Computation of Die Swell behind a Complex Profile Extrusion Die Using a Stabilized Finite Element Method for Various Thermoplastic Polymers

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The design of plastics-extrusion dies continues to rely on both experimental studies and the experience of the die designer. Direct die design is in general not possible; reason being the highly non-linear behavior of the plastics melt - a factor strongly tied to its viscoelastic properties. The latter cause the melt to swell severely and non-uniformly when exiting the shape-defining zone of the die. This final geometry of the product may resemble neither the size, nor the key features of the outflow die geometry. Considering the costly and time-consuming experiments that are presently needed in die design, it is advantageous to be able to predict the die swell behind industrially relevant extrusion dies on the basis of computational modelling. In this project, a suitable finite-element method is being developed. Regarding the modeling of the material properties, the focus rests on materials that can be represented by the Oldroyd-B model (e.g., LDPE or HDPE). The newly developed method focuses on two main ingredients: a stabilized discretization method and a boundary-conforming, highly resolved free-surface treatment capable of presenting die swell ratios of - after the first project phase - up to 170%.