Not dry enough
Vol. 18 #2, July 1991
When running hygroscopic hydrolytically degradable polymers (PET), one way to determine if the polymer is adequately dried is to look at the screw power.
If screw power (torque) reduces, then the polymer is being degraded. This reduction in MW can be inferred by a reduction in the drive power.
See also
PET dryer
PET dryer instrumentation