-free formation of droplets.

[0012] Measuring the temperature of the ink is therefore only an auxiliary means for regulating the ink viscosity, and this can lead to two practical disadvantages: since changes in the ink cannot be detected, preventive maintenance or warnings to the machine operator are not possible; and as soon as the ink viscosity changes, droplet formation is no longer smooth and production cannot continue.

[0013] The viscosity of the ink can be measured directly with a viscometer. However, within the framework of a feasibility study it was shown that installing a commercially available viscometer and mounting it on an inkjet printer would result in economically unacceptable additional costs.

[0014] Measuring the flow of ink has also been disclosed elsewhere. EP 3 222 429 A1 discloses a component or assembly associated with a "continuous ink jet" printer for measuring the flow rate of ink fed into an inkjet print head. Summary of the invention

[0015] It is therefore an object of the present invention to improve the prior art and in particular to enable permanent, trouble-free inkjet printing, in particular when the viscosity of the ink changes.

[0016] According to the present invention, the above-mentioned object is solved by a preferred technical solution of the present invention.

[0017] Advantageous and therefore preferred embodiments of the invention emerge from the alternative embodiments as well as from the description and the drawings.

[0018] The present invention proposes a method for regulating the temperature of ink during an ink printing process, wherein the ink is pumped through an inlet line and a return line of an ink distributor and between them by a print head to which the ink is supplied in parallel, wherein an actual temperature value is compared with a predetermined temperature setpoint value, and a regulating variable for a setpoint value of an ink heating device is calculated based on a setpoint / actual deviation, characterized in that a flow sensor is operated between the inlet line and the return line and in parallel with the print head and measures the flow of ink or generates a measurement signal related to the flow; the flow or the measurement signal is converted into a temperature auxiliary value via respectively predetermined characteristic curves; and the temperature auxiliary value is used as the actual temperature value or as the temperature setpoint value for regulation.

[0019] Advantageous design and effects of the present invention

[0020] The invention advantageously enables permanent, interruption-free ink printing, in particular in the event of changes in the viscosity of the ink.

[0021] The present invention utilizes the measurement of the ink flow or a measurement signal related to the flow. When the viscosity of the ink changes, the flow will also change. Therefore, the change in viscosity can be identified by this measurement. The present invention also converts the measurement result (measurement value or measurement signal) into an auxiliary value using a predetermined characteristic curve. This auxiliary value is called a "temperature auxiliary value" because it does not measure the temperature, but is used to adjust the ink temperature according to the present invention. Finally, the present invention uses this auxiliary value as an actual value or a rated value for regulation. Therefore, the present invention includes two variants. The temperature of the ink is changed by the adjustment, thereby changing the viscosity of the ink. Each of these two variants can achieve the expected change in the viscosity of the ink by changing the ink temperature, thereby keeping the ink viscosity within a (preferably predetermined and preferably narrow) range, within which the printed product can be produced undisturbed. In the preferred variant, the temperature auxiliary value is used as the temperature actual value.

[0022] According to the invention, the optimum conditions for droplet formation when the ink is ejected from the nozzle are adjusted by using the ink temperature calculated from the ink flow measured at at least one position instead of at least one ink temperature previously measured in the prior art. The calculation is performed by means of a characteristic curve, which is preferably determined by assigning the measured flow rate to the measured temperature at several interpolation points above and below the desired rated temperature. It should preferably be noted here that the ink filling used for calibration should have the correct viscosity. This is preferably ensured by measuring the viscosity of an ink sample close to the rated temperature. In addition, it should preferably be noted that during calibration the structural arrangement of all components and the thermal insulation of the flow sensor match the configuration after the machine is installed. It is also possible to use a flow-related measurement signal instead of the flow itself (flow value).

[0023] The invention has the following advantages: since changes in ink viscosity can be detected, preventive maintenance can be carried out or warnings can be issued to the machine operator; even if changes in ink viscosity at the rated temperature according to the ink specifications result in drop formation no longer running without interference, production can still continue, because the control system is preferably able to adjust the ink temperature that deviates from the rated temperature of the specified ink so as to achieve the viscosity required for proper drop formation.

[0024] Furthermore, in a variant of the invention in which the temperature auxiliary value is used as the temperature rated value for regulation, the ink temperature measured at the ink dispenser can continue to be incorporated into the regulation unchanged. Depending on the current pulse rate of the flow sensor (DFS), etc., the temperature rated value at the dispenser will change periodically compared to the rated value predetermined at the beginning (applicable to constant ink viscosity). To this end, the temperature rated value determined according to the pulse rate and the characteristic curve of the flow sensor is subtracted from the previously fixedly predetermined rated value, and the difference is then added to the previously predetermined rated value. The newly calculated rated value will replace the previously predetermined rated value. In the next cycle, the previous rated value will be overwritten. Two examples a and b are given for this purpose:

[0025] a) Rated value: 30°C; Temperature determined based on DFS signal and characteristic curve: 29°C; Difference: 30-29 = +1°C; New rated value: 30+1 = 31°C

[0026] b) Rated value: 30°C; Temperature determined based on DFS signal and characteristic curve: 31°C; Difference: 30-31 = -1°C; New rated value: 30-1 = 29°C

[0027] In order to keep the temperature difference between the ink temperature in the manifold and the ink temperature in the flow sensor as constant as possible even in the event of external disturbances (such as changes in environmental conditions) and thereby to make the corresponding deviation from the characteristic curve as small as possible, the flow sensor is preferably installed as close to the manifold as possible; and its housing and its pipelines are provided with the best possible thermal insulation.

[0028] In both variants of the invention (depending on whether the temperature auxiliary value is used as the temperature actual value or the temperature setpoint value for regulation), it is advantageous that: the two pressures ("manifold input / output"), i.e. their values ​​or their difference, are preferably continuously regulated (in particular via the pump speed); this regulation preferably corresponds to that during a previously performed calibration, i.e. using the same configuration, or the calibration is preferably only carried out in the installed machine.

[0029] The following example illustrates the functioning of the present invention when ink viscosity changes due to ink aging. Assuming that the viscosity of the ink has changed compared to the ink specification, for example, due to premature aging, the ink flow rate determined at the flow sensor has decreased by x%, then the following applies:

[0030] In the practice hitherto according to the prior art, the regulating system would continue to maintain the ink temperature required by the ink specification, thereby setting the viscosity of the ink to be printed too high when the droplets are formed.

[0031] - In the process of the invention, the ink flow rate determined at the flow sensor (reduced by x%) is converted into a correspondingly reduced ink temperature, which deviates downward from the setpoint temperature, according to the stored characteristic curve. This calculated (auxiliary) temperature is now incorporated into the control system as the actual temperature of the ink. This causes the control system to intervene and increase the actual temperature of the ink. As a result, the viscosity of the ink is reduced or the ink flow through the flow sensor is increased until the calculated temperature matches the setpoint temperature. This makes it possible to detect that the ink has changed and to ensure that droplet formation continues to function normally, because the viscosity has been correctly adjusted again by increasing the ink temperature.

[0032] For example, if it is discovered through remote maintenance of the printer that the temperature at the ink dispenser and the sensor signal of the flow meter in the machine no longer correspond to the stored characteristic curve, this can have two main different reasons: 1. The viscosity of the ink has changed; 2. The flow meter has aged. To determine the actual cause, an ink sample can be taken, its viscosity measured and compared with the ink specification. If the viscosity of the ink sample is correct, it is most likely that the flow sensor has aged. In this case, either the flow meter is replaced or (if it is not directly available) the aged flow meter is recalibrated (as a temporary solution) and the corresponding characteristic curve is replaced in the software of the regulation system. In this way, the production capacity of the printer can be maintained at least in the short term.

[0033] If, due to external influences (room climate, ambient conditions), the difference between the ink temperature at the flow sensor and at the manifold fluctuates too much to maintain the required tolerance for regulating the ink temperature, it is preferred to measure / calibrate the temperature near the flow sensor itself. For this purpose, it is preferred to install an existing temperature sensor or an additional temperature sensor in the immediate vicinity of the flow sensor (e.g. in the manifold of the flow sensor branch or in the branch itself directly before or after the flow sensor), or to use a flow sensor with an integrated temperature sensor. If the existing temperature sensor is retained in its existing position and the ink temperature is additionally measured near the flow sensor, deviations of the above-mentioned difference due to external disturbances can be corrected by using the additional data thus obtained.

[0034] Measuring the ink temperature in the manifold in the middle of the input line between the printhead branches has the advantage, compared to an arrangement directly at the branch of the flow sensor, that the average deviation of the ink temperature within the individual printheads is smaller. If a temperature measurement close to the flowmeter is required due to external influences, the advantages of the embodiment mentioned in 3.2. can be maintained by additionally performing a T measurement close to the flowmeter. However, this requires an additional T sensor (which can be at the branch or in the branch of the flowmeter or can already be integrated in the flowmeter).

[0035] Expansion scheme of the present invention

[0036] The following describes preferred developments of the present invention (developments for short). These developments can also be combined with one another if this is not technically impossible.

[0037] One development can be characterized in that the flow rate of the ink is determined as a measured value. One development can be characterized in that the measured value is determined in milliliters per minute or in liters per minute.

[0038] One development can be characterized in that the flow-dependent measurement signal is one of the following measurement signals: pulse rate, pulse interval or analog voltage.

[0039] One development can be characterized in that the flow sensor is designed as a paddle wheel sensor.

[0040] An extension may be characterized in that the characteristic curve is a characteristic curve between ink temperature measurement values ​​and ink flow rate, or a measurement signal related to the flow rate. An extension may be characterized in that the characteristic curve is determined by using ink of known viscosity. An extension may be characterized in that the characteristic curve is determined at a plurality of interpolation points above and below the temperature rating. An extension may be characterized in that a formula for determining the characteristic curve is provided to a device for regulating the temperature of the ink.

[0041] An extended solution may be characterized in that the pressure difference of the ink between the input and output of the flow sensor is kept constant. An extended solution may be characterized in that the pressure of the ink is generated by at least one pump and the pump is operated in a controlled manner. An extended solution may be characterized in that the speed of the pump is controllable. An extended solution may be characterized in that the pressure of the ink is regulated using at least one pressure sensor.

[0042] The features and feature combinations disclosed in the above technical field, summary and development sections as well as in the following exemplary embodiments section (in any combination with one another) represent further advantageous developments of the invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] Figure 1 and Figure 2 Preferred exemplary embodiments of the invention and further developments are shown. DETAILED DESCRIPTION

[0044] Figure 1 A schematic diagram shows a preferred apparatus for carrying out a preferred embodiment of the method of the present invention.

[0045] An industrially available inkjet printer 1 (only briefly shown) is used, for example, for printing sheets, webs or labels and comprises a plurality of print heads 2, each of which has a plurality of nozzles 3 for generating droplets 4 of liquid ink 5 according to an image. During the production process, the print heads 2 are preferably stationary and arranged as so-called print beams transversely to the transport direction of the substrate. The printer 1 also comprises a (digital) computer 40; this computer can control the printing process and can be used to adjust the ink temperature according to the invention.

[0046] The printer 1 comprises an ink circuit or a device 10 for cyclically supplying ink to the print heads 2, which has a storage container 11, a controllable ink heating device 12 and at least one controllable pump 13 for ink 5 (whose rotation speed or generated volume flow is controllable). The device 10 supplies the print heads 2 with ink 5 in parallel and continuously. For this purpose, an ink distributor 20 is provided, which has a supply line 21 and a return line 22 (a hose or preferably a pipe). The print heads 2 are arranged in parallel with each other between the lines 21 and 22. The ink distributor 20 is connected to the storage container 11 via a line 23 that closes the ink circuit.

[0047] According to the invention, the temperature of the ink 5 in the ink circuit and the temperature of the ink in the print head 2 is to be regulated, wherein the viscosity of the ink 5 required for the undisturbed generation of droplets can be regulated by the temperature. For the corresponding temperature control of the ink 5, an electric ink heating device 12 is preferably provided. The measured values ​​for the temperature regulation are preferably provided by at least one temperature sensor 30; the temperature sensor or its receptor is preferably arranged in the feed line 21. The corresponding temperature sensor 30 is connected to a computer 40 via a connection device 33a (only briefly shown); its measurement results can therefore be provided to the computer 40 for processing. The computer is connected to a device 41 for regulating the temperature of the ink; the device is in turn connected to the ink heating device 12 via a connection device 33b. The temperature sensor 30 is preferably arranged near the branch of the feed line 21 and the flow sensor 32; or approximately in the middle between the branch of the feed line 21 and the print head 3. Another temperature sensor (not shown here) can (as is known) be arranged near the heating device 12 and measure the temperature of the ink just heated by the heating device.

[0048] According to the invention, the regulation of temperature and viscosity is preferably carried out under the condition that the ink pressure in the ink dispenser 20 is substantially constant. Therefore, it is preferred to arrange pressure sensors 31 or their receptors in the input line 21 and the return line 22, respectively. Each pressure sensor 31 is connected to the computer 40 via a connection device 33a (only briefly shown); its measurement results can therefore be provided to the computer 40 for processing. The measurement results can be used to control the pump 13 via the connection device 33c, so that the required fluid pressure of the ink 5 is maintained at a constant level. Each pressure sensor 31 can measure the fluid dynamic pressure in the input line 21 and the return line 22, for example Figure 1 The measurement locations are shown in .

[0049] According to the invention, the device 10 comprises a flow sensor 32, for example a vane sensor, as a flow meter. It runs between the input line 21 and the return line 22 (in the "bypass") and is connected in parallel with the print head 5 and measures the flow velocity or flow rate or flow rate of the ink (for example, volume flow rate in ml / min or liter / min) or generates a flow-related measurement signal (for example, pulse rate or pulse interval or analog voltage). The flow sensor 32 is connected to the computer 40 via a connection device 33a (only briefly shown); its measurement results or measurement signals can therefore be provided to the computer 40 for processing.

[0050] The computer 40 or device 41 performs the following adjustment steps of the present invention: comparing the actual temperature value with the predetermined temperature rated value, and calculating the adjustment variable for the rated value of the ink heating device 12 based on the rated / actual deviation; converting the flow or measurement signal into a temperature auxiliary value through a characteristic curve that is respectively predetermined (and preferably stored in the computer 40 or device 41); and using the temperature auxiliary value as the actual temperature value or the temperature rated value for adjustment.

[0051] Figure 2 A flow chart showing a preferred embodiment of the method of the present invention has the following steps:

[0052] Step 50 : Predetermining the temperature setpoint of the present invention, preferably for a predetermined measurement location in the ink dispenser 20 .

[0053] Step 51: (indirect) measurement of the actual temperature value according to the invention, preferably at a predetermined measuring point in the ink dispenser 20. The indirect measurement is carried out by measuring the flow according to the invention and converting it according to a characteristic curve predetermined according to the invention (alternatively: by a measurement signal that is dependent on the flow).

[0054] Step 52: The computer 40 calculates another temperature setpoint value (which is different from the temperature setpoint value of the present invention) from the two values ​​in steps 50 and 51, preferably for a measurement location at the output of the ink heating device 12. To this end, the computer 40 will access a previously created and stored algorithm, which may be based on another characteristic curve (which is different from the characteristic curve of the present invention) if necessary. Since the measurement location in the ink dispenser and the measurement location at the ink heating device are spaced apart from each other, the algorithm preferably takes into account the temperature difference between these two locations.

[0055] Step 53 : The further temperature setpoint value calculated in step 52 is provided to the device 41 .

[0056] Step 54 : The device 41 regulates the ink heating device 12 or controls its (eg internal) control unit based on the further temperature setpoint value.

[0057] Step 55: Start the control unit of the ink heating device.

[0058] Step 56 : activating the ink heating device 12 so that it heats the ink 5 to the other desired temperature value.

[0059] Step 57: Measure another actual value of the temperature of the ink (for example with another temperature sensor), preferably at a measuring location at the output of the ink heating device 12. This value is provided to the device 41 for carrying out the regulation step 54.

[0060] If, according to the invention, the temperature auxiliary value converted by means of a predetermined characteristic curve is used as a temperature setpoint value for control purposes, this temperature auxiliary value is used in step 50 , ie, as the temperature setpoint value here.

[0061] If, instead, according to the invention, a temperature auxiliary value converted via a predetermined characteristic curve is used as the temperature actual value for control, this temperature auxiliary value is used in step 51 , ie as the temperature actual value here.

[0062] Reference numerals list

[0063] 1 Inkjet Printer

[0064] 2 Print heads / print heads

[0065] 3 one / multiple nozzles

[0066] 4 Droplets

[0067] 5. Ink

[0068] 10 Device for supplying ink

[0069] 11 Storage Container

[0070] 12 Ink heating device

[0071] 13 Pump for ink

[0072] 20 Ink Dispenser

[0073] 21 Input pipeline

[0074] 22 Return line

[0075] 23 One / multiple pipelines

[0076] 30 One / multiple temperature sensors

[0077] 31 One / multiple pressure sensors

[0078] 32 Flow sensor

[0079] 33a, b, c Connecting device

[0080] 40 Computer

[0081] 41 Device for regulating ink temperature

[0082] 50-57 Method steps.

Claims

1. A method for regulating the temperature of ink during an ink printing process, wherein ink (5) is pumped through a feed line (21) and a return line (22) of an ink distributor (20) and between them through a print head (2) to which the ink (5) is supplied in parallel, wherein an actual temperature value is compared with a predetermined desired temperature value and a control variable for a desired value of an ink heating device (12) is calculated as a function of a desired / actual deviation, characterized in that A flow sensor (32) is operated between the input line (21) and the return line (22) and in parallel with the print head (2) and measures the flow of ink (5) or generates a measurement signal related to the flow; the flow or the measurement signal is converted into a temperature auxiliary value via a respectively predetermined characteristic curve; and the temperature auxiliary value is used as a temperature actual value or a temperature setpoint value for regulation.

2. The method according to claim 1, characterized in that The flow-related measuring signal is one of the following measuring signals: pulse rate or pulse interval or analog voltage.

3. The method according to any one of the preceding claims, characterized in that The characteristic curve is a characteristic curve between the measured temperature of the ink (5) and the flow rate of the ink (5) or a measurement signal related to the flow rate.

4. The method according to any one of the preceding claims, characterized in that The characteristic curve is determined by using an ink (5) of known viscosity.

5. The method according to any one of the preceding claims, characterized in that The pressure difference of the ink (5) between the input end and the output end of the flow sensor (32) is kept constant.

6. The method according to any one of the preceding claims, characterized in that The pressure of the ink (5) is generated by at least one pump (13) and the pump (13) is operated in a regulated manner.

7. The method according to any one of the preceding claims, characterized in that At least one pressure sensor (31) is used to adjust the pressure of the ink (5).

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