Automated die optimisation for the coextrusion of recyclates
In coextrusion, interfacial flow instabilities frequently occur, leading to optical and mechanical defects in the final product. The Coex-Instabilities project investigates how such instabilities can be avoided by means of automated optimisation of coextrusion dies.
© IKVA decisive factor for product quality is a stable layer thickness distribution in the feedblock confluence region, where strong extensional flows or differences in viscosity may give rise to so-called wave instabilities, causing scrap and limiting process robustness. To improve flow stability, a comprehensive simulation environment is being developed in OpenFOAM. To this end, the viscoelastic properties of virgin and recyclate-based compounds are determined experimentally and represented using the Phan–Thien–Tanner material model. The simulations employ a volume-of-fluid approach to accurately resolve the layer interface within the die. The central methodological concept is an adjoint-based optimisation procedure that selectively adapts predefined geometric degrees of freedom of the feedblock. The total normal stress difference (TNSD) criterion is used as an assessment parameter. It identifies critical normal stress differences and thus indicates the onset of interfacial layer instabilities. On this basis, the simulations provide concrete guidance on how the die geometry should be modified in order to reduce such instabilities.
In addition, various material pairings are investigated systematically, generating a data set that reveals how properties such as extensional viscosity and relaxation times affect process stability. Selected operating points are subsequently validated in practical trials to ensure the transferability of the simulation results to industrial practice. The project thus contributes to the economical and high-quality use of recyclates in premium plastic products. Die optimisation and rapid, data-driven process set-ups help to reduce scrap and sustainably increase resource efficiency in packaging-relevant coextrusion.
Project data and funding
We would like to thank the DFG for funding the project (funding reference HO 4776/98-1) and the project partners for their cooperation.
Project duration: 01.03.2026 – 28.02.2027
