Numerical stimulation of stress-induced crystallization of injection molded semicrystalline thermoplastics
Department of Mechanical Engineering
Doctor of Philosophy
Narh, Kwabena A.
Gogos, Costas G.
Stress Induced Crystallization
Injection molded semicrystalline plastic products exhibit variable morphology along their thickness directions. The processing conditions have a significant effect on the crystallinity distribution in the final parts. However, because of the lack of sound theoretical models for stress-induced crystallization kinetics in thermoplastics, simulations of the injection molding process of semicrystalline plastics with the consideration of stress-induced crystallization have been scarce.
A stress-induced crystallization model for semicrystalline plastics is proposed based on the theory that stress induced orientation of polymer chains increase the melting temperature of the plastics, and hence, the supercooling which is the driving force for crystallization. By assuming that the effect of stress on crystallization is only by increasing the equilibrium melting temperature, the basic quiescent state crystallization equation can be directly applied to model stress-induced crystallization kinetics. A simple experimental technique such as rotational rheometric measurement, can be used to determine the melting temperature shift. The model predicts the most prominent features of stress-induced crystallization: with the application of shear stress, crystallization rate becomes higher, the crystallization temperature range is broadened and the peak of crystallization rate shifts to higher temperatures. The main advantage of the model is that the parameters in the quiescent state crystallization model do not change and the parameters in the equilibrium melting temperature shift model are easy to determine. And the unknown constants are kept to a minimum.
The injection molding process of semicrystalline plastics was simulated with the proposed stress-induced crystallization model. A pseudo-concentration method was used to track the melt front advancement. The simple Maxwell stress relaxation model in combination with WFL equation was used to investigate the importance of stress relaxation on the development of crystallinity during the injection molding. Simulations were carried out under different processing conditions to investigate the effect of processing parameters on the crystallinity of the final part. Other results such as skin layer build-up and mold pressure were also simulated. The simulation results reproduced most of the features that were obtained by the experiments reported in the literature.
njit-etd2001-046 (204 pages ~ 11,893 KB pdf)
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Created September 21, 2001