How Do You 3D Print Glass?

For centuries, humans have heated, blown, and stretched glass into everything from useful household items to intricate story-telling windows. Now researchers at the University of Notre Dame in Indiana are working to adapt artisan glass-blowing techniques to a robotic platform. Instead of a torch and a pair of flame-resistant gloves, they used a laser and a computer-controlled stage. And they’re showing that it works for creating the transparent solid structures needed in optical, microfluidic, and photonic devices.

Ed Kinzel is an associate professor of aerospace and mechanical engineering at the University of Notre Dame. His goal is to capture the geometric freedom of the handblown glass approach with the precision of modern 3D printing. “I would argue that there are no good ways to rapidly prototype with glass,” says Kinzel. If you want a glass geometry different than that which comes up in commercial chemistry apparatus, you would go to a scientific glass blower—and pay the price in both money and time. That’s not the case for metals and many polymers, where computer numerical control (CNC) can rapidly create individual parts with complicated geometries, one at a time. A machine that allows rapid manufacturing of glass can bridge that gap.

But the very properties that make silicate-based glasses desirable for scientific equipment and consumer products—high thermal stability, stiffness, optical transparency, and chemical inertness—put the material out of reach of most additive manufacturing processes. For example, high molten viscosity makes it hard to get rid of bubbles in powderbed processes, and glass’s transparency from near ultraviolet to near infrared wavelengths means that they do not absorb the high-energy continous wave lasers that are common in melt-and-solidify layering approaches.

How to 3D Print Glass

One additive approach starts with fully formed glass and fuses it under the application of heat—workable for glasses with lower processing temperatures, and for lower-resolution components. But the seams that form between adjacent tracks limit the transparency and make it difficult to create fully dense structures. Another successful approach, that’s even been commercialized, deposits silica-precursor slurries combined with a photopolymer into a pattern, then dries and densifies the resulting structure through prolonged heating. This approach has “created some awesome objects but may struggle to make larger or sparse structures,” says Kinzel.

The 3D printing technique can make smooth, bubble-free glass in complex shapes. University of Notre Dame

That’s where digital glass forming comes in. Kinzel and his colleagues developed a technique that uses a laser to heat the surface of a glass rod, an approach that some companies are already using to produce glass products. Other teams, including Kinzel and his colleagues, independently arrived at a similar approach in 2014, and they’re adapting it to use for specialized applications.

In Kinzel’s lab, a carbon dioxide laser locally melts a small-diameter glass filament, so it can be deformed. The researchers construct 3D shapes by moving a fused quartz substrate on a 4-axis CNC platform relative to the intersection of the filament and the laser beam. The molten glass is controllably shaped by the interaction with the substrate and pressure from the unheated portion of the filament , as well as by gravity and surface tension. Applying pneumatic pressure directed down the glass feedstock allowed the team to apply this process to hollow glass rods to create both on-substrate 2D shapes and free-standing 3D spiral structures.

Improving the 3D Printing Process for Glass

By carefully tweaking the properties of the scan rate, laser power, and the filament feed rates, Kinzel and colleagues Luis Deutsch Garcia and Horacio Ahuett Garza at the Monterrey Institute of Technology and Higher Education in Mexico have progressed from slowly printing hollow-tube structures to rapidly creating dense, perfectly transparent 3D solids. In volumetric heating, a laser beam in a coaxial configuration heats a soda lime glass filament that is itself transparent to the laser—but lacing it with dopants changes the optical penetration depth. Experiments showed it possible to fabricate fully dense, smooth, bubble-free 2D and 3D geometries on the scale of tens-of-millimeters at a deposition rate of 15 cubic millimeters per second. This was limited by current stage performance, and models predict that ultimate performance may be much higher.

“Precisely creating glass structures is hard,” says Kinzel. Lenses can, of course, be subtractively manufactured—magnetically or mechanically grinding a surface down to perfection—and tools exist for precision-forming glass lenses for high-volume manufacturing. But free-surface, free-form glass structures are generally limited to one-dimensional sheets or fibers. “We believe there are applications such as lightweight, high strength lattices, that would benefit from glasses properties but would be very challenging to produce by an artisan and would benefit from high precision,” says Kinzel.

The team has also demonstrated their ability to create optical waveguides, where 125-micrometer-diameter fiber feedstock deposited on a substrate maintain optical transmission around curves of a sufficient diameter. To create photonic circuits, however, two deposited fibers must be coupled together such that evanescent waves from one fiber core can be coupled into another. “We’ve been able to create simple optics and are working toward printing photonic structures,” says Kinzel.

The work was presented in July at Optica’s Advanced Photonics conference in Quebec. The researchers further discuss the technical details in a June paper in the Journal of Manufacturing Processes.