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A triangularly-arranged tri-screw extruder (TATSE) is a new piece of equipment for polymer processing, which has a dynamic center region. To learn about the special contributions of the dynamic center region and the circumfluence phenomenon just in cross section on the mixing mechanism of the TATSE, the 2D computational fluid dynamics (CFD) model was established to reduce the axial effect. The effects of screw clearances of the center region on mixing mechanism of the TATSE were studied. The mixing evaluations for the TATSE and twin-screw extruder (TSE) with different screw clearances were performed and compared. Moreover, the effects of different blend mass ratios of LLDPE and SBS on the mixing efficiency of the TATSE were studied. The results show that as screw clearances increase, the values of mean time-averaged and instantaneous efficiencies of the tri-screw extruder increase, but the normalized segregation scale decreases. The screw clearances even change the particle conditions from stretch to compression in the center region.

There is a special circumfluence in the center region of cross-section for a tri-screw extruder. To study the effect of the dynamic center region on the flow and mixing mechanism of the tri-screw extruder, 2D finite element modeling was used to reduce the axial effects. Based on the particle tracking technology, the nonlinear dynamics of a typical particle motions in the center region was carried out and the mixing process in the tri-screw extruder was analyzed with Poincaré maps. Moreover, mixing evaluations of the tri-screw and twin-screw extruders were analyzed and compared. The results show that there are many multiple and fractional excitation frequencies in the Fast Fourier Transform (FFT) spectrums, and it shows a chaotic motion in the center region. Poincaré section maps in the tri-screw show the better particles diffusive ability than those in the twin-screw extrude due to the excellent diffluent ability. Furthermore, the tri-screw extruder has the better dispersive, distributive mixing abilities and stretching mixing efficiency than the twin-screw extruder in the cross-section.

Three-dimensional finite element modeling of polymer melt flowing in a new co-rotating tri-screw extruder was established with mesh superposition technique. Based on the particle tracking technology, three typical particle trajectories in the tri-screw extruder were calculated using a 4th-order-Runge-Kutta method to study the dynamic motions of the particles. Then the flow visualizations in the local center region were carried out. Moreover, the dispersive, distributive and stretching mixing efficiencies of the tri-screw and twin-screw extruders were compared, respectively. The results show that when the particles move from one screw to another, there are great abrupt changes in the velocities and displacements, which induce the abrupt change in the stress magnitude. Most of particles, which are initially distributed in the inlet plane of the center region, fast flow out the outlet and don't pass through any screw. This special phenomenon induces a series of new characteristics in the residence time distribution (RTD), flow number, segregation scale and time averaged efficiency. In comparison with the twin-screw extruder, the tri-screw extruder has better mixing efficiency.