Porosity gradient control of 3D-printed hybrid foam structures

In material extrusion, while maintaining a constant mass flow rate, operations involving temperature-sensitive expandable materials show an increase in the volumetric flow rate at the nozzle exit. This research uses foamable filaments made of thermoplastic elastomers and thermally expanded microspheres (TEMs) to examine this phenomenon and focuses on continuous 3D printing of hybrid thermoplastic elastomer foam samples with density gradients. This is achieved by real-time adjustments to printing temperatures and layer heights. Using TEMs with a higher initial expansion temperature provides a linear density decline from 200 to 240 \(^{\circ }\) $_{}^{\circ }$ C, allowing for the fabrication of gradient density structures by controlling 3D printing parameters. The pressure–volume–temperature investigation indicates that the expansion ratio of the foamable filament increases with TEM weight percent and a reduction in pressure, but the mass remains the same. Based on the volume conservation principle, custom G-codes are developed for strand analysis and hybrid cube production. It is observed that the layer height has a linear connection with flow rate, which increases with temperature. The material flow rate is key to controlling the foamable filament expansion, through which parts with gradient properties are produced. Finally, hybrid cubes made of filament containing 4 wt% of TEMs have been successfully fabricated.