Basic Principles of polymer processing modeling

In the past, the development of new polymers was motivated by the search for new or improved properties. It was stipulated that forming processes would adapt via a few adjustments made through a trial-and-error approach. This worked in a number of cases, but in others, it was soon discovered that these new polymers, with their "marvelous" properties, were very difficult to process, either because the pressures or motor torques required were incompatible with existing processing machines, or because they exhibited flow or stretch instabilities at production speeds incompatible with their economic profitability. In this respect, we recall the development of the first metallocene polyethylenes, which showed extrusion defects at much lower shear rates than those observed with traditional polyethylenes.

More recently, polymer producers have integrated the process into the development of their new materials, using minimachines (extruders or injection molding machines) from the earliest stages of their development to identify potential processing problems with just a few hundred grams of polymer. Unfortunately, extrapolation to industrial-scale machines has sometimes proved hazardous, particularly as "scaling up" does not obey the same homothety depending on whether mechanical or thermal phenomena are considered.

This is why numerical modeling, which was mainly used in the field of injection mold design some twenty years ago, is now penetrating the various sectors of the plastics industry. But this only makes sense insofar as the numerical models developed are based on a relevant physical analysis of the phenomena encountered in a machine or processing tool. The aim of this article is to set out a few rules for adapting the model to the constraints of the manufacturing process, and to answer the questions that arise, before illustrating them with a few examples. First of all, however, it is necessary to give a brief "state of the art" of processes.