Research On Polymer Forming Mechanism In The Extrusion Process And Its Corresponding Simulation And Optimization

Polymer is a kind of important manufacturing material in national industry and defence construction which is called the four most important material system incooperating with wood, metal and silicate. With the development of polymer processing technology, the industrial system of polymer processing is established with many kinds of departments and it is playing an important role in national economy. Extrusion is one of the most important polymer processing technologies by which all the thermoplastic polymers and some thermosetting polymers can be molded, such as pipes, sheets, rods, nets, monofils, films, profiles, foaming profiles, composite products and coated products. In the extrusion process, the accumulative structure and chemical structure of polymer melts can be varied with the change of temperatue, pressure and stress. The processing conditions determine the material structure and ultimately affect the performance and quality of final products.It is hard to afford to traditional experimental method for the reason that the influencing factors are multiplex and it is also expensive, time-consuming and laborious. Currently, the related experiments on polymer melts flow are mainly conducted in the laboratory and the aim of which is to investigate rheological properties based on the simple model of shear or extensional flows. As for the experiments conducted in practical extrusion process, advanced testing method is diffucult to be directly introduced and general testing method is inevitably affect real flow patterns. It is hard to quantitatively but can only qualitatively reflect practical rheological behaviours of polymer melts by using experimental method. After several years' development, numerical simulation method gradually attracts the attention of scientific and technical researchers for its excellent solving ablility to complex physical and engineering problems. It is now widely adopted in mechanics, thermotics, material science and other fields which greatly accelerate the development of modern science and technology.The application of numerical simulation technology in fluid dynamics is discussed in the present research. The modeling theory and numerical method is studied and the mathematical model is established to investigate polymer rheological behaviours in the extrusion process. The distribution and changing law of some important field functions, such as velocity, stress and temperature, are discussed and analysed to explain the forming mechanism of polymer melts. The numerical simulation method is combined with optimal design theory and the optimization model and algorithm is established to realize the optimal control of extrusion process and products' quality.The mathematical model of three-dimensional non-isothermal flow of non-newtonian fluid is established under Euler frame on the basis of polymer rheology and fluid dynamics. The penalty finite elment method is adopted to avoid high-order interpolation which is usually used in the mixed finite element method so as to effectively make use of computational resources. The effects of shear rate and temperature on melts flow are considered and the coupled calculation of flow and heat transfer is realized based on the nonlinear viscosity model. The non-linear term is linearized by using linearation iterative algorithm and hence to reduce the effects of valiables' initial distribution. The stream upwind Petrove-Galerkin method is performed to enlarge the upwind effects by using asymmetry weight function to overcome the oscillation of convection terms dominated problems. The field variables' function in the pipe poiseuille flow of power-law fluid is deduced and the corresponding simulated results are compared with the analysed results to prove the reliability of current mathematical model and numerical algorithm for non-newtonian flow.The numerical method established above can not only be used in the analysis of general non-newtonian fluid flow problem but also can be adopted to solve complex engineering problem. Based on the proposed method, the mathematical modeling and processing analysis of a novel co-extrusion process of plastic profile with metal insert is performed in the study for the first time. The whole flow characteristic of polymer melts in flow channel is obtained by the calculation of velocity, stress, temperature and pressure. The influences of volume flow rate and metal insert moving velocity on the distribution of field variables is discussed and the corresponding advice on the processing design is put forward. The velocity relative difference is defined to jugde the outlet flow balance. The influences of both the inlet angle and the length of allocation region are investigated by the calculation of velocity relative difference.Polymer melts in the extrusion process not only have non-newtonian flow characteristics but also have strong viscoelastic flow characteristics. The mathematical model of three-dimensional viscoelastic non-isothermal flow is established and a stable numerical algorithm is proposed which has been successfully adopted in the analysis of polymer extrusion process. The PTT (Phan Thien-Tanner) model is adopted to depict such viscoelastic properties of polymer melts which can reflect the extensional flow characteristic better. The special energy consumption pattern of viscoelastic medium is considered and its energy conservation equation is deduced according to the nonequilibrium irreversible thermodynamical theory. A decoupled algorithm is adopted to realize stable calculation for the three-dimensional multi-variables field consisting of velocity, temperature and flow stress. The momentum equation will lose its ellipticalness when the stress term is taken as the quasi-body force term and the discrete elastic-viscous stress split algorithm is adopted to improve the stability of velcocity calculation by introducing the stabilization factor. The non-consistant stream upwind method is adopted to overcome the oscillation in the calculation of stress. The mathematical model and numerical method for viscoelastic flow simulation of polymer melts established in the study is introduced to the analysis of general profile extrusion process. The effects of calculation control parameters, such as mesh division, penalty factor and energy partitioning factor, are investigated. The viscoelastic flow characteristics of polymer melts in the profile extrusion process are analysed based on the simulated results of velocity, temperature and stress. The influences of processing conditions and die structure on flow characteristics are further discussed.Extudate swell is a common phenomenon for the reason of polymer melts' elastic deformation in the extrusion process which can severely influence the shape and dimensional precision of final products. Based on the viscoelastic flow simulation technology proposed in the study, the mathematical model and numerical algorithm for the simulation of three-dimensional extrudate swell is established and its finite element simulation program is worked out. The extrudate swell of an industrial LDPE is then investigated by both experimental method and numerical method. The distributions of stored-energy modulus and consumed-energy modulus in small amplitude oscillating shear flow and the distributions of shear viscosity and the first normal stress difference in steady shear flow are obtained by using the strain-controlled rheometer. Both linear and nonlinear viscoelastic rheological parameters of PTT model are obtained by using nonlinear regression method. The swelling ratios of LDPE through circular die under different volume flow rates are detected by using indirect measurement and they are compared with the simulated results. The distributions of flow velocity and stress in LDPE annular extrudate swell flow field obtained by simulation are analysed and the corresponding mechanism is further discussed.The numerical simulation of polymer extrusion process is a passive system whose results have to be judged and analysed by professional worker with corresponding experiences and knowledge. In the study, the numerical simulation technology is combined with optimal design theory to realize the automatic optimal design and control for products. An optimal design method for polymer extrusion process is put forward based on numerical simulation, artificial neural net and generic algorithm. The optimization model is established by using the outlet flow balance as the optimization object. The processing parameters and die structure parameters are taken as design variables. The neural net model is trained by using the sample database obtained by simulated results so as to reduce the calculated amount of numerical simulation. The optimal design is achieved based on the scientific analysis through the iteration of genetic algorithm and neural net model. The corresponding optimization program for processing parameters and die structure parameters is worked out and the optimal design both for the co-extrusion process of plastic profile with metal insert and for the sheet extrusion process are further achieved.The application of numerical simulation technology in the polymer processing engineering becomes one of the important branches in the computational rheology field. Although great progresses have been made in numerical methods on fluid dynamics in the recent years, its solving ability is still restricted to flow regions and computational stability. Researches on the complex mathematical modeling and the key technologies for numerical simulation are rarely reported especially for the solving of complex engineering and technology problem. The mathematical model and numerical method for the simulation of both viscous and viscoelastic characteristics of polymer melts in the study are of great interest to enrich the theory of computational rheology. It is also of much industrial interest to take research on rheological behaviour and the related forming mechanism in the polymer extrusion process based on the simulation and optimization technology.