Numerical And Experimental Study On The Polymer Crystallization Behavior During Injection Molding

Injection molding is the most widely used process for producing high quality polymeric products. The generated crystalline morphology during injection molding directly affects final products’ performance. However, by far, the polymer crystalline morphology control theories in injection molding still remained at the level of qualitative analysis. In industrial production, the control of crystallization morphology usually employed the inefficient "trial and error" method. To solve these problems, this dissertation focuses on the numerical and experimental study on the polymer crystallization behavior during injection molding. The main contents of this dissertation include the following four parts:(1) The simulation principles and application methods for the crystallization simulation package in the commercial software Moldflow were introduced. Take a bottle preform as an example, the relationships between the degree of crystallinity and the process parameters in the packing stage (such as packing pressure, packing time, and mold temperature) were studied by orthogonal Moldflow simulation experiments.(2) A non-isothermal shear stress-induced crystallization kinetics model was proposed based on the hypothesis that shear stress induced orientation of polymer chains increases the equilibrium melting temperature, and a finite volume method was employed to simulate the three-dimensional crystallization evolution behaviors in the filling stage of injection molding. The melt velocity, pressure, temperature and relative crystallinity were coupled in the simulation. Additionally, the crystallization evolution behaviors in the filling stage under different process parameters were analyzed.(3) A numerical simulation method was presented to predict the three-dimensional crystallization evolution behaviors in the packing stage of injection molding. In this method, the degree of crystallinity is calculated based on density, which is computed from pressure-specific volume-temperature (PVT) curve in the packing stage. Moreover, the crystallization evolution behaviors in the packing stage under different process parameters were analyzed.(4) Injection molding experiments were carried out. The relationships between the degree of crystallinity and the process parameters in the filling stage (mold temperature, injection temperature and injection time), the process parameters in the packing stage (mold temperature, packing pressure and packing time) were studied. The results of the injection molding experiments agree well with those of numerical simulations.The developed crystallization simulation software and the achievements in this dissertation can be applied to study the crystallization evolution behaviors during injection molding, and can be used to analyze the relationships between the process parameters and the performance of products. These are of great importance to control polymer crystalline morphology, and to produce materials with high performance. Meanwhile, in industrial application, the achievements in this dissertation can be adopted to solve the problems of process analysis, process evaluation, and process optimization etc. These also have a wide application perspective in controlling polymeric products’performance.