Temperature Profile And Crystalline Behavior In Cooling Stage Of Injection Moldings Of High-Density Polyethylene

The study is an attempt to formulate a new approach to the modeling of the temperature and crystallinity profiles in the cooling stage of the injection molding of a high-density polyethylene. On the basis of its fast crystallization rate, which can mostly result in the insignificant dependence of its aggregation structural formation on processing conditions, the research work mainly investigates the temperature profile during the cooling stage of the injection molding and crystallization behavior in the simulated cooling history.According to the difference of phase transformation between the crystallizing polymer and low molecular material, the method of modified phase transformation temperature with the inclusion of the crystallization induction time is firstly presented by modifying the moving phase transformation interface in Stefan's theories, and the governing equations are developed to describe the cooling temperature of solid, liquid, and solid-liquid interface, respectively. An enthalpy E and parameter T are employed to convert the energy equation into a non-linear equation successfully, which is then discreted by the control volume finite difference approach and solved with mathematical software MATLAB. The calculated temperature profile exhibits the existence of thermosphere and phase transformation and requires time 2 minutes for the cooling cycle,which are in agreement with those experimental data on the inner layer. Thus it can be applied to the numerical performance of Stefan problems.At the same time, non-isothermal crystallization kinetics of high density polyethylene is investigated via differential scanning calorimeter (DSC). Non-isothermal crystallization kinetics data obtained from DSC are employed to estimate the kinetic parameters of mathematical models describing the non-isothermal crystallization of HDPE. It is found that the non-isothermal crystallization kinetics of HDPE can be best described by Nakamura model with the inclusion of induction times. A linear regression method and trial-and-error method are presented using the Nakamura model to obtain crystallization rate equation parameters directly from non-isothermal crystallinity data. Thus, a one-dimensional finite integration model can be used to simulate the cooling stages of the injection molding. Furthermore, it is also discussed the effects of inlet melttemperature, mold temperature and product dimension on its cooling profile and crystallinity variations for optimizing processing conditions and producing the required physical properties.