Effect of Crystallization on Rheology

The injection molding process includes both molten polymer and solidification phase, where flow and crystallization may take place simultaneously. Therefore, for modeling injection molding, a rheological relation connecting the relative crystallinity a to flow parameters is needed. 

Doufas et al. (1999, 2000) proposed a two-phase model based on a modified Giesekus model for the amorphous melt phase and a rigid dumbbell model for the semi-crystalline phases. In the modified Giesekus model the relaxation time is a function of relative crystallinity a  

This two-phase model is able to predict the locking-in of stresses and the orientation of crystallites. 

Zheng and Keimedy (2004) followed the approach of a two-phase model, but in their model, the effect of crystallinity is not buUt-into the amorphous parameters. They view the system as a suspension of crystalhne phase in a matrix of amorphous phase. The physical properties of the amorphous phase are independent of a, while the viscosity of the whole system is a function of the relative crystalhnity. 

However, as pointed out by Tanner (2003), the stress law of (4.24) over simplifies the real picture of structure. The additive rule envisages parallel components of amorphous and crystalline phase at each point. Photographs of crystallizing polypropylene such as those shown by Koscher and Fulchiron (2002) and many others do not support this assumption. 

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In this chapter, we shall focus on two relevant topics the effect of flow on crystallization kinetics, and the effect of crystallization on rheological and thermal properties. 

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