Patent Application
20180194037
The present invention concerns a mixing device having the features of the classifying portion of claim 1.
The invention can be particularly preferably used in the production of foamed plastics (for example in the form of plastic seals). To achieve a desired microstructure of the plastic it is necessary for a pressure which is in a given pressure window to apply in the mixing device. If there is a gas nucleation of one of the two liquid plastic components the given pressure window also depends on the gas nucleation. If the desired discharge amount from the discharge opening varies that can result in the microstructure of the foamed plastic being of a severely fluctuating quality.
It is known that variations in the quality of the microstructure can also occur upon fluctuations in temperature or viscosity of the first and second liquid plastic components and due to contamination in the mixing device.
Measures are therefore already known to stabilize the temperature, to remove contamination and to keep the viscosity constant, and to use different nozzles for adaptation to different discharge amounts. Thus, DE 42 35 850 B4 discloses a mixing device of the general kind set forth, whose agitator performs a dual function, more specifically on the one hand the agitator function and on the other hand the function of closing the discharge opening. To perform the closure function the agitator is arranged in a mixing chamber of the mixing device in such a way that it can be raised and lowered. The outlet nozzle housing is interchangeable to be able to provide for adaptation to different discharge amounts.
EP 1 123 180 B1 also discloses a mixing device of the general kind set forth, whose agitator performs a dual function as an agitator device and a closure device. To avoid abruptly occurring opening and closing movements of the agitator, which in the case of processing sensitive materials, can give rise to declines in quality in the workpieces produced from the agitated mixture, the drive unit for raising and lowering the agitator is equipped with cam drive, the actuating cam of which is of a cam profile with a steady or jerk-free configuration.
The previously known mixing devices have not been concerned with the quality of the microstructure of the plastic foamed from the mixture so that, when the known mixing devices are used, the quality of the microstructure can in part vary greatly.
The object of the invention is to achieve a uniform quality of the microstructure of the foamed plastic.
That object is attained by a mixing device having the features of claim 1.
The invention is based on the surprising realization that it is possible to achieve a uniform quality for the microstructure of the foamed plastic automatically by keeping the pressure in the mixing device constant as in that way changes in the discharge amount, fluctuations in temperature and viscosity of the first and second liquid plastic components, contamination which in fact influence the pressure in the mixing device in an unpredictable fashion can be eliminated.
The level of the pressure in the mixing device is inter alia dependent on the size of the gap between the agitator and the discharge opening and on the size of the discharge opening. Further parameters which influence the pressure like for example the dimensioning of the nozzle can be viewed as being constant. By determining the pressure which prevails in the mixing device and adjusting the gap or the size of the discharge opening by means of sensors, a regulating device and an adjusting device, the pressure in the mixing device can be set to a desired predetermined value and can be kept substantially constant.
The proposed mixing device is particularly advantageous if the mixing device is operated in the low-pressure range with a pressure prevailing in the mixing device of below about 20 bars. In that pressure range a gas which for example is present in the first liquid plastic component—for example air dissolved in polyol—can already bubble out in the mixing chamber. The gas which has already bubbled out in the mixing chamber serves as a condensation nucleus and provides for uniform foaming of the mixture upon being discharged from the mixing device.
It was established by trials that, with the proposed mixing device, it is possible to achieve a uniform microstructure for a wide range of different discharge amounts. Thus for example up doubling or tripling of the discharge amount, it was possible to achieve a substantially unchanged microstructure and thus a uniform microstructure quality.
The first liquid plastic component can preferably be a viscous liquid. Examples are polyols, silicones, epoxy, polyester, acrylates and MS-polymers.
Examples of the second liquid plastic component are isocyanate and silicones.
Examples of the gas with which the first and/or second liquid plastic component can be mixed are air, nitrogen and carbon dioxide.
It can be provided that the gas is present in at least partially dissolved condition in the first and/or second liquid plastic component.
Further examples of systems comprising at least two components are:
According to a preferred embodiment it can be provided that the agitator is adapted to be displaceable relative to the discharge opening. In that respect it can be provided that the adjusting device is connected to the agitator, in which case the agitator is moveable relative to the discharge opening by the adjusting device.
It can be provided that the adjusting device includes a linear motor which is preferably arranged on a shaft of the agitator.
It can be provided that the discharge opening is adapted to be variable in size. In that respect it can be provided that the adjusting device is connected to the discharge opening, where the size of the discharge opening is variable by the adjusting device. The adjusting device can be for example in the form of an iris-like aperture, the discharge opening being formed by the central region, that remains free, of the iris aperture. Alternatively the adjusting device can be in the form of an elastic element in the manner of a hose clip, the discharge opening being formed by the internal width, that remains free, in the hose.
If the mixing device is of an elongated configuration it may be advantageous if the at least one feed opening for the at least one first liquid plastic component and the at least one further feed opening for the at least one second liquid plastic component are arranged at different axial positions of the mixing device. Because mixing of the two liquid plastic components first occurs in the region of that feed opening which is closer to the discharge opening no or only slight contamination occurs in the region disposed axially above same.
It can be provided that the discharge opening of the mixing device is connected to a nozzle.
Preferably it can be provided that the predetermined value for the pressure prevailing in the mixing device is less than 20 bars, preferably less than 10 bars, particularly preferably between about 2 and about 4 bars.
Protection is also claimed for an arrangement comprising a mixing device according to at least one of the preceding embodiments and a source for the at least one first liquid plastic component, that is connected to the at least one feed opening for the at least one first liquid plastic component, wherein preferably the at least one first liquid plastic component is mixed with a gas, and a source for the at least one second liquid plastic component, that is connected to the at least one further feed opening for the at least one second liquid plastic component, wherein preferably the at least one second liquid plastic component is mixed with a gas.
Embodiments of the invention are discussed with reference to the Figures in which:
To produce the mixture consisting of the first and second liquid plastic components the first liquid plastic component which is mixed with air in this example and which is provided by a source 10 is introduced through the first feed opening 2 and the second liquid plastic component which is provided by a further source 11 is introduced through the second feed opening 3 at different axial positions of the mixing device 1 which here is of an elongated configuration. Mixing of those two components is effected by the rotating agitator 7.
Provided adjacent to the tip of the agitator 7 is a discharge opening 4 for discharge of the mixture comprising the first and second liquid plastic components from the mixing chamber 12. The mixture can be discharged from the discharge opening 4 in the direction of a nozzle 9 by way of a gap 6 remaining between the agitator 7 and the discharge opening 4.
There is provided an adjusting device 5 by which a relative movement between the discharge opening 4 and the agitator 7 can be produced and thus the gap 6 between the agitator 7 and the discharge opening 4 is variable. The adjusting device 5 has a stepping motor which engages a gear by way of a shaft, the gear laterally engaging into a screw sleeve arranged concentrically with respect to the shaft 16. Rotation of the gear by the stepping motor causes axial displacement of the screw sleeve whereby height adjustment of the agitator 7 is effected, thereby setting the gap between the agitator 7 and the discharge opening 4. Measurement of the adjustment made can be implemented by way of the diagrammatically illustrated scale and communicated to the regulating device 14 (the corresponding signal lines are not shown). During the metering operations air is introduced into the region above the seal 15 in the direction of the diagrammatically illustrated arrow, which leads to lifting of the agitator 7 against the force applied by the force storage means 8 until reaching the abutment condition defined by the screw sleeve. When the air pressure is switched off the force storage means 8 moves the agitator 7 in the direction of the discharge opening 4 (downwardly in this view) to such an extent that the agitator 7 closes the discharge opening 4.
To determine a pressure prevailing in the mixing device 1 or the mixing chamber 12 thereof, there is provided a sensor 17 which is in signal-conducting relationship with a regulating device 14 by way of a signal line 18. The regulating device 14 in turn is in signal-conducting relationship with the adjusting device 5 by way of a control line 19 so that the regulating device 14 can set the gap 6 by way of the adjusting device 5 in such a way that the pressure prevailing in the mixing device 1 or its mixing chamber 12 is at a predetermined value of for example less than 20 bars. As an alternative to the illustrated positioning of the sensor 17 it is also possible to conceive other positions, for example in the line 10.
In the present embodiment the pressure conditions are so selected that there is a constant pressure in the mixing chamber 12, which is sufficiently low that a foaming agent present in the mixture can foam out.
If, during or after a process of discharging the mixture from the mixing device 1, the pressure in the mixing device 1 or its mixing chamber 12 changes that change in pressure can be detected by way of the sensor 17 and the regulating device 14 can actuate the adjusting device 5 in such a way that that change in pressure is counteracted by a corresponding relative movement as between the agitator 7 and the discharge opening 4 and the desired predetermined pressure is maintained in the mixing device 1. In particular suitable pressure adaptation can be implemented after each discharge process or after each metering operation.
A seal 15 (which can act sealingly in one direction or in both directions) sealingly surrounds the rotating shaft 16 of the agitator 7 and seals off the mixing chamber 12 in the region of the shaft 16 of the agitator 7. The seal 15 can for example comprise Teflon and can permit smooth axial movement of the rotating shaft 16. By virtue of the fact that the shaft 16 rotates and the seal 15 causes slight friction it is possible to achieve very fine positional adaptation of the agitator 7. In particular it is possible in that way to avoid unwanted pressure fluctuations due to abrupt axial displacement of the agitator 7 as a consequence of a stick-slip effect. The axial displacement of the agitator 7 is also facilitated by the rotation of the shaft 16. Preferably the seal 15 is in the form of a rotational and translational seal.
As a difference from the illustrated configuration the adjusting device 5 for example can also include a per se known cam drive, spindle drive or hydraulic drive.
| Number | Date | Country | Kind |
|---|---|---|---|
| A 436/2015 | Jul 2015 | AT | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/AT2016/050228 | 6/24/2016 | WO | 00 |
