The goal of the project iPrint at the Chair of Polymer Processing was to manufacture cranial implants during an operation right in the surgical suite via the FFF (Fused Filament Fabrication)-process. Filaments with defined and reproducible properties were required as source material for a reproducible FFF-process. The aim of this work, that was part of this project, was to provide filaments with high and assured quality. A good printability and a constant volume flow require absence of voids, a constant diameter (1,75 mm ± 0,05 mm) and a low ovality (± 0,05 mm), which are achieved through proper process management of the filament fabrication. Furthermore a bioactivation of filaments was planned by adding hydroxyapatite. The printing of a model of the implant out of the newly acquired filaments should ensure the evaluation of the quality. Those factors as well as their setting windows, that mostly influence the quality of the produced filament, had to be found in a variety of potential machine and process settings. Regarding the machine settings it was shown that the most important ones are the settings of the pumping gear. A speed-controlled status with a positive differential pressure between inlet and outlet pressure led to a round filament. The second important factor was a specific designing of the cooling process. Only a preferably slow and smooth cooling (low temperature gradient) of the filament could avoid internal stresses that arise otherwise either voids, if the surface layer is already frozen, or a higher ovality of the filament, if the surface layer still can be deformed. Therefore the cooling path had to be equipped with an airgap as long as possible and additional air cooling, f. ex. a fan, followed by a heated and sequentially temperature controllable water bath. Based on these settings it was possible to produce medical filaments with good quality out of unfilled polymers. Also a fabrication of filaments out of polymer/hydroxyapatite-compounds was feasible with these settings. As a last step a model of an implant was printed via the FFF-process to examine the printability of the bioactivated filaments. This test print was successful.