| The whole polymer extrusion production line is usually made up of the following stages: materials transportation in a twin screw extruder, the flow in the extruder header, the flow in the die and die swell and the subsequent molding process. It produces many important plastic and rubber products such as sheets, shaped extrudates, fibers, films and containers. During the processing process, the polymeric materials experience complex effects of heat and flow, thus the rheological feature and internal structure of the material will be changed, which will greatly affect the final properties of the products. To study the influence of processing conditions on the performance of the product and establish the relationship between the macroscopic property of the material and its internal mesoscopic structure is now one of the most important research topics in the polymer processing field.In this work, an advanced and high-effective computer simulation technology was adopted to study the flow behaviors of polymer materials in the polymer extrusion process. We focused on the material transportation process in a twin-screw extruder and three important molding technics, i.e. high-speed fiber spinning, blow-molding and film casting. The influence of process parameters on the extrusion process and final product quality were stressed and summarized. At the same time, to overcome the shortcoming that the classical continuum mechanics models were unable to describe the inner structure change of the material under the flow, a new kind of model, i.e. conformation tensor rheological model was introduced to simulate the flow in the complex processing flow fields. Both the viscoelastic behavior of the polymer melts and the orientation and stretch degree of polymer chains were calculated and so the simulation work was furthered into the internal structure of material. The main research work and conclusions are introduced as follows:(1) With the help of Ludovic software, the whole material transportation process in a twin-screw extruder was studied by the simulation method, including solid conveying, melting and melt conveying. A most appropriate melting model for PP and PS was chosen through the comparison of simulation results and experiments. The predicted pressure, temperature and residence time distribution along the extruder agreed well with the measurements, which confirmed the validity of the present simulation method. Moreover, the simulation also showed us the profiles of other important parameters in the extruder, including viscosity, shear rate, filling ratio, etc. It seemed that the simulation helped unclose the"mystic world"inner the extruder. The influence of feed rate, rotational speed and barrel temperature on the extrusion process was investigated experimentally and numerically. It was found that the melt's temperature inner the extruder was greatly affected by the rotational speed and barrel temperature, but less by the feeding rate. Simulation predicted that the melting of the solid particles occurred quickly in a twin-screw extruder, which was quite different from the case of a sing-screw extruder. Following that, the residence time distribution of the material in a twin-screw extruder was measured by an online fluorescence monitoring device and the influence of feeding rate and rotational speed on the RTD was studied and predicted by the simulation. The results indicated that the residence time was reduced when the rotational speed or feeding rate was increased. The shape of RTD curves changed only with feeding rate, but not rotational speed. Some important formulae describing the relationship of the residence time and the two processing parameters were obtained and they were useful to help to optimize the extrusion process.(2) Three extension-dominant processing processes, i.e. fiber spinning, blow-molding and film casting were simulated using the finite element method. Firstly, an axisymmetric two-dimensional model was set up for the circular fiber. Based on the physical model, a two-phase microstructrue model proposed by Doufas was calculated using the FEM to realize the simulation of the high-speed fiber spinning process. The method overcame the shortage of Doufas'cross-section-average approach and predicted both the axial and radial distribution of temperature. The comparison between the predictions and experiments on the temperature and velocity profiles approved the predictive ability of the present simulation method. On the basis of simulation results, two questions were discussed in detail: the cause of necking phenomenon in the high-speed fiber spinning process and the extensional degree of polymer chains along the spinline. Moreover, the spinning process of the non-circular fiber was also simulated and it was found that the surface tension was the most important factor that caused the shape change of fiber cross-section during the spinning process. Secondly, the method of simulating stretch-blow molding was established. An integral viscoelastic constitutive model, i.e. KBKZ, was used to describe the rheological characters of the polymer melt and an injection-stretch-molding process of an actual blow-molded product was simulated. The simulation result predicted a reasonable thickness profile of the final bottle comparing with the experiment. Also, the effect of stretch process and blowing pressure on the thickness profile of bottle was investigated numerically. The simulation method was then applied to the development process of a new blowing product with our modified UHMWPE material. According to the rheological analysis of the material, it was believed that this material could be processed by using the extruder, but the extrusion system should be re-designed in advance to avoid the trouble caused by the high-viscosity and slip. Computer simulation predicted that the lowest inflation pressure was 0.8MPa and the UHMWPE bottle was produced when the operation pressure was set as the numerical value. Furthermore, the pressure-maintaining process was also simulated and the quality of the mould could be evaluated based on the simulation results. Finally, computer simulation was carried out on the 3D non-isothermal viscous fluid and 3D isothermal viscoelastic fluid flow in a cast film process. The flow in the film thickness direction was considered in the simulation so that the flow field information was more accurate and complete. Computer simulation succeeded in predicting two basic defects in the steady film casting line, i.e. neck-in and edge-bead defects. The influence of air cooling, draw ratio, air gap length, self-weight and viscoelasticity on the final film geometry was discussed in detail. It was found that cooling and gravity would reduce the neck-in of the film and the non-uniform area near the edge. For a Newtonian fluid, the film shrinkage increased with both draw ratio and air gap length. But for a viscoelastic fluid, the relationship between the neck-in and draw ratio depended on the elasticity of the material. When the elasticity of material was small, similar with PET, neck-in of the film increased with the draw ratio first and then reached a equilibrium value; When the elasticity was as large as PE, a decreasing trend of neck-in was observed at large draw ratios, which was consistent with the experimental observation on the cast film production line of LDPE; When the elasticity of material continued to be increased to be similar with PP, an increasing trend occurred just after the plateau at large draw ratios, however, this predicted dependency was still lack of experimental validation. It was also found that the elasticity of material was helpful to suppress the edge bead defect and form a film with more uniform thickness.(3) To simulate the structure change of polymer chains in the complex flow fields, a new coupled solving system, i.e. velocity-pressure-conformation tensor formula, was proposed, which would take the place of the traditional velocity-pressure-stress tensor system. This was a novel simulation method because the structure of polymer chains in flow fields was first calculated and then the macroscopic variables were computed based on the structure change, representing the new simulation ideology that structure determines properties. Two benchmark problems, i.e. a planar contraction flow and a planar flow around a symmetrically-placed cylinder were solved by FEM using the proposed models. The simulation results were compared with the experimental data including the velocity and stress profiles, which were measured by the Particle Tracking Velocimetry and Flow Induced Birefringence, respectively. A reasonable agreement between them proved the validity of our model and simulation method. The proposed mesoscopic rheological model was coupled with the finite element method to develop a digital analysis platform named DAMPC (Digital Analysis of Morphology of Polymer Chains in Flow Fields), which was able to be used to calculate the structure and morphology evolution of polymer chain coils in complex flow fields and viscoelastic properties of polymer melt. It incorporated the progress of the conformation tensor model and its applications in the polymer processing process. The current work overcomes the shortcoming of the classical mechanical viscoelastic models and furthers the simulation of polymer processing process into the mesoscopic scale. The computational platform has been applied in some research fields such as phase separation, reactive processing and flow induced crystallization.Therefore, the main innovations of the present research work are listed as follows:1. The Doufas'work was extended. Based on their two-phase microstructure rheological model and the fiber radial flow was first considered, the high-speed fiber spinning process was then simulated. The flow information in the radial direction of the fiber was predicted accurately and more completely. According to the simulation results, the causes of neck-in and extensional degree of polymer chains during the high-speed fiber spinning process were analyzed. Besides, the spinning process of the non-circular fiber was also simulated by using FEM. It was first pointed out that the surface tension was the most important factor which caused the shape change of fiber cross-section.2. Substituting the conformation tensor for the stress tensor, a new governing system was established to describe the melt flow in the complex flow fields. The solving system was solved by FEM and the structure of polymer chains was first solved and the macroscopic viscoelastic of polymer melt or solution was then calculated based on the conformation distribution of polymer chains. A software which was able to be used to calculate the morphology evolution of polymer chains and macroscopic viscoelastic quantities in the simple and complex flow fields was developed by using Visual C++ language and named DAMPC. The digital analysis platform has been used in the simulation of polymer melt flow in some complex flows such as the contraction flow and the planar flow around a cylinder. The predicted velocity and stress distribution were consistent with the experimental data, which validated the proposed simulation method.3. Considering the free energy change of polymer melt under the flow, a new flow-induced crystallization model was developed incorporating the effect of orientation and stretch of polymer chains. The predicted induction time of crystallization under the shear flow agreed with the published experimental data well. Therefore the proposed model was believed to be correct and could be extended to predict the crystallization kinetics of polymer materials in the complex flows.
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