Most 3D printers, including light-based techniques, build up 3D objects layer by layer. This leads to a “stair-step” effect along the edges. They also have difficulties creating flexible objects because bendable materials could deform during the printing process, and supports are required to print objects of certain shapes, like arches.
Nicknamed the “replicator,” a new 3D printer can create objects that are smoother, more flexible, and more complex than what is possible with traditional 3D printers. It can also encase an already existing object with new materials — for instance, adding a handle to a metal screwdriver shaft — which current printers struggle to do. The technology has the potential to transform how products from prosthetics to eyeglass lenses are designed and manufactured since customizable geometry can be added to a metallic component or something from another manufacturing process.
The printer relies on a viscous liquid that reacts to form a solid when exposed to a certain threshold of light. Projecting carefully crafted patterns of light — essentially “movies” — onto a rotating cylinder of liquid solidifies the desired shape all at once. Basically, the system is an off-the-shelf video projector plugged into a laptop that projects a series of computed images while a motor turns a cylinder with a 3D printing resin in it. Subtleties include how the resin is formulated and how the images that are going to be projected are computed. Currently, the printer can make objects up to four inches in diameter.
The new printer was inspired by the computed tomography (CT) scans that can help doctors locate tumors and fractures within the body. CT scans project X-rays or other types of electromagnetic radiation into the body from all different angles. Analyzing the patterns of transmitted energy reveals the geometry of the object. In the new printer, that principle was reversed. It creates an object rather than measures an object, but the underlying theory can be translated from the theory that underlies computed tomography.
Besides patterning the light, which requires complex calculations to get the exact shapes and intensities right, the other major challenge was how to formulate a material that stays liquid when exposed to a little bit of light but reacts to form a solid when exposed to a lot of light. The 3D-printing resin is composed of liquid polymers mixed with photosensitive molecules and dissolved oxygen. Light activates the photosensitive compound, which depletes the oxygen. Only in those 3D regions where all the oxygen has been used up do the polymers form the crosslinks that transform the resin from a liquid to a solid. Unused resin can be recycled by heating it up in an oxygen atmosphere.
The technique generates almost no material waste and the uncured material is 100 percent reusable — an advantage that comes with support-free 3D printing. The objects also don’t have to be transparent. Objects that appear to be opaque can be printed using a dye that transmits light at the curing wavelength but absorbs most other wavelengths.