METHOD FOR FEEDBACK CONTROL OF THE TEMPERATURE OF AN INK IN INKJET PRINTING

Abstract

A method for feedback control of ink temperature in inkjet printing includes pumping ink through supply and return lines of an ink distributor and between these lines through print heads supplied in parallel with the ink, comparing a temperature actual value with a predefined temperature setpoint value and calculating a manipulated variable for a setpoint value of an ink heater from a setpoint/actual value deviation. A flow sensor is operated between the supply and return lines in parallel with the print heads and measures the flow of the ink 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 feedback control. This enables continuous unimpaired production by inkjet printing, particularly upon changes in viscosity of the ink in industrial inkjet printing.

Claims

1. A method for feedback control of the temperature of an ink in inkjet printing, the ink (5) being pumped through a supply line (21) and a return line (22) of an ink distributor (20) and between these through print heads (2) supplied in parallel with the ink (5), a temperature actual value being compared with a predefined temperature setpoint value and a manipulated variable for a setpoint value of an ink heater (12) being calculated from a setpoint/actual value deviation, characterized in that a flow sensor (32) is operated between the supply line (21) and the return line (22) and in parallel with the print heads (2) and measures the flow of the ink (5) or a measurement signal dependent on the flow is generated, in that the flow or the measurement signal is converted by means of a respectively predefined characteristic curve into a temperature auxiliary value and in that this temperature auxiliary value is used as the temperature actual value or as the temperature setpoint value for the feedback control.

2. The method as claimed in claim 1, characterized in that the measurement signal dependent on the flow is one of the following measurement signals: a pulse rate or a pulse spacing or an analog voltage.

3. The method as claimed in either of the preceding claims, characterized in that the characteristic curve is a characteristic curve between a measured temperature of the ink (5) and the flow of the ink (5) or the measurement signal dependent on the flow.

4. The method as claimed in one of the preceding claims, characterized in that the characteristic curve is determined by using an ink (5) with a known viscosity.

5. The method as claimed in one of the preceding claims, characterized in that the pressure difference of the ink (5) between the entry and the exit of the flow sensor (32) is kept constant.

6. The method as claimed in one of the preceding claims, characterized in that the pressure of the ink (5) is generated by at least one pump (13) and in that the pump (13) is operated under feedback control.

7. The method as claimed in one of the preceding claims, characterized in that at least one pressure sensor (31) is used for the feedback control of the pressure of the ink (5).

Description

BRIEF DESCRIPTION OF THE FIGURES

[0044] FIGS. 1 and 2 are block diagrams showing a preferred exemplary embodiment of the invention and of developments thereof.

DETAILED DESCRIPTION OF THE INVENTION

[0045] Referring now to the figures of the drawings in detail and first, particularly, to FIG. 1 thereof, there is seen a schematic representation of a view of a preferred apparatus when carrying out a preferred exemplary embodiment of a method according to the invention.

[0046] An industrially usable inkjet printing machine 1 (only rudimentarily represented), for example for printing on sheets, webs or labels, includes a multiplicity of print heads 2, each having a multiplicity of nozzles 3 for the generation of drops 4 of liquid ink 5 corresponding to an image. During production, the print heads 2 are preferably stationary and disposed as a so-called print bar transversely with respect to a printing material transport direction. The machine 1 further includes a (digital) computer 40 which may control the printing process and may be used for the inventive feedback control of the temperature of the ink.

[0047] The printing machine 1 includes an ink circuit, or a device 10 for the circulating supply of ink to the print heads 2, having a storage container 11, a controllable ink heater 12 and at least one pump 13 for the ink 5, which is controllable (in terms of its speed or its generated volumetric flow). The device 10 supplies the print heads 2 continuously with ink 5 in parallel. For this purpose, an ink distributor 20 with a supply line 21 and a return line 22 (tubes or preferably pipes) is provided. The print heads 2 are disposed parallel to one another between the lines 21 and 22. The ink distributor 20 is connected to the storage container 11 through lines 23 that close the ink circuit.

[0048] According to the invention, the temperature of the ink 5 is intended to be feedback-controlled in the ink circuit and therefore also in the print heads 2, it being possible to set the viscosity of the ink 5 that is required for the unimpaired generation of drops by using the temperature. The preferably electrical ink heater 12 is provided for the corresponding thermal adjustment of the ink 5. Measurement values for the temperature control are preferably delivered by at least one temperature sensor 30. This sensor or its sensing element is preferably disposed in the supply line 21. The respective temperature sensor 30 is connected to the computer 40 via a connection 33a (only rudimentarily represented). The measurement results of the sensors are therefore available to the computer 40 for processing. The computer is connected to a device 41 for feedback control of the ink temperature. This device 41 is in turn connected to the ink heater 12 via a connection 33b. The temperature sensor 30 is preferably disposed close to the branch point at the supply line 21 to the flow sensor 32. Alternatively, for instance, it is disposed in the middle of the supply line 21 between the branch points to the print heads 2. A further temperature sensor (not represented herein) mayas is already knownbe disposed close to the heating device 12 and may measure the temperature of the ink just heated by the latter.

[0049] The inventive feedback control of the temperature, and therefore of the viscosity, is preferably carried out at a substantially constant pressure of the ink in the ink distributor 20. A pressure sensor 31, or the sensing element thereof, is therefore preferably disposed respectively in the supply line 21 and in the return line 22. The respective pressure sensor 31 is connected to the computer 40 via a connection 33a (only rudimentarily represented). The measurement results of the sensor 31 are therefore available to the computer 40 for processing. The measurement results may be used in order to control the pump 13 via the connection 33c in such a way that the required liquid pressure of the ink 5 is kept at a constant level. A pressure sensor 31 in each case may measure the hydrodynamic pressure in the supply line 21 and the hydrodynamic pressure in the return line 22, for example at the measurement locations represented in FIG. 1.

[0050] According to the invention, the device 10 includes the flow sensor 32 as a flow meter, for example an impeller sensor. The latter is operated (in a bypass) between the supply line 21 and the return line 22, and in parallel with the print heads 2 located there, and the flow or flow rate or flow quantity of the ink is measured (for example a volumetric flow in ml/min or l/min) or a measurement signal dependent on the flow is generated (for example a pulse rate or a pulse spacing or an analog voltage). The flow sensor 32 is connected to the computer 40 via a connection 33a (only rudimentarily represented). Is measurement results or measurement signals are therefore available to the computer 40 for processing.

[0051] The computer 40 or the device 41 carries out the following inventive steps of the feedback control: a temperature actual value is compared with a predefined temperature setpoint value and a manipulated variable for a setpoint value of the ink heater 12 is calculated from a setpoint/actual value deviation; the flow or the measurement signal is converted by using a respectively predefined characteristic curve (preferably stored in the computer 40 or in the device 41) into a temperature auxiliary value; and this temperature auxiliary value is used as the temperature actual value or as the temperature setpoint value for the feedback control.

[0052] FIG. 2 shows a flowchart of a preferred exemplary embodiment of a method according to the invention, having the following steps:

[0053] Step 50: a temperature setpoint value according to the invention is predefined, preferably for a predefined measurement location in the ink distributor 20.

[0054] Step 51: an (indirect) measurement of the temperature actual value according to the invention is carried out, preferably at the predefined measurement location in the ink distributor 20. The indirect measurement is carried by using the inventive measurement of the flow and conversion according to the characteristic curve predefined according to the invention (alternatively, by using a measurement signal dependent on the flow).

[0055] Step 52: the computer 40 calculates a further temperature setpoint value (which is different from the temperature setpoint value according to the invention) from the two values of steps 50 and 51, preferably for a measurement location at the exit of the ink heater 12. For this purpose, the computer 40 retrieves a previously compiled and stored algorithm, which is optionally based on a further characteristic curve (which is different from the characteristic curve according to the invention). Since the measurement location in the ink distributor and the measurement location for the ink heater are spaced apart from one another, the algorithm will preferably take resulting temperature differences between the two locations into account.

[0056] Step 53: the further temperature setpoint value calculated in step 52 is available to the device 41.

[0057] Step 54: on the basis of the further temperature setpoint value, the device 41 feedback-controls the ink heater 12, or controls the (e.g. internal) control unit of the latter.

[0058] Step 55: the control unit of the ink heater becomes active.

[0059] Step 56: the ink heater 12 is activated in such a way that it heats the ink 5 to the further temperature setpoint value.

[0060] Step 57: a further temperature actual value of the ink is measured (for example by the further temperature sensor), preferably at the measurement location at the exit of the ink heater 12. This value is available to the device 41 in order to carry out the control step 54.

[0061] If, according to the invention, the temperature auxiliary value converted by using the predefined characteristic curve is used as the temperature setpoint value for the feedback control, the temperature auxiliary value is used in step 50, i.e. as the temperature setpoint value there.

[0062] If instead, according to the invention, the temperature auxiliary value converted by using the predefined characteristic curve is used as the temperature actual value for the feedback control, the temperature auxiliary value is used in step 51, i.e. as the temperature actual value there.

[0063] The following is a summary list of reference numerals and the corresponding structure used in the above description of the invention: [0064] 1 inkjet printing machine [0065] 2 print head/heads [0066] 3 nozzle(s) [0067] 4 drop [0068] 5 ink [0069] 10 device for ink supply [0070] 11 storage container [0071] 12 ink heater [0072] 13 pump for ink [0073] 20 ink distributor [0074] 21 supply line [0075] 22 return line [0076] 23 line(s) [0077] 30 temperature sensor(s) [0078] 31 pressure sensor(s) [0079] 32 flow sensor [0080] 33a, b, c connection(s) [0081] 40 computer [0082] 41 device for feedback control of the ink temperature [0083] 50-57 method steps