Macro-mesoscopic Simulation Of Short-fiber Reinforced Polymer Injection Molding

The process of short fiber reinforced injection molding is a typical multi-scale problem, it has experienced a rather complex process of deformations and phase changes, such as the non-isothermal and non-equilibrium process of fusion, melting flow and solidification, and also accompanied by complex physical and chemical changes of phase change, molecular orientation, possible crystallization and fiber orientation etc. As a result, it has become a cutting-edge issue of the field of polymer molding to study clearly the mesoscopic structures'changing rules in the whole molding processing and to establish the scientific quantitative relationship among mesoscopic morphology, molding process conditions and product macro-performance. On the basis of experimental analysis, this article finishes the mesoscopic simulation study of short-fiber reinforced polymer melt flow through dissipative particle method, and establishes the macro-mesoscopic double scale coupling model of short-fiber reinforced polymer melt's injection molding filling flow, which includes the following aspects:1. The experimental has studied the shear rate, temperature, fiber concentration, aspect ratio and other parameters' effect on short-fiber reinforced polymer melt's viscosity change rule and the viscoelastic properties of small amplitude oscillatory shear flow, steady shear flow, transient shear flow. The experimental results shows: Short fiber reinforced polymer melt is a pseudoplastic fluid, the addition of short fiber does not change the flow curve type of polymer matrix material; When short fiber content is low, the system viscosity did not change significantly; When the content exceeds 20%, the melt viscosity of the short fiber reinforced polymer began to increase; The greater of the aspect ratio the effect on melt viscosity is more obvious.2. To develope the dynamic equation of single fiber, the evolution equation of the fiber orientation distribution function with time and the evolution equation of orientation tensor, and to analyze fiber movement, the orientation and the flow field and its own parameter relation. The results of the study indicate that:(1) short fiber particles periodic motion is related with shear flows and aspect ratio. the more shear rate increases, the shorter rotation period; the greater of the aspect ratio, the longer rotation period; (2) In different shear rate flow field, the fiber orientation distribution has great changes. The more shear rate increases, the narrow orientation distribution, the more tendency to shear flow direction.3. Constructed the coarsening model of short fiber reinforced polymer molecular and molecular chain, the numerical simulation method of melt flow based on dissipative particle dynamics is proposed. Approximated short fiber reinforced polymer as fluid-solid two-phase structure containing cylindrical solid particles and established motion equation of dissipative particle dynamics and coarsening chain conformational change equation. In accordance with the third type solid and liquid boundary conditions solved the imbalance problems caused by melt particle through the wall. Simulation results show that:The stretching rate of polymer coarsening chain increases with the increase of shear rate, and become bigger with time steps cumulative; polymer coarsening chain orientation along the flow direction of the field, the greater of the shear rate, the more obvious of the orientation; short fiber also oriented along the flow direction of the flow field, the greater of the shear rate, the more obvious of the orientation.The simulation results match with "Skin-Core" structure exists in completed 30% of short glass fiber reinforced polypropylene injection molding product, initial achieved the qualitative description of structure, properties and dynamic behavior of short fiber reinforced polymer melt system.4. Established the macro-mesoscopic double scale coupling model of short fiber reinforced polymer injection molding filling flow. Based on the Lattice Boltzmann particle motion, it is proved the consistency of solving problem of meso and macro motion equation through Chapman-Ensko's Multi-scale Series. On this basis, approximated short fiber reinforced injection molding filling process as macro-mesoscopic double scale problem composed by macro-flow and small-scale short fiber movement, will the meso information of the small- scale short fiber through to macro flow field of large-scale, established a control equation of which the macroscopic flow field and the orientation of short-fiber coupled. Use finite element method simulated the size and variation of the physical quantities and short fiber orientation distribution in the flow field. The simulation and experimental results shows:Double-scale coupling method can describe the macro flow field's influence law on short fiber distribution more accurately.The research results above enrich and help to develop the theory of polymer molding and processing. It opens up new ideas of meso-scale and macro-mesoscopic double-sale simulation studies of polymer molding process, and is of some theoretical significance and engineering value of the guidance of polymer molding and processing.