WO1990010540A1

WO1990010540A1 - Process and device for optimising the pressure pulses in ink printers operated by thermal converters

Info

Publication number: WO1990010540A1
Application number: PCT/EP1990/000141
Authority: WO
Inventors: Ernst Goepel
Original assignee: Siemens Aktiengesellschaft
Priority date: 1989-03-14
Filing date: 1990-01-25
Publication date: 1990-09-20

Abstract

Optimising the pressure pulses for electro-thermal converter units in ink printing installations (bubble jet principle) is based on the temperature-dependent viscosity of the fluid ink. As the temperature rises and hence the viscosity falls, the internal friction in the fluid ink flowing through capillary tubes is reduced, as is also therefore the pressure needed to eject the drops as well as the pulse energy. As the electrical pulse energy depends upon the pulse voltage and duration, said pulse voltage and duration are altered according to the measured ink temperature so that, at an ink temperature higher or lower than a threshold value, the converter units are controlled at a reduced or increased pulse energy.

Description

Method and device for optimizing the pressure pulses in ink printing devices operated with thermal converters

The invention relates to a method and a device

for optimizing the pressure pulses in ink printing devices which operate according to the thermal converter principle, according to the features of claims 1 and 7.

Known ink print heads that work according to the thermal converter principle (bubble jet principle) and, for example, in the

DE-OS 30 12 698 are described, have a plurality of individual nozzles from which defined droplets are ejected under the action of an electronic control. Individual capillary ink channels are assigned to the individual nozzles, in which pressure waves are generated in the ink liquid by means of actuators. The procedure for

Pressure build-up in the ink liquid is based on the generation of micro vapor bubbles. In the bottom of each ink channel

is located at a certain distance from the outlet nozzle as an actuator an electro-thermal converter element (heating element) in the form of an electrical thin-film heating resistor, which is excited with a voltage rectangular pulse.

As a result, the ink liquid immediately above the transducer element is raised to a high level in a thin layer

Temperatures heated. In the course of the subsequent evaporation of the heated ink liquid, a vapor bubble (bubble) with high internal pressure is formed above the transducer element, the expansion of which causes a part of the ink liquid located in the corresponding ink channel to be expelled from the nozzle. Since the gas bubbles by briefly energizing the

Thin film heating resistors are produced, there is a functional Relationship between the electrical pulse energy and the pressure pulse required to eject individual droplets. The pulse energy is determined by that of the

Transducer element applied pulse voltage, by the electrical resistance of the transducer element (dot resistance) and by the pulse duration of the applied voltage. In order to ensure a safe droplet ejection from the nozzles, on the one hand the pulse energy must not fall below a predetermined value and on the other hand the heating of the ink print head due to the thermal converter principle should not be increased by unnecessarily high pulse energy.

The invention is therefore based on the object of specifying measures for optimizing the pressure pulses for the electrothermal transducer elements in ink printing devices of the type mentioned at the outset, which make it possible to provide a pulse energy adapted to the ink temperature for the transducer elements without the print quality being impaired in the process.

This object is achieved according to the features of claims 1 and 7. Advantageous refinements are characterized in the subclaims.

According to the invention, the temperature dependence of the viscosity of the ink liquid flowing in the capillaries is used. For this purpose, the temperature of the ink liquid is measured and compared with a predetermined limit. If the measured temperature is above this limit value, the amplitude of the pulse voltage provided for the control of the thermal converter elements or the pulse duration is reduced, so that the pulse energy supplied to the converter elements is reduced. The optimization of the pressure pulse can also be done by changing both the pulse amplitude as well as the pulse duration. Such an optimization of the printing pulses makes it easy to change the printing energy without impairing the printing quality. If the pressure energy is reduced at an ink temperature that is higher than the limit value, a reduction in the heating of the ink print head is achieved at the same time. This results in the advantage of a smaller ink print head cooling surface or an increase in the print data throughput, that is to say a larger one

Amount of data per unit of time can be processed without the ink printhead being heated inadmissibly. Such a reduced printing energy can significantly improve the ratio of the operating phase to the cooling phase of the ink print head.

The temperature of the ink liquid can be recorded particularly easily if a temperature sensor is arranged on the ink print head in the region of the ink reservoir. Because of the proximity and the good thermal coupling, the temperature of the ink liquid can be replaced by the temperature of the ink print head in the area of the ink reservoir with sufficient accuracy.

If a comparator is used to switch from a high pressure pulse energy to a lower pressure pulse energy

Bridge resistance network used, the output of which controls an adjustable voltage regulator that is already available for the control of the thermal converters, so the effort for the optimization is reduced to a few standard components, resulting in an overall inexpensive device.

The invention is explained below using an exemplary embodiment, for which reference is made to the illustrations. Show there

FIG. 1 shows the temporal profiles of the control voltage and the pressure pulse and, in a schematic manner, the generation of vapor bubbles and droplets,

FIG. 2 shows the temperature dependence of the kinematic viscosity of an ink liquid suitable for printing with thermal converters,

FIG. 3 shows the dependence of the pulse energy, pulse voltage and pulse duration on the ink temperature,

Figure 4 is a block diagram of a device for optimizing the pressure pulses and

Figure 5 shows another way to optimize the pressure pulses in a detailed representation. The left half of FIG. 1 shows a voltage square-wave pulse suitable for driving electrothermal transducer elements, which is characterized by its amplitude U and the pulse duration T. In addition, the time course of the pressure pulse P is drawn in an ink channel TK, which ultimately leads to the ejection of ink droplets from the nozzles of an ink jet print head. Individual selected points in time t ₁ to t _{7 are} marked on the time axis t, which can be found in the vapor bubble generation shown schematically on the right half of FIG. 1 with subsequent droplet ejection process. The side views of the ink channel TK associated with the individual times t ₁ to t ₇ are shown there in each case. The thermal converter element is represented by its electrical resistance R _D.

At time t ₁ , the pulse voltage is applied to the transducer element, this begins to heat up and the ink liquid TFL is heated. From time t ₂ , the pressure begins to build up within the ink channel TK and thus develop an ink vapor bubble TBL. This is growing continue until a time t ₄ , although the pulse voltage has already been _{switched off again} (time t ₃ ) _{. The} pressure pulse P _I had its maximum value at this time. The ink vapor bubble TBL collapses (times t ₅ , t ₆ ) and an ink droplet TTR is expelled. The ink vapor bubble TBL retracts the meniscus at the nozzle outlet when it collapses, and the capillary force of the meniscus causes the nozzle to be refilled with ink from the ink reservoir. At time t ₇ , the original state (time t ₁ ) is restored and a new droplet ejection can be initiated.

Since, on the one hand, the internal frictional force of the ink liquid flowing in the capillaries must be overcome by the pressure pulse during ink ejection and, on the other hand, there is a proportionality between the frictional force and the kinematic viscosity of the ink liquid, it becomes apparent that the frictional force decreases with decreasing viscosity. This reduces the pressure pulse required to eject droplets and thus the pulse energy. At the

Printing operation now heats up due to the thermal converter principle of the ink print head, whereby a temperature of the ink print head depends on the print throughput (= print quantity / time), the ambient temperature and the cooling surface of the ink print head. Since there is an spatially and therefore thermally connected ink reservoir (not shown) on the ink printhead directly next to the capillary nozzles, the temperature of the ink liquid in the reservoir increases to the same extent with increasing ink printhead temperature, i.e. the temperature of the ink print head can be set equal to the temperature of the ink liquid with sufficient accuracy.

In FIG. 2, the temperature dependence of the kinematic viscosity V _{T is} one for ink printing with thermal converters suitable ink liquid TFL shown. The kinematic viscosity V _T is the quotient of the

dynamic viscosity and the density of the ink liquid defined. It can be seen from the curve drawn that, for example, at an ink temperature θ _T of

20 ° C the kinematic viscosity V _{T is} 5.5 mm ² / s, while at an ink temperature of 45 ° C it has a value of 2.5 mm ² / s. This characteristic property, that the kinematic viscosity V _{T of} the ink liquid TFL falls with increasing temperature, is advantageously used according to the invention to optimize the pressure pulses. For a warmer ink printhead with a lower viscosity viscous liquid TFL, a lower pulse energy is therefore sufficient for the printing operation than for a colder ink printhead with a higher viscosity viscous liquid TFL, without the ink output thereby changing.

According to the relationship for the pulse energy E _I =

(U ² / R _D ). T _I , this optimization can be done by changing the amplitude U of the pulse voltage and / or the pulse duration T _{I to} match the ink temperature θ _{T of} the ink liquid TFL. The functional relationship between these quantities and the ink temperature is shown in FIG. 3. The abscissa shows the temperature θ _{T of} the ink fluid TFL in ºC and the ordinate shows the pulse duration T _I in μs, the amplitude square U ^{2 of} the pulse voltage, on different scales plotted in V ^2, and a relative pulse energy e _I / e _I in percent. For the following considerations, the use of an ink liquid is assumed, the temperature-dependent viscosity V _{T of which has} a profile according to the characteristic curve shown in FIG. 2. For example, if you operate an inkjet print head using thermal converter technology, its electrical resistance of the transducer elements R _Di = 80Ω, at a temperature of the ink printhead θ _TDK ≈ θ _T = 35 ° C, a pulse voltage with an amplitude U = 20 V and a pulse duration T _I = 6.5 becomes, according to FIG μs required. The relative pulse energy E _I '/ E _I is set at 100%. If the temperature of the ink print head and thus the temperature of the ink liquid during printing operation and / or due to the ambient temperature increases to, for example, 50 ° C., the pulse energy can be reduced by 15% according to FIG. 3 without the droplet output changing. However, this means that with a constant pulse voltage amplitude U = 20 V the pulse duration T _{I can go} from 6.5 μs to 5.5 μs or with a constant pulse duration T _I = 6.5 μs the amplitude U of the pulse voltage can be from 20 V 18.4 V can be reduced. Likewise, a joint reduction of the pulse voltage amplitude U and the pulse duration T _{I is} possible such that the decrease in the pulse energy does not exceed a value of 15%. In Figure 4 is a possible device for the top

Optimization of the pressure pulses P _{I described is} shown schematically in the form of a block diagram. One after the

Thermal printing principle working ink printing device, as described in detail in DE-OS 30 12 698, for example, has an ink printhead TDK, which has the thermal transducers shown only hinted here with their heating resistors R _Di. A temperature sensor TF is arranged on the ink print head TDK in the immediate vicinity of an ink reservoir, not shown here. With this temperature sensor TF, which can be implemented, for example, as a thermistor, PTC thermistor, silicon component or as a thin-film resistor, the temperature θ _{TDK of} the ink printhead TDK is detected. Because of the spatial proximity and thus the close thermal coupling of the temperature sensor TF For the ink liquid TFL in the ink reservoir, the detected temperature θ _{TDK of} the ink print head TDK can be used as a measure of the temperature of the ink liquid TFL (θ _T ≈ θ _TDK ). The signal of the temperature sensor TF is fed via an analog / digital converter AD in digitized form to a central control ZS of the ink printing device. The central control ZS can be a control device which is known per se and is necessary for the printing operation anyway, which electronically processes the data to be printed out and forwards it to an ink print head control TKS, which in turn activates the thermal converters via individual electronic switches (not shown here). In particular, the central control ZS can also be implemented by a microprocessor control known per se. 3, which is based on the characteristic shown in FIG. 2 and is stored in a memory SP of the central control system ZS, the pressure pulse energy is set as a function of the measured temperature θ _TDK . This is done by means of the parameters pulse duration T _I and / or the amplitude U of the pulse voltage, with which the thermal transducers are controlled via the ink printhead control TKS. The control voltage can be generated in a known manner via a voltage regulator.

5 shows a further embodiment of a device for switching from a high pressure pulse energy E _I to a lower energy E _I 'after reaching a predetermined limit temperature for the liquid ink. The solution according to the device described with reference to FIG. 4 is simplified here by a comparator with a bridge resistance network replacing the analog / digital converter and a pulse voltage source controlled by the comparator output. In the case of the ink-printhead control TKS, which is only shown in part here, only the shown those parts which are necessary for understanding the device according to the invention. In detail, these are printer-controlled electronic switches ES _i to ES _n provided according to the number of thermal converters R _Di for releasing the pulse voltage during the pulse duration, a pulse voltage source UBB and a comparison circuit K. The inkjet printhead TDK contains a large number of thermal converters which are known are integrated on a single substrate and have the heating resistors denoted by R _Dl to R _{Dn in} FIG. The individual heating resistors R _Di are assigned, via separate lines, for example in the form of conductor tracks, the individually controllable electronic switches ES ₁ to ES _n , which are connected to the actual print-generating device and have the task of converting the converter elements with their heating resistors R _Dl to R _{Dn to be} applied to the control voltage depending on the print character. For this purpose, a connection of the heating resistors R _{Di is made} to the collector of the corresponding electronic switch ES _i . The emitters of the electronic switches ES _i are connected to a reference potential 0 V, for example to the ground line. The other connections of the heating resistors R _Di are to all of the heating resistors R ~. common line connected via which they can be connected to a pulse voltage source UBB. The temperature sensor TF is additionally arranged on the ink print head TDK in the immediate vicinity of an ink reservoir (not shown here). In this exemplary embodiment, a thermistor is used as the temperature sensor TF, the resistance of which at a specific temperature of the ink liquid θ _{T is} referred to as the resistance R _TF . While one connecting wire of the temperature sensor TF is connected to the reference potential, its other connection is connected to the inverting input of a comparator K. This comparator K is with a operated with respect to the ground positive and negative supply voltage UCC. A series connection of two resistors R ₄ and R _{5 is} connected between the positive connection of the supply voltage UCC and the ground potential. An unspecified connection point between these two resistors is led to the non-inverting input of the comparator K. In addition, there is a resistor R ₆ between the positive connection of the supply voltage UCC and the inverting input of the comparator K. The output A of the comparator K is connected to the base of a switching transistor ST, the collector of which can be set via a resistor R ₂ to a setting input ADJ Voltage regulator SR is performed. The pulse voltage source UBB and the output terminal are connected to the input terminal V _{IN of} this voltage regulator SR

V _OUT is connected to the connection line common to all converter elements. In addition, a series connection of two resistors R ₃ and R _{1 is connected} to ground at the output V _{OUT of} the voltage regulator SR. The connection point of the two resistors R ₃ , R ₁ is led to the setting input ADJ. For example, the LM 317T module from Texas Instruments can be used as the voltage regulator SR

Find use. Since the device described is intended to reduce the pressure pulse energy after reaching a predetermined limit temperature for the ink liquid, this limit temperature (for example 50 ° C.) must first be set. This is done with the aid of a reference voltage divider consisting of the resistors R ₄ and R ₅ . The temperature θ _{T of} the ink liquid TFL is detected by the temperature sensor TF and is given by the measuring voltage divider consisting of the resistors R ₆ and R _TF . If the temperature of the ink liquid TFL is less than or equal to that of the counter If R ₄ and R _{5 were} set limit temperature, the output A of the comparator K is on a logical

State "low" corresponding level, the switching transistor

ST is thus blocked. The amplitude U of the pulse voltage at the output V _{OUT of} the voltage regulator SR is determined by the voltage divider resistors R ₃ and R ₁ (for example U = 20 V). If, for example, the temperature of the ink liquid TFL increases to a value which is above the limit temperature during the printing operation, the comparator K switches over and its output A is at a level corresponding to the "high" state. The switching transistor ST is thus driven and brought into the conductive state. Since the emitter of the switching transistor ST is connected to the reference potential, the resistor R _{1 is} additionally connected to the resistor R _{2 in} parallel. A greater voltage now drops across the voltage divider R ₁ , R ₂ , R ₃ thus modified, so that the amplitude of the pulse voltage U is reduced to a value U '(U', for example 18.4 V). As mentioned at the beginning and explained with reference to FIG. 3, the pressure pulse energy also decreases with the pulse voltage.

In the embodiment shown, the pulse duration T _I with which the thermal converter resistors R _Dl to R _Dn are activated in a pressure-controlled manner via the electronic switches ES ₁ to ES _{n was} kept constant (for example 6.5 μs). The output A of the comparator K can, however, in connection with the central control ZS of the ink printing device, not shown here, also for switching the pulse duration from T _I to T _I '

(For example, 5.5 μs) can be used, the amplitude U of the pulse voltage being kept constant, or else the pulse voltage and the pulse duration could be controlled together with the output A of the comparator K. The optimization of the pressure pulses has been presented in the present

Example described using a reduction of the pulse energy with increasing ink temperature. However, it is also within the scope of the invention to increase the pulse energy from a value corresponding to the ambient temperature, i.e. to increase the amplitude and / or the pulse duration. This could e.g. then be useful if the ink printing device is operated at extremely low ambient temperatures and the ink preheating provided to keep the ink constant is not sufficient to enable undisturbed printing operation.

Claims

1. Method for optimizing the pressure pulses (P _I ) in

Ink printing devices, their ink printheads (TDK)

have a large number of individual electrothermal transducer elements (RD _i ) which can be selectively controlled with an impulse voltage (UBB) via electronic switches (ES _i ), each of which is assigned an ink channel (TK), these transducer elements (RD _i ) being an ink liquid ( TFL) locally heat and thereby ink droplets (TTR) are ejected from nozzles of the ink channels (TK),

with the following features:

a) the viscosity (γ _T ) of the ink liquid (TFL) is compared to the temperature (θ _T ) of the ink liquid (TFL)

detected,

b) the temperature determined in this way (θ _T ) is compared with a predetermined limit value,

c) depending on the result of this comparison, the pulse energy (E _I ) and thus the pressure pulse (P _I ) of the measured temperature (θ _T ) of the ink liquid (TFL) is adjusted.

2. The method according to claim 1, d a d u r c h

characterized in that the adaptation of the pulse energy (E _I ) by changing the amplitude (U) of the pulse voltage (UBB) and / or the pulse duration (T _I ) is carried out in such a way that the temperature of the ink liquid (TFL) is higher than the limit Transducer elements (RD _i ) supplied pulse energy (E _I ) is reduced.

3. The method according to claim 1 or 2, d a d u r c h

characterized in that the temperature (θ _T ) of the ink liquid (TFL) is measured indirectly via the temperature (θ _TDK ) of the ink print head (TDK) by a temperature sensor (TF) arranged in the region of an ink reservoir.

4. The method according to claim 2, d a d u r c h

characterized in that the amplitude (U) of the pulse voltage (UBB) is changed by changing the external circuit resistances (R ₁ , R ₂ , R ₃ ) of an adjustable voltage regulator (SR).

5. The method according to claim 4, d a d u r c h

characterized in that the change in the wiring resistances (R ₁ , R ₂ , R ₃ ) from the output (A) one of the limit temperature and the measured temperature (θ _T ) of

Ink printhead (TDK) comparative comparator (K) is controlled.

6. The method according to claim 1, characterized in that the temperature signal (θ _T ) via a

Analog / digital converter (AD) is fed to the central control (ZS) of an ink printing device which contains a memory (SP) in which the values for the required pulse energy (E _I ) are stored depending on the measured ink temperature (θ _T ), with which the converter elements (R _Di ) are controlled via an ink printhead control (TKS).

7. Device for optimizing the pressure pulses (P _I ) in

Ink printing devices, the ink printing heads (TDK) of which have a large number of individual electrothermal transducer elements (R _Di ) which can be controlled selectively with a pulse voltage (UBB) via electronic switches (ES _i ), each of which is assigned an ink channel (TK), these transducer elements ( R _Di ) locally heating an ink liquid (TFL) and thereby dropping ink droplets (TTR) from nozzles of the ink channels (TK), with the following features:

a) on the ink printhead (TDK) is in the range of one

Inkπreservoirs arranged a temperature sensor (TF), the signals of which are fed to an ink printhead control (TKS),

b) the ink printhead control (TKS) contains one

Comparison circuit (K, R ₄ , R ₅ , R ₆ ), which with the

Temperature sensor (TF) measured temperature (θ _T ) of

Compares ink fluid (TFL) with a specified limit,

c) the comparison circuit (K, R ₄ , R ₅ , R ₆ ) is connected to a control device (SR) which supplies the pulse energy (E _I ) to the transducer elements (R _Di ) the measured temperature (θ _T ) of the ink liquid (TFL ) adapts.

8. The device according to claim 7, d a d u r c h g e k e n n z e i c h n e t that on the control device (SR)

Pulse voltage (UBB) is present and when the temperature (θ _T ) of the ink liquid (TFL) is higher than the limit, the amplitude (U) of the pulse voltage (UBB) is reduced.

9. The device according to claim 7 or 8, d a d u r c h

characterized in that the comparison circuit (K, R ₄ to R ₆ ) contains a comparator (K) with resistors (R ₄ , R ₅ , R ₆ ), the output (A) of which is via an electronic switch (ST) to the control input (ADJ ) the

Control circuit (SR) is performed.

10. Apparatus according to claim 7 or 8, characterized in that an adjustable voltage regulator is used as the control circuit (SR), the external resistance circuit (R ₁ to R ₃ ) is changed depending on the measured temperature (θ _T ) of the ink liquid (TFL).