Today's KNOWLEDGE Share: Crystallization kinetics

Today's KNOWLEDGE Share:

How does crystallization kinetics possibly influence the PvT data we use in all our molding simulations ?

PvT are typically obtained, in theory, under "thermodynamic equilibrium" conditions. This means at speed so low that kinetics should be irrelevant.

The molding process is the complete opposite ! Very fast cooling will shift crystallization T to lower values. And, as also depicted in the figure, the solid state density will be lower (so specific volume, the Y axis, higher for the red curve).

Implementing crystallization kinetics in Flow Simulation is therefore quite a big challenge since cooling rate will be different from part to part, molding condition to molding condition, as well as within the same part (thin vs. thicker areas cool at different rates for instance and temperatures are different throughout the melt).

People who have attempted to implement such "dynamic PvT" or "fast PvT" often neglect to account for the extremely strong nucleating effect of shear-stress in the outer layers. Any DSC (quiescent) crystallization kinetic data will fail to predict the true kinetics observed under the very high flow stresses of Injection Molding.

Pressure is also a well known nucleating "agent" (speeding up crystallization, as seen in the classical horizontal shift of the transition zone in PvT data) but will also increase the Glass Transition temperature of the material which results in the opposite effect of slower molecular diffusion (higher viscosity) and hence somewhat slower crystallization.

Quite some work ahead of us to further improve the accuracy of Injection Molding without actually deteriorating the simulation performance by forgetting important pieces of this complex Physics !.

Source:VITO LEO

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