Local activation of porosity in fused filament fabrication by in-nozzle physical foaming with supercritical CO2

Abstract Material extrusion of thermoplastics (fused filament fabrication, FFF) commonly deposits solid strands. Foams combine low density with functions such as damping and thermal insulation, making cellular strands a desirable extension of FFF. We demonstrate dissolution-assisted physical foaming inside the nozzle of an FFF system: an external gas booster supplies supercritical CO 2 through a lateral port into the melt, and a static mixing element upstream of the 0.60 mm nozzle promotes dissolution before foaming occurs at the nozzle exit. This transfers inline gas dosing and melt homogenization, established in foam extrusion, into an FFF printhead. We printed foamed strands and ring-shaped specimens from PETG and characterized them by microscopy and micro-computed tomography (µCT). Full-volume µCT analysis yielded an overall porosity of 75.3% (70.0–79.8% across segmentation thresholds), with a denser skin surrounding a more porous core. Using only gas, the polymer remains free of foaming additives, and we initiated pore formation locally on demand through the gas supply alone.