The Rheological Theories And Experimental Research On Polymer Micro Extrusion Process

With the rapid development of technology,products molded by micro extrusion technology are widely used in more and more fields such as automobiles, medical industry and spaceflight. Under the micro scale effects, the flow of polymer melts characterizes with scale effects and many factors including wall slip and rheological characteristics, which can be neglected in macro scale, have great influence on the flowing process.The traditional extrusion theories are not suitable to be used in micro extrusion process.Therefore, the study on micro extrusion theories has leading significance as well as practical value on disclosing the function of scale effects, building dynamic models for micro extrusion,instructing to the design of micro extrusion die and improving the quality of extrudate.The main research in this paper contains the follows.First of all, through the capillary rheometer, the wall slip of four polymer melts PP, HDPE, PS and PMMA during extrusion process is investigated.It is found that the velocities of PP and HDPE are positive and those of PS and PMMA are negative. Based on the entanglement-disentanglement mechanism,a new wall slip calculation model is built.With the proposed method, the velocities of PP and HDPE correspond with the experimental values very well and those of PS and PMMA are positive.Second,with the help of molecular dynamic simulation,the differences of the mechanisms of four polymer metls are explained.It is found that the combination energy between molecules of PP and HDPE and atoms of wall is smaller than that of PS and PMMA, which means that when wall slip occurs, the molecules of PP and HDPE are easier to slip along the wall.Instead,those of PS and PMMA are opposite.Therefore, the wall slip mechanism of PP and HDPE is adsorption-desorption mechanism and that of PS and PMMA is entanglement-disentanglement mechanism.After that, according to different mechanism of wall slip, the influence of wall slip on viscosity is studied.It is found that the viscosity measurement of capillary rheometer has some limitations.Base on entanglement-disentanglement mechanism,the shear rate is revised and a new viscosity model which is suitable for PS and PMMA is built.With the new model, the viscosity of polymer melts PS and PMMA decrease with the reducing of the diameters which is the same with that of PP and HDPE. Finally, according to the rheological models, through finite element analysis software POLYFLOW, the extrusion process of single-lumen catheter is simulated.In addition, the single-lumen extrusion die is designed and manufactured.Comparing the experimental pressure value with the result of simulation, it is found that the rheological theories are correct. Meanwhile, the roundness of the catheter and the uniformity of the wall are used to prove the reasonable design of the single-lumen extrusion die.