In situ mechanical foaming in fused filament fabrication

In the context of lightweight design and functional integration, the generation of foamed structures in additive manufacturing represents a key technological objective. Conventional foaming methods often rely on chemical blowing agents or physical foaming in downstream processes such as autoclaves, which require complex process chains and high energy input. To address these limitations, this work presents a first feasibility demonstration of a process-integrated mechanical foaming approach for material extrusion, ensuring continuous production in an in-line foaming process. A modular nozzle was developed, in which carbon dioxide is injected into the polymer melt under high pressure during extrusion. Gas enters the melt through a porous medium embedded in the nozzle, enabling controlled gas transfer while preventing melt backflow. This mechanism facilitates mechanical foaming within the nozzle itself, eliminating the need for separate process stages. Systematic material screening showed that metallic porous media with submicron pore diameters provide sufficient resistance to melt intrusion while allowing stable gas injection. Extrusion trials with polylactic acid confirmed that the resulting foam morphology depends on the gas-to-melt mass flow ratio, yielding uniform microcellular structures with porosities up to 25 % and mean pore diameters around 100 µm. The presented results demonstrate that stable foam extrusion based on mechanical foaming through in-nozzle gas injection is feasible, and they establish the foundation for further investigations aimed at process refinement toward finer microcellular structures and fully additively manufactured foamed components.