  This paper presents in-situ X-ray computed tomography (CT) experiments used
to study the flow behavior within a conventional hot-end of a fused filament
fabrication printer. Three types of experiments were performed to better
understand the melt and flow behavior. In one experiment, 360{\deg} CT scans
were conducted, focusing on the air gap between the filament and the nozzle
wall. In a second experiment, the flow profile inside the nozzle was studied
using radiography. To provide a good contrast to the surrounding nozzle
material, filament was prepared containing small amounts of tungsten powder as
a contrast agent. During a third test, the extruder forces were measured and
compared with the X-ray results and the predictions of a numerical simulation.
The CT scans showed that at higher filament speeds, less area of the nozzle
wall is in contact with the melt. This means that a larger part of the barrel
section is occupied by an air gap between the solid filament and the nozzle
wall. In contrast to the filament speed, the influence of the heater
temperature shows no discernible effect on the part of the nozzle filled with
melt. Radiographic evaluation of the velocity profile revealed a parabolic
distribution under the studied conditions, closely matching numerical
simulations modeling the flow as isothermal and non-Newtonian. The study's
findings offer potential for improving nozzle design. Furthermore, the
presented experimental method can serve as a valuable tool for future
validation of more complex numerical simulations.
