| Polymer melts are typical viscoelastic materials which are neither similar to elastic solids nor to viscous fluids.The viscoelastic characteristics are showed when the materials are effected by outer forces. With the different outer environments, the characteristics are different, for example, the frequency and magnitude of the force, the time of the action on the materials, the shift of the temperature and so on are all the facts to affect the polymer melts. To master the viscoelasticity of polymeric materials is very important. One hand, it is instructive for the mold design and the optimization of technics in the polymers molding process, the other hand, it gives the scientific facts to the research on the rheology theories, it can compare the viscoelastic constitutive equations and find the advantages and disadvantages in applying them. In this dissertation, the research on polymer melts is carried on both experiments and theories, at the same time, the author does some wok on the residual stress in the injection-molded parts, the methods of the calculation and measurement on residual stress are presented. The main work in the paper is as following:I.According to the typical rheology phenomena, some different viscoelasticity constitutive equations are reviewed, the advantages and disadvantages which illustrate the viscoelasticity are compared for every equation. By using the mechanics mold, the variable characteristics of the polymers are easily described. The theory of the rheology constitutive equation is very helpful to the development of the polymers science. Before the process of the molding in the factory, we can simulate the flow of the melts in mold cavity by numerical methods using the constitutive equations, we know, CAE of the mold is very important in the mold design and manufacture. The Maxwell model which is combined with springs and viscocity pots is wildly used in describing the linear viscoelasticity,it has two forms: differential form and integral form. From the differential Maxwell model, we can derive the Dewitt model that is used to describe the nonlinear viscoelasticity. From the integral Maxwell model, we can understand discrete relaxation units which is associated with the relaxation spectrum. The Bolzmann overlap principle is basic theory for Maxwell model. From the rubber elastics and statistics, the K-BKZ model is derived .Based on the non-equilibrium thermodynamics, the Leonov model iswidely used to describe the nonlinear viscoelasticity.2. To determine the viscoelasticity characteristics, some Theological experiments on different materials are done. With the different temperature or the different stress and strain that act on the melts, the viscoelasticity characteristics are different. The reliable experimental data are necessary for materials science, mold design and quality controlling in the products. The viscoelasticity parameters are gained by Theological experiments directly or indirectly. The samples are PP and PE, the instrument is ARES that is strain-controlled revolving rheology system. The steady, dynamic and transient tests are carried with different temperature, frequency and strain, the Theological data are gotten. The viscosity, the store modulus, the loss modulus, the normal force difference, the relax modulus are different with the outer impacts. From the steady rate ramp sweep data we can understand the rule that the viscosity changes with the temperature and the shear rate, this is instructive for the determination of the melts injection rate and injection temperature in the factory. From the dynamic frequency sweep data we can understand that the elasticity and the voscocity change with the frequency, by the time-temperatue superposition principle we can get the information that the viscoelasticity change in very wild frequency range which is not gained in the experiments. From the transient single measurement and the stress relaxation tests we can get relaxation and retardation information.3. The discrete relaxation... |
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