Simulation Of Non-isothermal Flow In Extrusion And Mixing Process For Rubber Material

Extrusion and mixing are the two basic manufacturing processes of the automotive tires industries,of which the screw extruder and internal batch mixer(IBM)are the corresponding production devices.The two processes are both complex thermal-mechanical coupling problems,which involves complicated internal moving parts as well as considerable viscous heating.It can be time-consuming and laborious to apply experimental research methods to study the foregoing problems.In such a condition,this dissertation adopted the numerical simulation to do some preliminary study on the two subjects.In this paper,the mathematical model characterizing the rheological behaviors of the rubber material and the numerical method for simulating the extrusion and mixing process were determined.Based upon the rheological properties of the rubber material,the Bird-Carreau law and the Arrhenius shear stress law were utilized to describe the shear-thinning behavior and temperature dependence,respectively.Besides,by combining with the mesh superposition technique(MST),the control equations of numerical simulation as well as the analysis process were built.In order to investigate the flow patterns in rubber extrusion process,the finite element model of non-isothermal flow in the channel of a complicated-geometry single-screw extruder which adopts a main-subsidiary-threads configuration was established and the related boundary conditions as well as the initial temperature were determined.Based on the foregoing model,the velocity field,the local shear rate field,the pressure field and temperature field at the screw speed of 20 RPM were solved.The numerical result of temperature has a good agreement with that of the experiment measurement,which verified the validity of the numerical model.Besides,the numerical results of velocity field and shear rate field verified that the hot-feeding extruders with the main-subsidiary-threads configuration do avoid the "dead zone"and provide greater shearing effect.Owing to the existence of the subsidiary thread located at the extrusion region of the extruder,the flow of the rubber material is hindered and a longer as well as stronger shearing is provided,which gives rise to a higher pressure and the highest temperature in the corresponding zone.Moreover,the pressure and the temperature in different screw speed(RPM)conditions were investigated,when the screw speed increase,the greatest pressure and the highest temperature are both increasing.At the same time,because of the shear-thinning property and the temperature dependence of the rubber material,the upward trend slows down.For the simulation of the mixing,the finite element model of non-isothermal flow in the twin-rotor internal mixer was established and the related boundary conditions as well as the initial temperature were determined.Based on the foregoing model,this paper solved the velocity field,the local shear rate,the pressure field and the mixing index.The velocity of the region nearby the inlet and the outlet of the mixer are relatively low,indicating that the corresponding areas are not well-stirred,The axial velocity field results verified the existence of the axial recirculation of rubber material during the mixing process.The results of the pressure field showed that the maximum pressure appears in the wedge-region of the spiral edge while the minimum pressure appears in the back-pressure region of the spiral edge.The mixing index results showed that the mixing zone between the two rotors is dominated by the elongational flow pattern,and the shear flow pattern is mainly between the spiral edge and the mixing chamber wall.To investigate the distributive mixing,component-transport method was utilized to define and solve the homogenization rate of the concentration field in the chamber of the mixer.The results showed that the homogenization rate of concentration of 0～12s is very fast and that of 12s～25s slows down.After 36th second,the concentration remains almost unchanged.In addition,the particle tracking analysis(PTA)method was also applied to study the distributive mixing and the numerical results agree well with those of the component-transport method.Nevertheless,the particle tracking analysis method does not need to consider the self-diffusion of components and the computational-cost of simulation is relatively small.In the end,this paper solved the viscous heating of mixing process and approximately calculated the overall temperature-rise curve of the rubber in the chamber.The results showed that the temperature of the rubber material increases linearly with time in mixing process and the overall temperature-rise at the end of the mixing process is about 32℃ which is in agreement with the measured results obtained from the factory.