Selective laser sintering (SLS), a process that uses a laser to heat powder materials to the point where the micron-scale particles fuse together to form a solid mass, is one of the most widely used additive manufacturing processes today. Until now, SLS technologies have been limited to printing with one material at a time. To enable printing with multiple materials using SLS, researchers at Columbia University have literally turned the process upside down. They have inverted the laser so that it points upward instead of downward into the heated print bed and have eliminated the need for a powder bed. Laser beam transmitting upward through glass. Courtesy of John Whitehead/Columbia Engineering. The researchers set up multiple transparent glass plates, each coated with a thin layer of a different plastic powder. They lowered a print platform onto the upper surface of one of the powders and directed a laser beam upward from below the plate and through the plate’s bottom. This process selectively sintered some powder onto the print platform in a pre-programmed pattern according to a virtual blueprint. The platform was then raised with the fused material and moved to another plate coated with a different powder, where the process was repeated. This method allows multiple materials to either be incorporated into a single layer or stacked. It removes the need for a large powder bed because it allows the sintering of different powders in a single layer. The researchers demonstrated their working prototype by generating a 50-layer-thick, 2.18-mm sample out of thermoplastic polyurethane (TPU) powder with an average layer height of 43.6 μm, and a multimaterial nylon and TPU print with an average layer height of 71 μm. These parts demonstrated both the feasibility of the process and the capability to make stronger, denser materials by pressing the plate hard against the hanging part while sintering. Multilayer, single-material print sample. Courtesy of John Whitehead/Columbia Engineering. “This technology has the potential to print embedded circuits, electromechanical components, and even robot components,” professor Hod Lipson said. “It could make machine parts with graded alloys, whose material composition changes gradually from end to end, such as a turbine blade with one material used for the core and [a] different material used for the surface coatings. “We think this will expand laser sintering toward a wider variety of industries by enabling the fabrication of complex multimaterial parts without assembly,” Lipson said. “In other words, this could be key to moving the additive manufacturing industry from printing only passive uniform parts, toward printing active integrated systems.” The researchers are now experimenting with metallic powders and resins in order to directly generate parts with a wider range of mechanical, electrical, and chemical properties than is possible with conventional SLS systems today. The research was published in Additive Manufacturing (www.doi.org/10.1016/j.addma.2020.101440). Researchers at Columbia University invented a new technique that could transform additive manufacturing processes, potentially enabling the printing of circuit boards, electromechanical components, and perhaps even robots. Courtesy of John Whitehead/Columbia Engineering.