Investigation On Melt Flow And Mixing Performance In A Miniature Conical Twin Screw Extruder

The conical twin-screw extruder has low shear rate characteristics due to the decreasing diameters of barrel and screw.Hence,it is very suitable for the possessing materials that are sensitive to shear and easy to decompose in the plasticized state.The conical twin-screw extruder possesses great potential in polymer processing field due to their good pressure establishment performance,forward conveying capacity and processing flexibility.Therefore,it is of great significance to investigate the flow and mixing performance of the conical twin-screw extruder,which can provide guidance for the design and optimization of the conical twin-screw extruder.The combination of the experimental technique and computational fluid dynamics(CFD)simulation has been adopted to study the melt flow and mixing performance in a continuous miniature conical counter-rotating twin-screw extruder.Achieved results are as follows:An on-line fluorescence detection device was designed and built based on the tracer fluorescent characteristic.Polypropylene(PP)was selected as the experimental material,the blend of PP and small amount of anthracene was used as the tracer.The residence time distribution(RTD)of MCTSE was measured by the on-line detection device.The reliability of the device was validated by reproducibility,where good linearity between the amplitude of response signal and the amount of tracer is attained.And the effect of the screw speed and feed rate on RTD was investigated.Results show that increasing screw speed shifts the RTD curve toward the shorter time domain,however,the width of RTD curve under different screw speeds tends to be similar.Increasing feed rate shifts the RTD curve toward the shorter domain,however,the width of RTD curves significantly narrowed,which was different from increasing screw speed.The three-dimensional finite element method(FEM)combined with mesh superposition technique(MST)was used to simulate the extrusion process of PP in the MCTSE in order to obtain the velocity magnitude distribution,shear rate distribution and mixing index distribution.Particle tracking technique was carried out to evaluate the mixing performance,such as the RTD,the length of stretch and the mixing efficiency.The influences of the screw speed,feed rate and screw structure on hydrodynamics and mixing performance of MCTSE were investigated.The RTD data by numerical simulation show good agreement with experimental results.The results show that velocity magnitude and shear rate decrease from inlet to outlet due to the decreasing diameters of screws.Velocity magnitude in the intermeshing zone is larger than that of region near the barrel wall.The intermeshing zone possesses lower shear rate than that of region near the barrel wall.The mean length of stretch increases exponentially from the inlet to outlet section,and the mixing efficiency remains positive,indicating that such kind of conical twin extruder has good laminar mixing performance.