Of the different mixing modules contained within the closely intermeshing, co-rotating twin-screw extruder (CICo-TSE), the trilobal elements and mixing discs are of major importance to the mixing efficiency. These two types of mixing element impose contrasting mixing behaviour on the viscous melt and are thus used selectively to perform differing mixing tasks. Problems encountered when solving the flow problem within these two mixing element zones arise from both the complex 3D geometry and the time dependent flow boundaries as each respective element type rotates about its fixed axes. Following on from the 2D results reported, the computational fluid dynamics package, Polyflow, was employed to investigate the mixing mechanisms exhibited by both a set of three trilobe element pairs and by a pair of staggered mixing discs, as utilised within the Betol BTS40 CICo-TSE. The flow fields were simulated using the ideal rheological properties of polypropylene and characterised in terms of velocity vectors, shear stresses generated and a mixing parameter λ, which quantifies the elongational and rotational flow components. Each flow field exhibited different characteristics for the configurations investigated and it has been found that the average flow field parameter values for the trilobe elements change periodically, whereas minimal changes are observed as the mixing discs rotate.
© 1997, Carl Hanser Verlag, Munich