Selective powder sintering for 3D printing is an increasingly affordable solution for manufacturing made-to-order elements of almost any shape or geometry. This technique involves heating a bed of powder (such as polyamide, PA12) to just below its melting point using an infrared light source to selectively melt a cross-section of the powder, then adding more powder and repeating to form a 3D object. To reduce costs and increase printing speed, a photothermal sensitizer is often added to the powders.
Typically carbon-based with a strong broadband absorption, adding these sensitizers to the polymer powders increases the conversion of incident light to heat, which means greater print speeds. Carbon-based sensitizers, however, can only produce black or gray objects. To create white or colorful prints, visibly transparent equivalents are needed.
A solution for overcoming color restrictions in this method uses plasmonic nanoparticles. Researchers designed gold nanoparticles coated with silica as a photothermal sensitizer to allow a rapid sintering of polymer powders into 3D objects by having the nanoparticles strongly absorb in the near-infrared, while only minimally interacting with visible light. At resonance, these composites showed greatly improved light-to-heat conversion compared with equivalent composites and could be sintered using low-power light sources. While these particles showed to be very efficient for the rapid fabrication of colored 3D objects, they proved to have certain limitations when trying to print 3D objects in pure white or multiple colors with high color fidelity across a large hue range, affecting the quality of the coloration of prints in high concentrations.
Thus, the researchers created a new sensitizer that has overcome these problems. The team used nanoparticles made of tungsten oxide (WO3) as the photothermal sensitizers for polymer powders. The nanoparticles comprise low-cost elements, which make them easy and cheap to fabricate. They are colorless at high concentrations and have a strong absorption in the near-infrared region, proving their capability of turning light into heat at a fast rate. In addition, they can be efficiently turned on or off with electricity or ultraviolet radiation. They are stable at very high temperatures and demonstrate a heating-to-color change rate superior to other available sensitizers. When mixed with other color inks, these nanoparticles have been able to reproduce the same shades of color as the original powders, maintaining the color purity of the samples.