Computational and Experimental Modelling of Microlayer Forming Processes
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This work deals with the numerical and experimental investigation of polymer flow in the microlayer
coextrusion process. In particular, high-viscous polyolefins are considered which can be
used for extrusion of thermoplastic pipes. As many pressure pipes are reinforced with glass fibres,
the influence of fibre length on the rheological characterization of these compounds is investigated.
Numerical simulations based on both the finite element and the finite volume method
are carried out to design extrusion dies capable of producing flat or annular parts which can
be further used for mechanical or thermo-mechanical testing. Moreover, a novel solver for the
OpenFOAM toolbox is developed which incorporates the viscous dissipation of polymer melts.
One of the major problems of three-dimensional simulations is their requirements in terms of
high computational power and large memory, since great precision is essential to obtain good
results. Therefore, the so-called Network Simulation Method (NSM) is employed which is based
on the approach that the flow problem in a complex geometry can be solved by subdividing the
manifold into smaller, geometrically simpler lining segments for which analytical formulae
based on conservation equations are available. The resulting two-dimensional flow resistance
network can be solved in a manner analogous to network analysis of electrical circuits. Several
slit-exit cross-section dies are compared in terms of uniformity of the velocity profile at the die
outlet, residence time distribution, shear rate and pressure drop. Both viscous heating and shear
thinning are taken into account.
Different geometrical configurations of mixing elements (multiflux static mixer, interfacial surface
generator) are compared in terms of layer homogeneity. To evaluate the layer-forming process,
a reduced order approach based on trajectory calculations for a large set of material points,
followed by a statistical analysis is applied. Viscoelastic simulations are carried out and the occurrence
of secondary motions in square channels is investigated. The results of this study provide
deeper insights into the layer-forming process of high-viscous or viscoelastic melts.
Finally, a multi-layer pipe extrusion die is optimized using numerical simulations and a prototype
die is developed which can be used in combination with tailor-made mixing elements to
produce microlayer pipes.