  Fused Filament Fabrication is an additive manufacturing technique in which
molten thermoplastic polymers are extruded through a nozzle. Therefore, the
interplay between the viscoelastic nature of the polymer melt, temperature,
printing conditions and nozzle shape may lead to inconsistent extrusion. To
improve the extrusion control and optimize the print-head performance, a better
understanding of the flow process of the polymer melt both in the nozzle and
the liquefier is needed. However, several challenges need to be overcome due to
the complexity of gathering experimental data on the melt pressure in the
nozzle and the lack of numerical models able to capture the full rheology of
the molten polymer. This research introduces an innovative approach for
monitoring the pressure within a material extrusion 3D printer's nozzle. This
method involves utilizing a pin in direct contact with the molten material,
which then transmits the applied force from the material to an externally
mounted load cell. The setup provides reliable, repeatable pressure data in
steady-state conditions for two nozzle geometries and at different extrusion
flows and temperatures. Moreover, the Giesekus model enabled capturing the
viscoelastic rheometric features of the melt, and the numerical predictions
have been compared with the experimental data. Results show that the numerical
model accurately describes the flow conditions in the nozzle and allows the
estimation of the behaviour of the melt in the liquefier zone, the area of the
print-head where the filament is molten. It could be concluded that the
backflow, which is the backward flow of the molten polymer in the gap between
the filament and the liquefier towards the cold end, caused significant
non-linearities in the total pressure drop measured in the feeders, which were
related to normal forces induced by shear in that region.
