3D printing has established itself in the 21st century as the process for producing prototypes and very small series. In the plastics sector, the fused filament fabrication (FFF) process is used in particular. A plastic filament is melted in a nozzle and a component is built up layer by layer. As with all manufacturing processes, there is an interest in continuously optimizing and improving the FFF process. One possibility is based on process simulations, which enable a better understanding of the entire process. Afterwards a validation of the simulation with the real process is always necessary. In the case of FFF, this validation was so far only possible to a limited extent. In this work, a method is presented that enables a non-destructive investigation of the melting behavior during the printing process. For this purpose, a 3D printer nozzle with an extruder was integrated into an X-ray computed tomography system. Thus, a computed tomography scan (CT scan) can be performed during the extrusion process. By using filaments with high absorbent tungsten, a sufficient contrast can be created between the metal nozzle and the plastic filament, which allows an analysis of the melting behavior. This setup allows to distinguish between the solid filament area and the melt area, as well as to determine contact between the filament and the nozzle wall. In this way, the simulations can be validated and nozzle geometries to be improved in the future by means of improved simulation tools.