EP3741571A1 - Method for monitoring and adjusting viscosity of the ink during operation of a continuous inkjet printer and continuous inkjet printer for carrying out such a method - Google Patents

Abstract

A method (100) is provided for monitoring and adjusting the ink viscosity during the operation of a continuous inkjet printer, comprising the steps of determining a target ink viscosity; Determination of an actual ink viscosity; Comparison of the actual ink viscosity with the target ink viscosity in order to check a correspondence of the actual ink viscosity with the target ink viscosity, at least temporarily improving the correspondence of the actual ink viscosity with the target ink viscosity by changing the ink temperature, and at least temporarily Returning the ink temperature change required to adapt the actual ink viscosity to the target ink viscosity by adapting the solvent content of the ink and a continuous inkjet printer (1000) for carrying out such a method.

Description

Process for monitoring and adjusting the ink viscosity during the operation of a continuous inkjet printer and continuous inkjet printer for carrying out such a process.

Inkjet printers are a widely used class of printers. A family of this class that is particularly suitable for industrial applications and has therefore achieved a high degree of penetration in this field are the so-called continuous inkjet printers.

A continuous inkjet printer prints with an ink that contains a variable amount of solvent. Accordingly, there is a mixing tank in which solvent from a solvent tank and the concentrated ink from an ink tank are mixed together to obtain the ink used for printing. When the term "ink" is used in the following, it means the liquid that is used for printing; the term "concentrated ink" is used for the liquid provided in the ink tank.

From the mixing tank, the ink is fed under pressure to a nozzle on the printhead, where the drops required for the actual printing process are created from the ink jet according to the basic principle of Rayleigh's decay of laminar liquid jets. The droplet formation and in particular the droplet size is controlled by a modulation that is impressed on the ink jet, for example, by piezo elements excited in a suitable manner.

The droplets produced in this way are electrically charged in a suitable manner and guided by deflection electrodes onto a desired trajectory which either guides them to a desired position on a substrate to be printed or, if no printing process is to take place at the moment, intercepted and recycled at the print head, i.e. be returned to the mixing tank.

An important variable in the operation of such printers is the viscosity of the ink, because it has a critical influence on the jet speed and the process of droplet formation. It tends to change over time because, as already mentioned, the ink has a solvent content which evaporates quickly compared to other ink components and in particular the components of the concentrated ink. As a rule, this leads to an increase in viscosity during operation, which changes the formation of drops and thus the print quality.

It is therefore known from the prior art, for example from the applicant's printer models of the Jet3 or Jet2 family, to monitor the viscosity of the ink by means of a falling ball viscometer and, if necessary, to reduce it again by adding solvent.

In practice it has been shown that adding solvents to regulate the viscosity does not lead to optimal results. Corrective action occurs relatively slowly because it takes a while for ink with the higher solvent content to reach the nozzle. It is precisely this that makes precise control difficult, because the monitoring of the ink viscosity at a point in time when a higher proportion of solvent is already present in the system, still the signal indicates that an additional solvent addition is necessary. In addition, a precise regulation is complicated by the return of unused ink (which then has at most the previous solvent content), which makes the analytical determination of the solvent content of the ink currently in the mixing tank more difficult and an exactly measured, analytically determined one Almost impossible to add solvents.

The object of the invention is therefore to provide a method for monitoring and setting the ink viscosity during the operation of a continuous inkjet printer, which method provides a result which is improved in particular with regard to the response time and the setting accuracy achieved. The task is also to provide a continuous inkjet printer for performing such a method.

This object is achieved by a method for monitoring and setting the ink viscosity with the features of claim 1 and a continuous inkjet printer with the features of claim 8. Advantageous further developments of the invention are the subject of the respective dependent claims.

The method according to the invention for monitoring and setting the ink viscosity during the operation of a continuous inkjet printer has at least the following steps:
First of all, a target ink viscosity, i.e. the viscosity value aimed at for the ink during printing, must be specified at least once. In addition to nozzle geometry and working pressure, this variable in particular determines the drop speed and it has an influence on the drop size. In practice, this determination is made by storing a corresponding value in a memory of the printer to which evaluation and / or control electronics have access.

During operation of the printer, the actual ink viscosity is then determined, preferably as continuously as possible, i.e. the viscosity that the viscosity currently used during printing is present, as well as, preferably as continuously as possible, a comparison of the actual ink viscosity with the target - Ink viscosity to check that the actual ink viscosity corresponds to the target ink viscosity. In practice, this value is determined by a sensor and the result is supplied to the evaluation and / or control electronics as an analog or digital signal, optionally after being temporarily stored in a memory of the printer, or made available for access by the evaluation and / or control electronics.

According to the invention, the improvement in the conformity of the ink viscosity is achieved by a combination of two measures:
On the one hand, an improvement in the correspondence between the actual ink viscosity and the target ink viscosity is at least temporarily brought about by changing the ink temperature, typically by controlling a heater. On the other hand, the change in temperature of the ink required to adapt the actual ink viscosity to the target ink viscosity is at least temporarily returned by adapting the solvent content of the ink. This can in particular be an addition of solvent; because of the combination with the quick-acting corrective action of increasing the ink temperature For the first time, however, it is also practicable to change the ratio between solvent and concentrated ink set for filling the mixing tank, which then only gradually adjusts this ratio to the ink in the mixing tank, which more reliably avoids too high a solvent content than the previous one Application of this correction mechanism.

In other words, depending on the temperature change required in each case or on the output level, solvent is supplied to a heater used to bring it about. This then has the effect that the required output level of the heating again approximates the basic state, which can exist, for example, when the heating is switched off.

The viscosity is a temperature-dependent variable which, as a rule, and particularly in the case of inks such as those used in continuous inkjet printers, decreases with increasing temperature. Accordingly, the viscosity of the ink can be adjusted to the desired value by local heating of the ink, with a mechanism that can be regulated quickly and precisely, which begins to act almost immediately and thus also the feedback through the comparison between the actual and target viscosity influenced much more quickly, because the variable that is too variable is typically the energization of the heating.

Despite these advantages, the use of the temperature for viscosity control appears to be problematic in the first access. An increased temperature tends to exacerbate the problem that volatile solvent evaporates; a permanent and significant increase in the temperature of the ink could also increase the efficiency of the ink gun and negatively affect the phase position of the charging signal. Therefore, according to the invention, combined with the temperature-based viscosity control, the adjustment of the actual ink viscosity to the required ink temperature change by adding solvent to the ink is essential and the interaction of these two measures is important for the success of the method according to the invention.

The additional mechanism of adjusting the solvent content of the ink ensures that the required temperature change does not become so great that the disadvantages of temperature-dependent viscosity control described above become noticeable and, in particular, it balances the negative effects of any solvent losses due to evaporation due to heating of the ink Application of this temperature-based correction mechanism.

The term “at least temporarily” with regard to the two measures mentioned means that the corresponding measure does not necessarily have to be applied continuously. The return of the ink temperature change required to adapt the actual ink viscosity to the target ink viscosity, which is brought about by adapting the solvent content of the ink and in particular the addition of solvent, does not necessarily have to take place continuously (even if this is basically possible), but it can also initiated only when a threshold is exceeded. The influencing of the ink viscosity by adjusting the ink temperature can optionally only be triggered when the deviation between the actual ink viscosity and the target ink viscosity is so great that a threshold value is exceeded.

As a result, when using the method according to the invention, a fast-acting correction mechanism and a slow-acting correction mechanism are superimposed, which on the one hand allows the need to use the fast-acting correction mechanism to subside over time and on the other hand can directly counteract any negative side effects of the fast correction mechanism. The particular advantage of the method according to the invention is based on this interaction between two corrective measures.

It is particularly advantageous if the change in the ink temperature is brought about by heating the nozzle. This can be implemented, for example, by an electrical heater arranged there, which is energized to different degrees depending on a signal from the evaluation and / or control electronics. The advantage of influencing the ink temperature at this point is that (unlike when the mixing tank is also basically heated) only a relatively small volume of ink needs to be heated, which leads to short reaction times and a quick effect of the corrective action. In addition, the regulation acts directly at the point at which the ink viscosity is a particularly critical variable.

In a preferred embodiment of the method it is provided that the at least temporary return of the change in ink temperature required to adapt the actual ink viscosity to the target ink viscosity is triggered by adapting the solvent content of the ink in that the necessary change in the ink temperature exceeds a limit value. In this way, rapid temperature fluctuations, which are not permanent, can be compensated and temperature increases reversed without being affected by the The viscosity changes caused by the slower correction mechanism of the addition of solvent must be reversed.

In a particularly advantageous development of the method, it is provided that the jet speed of generated ink drops is measured to determine the actual ink viscosity and that the comparison of the actual ink viscosity with the target ink viscosity is achieved by comparing the measured jet speed of generated ink drops with a target value of the Jet speed, which corresponds to the target ink viscosity, takes place. On the one hand, it is possible in this way to use other, more expensive viscosity measuring devices or sensors, e.g. relatively expensive spherical viscometers, avoid.

However, it is almost even more important that this measure, in conjunction with the embodiment of the method in which the temperature change is brought about by heating at the nozzle, also brings about a synergetic effect. This combination of features ensures that the ink viscosity is regulated and measured in the immediate vicinity of one another, so that every regulation of the ink viscosity has a direct and immediate effect on the measured ink viscosity, so that an overcorrection of the ink viscosity through the persistence of a correction signal, because the changed ink viscosity does not yet have an effect on the viscosity measurement, is almost completely avoided. The relationship between the viscosity and the drop speed can be derived from Hagen-Poisseuille's law.

Specifically, the measurement of the jet speed of the generated ink droplets can take place in that one or more Ink droplets are provided with an electrical charge and a signal generated by the ink droplet or droplets when flying past a detector electrode is used as a start and / or stop signal for a time-of-flight measurement. In particular, the start signal and the stop signal for the time-of-flight measurement can also be generated at the same detector electrode if two ink drops generated immediately one after the other are at a defined distance from one another and the signal generated by the first ink drop is used as the start signal and the signal generated by the following ink drop as the stop signal used.

If you control a heater provided on the nozzle to influence the ink viscosity so that the ink temperature at the nozzle is continuously above the ambient temperature, you can also create a possibility of correcting the ink viscosity not only in the direction of a lower ink viscosity, but also towards higher ink viscosity is possible.

A continuous inkjet printer according to the invention for carrying out such a method has, as is customary in such devices, a controller with a controller, a solvent tank, an ink tank, a mixing tank for mixing solvent from the solvent tank with concentrated ink from the ink tank, a device for determination the viscosity of the ink used for printing and a print head, these components being connected to one another by a hydraulic system. The printhead has an ink drop generator with a nozzle for generating ink drops with ink from the mixing tank, a charging electrode for applying charge to the generated ink drops, at least one deflection electrode for deflecting charged ink drops and a drip catcher to catch ink drops not used for printing, these ink drops being returned to the mixing tank through the drip catcher.

It is essential to the invention that the continuous inkjet printer furthermore has a heating element for influencing the ink temperature and that the controller for carrying out the steps furthermore has a device for determining the viscosity of the ink used for printing - for example a sensor that measures the viscosity or a this correlatable variable, whose data can usually be processed by the controller.

The control, which can also be referred to as control electronics, typically contains electronic components and circuits; it can in particular have at least one memory for storing data and program routines to be executed by the controller and at least one processor for processing data has optional interfaces for signal communication with sensors and is usually accessible via communication interfaces, e.g. Keyboard and display or a bus system for connection to a computer capable of being programmed by the user and of making data available to a user.

The print head of a continuous inkjet printer typically has at least one nozzle for ink from the mixing tank, a device for generating ink droplets when the ink emerges from the nozzle - for example a piezo element that can be activated by the controller with an electrical excitation signal Charging device for applying charge to the generated ink drops, typically a charging electrode, at least one deflection electrode for deflecting charged ink droplets, which can also be acted upon by the controller with a deflection voltage, and a droplet catcher for collecting ink droplets not used for printing so that they can be returned to the mixing tank. The latter implies that the hydraulic system has a return line from the drip catcher to the mixing tank.

It should be noted that the hydraulic system is usually not limited to a pure line system, but can have additional components, in particular filters, pulsation dampers and / or components that can be controlled by the controller, in particular valves and suction and / or pressure pumps. Of course, it can also contain passive, non-controllable valves.

• Festlegung einer Soll-Tintenviskosität
• Bestimmung einer Ist-Tintenviskosität
• Vergleich der Ist-Tintenviskosität mit der Soll-Tintenviskosität um eine Übereinstimmung der Ist-Tintenviskosität mit der Soll-Tintenviskosität zu prüfen,
• zumindest zeitweilige Verbesserung der Übereinstimmung der Ist-Tintenviskosität mit der Soll-Tintenviskosität durch eine Änderung der Tintentemperatur, und
• zumindest zeitweiliges Zurückführen der zur Anpassung der Ist-Tintenviskosität an die Soll-Tintenviskosität benötigten Tintentemperaturänderung durch Zusetzen von Lösungsmittel zur Tinte

eingerichtet ist, was einerseits eine geeignete Ausgestaltung der Hardware der Steuerung und andererseits eine geeignete Programmierung der Steuerung voraussetzen kann.A particular feature of a continuous inkjet printer according to the invention is that it also has a heating element for influencing the ink temperature and that its controller is used to carry out the steps

• Determination of a target ink viscosity
• Determination of an actual ink viscosity
• Comparison of the actual ink viscosity with the target ink viscosity in order to check whether the actual ink viscosity corresponds to the target ink viscosity,
• at least temporary improvement of the correspondence between the actual ink viscosity and the target ink viscosity by changing the ink temperature, and
• at least temporarily returning the required to adapt the actual ink viscosity to the target ink viscosity Ink temperature change due to the addition of solvent to the ink

is set up, which on the one hand may require a suitable design of the hardware of the controller and on the other hand suitable programming of the controller.

In particular, the target viscosity can be determined by querying such a value when the printer is set up by a user and storing it in a memory, and the actual viscosity can be done by querying data from a sensor that indicates the viscosity or a value this correlatable variable can be determined. The comparison between the target and actual viscosity is then carried out automatically by the control and a correction is initiated by it.

If there is a deviation from the target value or a predetermined tolerance range around the target value, the control signal to the heater or its energization can change the ink temperature so that the desired change in viscosity is brought about as an immediate corrective measure. in particular, the ink temperature will be raised if the ink viscosity is too high.

The ink temperature change required to adapt the actual ink viscosity to the target ink viscosity can be returned by adding solvent to the ink, in particular if the required heating power exceeds a limit value or has to be provided for longer than a predetermined period of time. If the controller is programmed accordingly, both can be monitored by the controller, which then also by control signals to a valve and / or a pump that influences the solvent supply into the mixing tank and thus effects the adjustment of the solvent.

In a particularly preferred embodiment of the continuous inkjet printer, the heater is arranged on the nozzle. Accordingly, a relatively small volume of liquid is specifically heated at the point where the viscosity plays an essential role, which guarantees a quick corrective effect.

It is particularly preferred that the device for determining the viscosity is a device for measuring the flight time which a charged ink drop needs to cover a defined distance. In particular, this is possible in that voltage pulses, which are induced when the charged ink droplet flies past detector electrodes, are used as a start or stop signal for timing, so that when the distance between the detector electrodes (if two are used) or between electrode end sections is known a long detector electrode can calculate the airspeed of the drop, from which the ink viscosity can then be calculated according to the law of Hagen-Poisseuille with known nozzle geometry.

Fig. 1:

eine schematische Blockdarstellung des Aufbaus eines Continous Inkjet Druckers;

Fig. 2:

einen vergrößert dargestellten Aufbau eines Teils des Druckkopfes eines Continuous Inkjet Druckers; und

Fig. 3:

ein schematisches Ablaufdiagramm eines exemplarischen Verfahrens zur Überwachung und Einstellung der Tintenviskosität.

The invention is explained in more detail below with reference to figures which show an exemplary embodiment. It shows:

Fig. 1:

a schematic block diagram of the structure of a continuous inkjet printer;

Fig. 2:

an enlarged structure of a part of the print head of a continuous inkjet printer; and

Fig. 3:

a schematic flow diagram of an exemplary method for monitoring and adjusting ink viscosity.

Figure 1 shows a schematic block diagram of the structure of a continuous inkjet printer 1000, with only hydraulic lines, but not electrical lines, with the exception of a data line 1034, being shown for the sake of clarity.

The continuous inkjet printer 1000 has a hydraulic system accommodated in a hydraulic housing 1016. Components of the hydraulic system are in particular a solvent tank 1002, an ink tank 1001 for concentrated ink, a mixing tank 1004, a main pump 1008 designed as a pressure pump and a suction pump 1005 as well as a metering device 1003, a main filter 1011 and a viscometer 1009 with an associated pump 1010 and hydraulic lines that connect these components to one another and, combined to form the schematically illustrated connection line 1019, to the print head 1014, the structure of which is explained in more detail below.

In the print head 1014, in particular, an ink drop generator 1013 for generating ink drops 1013 and a drip catcher 1015 for collecting and returning unused ink drops are arranged.

Further components of the continuous inkjet printer 1000 are included in an electronics housing 1017 with display 1018 (and thus in the Figure 1 not visible), in particular a power supply and control electronics, generally one Control electronics in the sense of this description are also able to regulate parameters, that is to say to be interpreted as control and / or regulating electronics. The display 1018 is used for communication with the user, as in Figure 1 shown, in particular a text box 1026 with the current operating parameters or status information and can, especially if it is designed as a touchscreen, also enables the input of commands for the user, but of course also via other input means of the continuous inkjet printer 1000, e.g. input keys , can be done.

In the mixing tank 1004, the ink used for the printing process consisting of solvent from the solvent tank 1002 and concentrated ink from the ink tank 1001, each supplied by hydraulic lines via the metering device 1003, which regulates the proportions of solvent and concentrated ink, is supplied to the mixing tank 1004, mixed and - if necessary with circulation - temporarily stored until use. Unused ink droplets are also returned to the mixing tank 1004 via the drip catcher 1015 and the suction pump 1005, as will be described in more detail below.

In order to supply the print head 1014 with ink, the ink is filtered in the main filter 1011 and fed to the print head 1014 by the main pump 1010 via a hydraulic line that is integrated in the connection line 1019 of the print head 1014. Ink that has not been used is returned from the print head 1014 to the mixing tank 1003 via a second hydraulic line integrated here in the connection line 1019 and the suction pump 1005. Another hydraulic line connects the ink drop generator 1012 to the mixing tank 1003 via a pressure sensor 1006 and a valve 1007.

The hydraulic system can also include further components, in particular pumps, valves and / or filters.

The electrical components of the printer are supplied with the necessary operating voltage by the voltage supply and controlled or regulated with the control electronics, which are in signal communication with them. In particular, the control electronics also convert the image to be printed into control signals for the print head 1014 in order to effect the printing of the image and is responsible for processing the signals from sensors of the continuous inkjet printer 1000.

Figure 2 shows an enlarged structure of part of the print head of a continuous inkjet printer, namely the ink drop generator 1012 with ink supply line 1027 and ventilation line 1028 and the electrode block 1032, in which, however, only the grounded first deflection electrode 1030 and not the second deflection electrode connected to positive voltage 1030, since the latter, if installed on its receptacles 1031, would cover the other electrodes.

From the in Figure 1 The mixing tank 1003 shown in the illustration, the ink is supplied under pressure, which is generated by the pump 1010, through the ink supply line 1027 to the ink drop generator 1012, shown in a simplified block-like manner. This includes, in particular, a nozzle 1022, on or behind which the ink droplets 12 required for the actual printing process arise from the initially continuously emitted ink jet according to the basic principle of Rayleigh's decay of laminar liquid jets. The ink drop formation and in particular the ink drop size is controlled by a modulation that is impressed on the ink jet by piezo elements excited by an electrical alternating voltage, which are not shown in detail because of the block-like simplification.

The ink droplets 1013 thus generated form an ink droplet jet. They initially fly through - with perfect adjustment or target jet position on the connecting line between the outlet opening of the nozzle of the ink cannon 10 and the inlet opening of the drip catcher 1015 - the charging electrode 1033, where they are electrically charged, and deflection electrodes 1030, with which in this case ink drops 1013 that are to be used for printing are deflected from the connecting line, while ink droplets 1013 that are not to be used for printing fly further along the connecting line to the drip catcher 1016 through a hydraulic line from the suction pump 1005 with a perfect adjustment or target jet position sucked in and returned to the mixing tank 1004 and thus recycled.

A time-of-flight measurement is used to determine the current viscosity of the ink, which is a critical parameter for the printing process, directly at the printhead 1014. For this purpose, two pairs of detection electrodes 1023, 1024 are arranged in the area between the charging electrode 1033 and the deflection electrodes 1030, seen in the direction of beam propagation of the undeflected ink droplets 1013, the second pair of detection electrodes 1024 relative to the first pair of detection electrodes 1023 in the direction of the droplet catcher 1015 is offset and wherein between the detection electrodes belonging to a pair of detection electrodes 1023, 1024, an offset in a direction perpendicular to a connecting line between Ink drop generator 1012 and drop catcher 1015, more precisely their respective centers. Ink drops 1013, which are already charged when they fly past the pairs of detection electrodes 1023, 1024, induce a signal on the detection electrodes. The signal on the pair of detection electrodes 1023 can serve as a start signal for a time-of-flight measurement and the signal on the pair of detection electrodes 1024 can be used as a stop signal for this time-of-flight measurement, so that the drop speed can be determined from this data by the control electronics and then the actual Viscosity at the print head can be calculated from the Hagen-Poisseuille law by the control electronics and compared with a viscosity setpoint.

If this comparison leads to a discrepancy, this can be counteracted immediately and promptly in that a heating element 1025 arranged in the area of the nozzle 1022 of the ink drop generator 1012 is appropriately regulated by the control electronics, so that the known temperature dependency of the viscosity is used to ensure it is quickly restored that the ink drops 1013 used for printing have the correct viscosity. In addition, the Continuous Inkjet Printer 1000 also has a second mechanism for regulating the ink viscosity, as this also depends in particular on the mixing ratio between solvent and concentrated ink. In the event of a greater and / or longer-term deviation from the setpoint, in particular if the temperature change that has to be brought about by the heating exceeds a limit value, the control electronics control the metering device 1003 in such a way that, through appropriately adapted supply of solvent from the Solvent tank 1002 and concentrated ink from the ink tank 1001 determine the viscosity of the ink in the Mixing tank 1004 reaches the desired area again and the heating of nozzle 1022 by heating element 1025 can accordingly be reduced again.

In particular, the charging electrode 1020 and the pairs of detection electrodes 1023, 1024 are designed as surface electrodes on a printed circuit board 1060 over which the ink droplets fly. A part 1061, shown schematically as a rectangle, of the evaluation electronics for the signals detected by the pairs of detection electrodes 1023, 1024 is arranged on the side of the printed circuit board 1060 opposite the side on which the pairs of detection electrodes 1023, 1024 are arranged. This ensures short signal paths.

The part of the evaluation electronics 1061 should contain at least one first amplifier stage 1061a and / or at least one A / D converter stage 1061b in order to be able to guarantee further processing of the data with as little interference as possible.

Figure 3 FIG. 10 shows a schematic flow diagram of an exemplary method 100 for monitoring and adjusting the ink viscosity. First, in step 110, the target ink viscosity stored in a memory element of printer 1000, in particular in the control electronics, is read out.

wobei p der Druck, l die Kanallänge der Düse und r der Düsenradius ist. In der Praxis kann man eine Kalibrierkennlinie aus im Drucker eingestellter Viskosität, die z.B. durch die Kugelfallzeit eines Kugelfallviskosimeters ermittelt wird, und der im Schreibkopf gemessenen Strahlgeschwindigkeit ermitteln und diese Kennlinie dann in der Software des Druckers hinterlegen. Die weiteren Parameter, die in die oben genannte Formel eingehen, sind während des Druckbetriebs konstant und gehen in die Steigung der linearen Kennlinie ein.In step 120, the jet speed, which is given by the speed of the ink droplets 1012, is then measured by dividing the quotient of the flow time between the first pair of detection electrodes 1023 and the second pair of detection electrodes 1024, which is derived from the time interval between the two the flying over of charged ink droplets 1012 results in induced signals t _DB2 -t _DB1 and the known distance between the first pair of detection electrodes 1023 and the second pair of detection electrodes 1024 s _DB2 -s _DB1 is formed. The linear relationship between the jet speed v _z and the viscosity η results from the formula $$v_z=\frac{r^2\times \mathrm{\Delta }\mathit{p}}{\mathrm{8th}\mathit{\eta l}}$$

where p is the pressure, l is the channel length of the nozzle and r is the nozzle radius. In practice, a calibration characteristic can be determined from the viscosity set in the printer, which is determined, for example, by the falling ball time of a falling ball viscometer, and the jet speed measured in the write head, and this characteristic can then be stored in the printer's software. The other parameters that go into the above formula are constant during printing and are included in the gradient of the linear characteristic.

In step 130, the comparison is made between the _intended target viscosity η and the actual _viscosity η of ink, which can be done simply by difference. No regulation is required within a tolerance range, which can be specified by a permissible deviation that is stored in a memory element in the continuous inkjet printer 1000, and a return to step 110 takes place. However, if the tolerance range is exceeded, step 140 carried out and by appropriate control of the heating element 1025 by the control electronics, the ink temperature at the nozzle 1022 is changed in order to bring the actual viscosity back to the target viscosity.

In this embodiment, the control of the heating element 1025, for example as control parameters can use a temperature change or a current intensity of the energization of the heating element, monitored by the control electronics and compared with a maximum permissible temperature change stored as a limit value in the continuous inkjet printer 1000.

As long as this limit value is not exceeded, there is a return to step 110 of the method, otherwise step 150 is activated before this return, in which the ink composition is modified by the control electronics by suitable control of the metering device 1003 in order to influence the viscosity of the ink that the temperature change caused by controlling the heating element 1025 can be reduced.

It should be noted that step 150 can also be activated in a variant of the method as soon as a certain change in the ink temperature at the nozzle 1022 was necessary continuously over a certain period of time by heating the heating element 1025, so that it is not just a short-term one Fluctuation acts.

List of reference symbols

100

Procedure

110

step

120

step

130

step

140

step

150

step

1000

Continuous inkjet printer

1001

Ink tank

1002

Solvent tank

1003

Dosing device

1004

Mixing tank

1005

Suction pump

1006

Pressure sensor

1007

Valve

1008

Main pump

1011

Main filter

1012

Ink drop generator

1013

Ink drop

1014

Printhead

1015

Drip catcher

1016

Hydraulic housing

1017

Electronics housing

1018

Display

1019

Connecting cable

1020

Hydraulic line

1022

jet

1023

Detector electrode pair

1024

Detector electrode pair

1025

Heating element

1026

Text box

1027

Ink supply line

1028

Vent line

1030

Deflection electrode

1031

Support for positive deflection electrode

1032

Electrode block

1033

Charging electrode

1034

Data line

1060

Circuit board

1061

Evaluation electronics

1061a

Amplifier stage

1061b

A / D converter stage

Claims (10)

Method (100) for monitoring and adjusting the ink viscosity during the operation of a continuous inkjet printer (1000) with the steps - Determination of a target ink viscosity - Determination of an actual ink viscosity - Comparison of the actual ink viscosity with the target ink viscosity in order to check whether the actual ink viscosity corresponds to the target ink viscosity, - At least temporary improvement of the correspondence of the actual ink viscosity with the target ink viscosity by changing the ink temperature, and - at least temporarily returning the ink temperature change required to adapt the actual ink viscosity to the target ink viscosity by adapting the solvent content of the ink. Method (100) according to claim 1,
characterized in that the change in the ink temperature is brought about by heating at a nozzle (1022) of an ink drop generator (1012) of the continuous inkjet printer (1000). Method (100) according to claim 1 or 2,
characterized in that the at least temporary return of the ink temperature change required to adapt the actual ink viscosity to the target ink viscosity is triggered by adding solvent to the ink in that the necessary change in the ink temperature exceeds a limit value. Method (100) according to one of claims 1 to 3,
characterized in that, to determine the actual ink viscosity, the jet speed of generated ink drops (1013) is measured and that the comparison of the actual ink viscosity with the target ink viscosity is achieved by comparing the measured jet speed of generated ink drops (1013) with a target value of the jet speed which corresponds to the target ink viscosity. Method (100) according to claim 4,
characterized in that the measurement of the jet speed of the generated ink droplets (1013) takes place in that one or more ink droplets (1013) are provided with an electrical charge and one of the ink droplets (1013) passes at least one detector electrode (1023, 1024) when it flies past. generated signal is used as a start and / or stop signal for a time-of-flight measurement. Method (100) according to claim 5,
characterized in that the start signal and the stop signal for the time-of-flight measurement are generated on the same detector electrode (1023, 1024) or the same detector electrodes (1023, 1024). Method according to one of the preceding claims,
characterized in that the ink temperature at the nozzle (1022) is continuously above ambient temperature. Continuous inkjet printer (1000) for performing the method (100) according to one of Claims 1 to 7 with a Control, a solvent tank (1002), an ink tank (1001), a mixing tank (1004) for mixing solvent from the solvent tank (1002) with concentrated ink from the ink tank (1001), a device for determining the viscosity of the ink used for printing and with a print head (1014), these components being connected to one another by a hydraulic system,
wherein the print head (1014) has an ink drop generator (1012) with a nozzle (1022) for generating ink drops (1013) with ink from the mixing tank (1004), a charging electrode (1033) for applying charge to the generated ink drops (1013), has at least one deflection electrode (1030) for deflecting charged ink droplets (1013) and a drip catcher (1015) for collecting ink droplets (1013) not used for printing, so that these are returned to the mixing tank (1004),
characterized in that the continuous inkjet printer (1000) furthermore has a heating element (1025) for influencing the ink temperature and that the controller for performing the steps - Determination of a target ink viscosity - Determination of an actual ink viscosity - Comparison of the actual ink viscosity with the target ink viscosity in order to check whether the actual ink viscosity corresponds to the target ink viscosity, - At least temporary improvement of the correspondence of the actual ink viscosity with the target ink viscosity by changing the ink temperature, and - At least temporarily returning the for adapting the actual ink viscosity to the target ink viscosity required change of ink temperature by adjusting the solvent content of the ink is set up. Continuous inkjet printer (1000) according to claim 8,
characterized in that the heating element (1025) is arranged on the nozzle (1022). Continuous inkjet printer (1022) according to claim 8 or 9,
characterized in that the device for determining the viscosity is a device for measuring the flight time which a charged ink drop (1013) needs to cover a defined distance.

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