As one of key factors that determine the quality of products, the homogenization of polymer melt is closely dependent on the mixing of polymers. The mixing of polymer melt in the homogenizing section by a single screw with torsion elements was analyzed with the computational fluid dynamics (CFD) simulation. The simulation results reveal that screws with such torsion elements arranged in a decentralized form have smaller segregation scale, distribution index and higher mixing efficiency, compared with the conventional screw. The decentralized arrangement is more effective for mixing than the centralized distribution. The rotational flow induced by the torsion elements can significantly enhance the mass transfer process and improve mixing and plasticizing. Compared with those in conventional screw, the distributions of temperature and viscosity are more uniform at the outlet of torsion screws, which also made the torsion element generally more effective and efficient for mixing and plasticizing.

Keywords: torsion element, distributive mixing, dispersive mixing, mixing efficiency, plasticizing quality

In both extrusion and injection molding, the homogeneity of polymer melt is determined for the quality of product,^1,2 thus various of screw geometries^3-5 and mixing elements^6,7 have been designed to improve the homogeneity of polymer melt. Mixing contributes to mass transfer, distributes physical property of ingredients more uniform including temperature, viscosity, etc. throughout three ways: disturbing flow, shear flow and elongation flow. The plasticizing effect can therefore be evaluated by looking at the mixing ability of the screw plasticizing unit to distribute ingredients and mass transfer enhancement.⁸
There are three main approaches to enhance mixing of polymer melts in the screw plasticizing unit. The first is by making use of disturbing flow or rotational flow. Among them the pin screw is one important mixing unit, and has drawn many explorations. Hwang et al.⁹ studied the influence of pin configurations on the screw’s mixing properties with dynamical systems theory, and results showed that the existence of the pin can generate a perturbation for large-scaled orbits of fluid particle, and generate elliptic rotations for small-scaled orbits. Yao et al.¹⁰ investigated the effect of the pin distance on the mixing of polymer melt in a pin screw extruder, and evaluated the mixing performance quantitatively from the aspect of integral mixing efficiency and residence time distribution (RTD). Li et al.¹¹ proposed nine arrays of pin mixing sections through orthogonal designing and systematically discussed the effects of their height and arrangements on the efficiency of mixing. The second way to promote mixing of polymer melt is through the enhancement of shear, such as the application of Maddock and spiral shearing sub-sections. Zitzenbacher et al.¹² presented a new calculation model to optimize the structure of fluted mixing sections, such as axial and spiral Maddock-ele-ments and Z-elements. Kubik et al.¹³ studied the mixing abilities of stratablend II mixer and two similarly designed Maddock mixers, and found that better mixing could be obtained with higher shear stress on average. Hu et al.¹⁴ designed a powerful shear mixer, and the results showed that the particles of T-ZnOw were dispersed more uniformly in the epoxy matrix mixed by the powerful shear mixer compared with those by traditional methods. Feng et al.¹⁵ introduced a vibration force field into the extrusion process by the axial vibration of screw, which resulted in the increasement of total shear strain of melt and was favorable for melt mixing.
The third way to improve mixing is by incorporating elongational flows. Bouquey et al.¹⁶ built a laboratory-scale mixing device and indicated that the elongational flow mixer had more uniform size distribution of dispersed particles than a rotary mixer with equivalent input of specific energy. Qu et al.^17-19 proposed a novel vane extruder based on elongational rheology and volume transportation. Their results showed that the vane extruder had good mixing ability and plasticizing capacity. Wen et al.^20,21 also studied the mixing abilities of vane extruder by numerical analysis found that there existed strong stretching of melt in the vane extruder. Yin et al.²² successfully prepared polypropylene nanocomposites with halloysite nanotubes (HNTs) through a vane extruder. Their results indicated that HNTs were well dispersed in polypropylene (PP) matrix and the novel vane extruder had well mixing capacity.
As to the evaluation of mixing, methods have been proposed over the past years. However, no one of them is able to quantify all aspects of mixing for every process, for example, segregation scale, distribution index and mixing efficiency are restricted to distributive mixing while mixing index is limited to dispersive mixing. So it is needed to combine these multiple methods to analyze the mixing performance comprehensively. Dhakal et al.²³ found that the commonly used mixing index was actually unable to predict the ranking of dispersive mixing performance because it did not reflect the magnitude of stress or the probability of particle passing the local region around the node for computing the index. Vyakaranam et al.²⁴ evaluated the air bubble dispersion by combining mixing index and shear stress, and the highest shear rate values were found in the simple shear flow region (mixing index=0.5) while the local shear rate values are much lower in the other regions (mixing index<0.4 and mixing index> 0.6). Liu et al.²⁵ comprehensively analysed residence time, segregation scale, mixing index, and instantaneous efficiency between pin unit and screw unit with or without vibration, and found that the pin unit and the vibration field could lead to alternating diffluence and rearrangement, and good flow states, respectively. Therefore, the pin unit with vibration field showed better performance of fluidity and plasticization for polymer melt particles consequently compared with the others. This comprehensive approach provided a reference for the evaluation of newly designed mixing or plasticizing devices.
More recently, we developed a novel set of torsion elements, and analyzed their heat transfer characteristics in the homogenizing section of screw using three-dimensional (3D) finite element method (FEM).^6,26 In this work, we aim to analyze and evaluate the homogenization of polymer melt by the single screw with torsion elements in homogenizing section using CFD. The case of single screw without torsion elements was also simulated for comparison.