process simulation | CAE Figure 1:
Results of a 1D simulation using ZSKalc to obtain temperature, pressure, degree of fill, and specific energy input over the extruder length.
Image: Coperion
equipment can verify whether the model accurately predicts real-world results. These challenges do not mean, however, that it is
not worthwhile to broaden the use of simulation software. One incentive that justifies investment in simulation is the ever-increasing manufacturing cost and dwindling lot size, says Dreiblatt. “These two factors mean that average runs are now very short and changeover time becomes critically important for profitability. When scaling-up a new product to the production line, start-up at or near the correct operat- ing conditions is enabled with 1D simulation software. Without such software, it can take hours or days of plant trials and wasted material; the cost of plant trials
is now approaching, and in some cases exceeding, the cost of the software.” For global companies, simulation provides a model
that can be used to accurately predict capacity and quality for producing the same compound at any location. And for any compounder, with the right data and process understanding, simulations can replace expensive and time-consuming experiments on an extruder. The following sections give some examples of how simulation is being successfully used in the compounding industry.
Scaling to production Scale-up from laboratory or pilot to commercial scale is typically based on experience and classical scale-up rules, but it can be aided by simulation software. Coperion uses its own proprietary 1D-simulation software, ZSKalc, as well as the SIGMA 1D software for comparison. Engineers also use commercial 3D CFD programs (Fluent, Star CCM, Polyflow and XimeX, for example) to simulate local process conditions as input for the 1D simulation. The most important raw material characteristics to have are melt density, heat transfer coefficient as a function of temperature, and viscosity at a range of shear rates, according to Anderson and Lechner. In some cases, additional information regarding the elasticity of polymer melts (which are dependent on melt temperature and shear rate) is important, adds Schmudde. Detailed mechanical dimensions of the extruder are also important for an accurate simulation.
Figure 2: This chart of mechanical power input versus power for an MFI 5 PP running at 600rpm was created using PolyTech’s WinTXS software and shows a decrease of specific energy input as feed rate is increased Source: CPM Century Extrusion
18 COMPOUNDING WORLD | March 2017
Optimising downstream screw design With accurate input data, simulation of flows after the melting and mixing sections is quite accurate, says Schmudde, and simulation can be used to optimise the
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