A Brookfield viscometer could do the job of quality control by checking whether the solution viscosity remains the same under identical testing conditions. If quality control is sought through melt viscosity of the PET flakes instead of solution viscosity, then determining the Melt Flow Index would be ideal.

Information in the following book may help in that case:

• A. V. Shenoy and D. R. Saini, Thermoplastic Melt Rheology and Processing, Marcel Dekker Inc., New York (1996).