Numerical Simulations Of Fiber Orientation,Residual Stress, Warpage And Hydrothermal Ageing Of Injection Molded Fiber-reinforced Plastics

Short fiber reinforced polymer composites are of excellent mechanical and physical properties, and applied widely in automobiles, machinery equipment, electronic and electrical products. Injection molding is the most effective processing method for the composites. The process of injection molding contains kinds of physic-chemistry changes, such as movement of free boundaries, the interaction between polymer and fiber, thermal conduction and phase change. It is very complicated. Therefore, the obtainment of qualified injection-molded plastics is a challenging task. The difficulties and particularities are:1) the fiber orientation varies during processing and plays an important role in plastic’s properties, but its mechanism is complex and difficult to control;2) the residual stresses derived from cooling will result in warpage, which as residual stresses is affected nearly by all the process parameters, and there are difficulties in quantitative analysis and optimization of residual stresses and warpage;3) Hydrothermal ageing will deteriorate the performance of plastics, and fiber orientation and level of residual stresses have influence on the process, but the influence factors of moisture diffusion and its law are still not distinct, rendering it is hard to predict water absorption of composites.Based on surface mesh model, the study of simulation of fiber orientation, residual stress, warpage and moisturediffusion focus on the corresponding difficulties, and the specific contents are listed as following.The single fiber orientation is represented by unit vector, and the orientation variation equation is built based on dynamics. And the fiber distribution in composites is described by probability density function, which needs to fulfill the Fokker-Planck equation. With above-mentioned orientation variation equation and Fokker-Planck equation, fiber orientation sensor evolution equation (labeled as Folgar-Tucker equation) can be deduced. Solving the Folgar-Tucker equation numerically, the fiber orientation field can be obtained. The only difficult point is the stability of algorithm. Aiming at the point, a stable algorithm is purposed, in which the IBOF closed approximation approach is adapted, the Folgar-Tucker equation is discretized by finite element/finite difference method, and the Lagrange multiplier method is used to ensure stability. Compared to experimental results, the algorithm can give remarkable solutions.After obtaining fiber orientation distribution, anisotropic residual stresses can be calculated. The difficulty is building anisotropic thermoviscoelastic constitutive equation containing strain history. Concerning on the difficulty, the computing model of residual stress of injection molded plastics is purposed with thermo-viscoelastic anisotropic constitutive equation. Introducing appropriate assumptions and primary boundary conditions, the residual stresses can be obtained. And the model is validated by hole-drilling method measured stresses.Warpage of plastics can be obtained with residual stresses as load. Warpage comes from unbalanced residual stresses. The troubles of warpage computation with the surface mesh model are formulations of high-performance eccentrical flat shells and obtainment of reasonable multi-point constraint equation. To overcome the troubles, adapting the shape functions of membrane element OPT and plate bending element DKT, the eligible eccentrical flat shell EOPT-EDKT is developed. Based on the mathematical theory of domain decomposition and the continuum mechanics theory, the same multi-point constraint equations are derived, respectively. Through experimental verification, the accuracy of the purposed model can always be ensured.Injection molded short fiber-reinforced plastics are in service under hydrothermal environment, which plays an essential role on its properties. The process can be divided into two parts:moisture diffusion and the influences of a certain amount of absorbed water on mechanical properties. The difficulties are the indistinction of influence rules of environmental factors on diffusion parameters and quantifying the influence of moisture on properties. Contraposing abovepoints, with experiments, the influences of temperature, relative humidity and level of fiber loading on moisture diffusion are analyzed and corresponding models are purposed to describe the influences. Based on the experimental results, the numerical computation model for moisture diffusion is built with the Fickian model and tested by case. Besides, the water-resulted degradations of quasi-static and dynamic mechanical properties are studied by the tensile test and DMA, respectively.Based on above research, the software modules of fiber orientation, residual stresses, warpage and moisture diffusion have been developed and integrated into the software system HsCAE3D7.5. The proper use of these modules can not only effectively optimal fiber orientation, residual stresses and warpage, reduce moisture absorption and raise the product quility, but also improve the efficiency of mold design, reduce mold test times and cut cost at the same time.