Multi-Material Interpenetrating Lattices via Sacrificial 3D Printed Molds

Additively manufactured (AM) polymer lattice structures are highly tailorable geometries used to achieve a tunable compressive mechanical response, leading to emerging commercial applications such as helmet impact pads and shoe soles. AM polymer lattices can be directly manufactured using a variety of techniques including vat photopolymerization (VPP) and fused filament fabrication (FFF), however, present AM methods are limited in the range of mechanical properties available (e.g., durometer, strain to failure) for printable feedstocks, and integrating multiple elastomers into the same lattice can prove challenging. In the present study, dissolvable polymer molds fabricated via FFF are used to cast lattices from conventional elastomeric resins that are not generally amenable to direct 3D printing. Because the molds are dissolved after casting, complex and interpenetrating lattices can be formed. Furthermore, interpenetrating lattices comprising multiple elastomer material grades can also be implemented. A range of polymers and solvents (i.e., polyvinyl alcohol & water; butenediol vinyl alcohol copolymer & water; and high impact polystyrene & d-limonene) are evaluated. Lattice designs are parameterized to consider unit cell size, truss angles, and spacings. The interpenetrating lattice molds feature ports to infuse resin from a syringe with vacuum assistance to produce fully dense lattices; print and fill conditions designed to eliminate air or casting leaks; and are mechanically robust to withstand vacuum pressures. Both urethane and silicone casting resins are considered. Lattices are subject to compressive testing to determine mechanical response and downselect to designs that exhibit stress-strain behaviors that are advantageous for impact energy absorption. This study shows how dissolvable printed molds can be used to efficiently create complex functional shapes from materials that are not specifically formulated for 3D printing.