Automated optimization of coextrusion dies using the method of Lagrange multipliers for the minimization of flow instabilities

• Applicant: Professor Dr.-Ing. Christian Hopmann
• Subject Area: Plastics Engineering
• Term: since 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 541089602

Extrusion dies are used to continuously form products and semi-finished products. In the case of films that find their use as food packaging, several layers are required in order to meet the high demands on, e. g. transparency, barrier properties, sealability, mechanical properties and cost-effectiveness of the packaging. In most cases, the coextrusion process is suitable for the production of such film composites for economic reasons. Another application for coextrusion has been increasing in recent years as a result of demands for sustainability in plastics processing. Since recycled plastic is often visually and mechanically inferior to virgin material and is subject to stricter regulations when in direct contact with certain substances, coextrusion is used here to convert a former monomaterial product into a coextruded product in which the core of the product is made of recyclate that is encased in an outer layer of virgin material. One of the biggest challenges to the economics of coextrusion processes is flow instabilities at the interface. There are different types, one of which is the so-called wave instability, which occurs as a result of unsteady normal stresses in the confluence region of the die. It is - as of today - not known how wave instabilities can be reliably detected and prevented in the coextrusion process. In the proposed research project, essential basic knowledge will therefore be created by examining, on a simulation-driven basis, various material pairings and operating points regarding their tendency to wave instabilities. The aim is to find out which changes in the feedblock configuration lead to a reduction of wave instabilities. To achieve this goal, a simulation environment must first be created in which the coextrusion process is modeled and in which interface instabilities are predicted using the Total Normal Stress Difference (TNSD) criterion. Furthermore, this simulation environment will be extended by algorithms for the automatic optimization of the flow channel geometry regarding interface instabilities. The TNSD criterion and the surface sensitivities are used to indicate by which adjustment of the geometry the boundary layer instabilities can be reduced. In laboratory experiments, the optimization method is validated and a method is developed to adjust the feedblock according to the optimization results. Finally, this simulation environment is used to create a data set that can provide information on which material pairings tend to exhibit which TNSD behavior. From the data set, characteristic diagrams and design guidelines are derived.

• DFG Programme: Research Grants