  In twin-screw extrusion, the geometry of a mixing element mainly determines
the basic flow pattern, which eventually affects the mixing ability as well as
the dispersive ability of the mixing element. The effects of geometrical
modification, with both forward and backward pitched tips, of a conventional
forwarding kneading disks element (FKD) in the pitched-tip kneading disks
element on the flow pattern and mixing characteristics are discussed. Numerical
simulations of fully-filled, non-isothermal polymer melt flow in the
melt-mixing zone were performed, and the flow pattern structure and the tracer
trajectories were investigated. The pitched tips largely affects the inter-disc
fluid transport, which is mainly responsible for mixing. These changes in the
local flow pattern are analyzed by the distribution of the strain-rate state.
The distribution of the finite-time Lyapunov exponent reveals a large
inhomogeneity of the mixing in FKD is suppressed both by the forward and
backward tips. By the forward tips on FKD, the mixing ability is relatively
suppressed compared to FKD, whereas for the backward tips on FKD, the mixing
ability is enhanced while maintaining the same level of dispersion efficiency
as FKD. From these results, the pitched tips on the conventional KD turns out
to be effective at reducing the inhomogeneity of the mixing and tuning the
overall mixing performance.
