Structure development in polymers - Applications

Structure development, the general concepts of which were the subject of the article [AM3120] , is an increasingly important field in polymer shaping, with the aim of controlling the final properties of products. By structure development we mean, in the broadest sense, all the phenomena occurring during the transition from the initial molten polymer to the final, very often semi-crystalline, state, including :

• crystallization, which takes place under complex geometrical (tool walls), mechanical (flow, pressure) and thermal (cooling rates, thermal gradients) conditions;

• orientation; from the orientation of the molten phase to the final orientation of the amorphous and crystalline phases.

The many experimental results obtained in laboratory experiments, characterization of finished products or in-line measurements make it possible to propose a general description of polymer crystallization in shaping processes. The general concepts outlined in [AM3120] are applied in this article to four processes, corresponding to different influences of forming parameters on microstructure. In flat film extrusion, crystallization occurs after the flow has stopped. In contrast, in extrusion or sheath blow molding, crystallization occurs during the flow. In the injection molding process, very different situations are encountered depending on the location inside the part.

Numerical simulation is an effective tool for interpreting and predicting the coupled phenomena involved in the development of structures. It involves introducing a crystallization law into a thermomechanical process model. The example presented here concerns flat film extrusion.