| Undergoing several engineering units such as solid conveying, melting, melt homogenizing etc., polymer pellets pass through die and are formed into products. Therefore, the extrusion theory can be divided into solid conveying theory, melting theory and melt conveying theory, among which, the melting theory is the most intricate because it have to deal with the non-linear viscoelastic rheologic characteristic of polymer melt in the complex stress field, and the phase transition which is always unstable, and must be solved by combining kinetics and thermodynamics theory.Most extruder screws are equipped with some form of cooling system to server as an additional method of thermal control when required. Cooling screw can effectively reduce the screw temperature and avoid pre-melting of pellets in solid conveying zone and degrading of melt in the metering zone, improve the efficiency of solid conveying, help to release the gas and lower melting temperature volatile in the continuous solid bed, eliminate the break-up of solid bed, and will absolutely help to stabilize the extrusion and improve the product quality. However, the advantages of cooling screw have not been extensively realized because few studies have been made for cooled screw extrusion and the manufacturers are not reliable to adopt the process strange to them. This thesis combined experimental researches with theoretical analysis, studied the melting mechanism on the basis of visualization experiments and focused on two subjects: physical and mathematical model in melting zone, and phase transition in the extrusion. The studies can be described in details as following:(A) Analyzed the experimental and numerical methods used in studying extrusion theory. The analysis showed that, the real-time visualization technology had great advantage in experimental research, and finite difference method was widely used in numerical computation and showed more advantage in this study, numerical schemes were drawn out to solve the couple of energy equation and the moving boundary between solid bed and melt pool. (B) The visualization single screw extrusion with cooled screw was studied, and the result was used to improve the physical model in melting zone which is former founded, the photos which were taken from the transparent windows showed that: solid bed kept continuous and would not break up in any melting stage in barrel, the lower molten film and side molten film between the solid bed and surface of screw which always appeared in extrusion without cooled screw did not appear in this study. The experiment also showed that there were energy and mass exchange between the solid bed and melt pool.(C) Mathematical melting model of extrusion with cooled screw was carried out for the first time and computational method was used to solve the control equations of different zone in melting, Visual C++ was used as computer code to solve the control equation of melting mathematical model, numerical solution were carried out.(D) The precision of numerical solution is supported by the visualization experiment. the pressure profiles and width of solid profiles along the screw channel were carried out. (E) The relationship between velocity and deformation of solid bed are analyzed, and the solid bed is looked as two parts: a core which has already compressed into a rigid plug in solid conveying zone and a crust in which the clearance among the pellets was filtered with melt. The modulus of the solid bed should been looked as the combination of these two parts. The ratio coefficient of solid bed velocity to barrel velocity in z direction was defined. Tracers were added into polymer to measure the residence time. empirical regression equation of the coefficient was carried out with the experimental data, and the velocity of solid bed at any position can be easily drawn from the equation.(F) Research on the phase transition was deduced from explanation on a phenomenon that former researchers could not interpret. The factors that would affect the unstab...
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