SIMULATION | PROCESSING
the preparation and modification of highly filled, temperature and shear-sensitive plastics, especially formulations with stringent mixing specifications. However, determination of the optimum design and arrangement of the kneading elements in each case is usually based on experience or trials in a pilot plant. Due to the process complexity, Buss says there have only been a few systematic investigations on which to base analytical and numerical models for computer-aided optimisa- tion. The problem, the company says, lies in precisely defining and mathematically modelling the processes taking place in the co-kneader, where longitudinal mixing is axially superimposed on radial mixing due to the combination of oscillating screw movement and spatial interrup- tion of the screw flights.
Validation models Investigations with two different matrix materials on a Buss laboratory Kneader MX30 served as the basis for the validation of modelling and simula- tions. Screw elements with different flight geom- etries were examined, and the number of kneading pins, speeds and throughput varied. To obtain a deeper understanding of the processes involved, melt transport, full fillings, backflow lengths and residence times were recorded using measure- ments of the local filling level (which influences both dwell time and extruder performance). Alongside this, numerical flow simulations were
used to resolve experimentally undetectable variables, such as flow direction and shear rate. Analytical calculation models delivered process variables, such as temperature and input power along the extruder. Within the scope of numerical modelling, different kneading flight positions were sectioned and simulated, and the superimposed oscillating screw stroke at constant rotary speed was also taken into account. In total, two different kneading elements were simulated for each of two machine types and then compared with regard to the resulting velocity fields and shear rates. For analytical modelling, the geometry was split
into short sections and then described with param- eters such as number, height and width of screw channels, number of pins, and width of the interrup- tion between kneading flights. Together with melt material data, these parameters were used to calculate process variables. The model considered several variables relevant to the design, such as dwell time, melt temperature, filling level, power input and pressure over the entire extruder length. The numerical simulations were evaluated qualitatively based on the velocity field, and
www.compoundingworld.com
quantitatively by calculating the mean shear rate. The assumptions made were said to reflect practical experience, which means that initial tendencies can be predicted using quantitative data. For the analytical calculation, a tool was created
that enabled one-dimensional simulations with the developed co-kneader models on the basis of melt material, geometry and processing parameters. Comparing the calculated pressure, filling level and mixing characteristics with the test results showed good agreement. The researchers say that – taking into account the simplifications made for modelling – there was also good agreement between calcula- tion and test data with regard to dwell times, power input and mass temperature. According to the consortium partners, the
investigations enable identification of significant process parameters, thereby clarifying the interac- tions in the co-kneader between machine configu- ration, kneading characteristics and materials. The simplified calculation models are able to depict initial tendencies in variation of geometry and influencing factors and, for the first time, their implementation in a simulation tool enables important process variables in the co-kneader to be predicted in a few seconds. Even though only the simulations for the laboratory co-kneader have been validated, results are already said to be enabling the use of simulations for design configu- ration of co-kneader compounding processes.
CLICK ON THE LINKS FOR MORE INFORMATION: �
https://ktp.uni-paderborn.de (KTP) �
www.scconsultants.com �
www.ianus-simulation.de �
www.busscorp.com �
www.ikt.uni-stuttgart.de �
www.skz.de
March 2021 | COMPOUNDING WORLD 45 Above:
Modelling the superimposed combination of longitudinal and radial mixing in a kneader extruder is a challenge
IMAGE: BUSS
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62
