  Polymer rheology profoundly influences the intricate dynamics of material
extrusion in Fused Filament Fabrication (FFF). This numerical study, which uses
the Giesekus model fed with a full rheometric experimental data set,
meticulously examines the molten flow patterns inside the printing nozzle
during FFF. Our findings reveal new insights into the interplay between elastic
stresses and complex flow patterns, highlighting their substantial role in
forming upstream vortices. The parametric map $\alpha$-$\lambda$ from the
Giesekus model allowed us to sort the materials and connect the polymer
rheology with the FFF nozzle flow dynamics. The identification of elastic
instabilities, the characterization of flow types, and the correlation between
fluid rheology and pressure drop variations mark significant advancements in
understanding FFF processes. These insights pave the way for tailored nozzle
designs, promising enhanced efficiency and reliability in FFF-based additive
manufacturing
