Study On Mixing Mechanism And Experimental Study Of Single Screw Chaotic Extrusion Process

The single-screw extruder is widely used in polymer processing for its merits, such as sample structure, low manufacturing cost and easiness design, straightforward operation and so on. At present, much attention is paid to mixing performance enhancement in single-screw extrusion processes. Chaos screw as a mixing-enhancing device for the single-screw extrusion process by a simple modification of a conventional screw is developed. Chaotic mixing can break closed shells streamlines in regular flows, and has been accepted as the best method for laminar flows. The basic mechanism of chaotic mixing in a single-screw extruder is the repetition of stretching and folding, which the material undergoes while moving along the down channel direction due to the spatially periodic baffles. Some researches about the chaos theory in polymer processing are published abroad, but it is still very few in domestic. It can be found that the chaotic convection can greatly improve the performance of polymer product. Undoubtedly, it would be of great value to study on enhancing the mixing performance via chaotic mixing in polymer processing.The present article tries to insert spatially periodic intermittent baffles into the channel to enhance the mixing performance, which bring in periodic perturbation and induce the chaotic mixing. The physical and mathematical models of unwound screw channel and3D of the novel types of single-screws are proposed. The corresponding incompressible fluid obeys the Carreau flow model. With the help of the mesh superposition technique provided by fluid dynamics software Polyflow, the flow performances of polymer melt in such new types of extruders and conventional single-screw extruder are numerically simulated. Adopting the particle tracking technique and statistical methods, the mixing performances are investigated and characterized in terms of different parameters such as residence time distribution, segregation scale, mixing efficiency and mixing index. Details of the influence of the geometrical parameters of intermittent baffles are obtained.The performance of the perturbation on the flow field by intermittent baffles in the screw channel is clearly indicated by the particle track method. And the numerical simulations results show that in comparison with the conventional screw, the novel geometries not only make material particle segregation scale variation stable decrease, but also lead to broadening of RTD much wider along with higher mixing index and higher mixing efficiency of stretching, which are helpful to improve the dispersion and blending property of the material, get better dispersion mixing effect.The extrusion experiments are carried out on the novel types of single-screws using the polymer LLDPE filled with CaCO3and a typical extremely incompatible blend polyethylene/polycarbonate (HDPE/PC) system. The microstructure of brittle fracture face are observed by electron scanning microscope, which present that the more efficient mixing are provided by new screws and agreement with the numerical simulation results.