Additive manufacturing, or 3D printing, uses digital manufacturing processes to fabricate components that are light, strong, and require no special tooling to produce. One of the most widely used manufacturing processes, selective laser sintering (SLS), prints parts out of micron-scale material powders using a laser — the laser heats the particles to the point where they fuse together to form a solid mass.
Selective laser sintering traditionally has involved fusing together material particles using a laser pointing downward into a heated print bed. A solid object is built from the bottom up, with the printer placing down a uniform layer of powder and using the laser to selectively fuse some material in the layer. The printer then deposits a second layer of powder onto the first layer, the laser fuses new material to the material in the previous layer, and the process is repeated over and over until the part is completed.
This process works well if there is just one material used in the printing process. But using multiple materials in a single print has been very challenging because once the powder layer is deposited onto the bed, it cannot be unplaced or replaced with a different powder.
Researchers developed a new approach to overcome SLS limitations. By inverting the laser so that it points upwards, they invented a way to enable SLS to use — at the same time — multiple materials.
In a standard printer, because each of the successive layers placed down is homogeneous, the unfused material obscures the view of the object being printed until the finished part is removed at the end of the cycle. This means that a print failure won't necessarily be found until the print is completed, wasting time and money.
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 up from below the plate and through the plate's bottom. This process selectively sinters some powder onto the print platform in a pre-programmed pattern according to a virtual blueprint. The platform is then raised with the fused material and moved to another plate, coated with a different powder, where the process is repeated. This allows multiple materials to either be incorporated into a single layer or stacked. Meanwhile, the old, used-up plate is replenished.
The team generated a 50-layer-thick, 2.18-mm sample out of thermoplastic polyurethane (TPU) powder with an average layer height of 43.6 microns and a multi-material nylon and TPU print with an average layer height of 71 microns. 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.
This technology has the potential to print embedded circuits, electromechanical components, and even robot components. 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.
The researchers are 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.