TECHNOLOGY | PROCESS MODELLING
example LDPE, PP, PA) and a melt flow value to create a new material file. In such instances generic reference values are used for properties such as melt density or thermal behaviour and a suitable rheological model implemented to create the material file using only melt flow. “While the resulting material file is not 100%
accurate, multiple material files can be created quickly and easily to run simulations. For scale-up and screw design optimisation, use of a material file that approximates the actual machine torque and melt temperature provides acceptable results since the same material is used to model two configura- tions or two different machines,” says Dreiblatt. Polytech’s WinXTS software is distributed in the
Figure 4: The “Generic Property” specification tool in PolyTech’s WinTXS software allows users to specify polymer and melt flow value to quickly create a material file for comparing/optimising screw designs or scale-up between machines Image: PolyTech
Right: 3D model of a screen fitted above the support plate for CFD calculation
where can we get all of the data to run simulations? We are using more and more recycled resins, where do I find the rheological data?’. This part of setting up simulations is unfamiliar to most process staff, who may know something about the resin melt index, but not much more.” Although data can be obtained from resin suppliers or from material testing, it is not realistic for a typical compounder to collect all of the data required for all of the blends that are produced on each of their machines, says Dreiblatt. Faced with such a data challenge, some com- pounders will assume that simulation is too much work. PolyTech hopes to address this through the creation of cloud-based materials databases that will make it easier for compounders to use simula- tion programs. Online materials databases already exist for properties, but these databases do not have all of the required property data necessary for simula- tion, says Dreiblatt. This new capability is currently under development at PolyTech and is expected to be available in 2019.
Generic materials In the meantime, users of PolyTech’s WinTXS program can create new material files quickly using the “Generic Viscosity” tool (Figure 4). This feature allows users to specify any polymer type (for
36 COMPOUNDING WORLD | March 2018
US by Manufacturing Productivity Solutions and is available in Europe from Germany-based Extricom, which was acquired by CPM Century Extrusion early last year.
Filtering results Another use of simulation software – beyond the melting, mixing, and conveying zones – is to model the melt filter and die at the end of the extruder. Coperion is working on improving its models for flow through melt screens and on validating its existing models. “The pressure drop in the discharge equipment can be critical to the whole process,” says Markus Schmudde, Head of Research and Development at the company in Germany. “Melt filters are still a bit tricky to simulate due to multiple flow channels within the screen changer and the mesh itself. Especially in the screen area, elongational flow has to be considered. Simulating that elongational flow is challenging, and good material data is needed to set up the right model.” Coperion’s research enables its engineers to make more accurate calculations and to better understand how models may deviate from real equipment, so that deviation can be taken into considera- tion when optimising melt screens. The company uses FEM to design heating of die heads to
guarantee an even flow, which helps produce uniform
pellets. Modelling helps optimise designs in the development phase and enables calculation of the best position for temperature sensors that control die heating. During the design stage, modelling can also calculate the
www.compoundingworld.com
PHOTO: COPERION
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