Design of thermoset composites for high-speed additive manufacturing of lightweight sound absorbing micro-scaffolds

Low printing speeds in extrusion-based additive manufacturing of thermosets, such as direct ink writing, are an obstacle for industrial-scale production. This work aims at developing lightweight thermoset composite materials compatible with the high-speed direct ink writing of porous microstructures featuring a sound absorbing functionality. The developed materials are blends of an aerospace-grade epoxy matrix filled with different loadings of hollow glass microspheres and fumed silica nanoparticles. The printability of the blends was assessed through microscopy and tomography scans, by studying the shape retention for different filler loadings and nozzle configurations. Oscillatory, rotational, and capillary rheological studies were conducted to obtain viscosity models relating extrusion pressures and printing speeds. Three promising blends containing a combination of 0-10 wt% hollow glass microspheres and 0-12 wt% fumed silica were selected based on their printability and shape retention. Printing speeds ranging from 110 to 175 mm.s(-1) were attained using 250 mu m tapered nozzles, which is over three time the highest speeds reported in the literature. An acoustic absorption characterisation using a Kundt's tube was carried out for printed porous microstructures resembling a log-pile network with 300 mu m square pores. Average acoustic absorption coefficients above 0.6 were achieved for frequencies from 500 to 6000 Hz.