Friction‐Corrected In‐Situ Viscosity Measurement in Material Extrusion: A Force‐Based Method Validated Against Capillary Rheometry Using Identical Nozzle Geometry

The extrusion force in fused filament fabrication carries information about the melt state, but published force‐to‐viscosity conversions attribute the entire measured force to the nozzle and neglect the friction of the solid filament in the cold end. This work quantifies that contribution and its consequences for in situ rheometry. Extrusion forces were recorded for PET and PETG at three nozzle temperatures each, covering apparent shear rates of 77–2300 s −1 with a 0.4 mm nozzle, and a dedicated open‐bore reference isolated the friction force. Friction accounts for 52%–92% of the total force at the lowest feed speed and falls below 10% only above 4–5 mm/s, so that the speeds typical for perimeter printing coincide with friction fractions of up to 80%. Subtracting it changes the apparent viscosity by 35%–170% at the lowest equilibrated shear rates and also qualitatively: in the uncorrected data the decaying friction fraction masks the increase in flow resistance that the corrected data reveal. The corrected viscosity was compared with high‐pressure capillary rheometry using an adapter that mounts the identical nozzle on the laboratory instrument. With increasing shear rate the in situ curves approach isotherms of successively lower temperature, indicating that the effective melt temperature decreases as throughput increases.