process simulation | CAE
back pressure of flow channels in discharge parts. This tool is used for the layout of extrusion lines as well as for optimisation of discharge parts.
Figure 3: Extruder throughput versus screw speed simulation data for Woodforce wood fibre filled PP carried out using SCC Ludovic software for two different screw profiles. Source: Scion
design of the devolatilisation and pressure build-up sections at the downstream end of the extruder. Simulation can be used to calculate the optimum screw pitch in the pressure-building sections of the screw and to calculate backup length (the length of screw that is fully filled in the pressure build-up section) for a given throughput and screw speed, so that vents can be appropriately placed in devolatilisation sections.
Modeling discharge back pressure Simulation is also valuable for modelling the pressure drop in discharge equipment (such as screen changers, die plates and pelletisers). Particularly for large extruders, discharge parts can be too big and too expensive to produce prototypes for tests, says Schmud- de. Coperion developed a simulation tool to calculate the
Predicting specific energy values Coperion uses simulation to optimise specific energy input by comparing one screw design to another. Variables such as screw pitch do affect energy input, particularly for large twin-screw extruders used by polyolefin resin producers. Because these extruder designs tend to have fewer mixing elements (kneading blocks), small changes have a stronger influence compared to changes in the designs typically used for compounding of specialty or filled compounds. At the Compounding World Forum in December
2016, Dreiblatt demonstrated how 1D-simulation can be used to investigate the role of specific energy (a single value that represents the mechanical energy needed for melting, mixing, conveying, and pressurisation in a twin-screw extruder) in a typical specialty compounding extruder design. Using the PolyTech WinTXS simulation software, he said it was found that 70-80% of the total specific mechanical energy is consumed in the first kneading elements to raise the temperature of the solid polymer to the melting point. He concluded that, for a typical compounding screw design, energy cannot be reduced dramatically by changing screw designs after the melting stage because the energy is primarily needed for melting the polymer. The simulation also showed that machine operating conditions can make some difference—there was a small increase in energy with increasing screw speed, some decrease in energy with increasing feed rate (Figure 2), and virtually no change with changes in barrel temperature.
Six steps to optimised production
Ludovic, a 1D simulation program from Sciences Computers Consultants, is used to identify scale-up conditions and to optimise the compounding process for a given product or recipe. The software can be thought as “trial scheduler” that dramatically reduces the number of physical trials that need to be run, according to Philippe David, General Manager at SCC. A simulation can help increase
understanding about the process and can help in decision making. He explains that a simulation procedure for a new setup
20 COMPOUNDING WORLD | March 2017
should generally follow these steps: l Minimal experimental data is meas- ured for an initial configuration (for example, product temperature at die exit, pressure sensor data). l This initial configuration is simulated based on physical models, geometry simplification, and material mechanical and thermal data. l The simulation is compared with the experimental data to identify trends (not to achieve a perfect correlation). l Several numerical trials are simulated, making assumptions for variables such
as flow rate, screw profile, and barrel temperatures, to identify good candidates for further evaluation. l A Design of Experiments (DoE) can be run to further explore the operating domain and the dependency and sensitivity of process conditions. This DoE can include hundreds to thousands of simulations, with only a few seconds needed per simulation. l Run real trials (pilot scale) to confirm the best conditions suggested by the simulation. ❙
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