16 Jan 2019
Resin solidified using two wavelengths to control process; resulting products are more durable, say Michigan developers.University of Michigan (U-M), creates complex shapes from a vat of liquid at up to 100 times faster than conventional 3D printing processes — such as building up plastic filaments layer by layer.
The team says that the most familiar form of 3D printing, which builds 3D objects with a series of 1D lines, has not been able to reduce its typical production timescales of a week or two.
“Using conventional approaches, that’s not really attainable unless you operate hundreds of machines,” said Timothy Scott, U-M associate professor of chemical engineering who co-led the development of the new 3D printing approach with Mark Burns, the T.C. Chang Professor of Engineering at U-M.
The new method solidifies the liquid resin using two wavelengths of light to control where the resin hardens — and where it stays fluid. This enables the team to solidify the resin in more sophisticated patterns.
They can make a 3D bas-relief in a single shot rather than in a series of 1D lines or 2D cross-sections. Their printing demonstrations include a lattice, a toy boat and a block M, pictured above.
“It’s one of the first true 3D printers ever made,” said Burns, professor of chemical engineering and biomedical engineering.
But this so-called "true 3D approach" is necessary to overcome the limitations of previous vat-printing efforts. Namely, the resin tends to solidify on the window that the light shines through, stopping the print job just as it gets started.
By creating a relatively large region where no solidification occurs, thicker resins — potentially with strengthening powder additives — can be used to produce more durable objects. The method also improves on the structural integrity of filament 3D printing, as those objects have weak points at the interfaces between layers. “With our new method, we can produce much tougher, much more wear-resistant materials,” Scott said.
The work is described in the journal Science Advances, in a paper entitled “Rapid, continuous additive manufacturing by volumetric polymerization inhibition patterning.” The specifics of the two-wavelength illumination system are as follows:
Blue light is provided by a collimated LED–based illumination source (Thorlabs M470L3-C1) with an output centered on 470nm used in combination with a current-adjustable LED driver (Thorlabs LEDD1B) for intensity control. UV light is provided by a UV spot curing system (OmniCure LX500, Excelitas Technologies) equipped with an OmniCure LED MAX head with an output of 365nm. Irradiation intensities are measured using an International Light IL1400A radiometer equipped with a GaAsP detector (model SEL005), a 10× attenuation neutral density filter (model QNDS1), and a quartz diffuser (model W).
Early approach modified
An earlier solution to the solidification-on-window problem was a window that lets oxygen through. The oxygen penetrates into the resin and halts the solidification near the window, leaving a film of fluid that allows the newly printed surface to be pulled away.
But because this gap is only about as thick as a piece of transparent tape, the resin must be very runny to flow fast enough into the tiny gap between the newly solidified object and the window as the part is pulled up. This has limited vat printing to small, customized products that will be treated relatively gently, such as dental devices and shoe insoles. By replacing the oxygen with a second light to halt solidification, the Michigan team can produce a much larger gap between the object and the window — a few millimeters thick — allowing resin to flow in thousands of times faster.
The key to success is the chemistry of the resin. In conventional systems, there is only one reaction. A photoactivator hardens the resin wherever light shines. In the Michigan system, there is also a photoinhibitor, which responds to a different wavelength of light. Rather than merely controlling solidification in a 2D plane, as current vat-printing techniques do, the Michigan team can pattern the two kinds of light to harden the resin at essentially any 3D place near the illumination window.
U-M has now filed three patent applications to protect the multiple inventive aspects of the approach, and Scott is preparing to launch a startup company. A paper describing this research will be published in Science Advances, titled, “Rapid, continuous additive manufacturing by volumetric polymerization inhibition patterning.”
The U-M video, below, shows the new way to perform 3D printing. The University of Michigan College of Engineering, one of the world’s top engineering schools, is home to 12 highly-ranked departments, and its research budget is among the largest of any public university. This project was co-led by Timothy Scott, associate professor of chemical engineering and Mark Burns, T.C. Chang Professor of Engineering and professor of chemical engineering and biomedical engineering.