Method for controlling ink temperature during ink printing

Abstract

To enable continuous unimpaired production by inkjet printing, particularly upon changes in viscosity of the ink.SOLUTION: A method for closed-loop control of ink temperature in ink printing includes pumping ink (5) between a supply line (21) and a return line (22) through print heads (2) supplied in parallel with the ink (5), comparing a temperature actual value with a predefined temperature setpoint value and calculating a manipulated variable for a setpoint value of an ink heater (12) from a setpoint / actual value deviation. A flow sensor (32) is operated between the supply line (21) and return line (22) in parallel with the print heads (2) and measures the flow of the ink (5) or generates a measurement signal dependent on the flow. The flow or the measurement signal is converted by a predefined characteristic curve into a temperature auxiliary value used as the temperature actual value or temperature setpoint value for closed-loop control.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a method for closed-loop control of ink temperature during ink printing with the features of the preamble of claim 1.

[0002] FIELD OF THEINVENTION The invention is based on the technical field of the graphic industry and, more particularly, on the technical field of high-productivity ink printing (inkjet) on industrial, i.e. flat, substrates, i.e. the imagewise application of fine droplets of liquid ink to a sheet-, web-, sheet- or label-shaped substrate, preferably made of paper, cardboard, cardboard, plastic, metal or composite material. In particular, the invention is based on the subdivision of a circulatory supply of liquid ink to a print head which ejects the ink droplets, with a closed-loop control of the temperature of the liquid ink.

[0003] Background technology It is already known in industrial ink printing to provide an apparatus for the circulating supply of ink liquid to a number of ink print heads which are fed in parallel, i.e. an ink circuit, which includes, besides a reservoir for the ink, an ink distributor (also called a "manifold") with supply lines to the heads and return lines to the reservoir, and at least one pump for pumping the ink through these lines.

[0004] It is already known in industrial ink printing to continuously measure the hydrodynamic pressure of the ink liquid in an ink distributor and to keep it at a preset target pressure by means of a closed-loop control, the ink distributor being provided with at least one pressure sensor for this purpose.

[0005] It is also already known in industrial ink printing to regulate the temperature, in particular to heat, the ink liquid circulating during the printing operation, i.e. to continuously measure the temperature and keep it at a preset target temperature by closed-loop control. This is done, among other things, in order to set the viscosity of the ink required for trouble-free printing. For this purpose, at least one temperature sensor is provided in the ink distributor.

[0006] DE 10 2022 109 615 A1 describes a method for continuously determining and varying the viscosity of an inkjet ink, in which the ink temperature is used as the manipulated variable for closed-loop control of the pump speed in the ink circuit. EP 3 741 571 A1 describes a method for monitoring and setting the ink viscosity during operation of a so-called continuous inkjet printer.

[0007] DE 69213542 A1, a translation of a European patent, discloses an apparatus with a closed control loop circuit for closed loop control of the temperature of an ink or thermal printhead.

[0008] WO 2023285291 discloses a closed-loop control that reads the volumetric flow rate of the feed pump, but this can cause problems if the feed pump speed is too inaccurate for this.

[0009] In ink printing machines available on the market, such as the "Gallus Labelfire" or "Gallus One" from Gallus Ferd. Rueesch AG, Switzerland, the temperature required for successful droplet formation is controlled in a closed loop using the ink temperature, which is measured directly at two different points in the ink distributor.

[0010] Closed-loop control of ink temperature, using only the temperature of the ink, measured at various points, as an input quantity, can have the disadvantage that it does not detect, and therefore cannot correct, changes in the rheological properties of the ink (e.g., due to variations in production between manufacturers' lots, or due to the ink aging faster than expected), however, it is the rheological properties of the ink that are important for successful drop formation.

[0011] The measurement of the ink temperature is thus merely an auxiliary means for setting the ink viscosity, which can lead to two practical drawbacks: Proactive maintenance or warning the machine operator is not possible, since changes in the ink are not detected. As soon as the drop formation no longer functions without problems due to changes in the ink viscosity, production is no longer possible.

[0012] The viscosity of the ink can be measured directly using a viscometer. However, during feasibility studies, it was found that incorporating commercially available viscometers and integrating them into an ink printing press would result in an economically unacceptable increase in cost.

[0013] Another aspect already known is the measurement of ink flow rate: EP 3222429 A1 discloses a component or device for measuring the flow rate of ink directed to an ink printhead in the context of a "continuous ink jet" printer.

[0014] Technical challenges It is therefore an object of the present invention to provide an improvement over the state of the art which allows for a sustained, trouble-free production by ink printing, in particular when the viscosity of the ink changes.

[0015] Solution of the Problem by the Invention The above object is achieved according to the invention by a method according to claim 1. Advantageous, therefore preferred, developments of the invention follow from the dependent claims and from the following description and drawings.

[0016] A method for closed-loop control of the temperature of ink during ink printing, in which ink is pumped through a supply line and a return line of an ink distributor by a print head to which the ink is supplied in parallel between the supply line and the return line, in which the actual temperature value is compared with a preset temperature setpoint and an operating variable for the setpoint value of the ink heating unit is calculated from the setpoint-actual deviation. The method according to the invention is characterized in that a flow sensor is operated between the supply line and the return line and in parallel to the print head, which measures the ink flow rate or generates a measurement signal that is dependent on the flow rate, converts the flow rate or the measurement signal into an auxiliary temperature value via a preset characteristic curve, respectively, and uses this auxiliary temperature value for the closed-loop control as the actual temperature value or as the temperature setpoint.

[0017] Advantageous embodiments and advantages of the present invention The invention advantageously enables continuous, trouble-free production with ink printing, especially when the viscosity of the ink changes.

[0018] The invention utilizes a measured value of the ink flow rate or a measurement signal that depends on the flow rate. If the viscosity of the ink changes, the flow rate changes. Thus, the change in viscosity is identified via this measured value. The invention further utilizes a predefined characteristic curve in order to convert the result of the measurement (measured value or measurement signal) into an auxiliary value. This auxiliary value is called "temperature auxiliary value" since it is not the measured temperature but is used according to the invention for the temperature closed-loop control of the ink. The invention finally utilizes this auxiliary value as actual value or as target value for the temperature closed-loop control. Thus, the invention includes two variants. Via the closed-loop control, the temperature of the ink and thus the viscosity of said ink are changed. Each of the two variants can be used to change the ink viscosity as desired via a change in the ink temperature and thus to keep the ink viscosity within a (preferably predefined and preferably narrow) range in which a problem-free print product can be produced. In this preferred variant, the temperature auxiliary value is used as the temperature actual value.

[0019] According to the invention, at least one of the ink temperatures measured so far in the prior art is replaced by an ink temperature calculated from the ink flow rate measured at at least one location and is introduced into the closed-loop control instead of the measured temperature, thereby providing a closed-loop control of the optimal conditions for droplet formation when the ink is ejected from the nozzle. This calculation is carried out via a characteristic curve, which is preferably determined by associating the measured flow rate with temperatures measured at a number of sampling points above or below the desired target temperature. Care is preferably taken here that the ink fill used for this calibration has the correct viscosity. This can preferably be ensured by measuring the viscosity of a sample of the ink close to the target temperature. It should preferably be further taken care that the structural arrangement of all components during the calibration and the thermal insulation of the flow sensor correspond to the later configuration after the machine installation. Instead of the flow rate itself (flow rate value), a measurement signal dependent on the flow rate can also be used.

[0020] The present invention provides the following advantages: changes in ink viscosity are detected, allowing proactive maintenance or warning to the machine operator; continued production even if drop formation no longer functions without problems due to changes in ink viscosity at the target temperature according to the ink specifications, since the closed loop control adjusts the ink temperature deviating from the specified ink target temperature so that the viscosity required for proper drop formation is achieved;

[0021] In one variant of the invention, in which the auxiliary temperature value is used as the temperature setpoint in the closed-loop control, the ink temperature measured at the ink distributor can be subsequently and unchanged introduced into the closed-loop control. The setpoint of the temperature at the distributor is regularly changed relative to a setpoint preset at the start (valid if there are no changes in the viscosity of the ink) depending, for example, on the actual pulse rate of the flow sensor (DFS: Durchfluss-Sensor). For this purpose, a temperature setpoint determined from the pulse rate and the characteristic curve of the flow sensor is subtracted from the initially fixedly preset setpoint and the difference is added to the initially preset setpoint. This newly calculated setpoint replaces the initially preset setpoint. In each subsequent turn, the respective previous setpoint is overwritten. Two examples a and b are given in this regard, namely: a) Target value: 30°C; Temperature calculated from DFS signal and characteristic curve: 29°C; Difference: 30-29=+1°C; New target value: 30+1=31°C b) Target value: 30°C; Temperature calculated from DFS signal and characteristic curve: 31°C; Difference: 30-31=-1°C; New target value: 30-1=29°C There is.

[0022] In order to keep the temperature difference between the ink temperature at the manifold and the ink temperature at the flow sensor as constant as possible even in the event of disturbances (e.g. changes in ambient conditions) and thus the corresponding deviation from the characteristic curve as small as possible, the flow sensor is preferably provided with an installation position as close as possible to the manifold and the housing of the flow sensor and its lines are provided with as good insulation as possible.

[0023] In two variants of the invention, it is advantageously noted (depending on whether the temperature auxiliary value is used for the closed-loop control as temperature actual value or as temperature setpoint value): the two pressures (manifold in / out), i.e. their values ​​or their difference, are preferably constantly closed-loop controlled (in particular via the pump speed), which preferably corresponds to the closed-loop control in a previously performed calibration, i.e. in the same configuration, or preferably this calibration is performed for the first time on the installed machine.

[0024] Below is an example of the functioning of the invention in the case of a change in ink viscosity due to ink aging. For example, under the assumption that the ink viscosity changes due to faster aging than the ink specification, the ink flow rate determined at the flow sensor is reduced by x%, and the following holds: - in the previous approach according to the prior art, the closed-loop control still maintains the ink temperature required by the ink specification, and thus the viscosity of the ink to be printed is set too high at the time of drop formation; - in the approach according to the invention, the ink flow rate determined at the flow sensor (reduced by x%) is converted into a correspondingly reduced ink temperature, which deviates downwards from the target temperature, according to a stored characteristic curve; this (auxiliary) calculated temperature is then introduced into the closed-loop control as the actual temperature of the ink, which then intervenes to increase the actual temperature of the ink; this then reduces the ink viscosity or increases the ink flow rate through the flow sensor until the calculated temperature and the target temperature coincide. The ink is identified as having changed and the viscosity is again properly set by increasing the ink temperature so that droplet formation continues to occur properly.

[0025] If, for example, it is ascertained via remote maintenance of a printing press that the temperature of the ink distributor and the sensor signal of the flow meter at the machine no longer coincide with the stored characteristic curve, this can primarily have two different reasons: 1. the viscosity of the ink has changed, 2. the flow meter has deteriorated. To determine the actual cause, an ink sample can be taken, its viscosity measured and compared with the ink specifications. If the viscosity of the ink sample is correct, there is a high probability that the flow sensor has deteriorated. In this case, the flow meter is replaced or (if not directly available) the deteriorated flow meter is newly calibrated (as a temporary solution) and the corresponding characteristic curve in the closed-loop control software is replaced. This allows the production capacity of the printing press to be maintained at least for a short period of time.

[0026] In order to be able to keep the required tolerances for the ink temperature to be controlled in a closed loop, if the difference between the ink temperature at the flow sensor and the ink temperature at the manifold fluctuates too much due to external influences (indoor climate, ambient conditions), it is advantageous to measure / calibrate the temperature close to the flow sensor itself. For this, preferably, an existing or additional temperature sensor can be installed in the immediate vicinity of the flow sensor (for example in the manifold, at the branch point leading to the flow sensor or at the branch itself, immediately before or after the flow sensor), or a flow sensor with an integrated temperature sensor can be used. If the existing temperature sensor remains in its previous location and the ink temperature is additionally measured close to the flow sensor, the additional data obtained thereby can be used to correct the deviations of the above-mentioned difference caused by disturbances.

[0027] Measuring the ink temperature in the manifold, in the middle of the supply line between the branching points to the print heads, has the advantage that the average deviation from the ink temperature within the individual print heads is smaller than if it were located directly at the branching point to the flow sensor. If a temperature measurement close to the flow meter is required due to disturbances, the advantages of the embodiment listed in 3.2. can be maintained by additionally performing a T measurement close to the flow meter. However, this requires an additional T sensor (at the branching point to the flow meter, at the branch of the flow meter, or already built into the flow meter).

[0028] Developments of the invention In the following, preferred developments of the invention (developments for short) are described, which can be combined with one another, unless they are technically mutually exclusive.

[0029] A further development may be characterized by specifying the flow rate of the ink as the measured value. A further development may be characterized by specifying the measured value in ml / min or l / min. A further development may be characterized by the flow rate-dependent measurement signal being one of the following measurement signals: a pulse rate or a pulse interval or an analog voltage.

[0030] One development may be characterized in that the flow sensor is configured as an impeller sensor.

[0031] A development may be characterized in that the characteristic curve is a characteristic curve between a measured temperature of the ink and a flow rate of the ink or a measurement signal that is dependent on the flow rate. A development may be characterized by determining the characteristic curve using an ink of known viscosity. A development may be characterized by determining the characteristic curve at a number of sampling points each above or below a temperature target value. A development may be characterized by determining an equation for the characteristic curve. A development may be characterized by providing an equation for an apparatus for closed-loop control of the temperature of the ink.

[0032] One development may be characterized by keeping a constant ink pressure differential between the input and output of the flow sensor. One development may be characterized by generating the ink pressure by at least one pump and operating the pump with closed-loop control. One development may be characterized by closed-loop control of the pump speed. One development may be characterized by using at least one pressure sensor for the closed-loop control of the ink pressure.

[0033] The features and feature combinations disclosed in the above sections of the technical field, the invention and developments and in the following sections of the embodiments, in any combination with one another, are further advantageous developments of the invention.

[0034] Examples and drawings of the present invention 1 and 2 show preferred embodiments of the invention and developments. [Brief description of the drawings]

[0035] [Figure 1] 1 is a schematic diagram of a preferred apparatus for carrying out a preferred embodiment of the method according to the present invention; [Diagram 2] FIG. 1 is a flow chart of a preferred embodiment of the method according to the present invention.

[0036] FIG. 1 shows a schematic diagram of a preferred apparatus for carrying out a preferred embodiment of the method of the present invention.

[0037] An industrially usable ink printing machine 1 (shown only as a guide) for printing, for example, on sheets, webs or labels, comprises a number of print heads 2 each with a number of nozzles 3 for producing droplets 4 of liquid ink 5 as an image. The print heads 2 are preferably positionally fixed during production and arranged transversely to the substrate transport direction as so-called print bars. The machine 1 further comprises a (digital) computer 40, which can control the printing process and which can be used for closed-loop control of the ink temperature according to the invention.

[0038] The printing press 1 comprises an ink circuit or device 10 for supplying ink in a circular manner to the print heads 2, comprising a storage container 11, a controllable ink heater 12 and at least one controllable (in terms of speed or the volumetric flow rate it produces) pump 13 for the ink 5. The device 10 supplies the print heads 2 with ink 5 in parallel and continuously. For this purpose, an ink distributor 20 is provided with a supply line 21 and a return line 22 (hose line or preferably a pipe line). The print heads 2 are arranged parallel to one another between the lines 21 and 22. The ink distributor 20 is connected to the storage container 11 via a line 23 which closes the ink circuit.

[0039] According to the invention, in the ink circuit and thus in the print head 2, the temperature of the ink 5 is controlled in a closed loop, via which the viscosity of the ink 5 required for the production of uninterrupted droplets can be set. For the corresponding temperature regulation of the ink 5, a preferably electrical ink heating unit 12 is provided. Preferably at least one temperature sensor 30 provides measured values ​​for the temperature closed loop control, the temperature sensor 30 or the sensing unit of the temperature sensor 30 being preferably arranged in the supply line 21. Each temperature sensor 30 is connected via a connection line 33a (shown only as a hint) to the computer 40, so that the measurement results are provided to the computer 40 for processing. The computer is connected to a device 41 for the closed loop control of the ink temperature, which device 41 is itself connected via a connection line 33b to the ink heating unit 12. The temperature sensor 30 is preferably arranged near the branch point of the supply line 21 to the flow sensor 32, or alternatively approximately in the middle of the supply line 21 between the branch points to the print head 3. A further temperature sensor (not shown in this figure) may be located near the heating element 12, as is already known, to measure the temperature of the ink that has just been heated by said heating element.

[0040] The closed-loop control according to the invention of the temperature and thus of the viscosity is preferably carried out at a substantially constant pressure of the ink in the ink distributor 20. Therefore, preferably a pressure sensor 31 or its sensing part is arranged in each of the supply line 21 and the return line 22. Each pressure sensor 31 is connected via a connecting line 33a (shown only as a guide) to the computer 40, so that its measurement results are supplied to the computer 40 for processing. The measurement results can be used to control the pump 13 via the connecting line 33c, so that the required hydraulic pressure of the ink 5 is kept at a constant level. By means of each pressure sensor 31, the hydrodynamic pressure in the supply line 21 and the hydrodynamic pressure in the return line 22 can be measured, for example at the measuring points shown in FIG. 1.

[0041] According to the invention, the device 10 includes a flow sensor 32 as a flow measuring device, for example an impeller sensor. The flow sensor 32 is operated between the supply line 21 and the return line 22 (in the "bypass") and there in parallel to the print head, measuring the ink flow rate or flow rate or flow amount (for example volume flow rate in ml / min or l / min) or generating a flow-dependent 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 line 33a (shown only as a hint) so that its measurement results or measurement signals are supplied to the computer 40 for processing.

[0042] The computer 40 or device 41 executes the following steps of the closed-loop control according to the present invention: compare the actual temperature value with a preset temperature target value, calculate the manipulated variable for the target value of the ink heating unit 12 from the target-actual value deviation, convert the flow rate or the measurement signal into an auxiliary temperature value via a characteristic curve that is preset (and preferably stored in the computer 40 or device 41), respectively, and use this auxiliary temperature value in the closed-loop control as the actual temperature value or as the temperature target value.

[0043] FIG. 2 shows a flow chart of a preferred embodiment of the method according to the invention, which comprises the following steps: Step 50: Preferably, a temperature target value according to the present invention is pre-defined for a pre-defined measurement point in the ink distributor 20. Step 51: An (indirect) measurement of the actual temperature value according to the invention is carried out, preferably at a predefined measuring point in the ink distributor 20. The indirect measurement is carried out via the inventive measurement of the flow rate and a corresponding conversion to a predefined characteristic curve according to the invention (alternatively via a measurement signal that is dependent on the flow rate). Step 52: The calculator 40 calculates from the two values ​​of steps 50 and 51 a further temperature setpoint (which is different from the temperature setpoint according to the invention), preferably for the measuring point at the output of the ink heating section 12. For this purpose, the calculator 40 accesses a previously created and stored algorithm, possibly based on a further characteristic curve (which is different from the characteristic curve according to the invention). Since the measuring point at the ink distributor and the measuring point at the ink heating section are separated from each other, the algorithm preferably takes into account the temperature difference occurring between the two points. Step 53: The further temperature target value calculated in step 52 is provided to the device 41. Step 54: The device 41 performs a closed-loop control of the ink heating section 12 based on another temperature target value, or performs an open-loop control of its (eg, internal) control unit. Step 55: The control unit activates the control unit of the ink heating section. Step 56: Activating the ink heating unit 12, thereby causing the ink 5 to be heated by the ink heating unit 12 to another temperature target value. Step 57: A further measurement of the actual temperature of the ink is carried out (for example by means of a further temperature sensor), preferably at a measuring point at the output of the ink heater 12. This value is supplied to the device 41 for carrying out the closed-loop control step 54.

[0044] If, according to the invention, the temperature auxiliary value converted via a preset characteristic curve is used as the temperature setpoint in the closed-loop control, the temperature auxiliary value in step 50 is used, i.e., as the temperature setpoint therein.

[0045] If, instead, according to the invention, the auxiliary temperature value converted via a predefined characteristic curve is used as the actual temperature value for the closed-loop control, then the auxiliary temperature value in step 51 is used, i.e. as the actual temperature value therein. [Explanation of symbols]

[0046] 1 Ink printing machine 2 Print Head 3 Nozzles 4 droplets 5. Ink 10. Apparatus for supplying ink 11 Storage Containers 12 Ink heating section 13 Ink pump 20 Ink distributor 21 Supply pipeline 22 Return pipe 23 Conduit 30 Temperature Sensor 31 Pressure Sensor 32 Flow Sensor 33a,b,c connecting line 40 calculator 41 Apparatus for closed-loop control of ink temperature 50~57 steps

Claims

1. A method for closed-loop control of ink temperature during ink printing, in which ink (5) is pumped by a print head (2) to which ink (5) is supplied in parallel through a supply line (21) and a return line (22) of an ink distributor (20) between the supply line (21) and the return line (22), the method comprising: comparing an actual temperature value with a preset temperature target value; and calculating an operation amount for a target value of an ink heating unit (12) from a difference between the temperature target value and the actual temperature value, a flow sensor (32) is operated between the supply line (21) and the return line (22) and in parallel to the print head (2), measuring the flow rate of the ink (5) or generating a measurement signal that is dependent on the flow rate, converting said flow rate or said measurement signal into a temperature auxiliary value via a predefined characteristic curve, respectively, and using said temperature auxiliary value as the actual temperature value or as the temperature setpoint value for the closed-loop control.

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

3. 3. The method according to claim 1, wherein the characteristic curve is a characteristic curve between the measured temperature of the ink (5) and the flow rate of the ink (5) or the measurement signal which is dependent on the flow rate.

4. 3. The method according to claim 1 or 2, characterized in that the characteristic curve is determined using an ink (5) of known viscosity.

5. 3. The method according to claim 1, characterized in that the pressure differential of the ink (5) between the input and output of the flow sensor (32) is kept constant.

6. 3. Method according to claim 1 or 2, characterized in that the pressure of the ink (5) is generated by at least one pump (13), said pump (13) being operated under closed-loop control.

7. The method of claim 6, further comprising using at least one pressure sensor (31) for closed-loop control of the pressure of the ink (5).

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