A Co-extrusion Additive Manufacturing Process with Mixer Nozzle to Dynamically Control Blowing Agent Content and Print Functionally Graded Foams

A unique approach to 3D-print functionally graded foams (FGFs) via dynamic control of the blowing agent content is demonstrated. The approach utilizes a co-extrusion additive manufacturing process equipped with a static mixer nozzle (SMN) and thermally expandable microspheres (TEMs) as the foaming agent. The nozzle consists of two flow paths, one longer than the other, to facilitate the feeding of two different filaments. It is also equipped with layer multiplying elements (LME) for the mixing of the incoming melt streams. The first incoming filament was the expandable polylactide acid loaded with 8.0 wt % TEM (ePLA) to be mixed with the second filament made of neat PLA. The mixing of the two filaments at various ratios was successfully achieved, resulting in foams with uniform cellular morphologies at various densities. The choice of flow path also had a significant effect on the foam density. When ePLA was fed through the longer flow path, a greater degree of foaming was obtained due to a longer residence time. The FGF flexural samples, printed through this method, demonstrated a superior mechanical performance compared to their single density foam and solid unfoamed counterparts. The results reveal that this approach of foam additive manufacturing process provides a capable method to manufacture complex and functionally graded structures with programmable density profiles with specific gravities varying between 0.43 and 1.21 g cm^-3 on demand.