Numerical Simulation Of The Crystalline Morphology Evolution In Injection Molding

Polymer can only partially crystallize which is different from the materials with low molecular weight, so it is called semi-crystalline polymer. Under quiescent conditions and for relatively low cooling rates, crystallization is a sufficiently well understood phenomenon. As flow conditions are considered, however, the evolution of crystallization is different. First, the crystallization kinetics under flow are accelerated with respect to quiescent conditions, the number of activated nuclei is increased, and the induction time, half crystallization time are shortened. Second, the final crystalline morphology generated under flow can be also completely different. On the contrary, the evolution of crystallization has a dramatic influence on material rheology. Injecton molding is one of the most widely employed method of polymer processing. The flow of polymer melts in a cold cavity is a typical example of an unsteady, non-isothermal flow of viscoelastic fluids. Every particle of the material experiences a complex thermo-mechanic history which results in the molded product with an intrinsic heterogeneous microstructure, featuring a gradual and hierarchical variation of crystalline morphology. Such a microstructure, in turn, contributes often crucially to determine both the processing behaviour and the final properties of the plastic part such as dimensional accuracy,dimensional stability, thermal conductivity and strength.The main research focus on crystallization kinetics and crystalline morphology through experimental observations and numerical simulation. The objective of this thesis is to present a numerical simulation for the shear-induced crystallization with regard to the experimental phenomenon. The main work is:(1) The effect of shear on crystllization is considered through the mathematical relationship between the additional number of nuclei induced by shear treatment and the first normal stress difference. Leonov viscoelastic model and Avrami model are used to describe the normal stress difference and the crystallization kinetics, respectively. The effect of different shear conditions on the crystal Unity, half crystallization time, induction time are discussed. The effect of crystallization on melt viscosity and shear stress are also considered.(2) Consider the crystallizing system as a suspension of semi-crystalline entities growing and spreading in a matrix of amorphous material. Flow make the crystalline entities rotate and orientate. All the interactions with the solvent and the crystalline phase are associated with the frictional facor. A second-order orientation tensor can be calculated in terms of the flow conditions, crystallinity and relaxation time. Then we can obtain the orientation factor which measure the semi-crystalline orientation with respect to the flow direction.(3) Every particle of the polymer melt experiences a complex thermo-mechanical history in injection molding. It is important to trace its flow path in order to obtain its thermo-mechanical information. The thermo-mechanical profile of the mold cavity is calculated with Euler method through governing equations, and trace the tlow path with Lagrangical method. The evolution of temperature and shear rate of nine points on different cross sections under different melt temperature and mold temperature are considered.(4) According to the thermo-mechanical history of the material and the relationship between the crystalline morphology and the chain stretch, the skin ratio, spherulite diameter and the total length of the shish per unit volume under different melt or mold temperature are calculated. With the increase of melt or mold temperature the skin ratio decrease, the spherulite diameter increase, and the total length of the shish per unit volume decrease.