How to 3D print optical-grade glass

Ancient material meets advanced technology. Researchers in Germany have developed a way to make clear, smooth, and intricate glass structures using 3D printing.

The work, described in today’s issue of Nature, means optical-grade glass can be 3D printed for the first time, and opens up a host of new possibilities.

Glassmaking is at least 5,000 years old, and produces a material so ubiquitous you probably never think about it. Glass is used everywhere we need to bend or transmit light; in our windows and spectacles, in lenses and mirrors, from the optical fibres that transmit data around the world to the ultra-smooth mirrors on the Hubble Space Telescope.

Simple glass objects can be made by pouring molten glass into moulds. But a common way to form complex structures is still by glass-blowing, the ancient art where a skilled craftsperson blows air down a tube to inflate a molten ball of glass, which they can then bend and twist to form. Making fine details usually involves etching with acid.

That’s distinctly different from the picture of modern manufacture, driven by robotic assembly, and, increasingly, 3D printing – a term describing the range of automated processes where machinery builds up a structure, layer-by-layer, by depositing material.

While 3D printed plastic is common and cheap, 3D printers can also work with metal, ceramics, concrete, and even living cells. Adding glass to this palette of “inks” has been a goal of materials scientists for a decade. 

In 2015, researchers at MIT developed the first method to 3D print transparent glass by melting and extruding it through a nozzle. But the printed parts had distinctive layers (a bumpy surface), which may rule out a lot of important uses, such as in optics.

Now, Bastian Rapp and his team from Karlsruhe Institute of Technology have worked out a completely new way to 3D print glass structures that are clear and smooth. 

Although 3D printing hit the mainstream consciousness only recently, it’s actually been around since the early 1980s, when American inventor Chuck Hull worked out how to draw patterns in a light-curable resin. The new glass-printing technique is a variation of Hull’s original process, except they load the resin with tiny particles of glass.

After the 3D structure comes out of the printer, the team then place it in an oven for two heating steps. One to burn off the resin, and another, at 1300 °C, to fuse the glass particles together. The result is a clear, continuous material as transparent as the glass in your window. The team also found they could also colour the glass by doping it with metal salts before the heating step.

The new technique can run on printers that are available commercially for just a few thousand dollars, typically for making jewellery prototypes. But the team also built their own printer, able to make extremely fine features. To show it off, the team printed a miniature glass castle, about 1mm high, complete with tiny turrets. Each spiky crennelation on the turret wall was about the diameter of a human hair.

On the more practical side, the team made some tiny channels for fluid mixing, and a couple of optical devices, including an array of microscale lenses.

3D printing is still relatively slow, more useful for making single, once-off or personalised shapes, rather than mass manufacture. Still, as Rapp and his team conclude, their process “makes one of the oldest materials known to mankind accessible to modern 3D printing techniques”.