Validated Modelling of Complex Geometry Dies for Extrusion Foaming of Starch-Based Mixtures

Biobased materials offer a good alternative to the widespread use of fossil-based plastic. Foamed plastics used in various industries such as packaging, automotive, marine, and aerospace may result in sea, soil, and air pollution due to insufficient waste management systems. One of the most abundant biobased materials is starch derived from natural sources such as potatoes, tapioca, and corn. This class of materials can be handled as a thermoplastic material and employed in the foaming extrusion process by using plasticizers (such as water and glycerol) and other additives. Starch foams as a biobased and biodegradable product could be a promising alternative to expandable polystyrene (EPS) or expanded polypropylene (EPP). Their application remains, however, difficult due to issues related to moisture content and irregular swelling at the die exit, both of which affect the final product properties and well-defined shape of an extrudate. Typically, to obtain the required shape of the extrudate, a die with an approximate shape is created and iteratively adjusted based on the quality of the extrusion until the shape matches the desired one, which is still considered an art. This work aims to develop and validate a fluid dynamics model for starch-based mixtures flowing in a die and relate the shape of the foamed extrudate cross-section with the predicted die geometry and estimated flow conditions to reduce trial and error iterations. The calculated pressure drop and velocity profile are compared to experimental data. Also, the die swell phenomenon (or Barrus effect) is investigated by numerical simulation to predict the final shape of the extrudate.