Orientation-Graded Morphologies in Microcellular Foams through Additive Manufacturing

We present experimental findings and modeling insights into the expansion of bubbles within a Newtonian fluid, observed during an advanced process integrating additive manufacturing and physical foaming techniques to fabricate complex microcellular foams with orientation-graded morphologies. A physical and sustainable blowing agent (CO2) is solubilized into a biopolymer (PLA) that is 3D-printed through a cylindrical nozzle, and, at the nozzle outlet, the blowing agent foams inside the polymeric strand due to pressure drop or/and temperature rise. The experimental results show that, by engineering the temperature gradient at high printing velocities, corresponding to values of the Graetz number (i.e., the ratio of heat diffusion time and residence time inside the printer hot-end) larger than one, the microcellular foamed strands have a microstructure characterized by anisotropic bubbles oriented along two different directions. The microbubbles at the center of the strands are stretched in the extrusion direction, whereas those in the periphery are stretched radially. A foam morphology with microbubbles having two different orientations, smoothly changing within the cross section, has never been reported before. We investigate the formation mechanism of such a morphology by simplified modeling and numerical simulations. Simulation results support the experimental findings and rationalize the effects of the Graetz number on the microcellular foamed strands and on the expansion of gas bubbles in a Newtonian fluid, suggesting that the different orientations of the bubbles are due to the combined effect of high Graetz number and the radial expansion of the strand.