Viscoelastic effects on drag forces and mixing within a mini-extruder used for polymer-nanocomposite compounding
The flow of a viscoelastic polymer through a conical co-rotating twin-screw mini-extruder commercially available is analyzed by means of dimensional analysis, experimental measurements and numerical simulations. The objective is to identify how viscoelasticity influences forces and mixing. Additionally, a proper relation is sought between the force exerted over the shell containing the extruder and mixing, as this force is the only operating variable that can be measured to control the process. For numerical simulations the Giesekus differential viscoelastic model is used. This model is implemented in COMSOL using a Galerkin least square approach. The model implementation is validated using results from analytical and numerical test cases present in the literature. Experimental measures of forces in the mini-extruder are collected for two polymers (HDPE and PP), of which rheometric properties were characterized. Numerical forces using the COM-SOL model and the experimental forces have a very good agreement. The numerical model for the simulation of the mini-extruder is used to analyze mixing properties by means of volume averaging of the standard deviation of concentration of a passive scalar at several mass Peclet numbers. Results show that viscoelastic properties of polymer blends (higher Deborah numbers) reduce the force over the shell for a given rotational Reynolds number and enhance mixing for a given mass Peclet number.