When a polymer is extruded at high temperatures, it is melted, strong shear forces are applied to it, and then it leaves the extruder through a die in order to get dried in a given mold and take a desired shape.
As it solidifies in the mold, the polymer can swell. Is there any physical parameter associated to the pre-extruded polymer, that allows the prediction of how much a polymer is going to swell under any given conditions? Is this just a temperature dependent re-structuration of the polymer matrix?
How die swell could be predicted through the knowledge of the Melt Flow Index (MFI) has been shown by Nakajima (Reference: Nakajima, N., Can die swell be predicted? Rheol. Acta, 13, 538-541, 1974). He used the time constant versus shear rate curves to get a measure of the die swell. The values of die swell thus calculated for over 100 observed data showed a variation from the actual within ±3.5% at 50% confidence level. However, this cannot be taken as a general method to correlate MFI with die swell. It is known that die swell is sensitive to molecular parameters such as molecular weight distribution, branching, and so forth, whereas MFI is not. Hence, Nakajima’s method needs to be extended to incorporate these parameters on the lines done by Shenoy and Saini (Reference: Shenoy, A. V. and Saini, D. R., Upgrading the melt flow index to rheogram approach in the low shear rate region, J. Appl. Polym. Sci., 29, 1581-1593, 1984).
Some other details related to this topic are available in section 9.2.1 of the book A. V. Shenoy and D. R. Saini, Thermoplastic Melt Rheology and Processing, Marcel Dekker Inc., New York (1996).
Determinate the polymer sweeping is quite hard, its dependents of temperature, mm, mm distribution, branches, real shear stress, moisture etc... the best way to obtain is performing a melt flow index and measure the difference between geometry of die and the polymer, better than mfi is capilar rheometry test, where you can change temperature and die shape...
The swelling depend on the extrusion rate (shear rate), normal stress, temperature and the morphology of the polymer. I recommend to do some experiments using capillary rheometry.
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