RU2350926C1 - Method for determination of paint viscosity in electric drop jet marking device - Google Patents

Abstract

FIELD: physics, measurement.

SUBSTANCE: invention is related to metering equipment and may be used in electric drop jet marking printers. Method for paint viscosity definition in electric drop jet marking device includes periodic supply of identical paint portions by pump for the time of pump suction and discharge into hydraulic load, measurement of average pressure P_av at pump discharge, stabilisation of specified pressure level by means of selection and measurement of pump action period duration T_pump required for that by means of control unit connected to pressure detector and pump drive, and paint viscosity η_paint is defined according to formula η_paint=K×P_av×T_pump.

EFFECT: higher accuracy, fast action and frequency of viscosity measurement.

3 cl, 3 dwg

Description

The invention relates to measuring equipment and can be used in electro-inkjet marking printers.

To ensure high-quality printing and reliable operation of the electro-ink jet technology, it is necessary to constantly measure the viscosity of the working fluid (inks, ink) and control this parameter, for example, stabilize it by dosing topping up the solvent.

There are two main methods for measuring the viscosity of a liquid both in laboratory conditions during the study or preparation of paints, and directly in electro-ink jet printers during their operation to control the parameters of the liquid.

The first method for determining the viscosity of a liquid is based on the Stokes law

F _Tr = 6πηrυ,

where F _Tr is the friction force in the liquid, υ is the speed of the ball relative to the liquid, r is the radius of the ball, η is the dynamic viscosity.

The determination of viscosity based on a Happler viscometer is based on this law. At the same time, a ball is lowered into a tube of a certain diameter filled with the test liquid and its fall rate or the time of flight of certain marks on the tube are measured, which are a measure of the viscosity of the liquid. It is taken into account that during laminar flow around a falling ball, the friction force is balanced by the weight of the ball minus the static lifting force / (1) - p. 129; (2) - p. 202 /.

The disadvantages of this method of measuring viscosity include the significant dimensions of the measurement setup and the difficulty of miniaturizing it, the difficulty and complexity in automating the measurement process, and the significant time required for the measurement process.

The second method for measuring viscosity is based on Newton's hypothesis, later substantiated by prof. N.P. Petrov, which characterizes the friction force between the fluid layers / (3) - p. 11; (1) - p. 126; (2) - p. 188 /

F _Tr = ηFdυ / dy,

where F _Tr is the friction force between the fluid layers, F is the area of the contacting layers, dυ / dy is the velocity gradient in the boundary layer, or if the plate is moved parallel to a flat wall in the fluid, at a distance between the plates “a” less than the boundary layer, then

F _Tr = ηFυ / a.

This method is based on the so-called rotary or drum viscometers, in which, for example, the glass rotates at a given speed in a fixed glass, between the walls of which a test fluid is located in a thin layer. In this case, the force of action on one of the glasses is dynamically recorded. The configuration of bodies immersed in a liquid can be different, as well as the method of measuring the friction force or directly viscosity, as, for example, in / 4 /.

This method has all of the aforementioned disadvantages: cumbersomeness, automation complexity, long measurement duration.

There is also a method of measuring the viscosity of a liquid and a device for measuring the viscosity of a liquid by this method, while the known method is based on the Poiseuille formula / (2) - p. 201; (1) - p. 128 /, which describes the process of laminar flow of liquid through a thin tube (capillary):

where V is the volume of fluid flowing through the tube over time t, R is the radius of the tube, Δp is the pressure difference at the ends of the tube, l is the length of the tube, η is the dynamic viscosity. In particular, this method of measuring viscosity is implemented in industrial viscometers of capillary glass type VPZH / (5); (6) /.

The method involves filling (pumping) a test liquid into a tank of a certain volume, and then the time of expiration of a specified volume of liquid under the influence of hydrostatic (gravitational) pressure is measured, i.e. by gravity through a calibrated capillary glass channel with defined dimensions.

The dynamic viscosity of a liquid is determined by the ratio

η = κρt,

where η is the dynamic viscosity, κ is the constant of a particular viscometer, ρ is the density of the liquid, t is the time of fluid outflow.

The disadvantages of this method and device for measuring the viscosity of a liquid are cumbersome, the complexity of embedding in a printer and the complexity of automating the measurement process, as well as the significant measurement time.

The so-called exponential method for measuring the viscosity of a liquid is known / 7, p. 146; 8/.

Its essence lies in the fact that in the process of measuring the viscosity of a liquid, including pumping liquid into a battery (receiver), its subsequent outflow through a capillary channel and measurement of flow parameters, which determine the viscosity of a liquid, the liquid is pumped into the battery by means of a pulse pressure source, and the process of fluid flow through the capillary channel measures the pressure drop between two pressure levels, which determine the instantaneous (current) viscosity of the fluid.

The pressure decay time is measured periodically with the frequency of the pressure pulses when the liquid is injected into the battery.

The device for measuring the viscosity of a liquid in the hydraulic system of an electro-inkjet printer according to this method consists of a pulse pressure source and a battery connected in series, and a capillary channel and a pressure decay time measuring device connected to the battery are additionally introduced, for example, a piezoresistive pressure sensor.

The liquid under pressure from a pulsed source of overpressure, for example, is supplied periodically or once to several atmospheres to a battery (receiver), for example, of a spring type, which can be considered as a process of charging a hydraulic tank, and then the tank spontaneously discharges through the capillary channel, i.e. . hydraulic resistance. In this case, the time of a short-term pressure drop is measured, i.e. part time of the transition process between two pressure levels. The measurement is automatically carried out, for example, using a piezoelectric pressure sensor in the measuring unit, and the measured part of the transient is one or tens of milliseconds.

In this case, the battery discharge transient is described by the relation

p (t) = P _max e ^{-t / τ} _per ,

where p (t) is the current pressure value, P _max is the maximum pressure, e = 2.718, τ _per is the transient time constant, which is measured.

Moreover

τ _lane = C _g R _g = kη,

where C _g , R _g - respectively, the hydraulic capacity of the battery and the hydraulic resistance of the capillary channel, k is the constant of the measuring circuit, η is the dynamic viscosity of the liquid.

Known hydraulic systems for electro-ink jet markers / (7, p. 149), (9) /, which include a discharge circuit (ink tank, filter, pump, battery, pressure sensor, valve, droplet generator in the print head), suction circuit (print trap heads, a filter, a pump — a source of rarefaction, a paint tank), a viscometry circuit (including a measuring receiver, a bypass channel, a pressure sensor), a chain of dosed solvent topping from the solvent tank to the paint tank, for example, using a pump. All processes in the hydraulic system of the electro-ink jet marker are controlled from the control unit.

The disadvantages of the known hydraulic systems with viscometry and dosed topping up the solvent are insufficient accuracy, speed and reliability of devices that measure viscosity, and the significant dimensions of these devices.

The proposed technical solution is aimed at eliminating the disadvantages of the considered methods for determining the viscosity of paints in the markers.

The essence of the proposed technical solution is as follows.

The method for determining the viscosity of the paint in an electro-ink jet marker includes periodically supplying the pump with the same portions of paint during suction and forcing the pump into the hydraulic load, measuring the average pressure P _cf at the pump outlet, stabilizing the set pressure level by selecting and measuring the duration of the pump action period T _us by means of a control unit associated with a pressure sensor and a pump drive, and the paint viscosity η _cr is determined by the formula

η _cr = K × P _cp × T _us ,

where K is the constant coefficient of the hydraulic circuit.

As a hydraulic load, a parallel bypass channel and nozzle are used, and pressure pulsations at the pump outlet are smoothed by the receiver.

The proposed method for determining the viscosity of the paint can be justified using Ohm's law for the hydraulic circuit

where R _g is the resistance of the hydraulic circuit, P is the pressure, Q is the volumetric flow.

Using Pusail's law, one can obtain

where V _us , T _us - the portion size of one pumping stroke (constant set value) and its period, K ₁ - constant coefficient of the hydraulic circuit, depending on geometric parameters.

From this we obtain the basic relationship for determining the viscosity of the paint

η _cr = K × P _cp × T _us ,

or more strictly

Thus, knowing the value of the average pressure during the pump period and the value of the pump period T _us , i.e. of the total time of pumping t _naked and suction t _suction , as well as the geometric constant, you can uniquely calculate (determine) the absolute value of the dynamic viscosity of the paint η _cr .

The proposed technical solution is simple and economical to implement, since the elements already existing in the hydraulic circuit of the electro-ink jet marker are used: pump, pressure sensor, hydraulic load.

Figure 1 shows a schematic diagram of a hydraulic circuit of an electro-ink jet marker; figure 2 is a timing diagram of pressure, figure 3 is a structural diagram of a hydraulic circuit.

Figure 1 is an equivalent diagram of a hydraulic circuit including a paint reservoir 1, a pump 2 with a drive in the form of a stepper motor, a collector motor or an electromagnet, a receiver (battery) 3, for example, with an elastic rubber wall, with a spring or with an air chamber, a pressure sensor 4, for example, of the piezoelectric type, a hydraulic load 5, 6, consisting of a parallel bypass channel R _pc (0.25 mm in diameter) and a nozzle element channel R _c (60 μm in diameter).

The circuit works by analogy with the electric circuit (p≡U, Q≡I, R _g ≡R).

Figure 2 shows a timing chart of pressure changes in the hydraulic load circuit. The pump period includes the discharge time and the suction time. The instantaneous value of pressure changes exponentially. The average value for the period is determined by the parameters of the circuit R, C, i.e. the receiver’s hydraulic capacity C _g and the equivalent hydraulic load resistance R _g , which is represented by a parallel bypass channel and a nozzle channel (R _p << R _e ).

Figure 3 shows the structural (functional) diagram of a real hydraulic circuit of an electro-ink-jet marker. It includes a paint injection circuit: paint tank 7, filter 8, injection cylinder 9, receivers 10, 11 for smoothing pressure pulsations, pressure sensor 12, filter 13, solenoid valve 14, nozzle channel 15 (shown conditionally). The bypass circuit and the reset circuit starts from the valve 14 (located in the printhead) through the manual valve 16, the bypass channel 17 and ends in the paint tank. The trap chain consists of a trap of unused drops 18 (shown conditionally), a filter 19, a suction cylinder 20 and ends in a paint tank.

The discharge, suction, bypass and discharge circuits operate according to the standard scheme, i.e. as in a conventional electro-ink jet marker.

To determine the viscosity of the paint, an existing pressure sensor 12 (DD) can be used, for example, of the piezoelectric type (a liquid pressure sensor developed by the applicant based on a MOTOROLA gas pressure sensor). A step drive or a sync disk with a collector motor make it easy to determine the instantaneous (current value) period of the pump. The control unit of the entire marker (not shown), which, in addition to other typical connections, is connected to a pressure sensor and pump drive, allows you to stabilize the pressure at a given level by changing the period (or frequency) of the pump and constantly determine the instantaneous viscosity of the paint by the formula η _cr = K × P _Wed × T _us .

When the viscosity increases above a predetermined critical level, automatic topping up of the required amount of solvent is activated (the auto-feeding circuit is not shown).

At constant pressure in the hydraulic chain, the current viscosity (in relative units) can be the period or frequency of the pump, or the number of revolutions of the drive (or strokes), or the frequency of the sync disk pulses on the drive shaft, etc. This further simplifies the process of determining paint viscosity.

In real continuous electro-inkjet printers using high-pressure droplet emission with ultrasonic crushing / 7 /, in the case of using fast-volatile paints based on alcohol, methyl ethyl ketone, etc. it is necessary to constantly monitor the viscosity of the paint in the system and in the case of critical thickening of the paint, i.e. reaching the threshold viscosity, it is required to automatically carry out a metered addition of solvent into the paint.

The proposed method for determining the viscosity allows with high accuracy, promptly, with a pump frequency (2-10 Hz), i.e. for each period, to control, measure and adjust (stabilize) the most important printing parameter - ink viscosity.

Thus, the proposed technical solution, in contrast to the known solutions, provides the following:

- improves accuracy, speed, frequency of viscosity measurement;

- allows you to optimize this process, reduce the size of structures, which allows you to embed a viscometer in the electro-ink jet markers, in this case, the print quality and the reliability of printers are improved;

- at the same time, it allows the use of the available parts of the electro-ink jet marker (pressure sensor, pump, bypass channel), i.e. the process of determining the viscosity of the paint can be carried out without complicating the design and cost of the marker.

Sources of information taken into account

1. Kuhling X. Handbook of physics. M., World, 1982

2. Paul R.V. Mechanics, acoustics and the theory of heat. M., Science, 1971

3. V.G. Geyer, V.S. Dulin, A.G. Borumensky, A.N. Zarya. Hydraulics and hydrodynamics. M., Nedra, 1981

4. The brochure of the rotary viscometer Rheomat 108 of the Swiss company DONAU (copy attached).

5. GOST 10028-81. Glass capillary viscometers, specifications. M., USSR State Committee for Standardization.

6. Viscometer capillary glass VPZh-3. Passport, M., Ministry of the USSR, 1982 (copy attached).

7. Bezrukov V.I. Basics of electro-jet technology. SPb., Shipbuilding, 2001.

8. RF patent No. 2196317.

9. RF patent No. 2212633.

Claims (3)

1. A method for determining the viscosity of a paint in an electro-ink jet marker, including the periodic pump supplying the same portions of paint during suction and pumping of a pump into a hydraulic load, measuring the average pressure P _cf at the pump outlet, stabilizing a given pressure level by selecting and measuring the desired duration of action pump T _us , through the control unit associated with the pressure sensor and the pump drive, and determining the viscosity of the paint η _cr by the formula
η _cr = K · P _cp · T _us ,
where K is the constant coefficient of the hydraulic circuit. 2. The method according to claim 1, characterized in that the pressure pulsations at the pump outlet are smoothed out by the receiver. 3. The method according to claim 1, characterized in that as a hydraulic load using a parallel bypass channel and nozzle.

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