FLOW-FIELD ANALYSIS OF BOTH THE TRILOBAL ELEMENT AND MIXING DISC ZONES WITHIN A CLOSELY INTERMESHING, CO-ROTATING TWIN-SCREW EXTRUDER

Of the different mixing modules contained within the closely intermesh ing, co-rotating twin-screw extruder (CICo-TSE), the trilobal elements and mixing discs are of major importance to the mixing efficiency. Th ese two types of mixing element impose contrasting mixing behaviour on the viscous melt and are thus used selectively to perform differing m ixing tasks. Problems encountered when solving the flow problem within these two mixing element zones arise from both the complex 3D geometr y and the time dependent flow boundaries as each respective element ty pe rotates about its fixed axes. Following on from the 2D results repo rted, 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 uti lised within the Betol BTS40 CICo-TSE. The flow fields were simulated using the ideal rheological properties of polypropylene and characteri sed in terms of velocity vectors, shear stresses generated and a mixin g parameter lambda, which quantifies the elongational and rotational f low components. Each flow field exhibited different characteristics fo r the configurations investigated and it has been found that the avera ge flow field parameter values for the trilobe elements change periodi cally, whereas minimal changes are observed as the mixing discs rotate .