Theoretical Analysis And Experimental Study Of Axial Circle Flow In Twin-Screw Extrusion

Twin-screw extruders have many advantages over other mixers, such as modular design, better feeding and mixing. In recent years, they have been applied to various kinds of polymer processing. It has been important to find their optimum geometrical configuration and operational processing conditions for the best performance of extrusion and products. But traditional non-intermeshing, intermeshing co-rotating and counter-rotating twin-screw extruders transport material ahead with the rotation of two screws, in this thesis the concept of axial circular flow was introduced. While as axial circular flow occurring, some region on one screw transports material forward, whereas the corresponding region on the other screw transports material backward.In this paper, three-dimensional flow simulations, of usual flight elements and combined elements (usual flight elements + axial circular elements) in melt-conveying zone, are performed. In non-intermeshing, intermeshing co-rotating and counter-rotating twin-screw extruder, the physical models and finite element models of usual screw element flow region and the combined flow region with axial circular flow used are built. After numerical calculation, the pressure field, velocity field and viscosity field were obtained. At the same time, unwound pressure profile, axial velocity profile, shearing rate profile, shearing viscosity profile and shearing stress are acquired by postprocessing. The effects of geometrical parameters, material parameters and operating conditions to axial circular flow are analyzed. Then the effect of introduction of axial circular flow on the transporting character and mixing capability of non-intermeshing, intermeshing co-rotating and counter-rotating twin-screw extruders is studied too.The simulation results indicate that the axial circular flow can occur in non-intermeshing, intermeshing co-rotating and part-intermeshing counter-rotating twin-screw extruders, but not in full intermeshing counter-rotating twin-screw extruder. With the axial circular flow occurring in the twin-screw extruding process, the transporting capability decreases while the distributive mixing property increases greatly. At the same time, the shearing stress of the whole flow field with axial circular region reduces a little, the dispersive mixing capability decreases, because the screws in the axialcircular region do not intermesh and have lower shearing rate and shearing stress than in usual screw region.The experiments were performed to validate the numerical simulations, which use usual screw and the screw combined with the axial circular region in non-intermeshing, intermeshing co-rotating and counter-rotating twin-screw extruders.The experimental results of pressure and flow rate shows that the models of melt-conveying in non-intermeshing, intermeshing co-rotating and counter-rotating twin-screw extruding are corrected, and the simulation of twin-screw extruding with the finite element numerical analysis method is feasible. The numerical results were in good agreement with the experimental observations.The experimental results of residence time distribution (RTD) indicate that after the introducing of axial circular flow, both the axial mixing quantity and the distributing mixing capability increase.The experimental results of the forming of axial circular flow shows that axial circular flow can be realized in non-intermeshing, intermeshing co-rotating twin-screw extruders, but not in intermeshing counter-rotating twin-screw extruder. But the axial circular is not obvious and can be formed only in little region because the screws intermesh fully in intermeshing co-rotating twin-screw extruder.The experiment results show that the increasing of screw rotating speed and the elongating of the axial circular region are propitious to the forming of the circular flow, whereas the increasing of feeding and die pressure are not in favor of the circular flow.Under the same operating parameters, the power consuming per kilogram and the torque of screw of the combined screw with the axia...