Study On Characteristics Of Solids Conveying In Vane Extruder

Traditional polymer process machinary uses the screw as the plasticating and conveyingunit of solids on the basis of the friction dragging to solids as the screw turns on. This methodis based on the shear rheology, in which the flow and deformation of solids are domniated bythe shear stress. However, there are some disadvantages in this polymer materials process,such as long thermo-mechanical, high energy consumption and too big equipment structure,etc. In order to overcome these problems, polymer processing center of South ChinaUniversity of Technology originally invented the vane extruder, based on the tension flowfield, different from the screw plasticating and conveying. The vane extruding molding canchang the tradional polymer process theory and application thoroughgoing, and thismachinery increases the kinds of polymer process equipment. Therefore, studies on theconveying characteristics of solid materials in vane extruder will bring great theory andpractice significance.On the basis of the forces on the differential element in the solids, a function between abulk density and pressure and the non-isotropic pressure distribution in the element, anexpression of pressure in the circumferential direction was derived. Without directlycalculating the rotor torque, the total power introduced through the vane conveying and thefriction dragging conveying was also deduced by the rotor torque and velocity. Experimentaldata were obtained using a self-developed simplified vane extruder with a adjustable rotoreccentricity to the stator. The solids conveying characteristics in a vane extruder wastheoretically and semi-empirically discussed. Besides, the various portions of total powerconsumption including the power dissipated on the surfaces of stator and baffles, the powerused to build pressure and the power converted into rotational kinetic energy were alsoderived. The effects of the eccentricity and friction coefficients on the conveyingcharacteristics were discussed theoretically and semi-empirically. Both of the calculatedvalues and the semi-empirical data show that the vane conveying is significantly prevailingafter the extruder geometry is optimized. Compared with the conventional screw extruderwith the friction dragging conveying, the vane extruder can shorten the thermo-mechanicalhistory, improve the adaptability of materials and reduce the energy consumption.The effects of the device parameters(eccentricity and friction coefficient), the operatingparameters(temperature and velocity) and polymer properties(different materials and differentshapes) on the compressibility of polymer solids are reported in terms of compactioncoefficient as a function of rotor torque. The results show that a relatively larger eccentricity or a lower external temperature or relatively rough metal surface can bring a smallercompaction coefficient, which results in higher pressure in the solids conveying zone of thevane extruder. Rotor velocities almost have no influence on the compaction coefficient underthe same condition. However, when the velocity is lower than10rpm, the influence of frictionbetween particle-wall and particle-particle should also be considered. the difference isobserved between the effects of the semi-crystalline polymers(such as LDPE) and thenon-crystalline polymers (such as GPPS) on the compaction coefficient, and also exploredbetween the effects of the crystalline polymers(such as PP) and the non-crystalline polymers(such as ABS, PVC and HIPS) on solid materials compressibility, these phenomena should betaken into consideration in vane extrusion processing operations. In addition, it is seemed thatthe effects of the solid material shapes on the compaction coefficient can be ignored.Although the compaction coefficient is an experimental or a semi-empirical value, it isimportant for the optimizing design of vane extruder and the optimization of vane extrusionprocessing, and it reveals the polymer particles compressibility in plasticating processingdevice.In addition. A mathematical model of the pumping capacity of polymer solids in the vaneextruder is established based on its structure and conveying mechanism. The minimum bulkdensity under atmospheric pressure and leakage coefficient in this model are obtainedexperimentally. Four general polymer materials are chosen to investigate the pumpingconveying capacity of polymer solids in the vane extruder. The experimental results of thefirst vane plasticization unit show that the pumping conveying capacity increases almostlinearly with the rotor velocity and is only slightly influenced by the material properties.Furthermore, the leakage coefficient increases when the temperature and velocity in the vaneextrusion process are set inappropriately; this reduces the pumping conveying capacity ofpolymer solids in the vane extruder.The vane extruder based on elongational flow is a novel machinery for polymer processing,conveying charateristics of solids conveying in the vane extruder are studied. This studyprovide an important theoretical basis for the promotion of the vane extruder.