Experimental validation of non-Newtonian stratified co-extrusion prediction models using a digital process twin

In the co-extrusion of plastics, pressure-throughput behavior, layer distribution, and residence time are crucial parameters, modeling of which contributes to manufacturing high-quality products at optimized process efficiency and significantly shortens development times for die systems. In previous work, we have presented symbolic regression models to predicting the (i) pressure-throughput behavior, (ii) position of the interface, (iii) interfacial shear stress, (iv) ratio of volume flow rates, and (v) interfacial flow velocity for isothermal two-layer co-extrusion flows through rectangular ducts. These regression models are mathematically simple and capable of capturing the shear-thinning nature of polymer melts without the need for numerical methods. Here, we present an experimental study validating the proposed models against co-extrusion process data and comparing them to existing theories. To this end, a two-layer co-extrusion demonstration die instrumented with an optical coherence tomography sensor for detecting the interfacial position was used. To accurately set up and evaluate the die flows, the overall co-extrusion process was represented by means of a digital process twin. Industrially relevant combinations of materials were tested under a wide range of processing conditions. Comparisons of pressure losses and interfacial positions to the predictions showed excellent agreement and the results outperformed the concept of representative viscosity.