| In the extrusion processing, the melting process means that the polymer is transformed from solid to liquid, and is believed to be the critical step for the quality and quantity of the extruded products.The classical extrusion theory involves how the solid plastic masses reduce gradually and dissolve finally in a whole-body melting under the heat exertion, with 10 mm of macroscopic material order as its observation dimension. At present, the development of science and technology requires that materials should be qualified with even better structure and property, while the structure and property depend on the processing technology. So the interesting objective of study in extrusion processing becomes even smaller from previous masses to pellets and powders nowadays. Generally speaking, the material size is no more than 3 mm. Therefor, the existing extrusion theory is not useful enough to explain the melting process of material pellets.Under the guidance of Professor Zhu Fuhua, the Extrusion Theory Study Group found that the shearing transformation and peripheral heat-conducting melting takes place in each pellet during the visualization study in the relationship between the structure and property and the processing equipment and technique of polymer materials. In addition, just when the different polymers are co-mixed, micro-structural changes and interface phase formation begin to come out onto the peripheral melting interfaces of individual pellets. Meanwhile there arises such the physical-chemical process as the infiltration between resin and various additives and fillers. Above-mentioned phenomena need to be explained by a new extrusion theory, which deals with the melting process in a smaller dimension of material size. But up to today, it is empty in this research field. For that reason, the National Foundation Committee of Natural Sciences granted the project "submacroscopic extrusion theory and submacroscopic melting theory" in year 1998.As part of that project, the present thesis took the submacroscopic melting theory its main content. And the polymer pellet is taken the objective of study, including pellet melting, molten permeating and its effects on the breakup of solid bed in order to make a deep insight into the melting process and to provide the optimization of extrusion process and extruder design with a theoretical basis.The combination of experimental with theoretical studies is used as the methodology. Above all, the existing macroscopic visualization system was remade into submacroscopic visualization system through self-design for meeting the needs of submacroscopic dimension observation. The second is the along-line sampling technique. Then, single-phased and double-phased mixing systems were studied in PP and LDPE polymer materials through submacroscopic visualization and along-line sampling techniques. On the basis of a serious of experimental data, the physical-mathematical model of pellet melting was established. The theoretical values were compared with the experimental values.Completed in the present thesis, the main works and conclusions read as follows:1. Reform the observing and photographing units of previous macroscopic visualization system to fulfil the requirement of sub-macroscopic visualization. Experimentally observe the melting process of single-phased and multi-phased polymer systems, and confirm the existence of pellet melting. The concept "peripheral melting" is put forward because the pellet melting process begins with the periphery.2. Based on the peripheral melting, the concept "molten permeating" comes into being. Analyze the geometric and physic conditions with which the molten permeating takes place, and set forward the physical-mathematical model which reflects permeating exertion in initial stage of the melting zone. The calculated results were compared with experimental results.3. Explain the weakening effects of permeating exertion on solid bed first time. In line with the shearing stress produced by permeating exertion,...
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