DE102013111188A1 - Extruder with a degassing dome and a level sensor - Google Patents

Abstract

The invention relates to an extruder (10) having a degassing dome (18) and a filling state sensor which is arranged to determine a filling level (F) in the degassing dome (18). According to the invention, it is proposed that the filling level sensor (22) is designed to measure a change in the dielectric constant (εr).

Description

The invention relates to an extruder with a degassing dome and a level sensor, which is arranged to determine a level in the degassing dome.

Extruders are used, for example, to produce a sprayable mass from a granulate. To avoid bubbles, most extruders contain a degassing dome. This is a device by means of which a vacuum can be applied to the mass processed in the extruder, the extrudate, so that gas bubbles contained therein can be removed.

If the conveying speed of the extruder or the viscosity of the material to be extruded changes, it is possible that liquid or plasticized material or extrudate foam of gas and extrudate rises in the degassing dome extrudate. This is undesirable because of the risk that plastic material penetrates into the vacuum system, by means of which the degassing dome is subjected to negative pressure, penetrates and damages it.

From the DE 32 42 239 A1 a device for level monitoring of a degassing is known in which nitrogen is introduced via a feed line into the degassing dome with a slightly higher pressure than the prevailing pressure in the degassing dome. As the melt rises, it closes the nitrogen supply line and thus the pressure rise within it. From a certain backpressure pressure reacts a pressure switch and sends to a signal to a drive unit. A disadvantage of this method is that the pressure rise measurement is not a completely reliable method by means of which the rise of molten foam is detectable. If the rising foam is open-pored, nitrogen can continue to be flushed into the degassing dome, and there is nevertheless no increase in pressure and no pressure difference, so that the rising of the foam is not detectable.

From the DE 38 16 981 A1 a device is known in which a thermocouple registers the rising of plastic melt in the degassing dome. The disadvantage of this is that an increase of melt can be detected only relatively slowly. Another disadvantage is that with rising foam, the temperature difference between the atmosphere in the degassing dome and the melt can be so low that no usable signal is obtained. Ascending foam can therefore not be reliably detected.

The object of the invention is to determine the fill level in the degassing dome, both with respect to the melt and with respect to foam, with a higher process reliability.

The invention solves the problem by a generic extruder, wherein the level sensor is designed for measuring the dielectric constant.

An advantage of this extruder is that both rising melt and rising molten foam can be detected with high process reliability. This reduces the risk that extrudate, whether in the form of liquid or in the form of foam, will penetrate into the vacuum system and damage it.

It is also advantageous that the advantage of significantly improved process control does not have to be compensated with an increased effort. It is also favorable that for the detection of the level no consumable material, such as nitrogen gas, is necessary. The level detection is therefore particularly simple and process-safe. It is also favorable that such a level sensor can also be used in potentially explosive areas.

It is a further advantage that changes in the fill level can be detected quickly and sensitively. This allows for improved process control.

Synergistically, the above advantages result in the extruder having increased availability and improved product quality.

In the context of the present description, an extruder is understood to mean a device which comprises at least one motor-driven screw for heating, possibly kneading, and conveying extrudate. It is favorable if the extruder additionally comprises a mixing device by means of which a plurality of starting components, for example granules and / or fillers, can be mixed with one another. Often, extruders also include a heater.

According to a preferred embodiment, the extruder according to the invention has a vacuum device, by means of which a negative pressure can be applied to the degassing dome.

The feature that the filling level sensor is designed to measure a change in the dielectric constant is understood in particular to mean that the filling level sensor emits a signal, in particular an electrical signal, from which a change in the dielectric constant of the material in the degassing dome can be determined. It is possible, but not necessary that the dielectric constant is encoded directly readable, it is decisive that a change in the dielectric constant from the output measurement signal can be determined. Preferably, the level sensor is designed to output a signal from which the dielectric constant itself can be determined.

The dielectric constant is a material constant. Changes the composition of the material in the degassing, because, for example, in

Degassing dome more melt in the form of liquid or gas is present, then the dielectric constant of the total material changes in Entgasungsdom. In other words, the level sensor is configured to measure a change in the total dielectric constant of the material or mixture of materials present in the degassing dome. Alternatively or additionally, the level sensor is configured to measure the total dielectric constant of the material or mixture of materials present in the degassing dome.

It is possible, but not necessary, for any change in the dielectric constant of the material in the degassing dome to result in a change in the signal that the level sensor emits. For example, it is possible that fill level changes that occur below a predetermined level are not detected. It is generally sufficient that an increase in the fill level to a value that is greater than a desired fill level is detectable.

By way of example, the fill level sensor comprises a sensor element and a drive unit, which is designed to deliver an alternating electrical current to the sensor element and to determine the dielectric constant from a phase shift between a current of the alternating current and a voltage of the alternating current.

The reactance of a capacitor depends on the capacitance of the capacitor. The capacitance of the capacitor in turn depends on the dielectric constant of the material between the poles of the capacitor. The level sensor is inserted into the degassing dome so that it forms a condenser with the degassing dome. From the phase shift between the current intensity and the voltage of an alternating electric current applied between the filling level sensor and the degassing dome, it is thus possible to determine the capacitance change of this capacitor and, therefrom, the change in the dielectric constant. Of course it is advantageous, although not necessary, if not only a change in the dielectric constant can be determined, but the dielectric constant itself.

Common plastics to be extruded have a dielectric constant between ε _r = 2 and ε _r = 3, and it has been assumed that changes of such small dielectric constants as are caused by extrudate foam can not be reliably detected. However, it has been found that a sufficiently sensitive detection is very well possible, so that the filling level can be measured sufficiently reliably even when filled with extruded foam.

According to a preferred embodiment, the filling level sensor is also designed to measure the electrical conductivity. Although it is in the extrudate, ie the material that is processed by the extruder and may be a plastic or a polymer or other extrudable material, usually an electrical non-conductor, but that does not mean that not a small electric current can be measurable. This is especially true if the extrudate is provided with a filler, for example, so that the electrical conductivity differs sufficiently clearly from the conductivity of the air otherwise present in the degassing dome or the corresponding gas present in the degassing dome. By measuring the electrical conductivity, the significance of the measurement result is increased.

It is particularly advantageous if the fill level sensor is an admittance sensor, that is to say that the measurement result measures the complex electrical resistance or the complex electrical conductance of the material between the fill level sensor and the degassing dome.

According to a preferred embodiment, the filling level sensor comprises an electrical evaluation unit which has a digital memory. In the digital memory, a map is stored, which encodes a dependence of the dielectric constant or the electrical resistance or the admittance between the level sensor and the degassing on the one hand with the level of the degassing.

The map is determined for example by preliminary tests. For example, the extrudate is provided with a liquid or a gas. In question comes as a liquid water or an organic solvent. As the gas, for example, air or an inert gas can be used. The addition of this fluid causes the formation of an extrudate foam in the degassing dome. The fill level of the degassing dome can be determined, for example, visually through a window in the degassing dome. With this optically determined Level are assigned to the dielectric constant and / or the admittance. By varying amounts of added fluid, the fill level in the degassing dome can be varied so that a characteristic diagram for a multiplicity of fill levels and associated dielectric constants or admittances is obtained.

During operation of the extruder, the dielectric constant and / or the admittance is measured at regular intervals, for example every 100 milliseconds, but at least once every 2 seconds. From the measurement result thus obtained, the fill level is determined on the basis of the characteristic map and compared with a desired value or a desired interval. If the determined value for the level is outside the desired interval or above the desired value, a message is output. Since the dependence of the level on the dielectric constant and / or the admittance depends on the extrudate, the map must be re-recorded usually for each new extrudate. But that usually represents a relatively low cost.

According to a preferred embodiment, the extruder comprises a second level sensor, which is arranged in the same degassing dome as the first level sensor. In this way, a more accurate measurement result is obtained. It is possible, but not necessary, for the two level sensors to be read out independently of each other. For example, it is possible that at the same time at most one of the level sensors is supplied with a high-frequency signal. But it is also possible that both level sensors are acted upon simultaneously with a high-frequency signal. Of course, it is also possible that more than two level sensors are present, for example, three, four or more.

According to a preferred embodiment, the filling level sensor comprises a heater. This has the advantage that plastic vapor can not deposit on the level sensor. This advantage is particularly great in polyolefins.

Alternatively or additionally, the fill level sensor has a circulation device for flushing the fill level sensor with a gas, in particular an oxygen-free gas. For example, the gas may consist of more than 90% by volume of nitrogen.

In the following the invention will be explained in more detail with reference to the accompanying drawings. It shows

1 a schematic view of an extruder according to the invention according to a first embodiment and

2 a cross section through an extruder according to the invention according to a second embodiment.

1 shows an extruder 10 with a snail 12 that of a schematically drawn drive 14 is driven in the form of an electric motor. In the present case, it is a single-screw extruder, but the number of screws is irrelevant to the invention. The snail 12 extends along a longitudinal axis L.

In a section A of the snail 12 in a snail shell 16 is arranged, the extruder includes 10 a degassing dome 18 , In the present case, the degassing dome 18 is substantially cylindrical and extends along a cylinder axis Z, which is perpendicular to the longitudinal axis L. However, it is not necessary that the degassing dome 18 is cylindrical, nor that any existing cylinder axis Z extends perpendicular to the longitudinal axis L. The location of the degassing dome 18 However, it is irrelevant for the measurement of the level. The degassing dome 18 can in any position, for example, top, bottom and / or side, relative to the extruder 10 be arranged.

In the degassing dome 18 is a sensor element 20 a level sensor 22 arranged. The sensor element 20 extends over an active length B in the degassing dome 18 , To an envelope H of the screw 12 has the sensor element 20 a distance d that is small against the active length B. The envelope H is the area on which all the points of the screw that are too extreme are located 12 move when the worm turns. Figuratively speaking, the envelope is the area that the snail 12 would work out of a soft body if the snail were surrounded by this. The envelope surface is usually a cylindrical surface and corresponds at the points where the degassing dome 18 is not formed, to a good approximation of the inner surface of the bore in the screw housing 16 in which the snail 12 running. For example, the distance d is less than one tenth of the active length B.

The level sensor also includes a drive unit 24 , on the one hand with the sensor element 20 and on the other hand with the degassing dome 18 electrically connected. The degassing dome is usually earthed. The sensor element 20 is opposite the degassing dome 18 electrically isolated. The drive unit 24 outputs an alternating current with the frequency f = 100 kHz, and the voltage U = 2 volts. Favorable frequencies between 50 kHz and 300 kHz and voltages between 0.8 and 5 volts, in particular 1 and 3 volts.

From the phase shift φ between the current I and voltage U and from the current I and the voltage U, an admittance Y is calculated. The admittance is a complex variable whose real part is the conductance and whose imaginary part is the susceptible.

The extruder 10 includes an evaluation unit 26 connected to the control unit 24 connected to signal forwarding. Alternatively, it is also possible that the drive unit 24 Part of the evaluation unit 26 is. It is also possible that the evaluation unit 26 Part of an in 2 schematically drawn machine control of the extruder 10 is. The evaluation unit 26 determined from the electrical measurement data that the drive unit 24 recorded, the admittance Y.

The evaluation unit 26 includes a digital memory 28 in which a map K is stored. The map K is an embodiment of a mathematical mapping that assigns a fill level F _i to a plurality of admittances Y _i (i = 1, 2. The level F is a measure of how high an extrudate 30 that of the snail 12 is promoted in the degassing dome 18 stands.

In 1 is the level F = 0. However, the zero point of the filling level can be set arbitrarily. For example, it is also possible that F = 0 that level is set, above which a control intervention is necessary to a schematically drawn vacuum system 32 to protect, by means of the degassing dome 18 is kept at negative pressure.

1 also shows that the extruder 10 a second level sensor 34 has, which also with the drive unit 24 is connected and in the same way as the level sensor 22 works. The evaluation unit 26 calculated from the measured values of the two level sensors 22 . 34 a common measured value for the level F.

2 shows a cross section through a second embodiment of an extruder according to the invention 10 , It can be seen that the degassing dome 18 a degassing dome cover 36 which is releasably secured to a Entgasungsdomgrundkörper. It is also possible that the Entgasungsdomdeckel comprises a window, so that the level F is detected from the outside. In this way, the level sensor 22 be calibrated.

For calibrating the level sensor 22 in other words, to determine the map K, the extrudate 30 added a substance that causes a schematically drawn extrudate foam 38 forms. For example, the liquid is water. Through the degassing dome cover 36 Optically, the level F is determined. By means of the level sensor 22 At the same time, the admittance Y and / or only the electricity constant ε _r or the ohmic resistance W are measured. The measured variable or the measured quantities are linked together in the map K. After a sufficiently large number of measurements, the admittance Y at the level F can be deduced. For this purpose, the map K is interpolated. For example, more than 50 admittances for different levels F are recorded.

In operation, the evaluation unit determines 26 in a short time interval, for example, all Δt = 0.1 sec, the admittance Y and from it the level F. If the level F exceeds a target level F _soll so gives the evaluation unit 26 an alarm signal off. This alarm signal is sent to the machine control, for example 27 the extruder sent, which then the speed of the screw 12 elevated. Alternatively or additionally, the alarm signal is sent to a granule feeding device 40 sent, which then throttles the volume flow of polymer material. Again alternatively or additionally, the alarm signal is sent to the vacuum system 32 sent, for example, the negative pressure in the degassing dome 18 decreases or a vacuum pump turns off to prevent further increase in the level and to prevent suction of extrudate in the vacuum pump

It is also possible that two or three of the above measures will be taken. It is also possible that further, not listed here measures are taken.

Not shown in the figures is an electric heater with which the sensor element 20 is heated. In this way it is prevented that gaseous extrudate or constituents thereof on the sensor element 20 embezzled.

LIST OF REFERENCE NUMBERS

10

 extruder

12

 slug

14

 drive

16

 snail shell

18

 degassing dome

20

 sensor element

22

 level sensor

24

 control unit

26

 evaluation

28

 digital memory

30

 extrudate

32

 vacuum system

34

 second level sensor

36

 Entgasungsdomdeckel

38

 Extruded foam

40

 Granule feeder

A

 section

B

 active length

d

 distance

F

 level

H

 envelope

K

 map

L

 longitudinal axis

W

 ohmic resistance (real size)

Y

 Admittance (complex size)

Z

 cylinder axis

ε _r

 permittivity

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• DE 3242239 A1 [0004]
• DE 3816981 A1 [0005]

Claims (10)

Extruder ( 10 ) with (a) a degassing dome ( 18 ) and (b) a filling state sensor, which is arranged to determine a filling level (F) in the degassing dome (US Pat. 18 ), characterized in that (c) the level sensor ( 22 ) for measuring a change in the dielectric constant (ε _r ) is formed. Extruder ( 10 ) according to claim 1, characterized in that the level sensor ( 22 ) is designed for measuring the electrical conductivity. Extruder ( 10 ) according to one of the preceding claims, characterized in that the level sensor ( 22 ) - a sensor element ( 20 ) and - a drive unit ( 24 ), which is designed to deliver an alternating current to the sensor element ( 20 ) and determining the dielectric constant (ε _r ) from a phase shift between a current (I) of the alternating current and a voltage (U) of the alternating current. Extruder ( 10 ) according to one of the preceding claims, characterized in that the degassing dome ( 18 ) with the level sensor ( 22 ) forms a capacitor and that the level sensor ( 22 ) is arranged to measure a capacitance (C) of the capacitor and / or an electrical resistance between the level sensor ( 22 ) and the degassing dome ( 18 ). Extruder ( 10 ) according to one of the preceding claims, characterized in that the level sensor ( 22 ) is an admittance sensor for measuring the admittance (Y). Extruder ( 10 ) according to one of the preceding claims, characterized by an electrical evaluation unit ( 26 ), which has a digital memory ( 28 ), in which a characteristic diagram (K) is stored, which has a dependence of the dielectric constant (ε _r ) and / or the ohmic resistance (W) on the fill level (F) of the degassing dome ( 18 ). Extruder ( 10 ) according to one of the preceding claims, characterized in that the evaluation unit ( 26 ) is adapted to automatically determine the level (F) from the level sensor ( 22 ) measured admittance (Y), wherein the evaluation unit ( 26 ) is particularly adapted to automatically output a signal when the level (F) is beyond a predetermined target level. Extruder ( 10 ) according to one of the preceding claims, characterized by at least one second filling level sensor ( 22 ) in the same degassing dome ( 18 ) like the first level sensor ( 22 ) is arranged. Extruder ( 10 ) according to one of the preceding claims, characterized in that the level sensor ( 22 ) has a heater and / or a Umspülvorrichtung for flushing the level sensor ( 22 ) with a gas, in particular an oxygen-free gas possesses. Use of an admittance sensor as fill level sensor ( 22 ) in a degassing dome ( 18 ) of an extruder ( 10 ).

Images