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3D Printing Optics and Microfluidics from Glass

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3D printing can be used to create complex three-dimensional objects, and has proved to be highly useful in the creation of complex components, such as those used in aerospace, and even artificial limbs. However, the range of materials that can be 3D printed has, until recently, been limited. Now, new advances in the technology mean materials such as metals and glasses can be used in 3D printing.

3D printing uses computer-aided design processes to build objects layer by layer. Items can be of any shape. This contrasts with material removed from a blank in conventional machining or moulding of products in injection moulding processes. The era of desk-top manufacturing is well and truly underway.

Although materials such as metal, plastic, clays and even biological cells have been used in 3D printing, the 3D printing of glass has remained elusive. However, new research has broken this convention and glass is now being 3D printed to a high quality. It is expected that this is going to have a big impact on the production of laser optics, lenses and other complex glass components. 

Why is it Difficult to 3D Print Glass?

Glass is an important high-performance material mainly due to its unsurpassed optical transparency, mechanical, chemical and thermal resistance and its electrical insulating properties. The problem is that glass, and particularly high-purity glass such as fused silica glass, is notoriously tough to shape into required forms.

The traditional process requires high-temperature melting and casting processes for large objects or the use of caustic chemicals to etch microscopic features. These weaknesses have made glasses inaccessible to 3D printing. Most methods for 3D printing glass require the glass to be melted first and cooled down later, which has the potential for residual stress and cracking.

Researchers have established some 3D printing techniques for glass objects but these have substantial drawbacks. Binder jetting has been applied to glass to overcome the high melting temperatures and high viscosity, and sintered glass objects from this method are on the market. However, they are very fragile and can appear opaque due to light scattering from the glass powders used.

There can also be filaments formed in the glass, which could be the focus of thermal or mechanical stress. A manual wire feeding technique is a little better, however it limits control and automation. All-in-all the glass structures produced by early 3D printing methods have been opaque, porous, and with non-uniform internal structures, making them of a lesser quality than glass structures produced using conventional methods.

Using Molten Glass as the Medium

The Mediated Matter research group at MIT have been investigating the 3D printing of glass. The team have used a different approach to develop the first molten glass material 3D extrusion system, which they can use to produce optically transparent items.

Their novel process takes glass from the molten state which is then printed, using a ceramic nozzle to withstand the high temperatures, and then annealed. The printing parameters and process are under full digital control, permitting a high repeatability and good dimensional stability. Developing the process was difficult – the viscosity of the glass had to be modeled and new systems of varying the temperature in glass kilns had to be developed. 

The process has been fully modeled, studied and optimized and this has involved a ceramic nozzle of controlled geometry, modeling glass viscosity, controlling glass levels, adjusting the temperature distribution in different kilns, as well as varying layer height and feed rate. Integration of colors was achieved as well as the production of coiling patterns to produce objects of different length scales.

Characterization of the produced 3D glass objects showed strong adhesion between layers and a substantial strength increase when the process was performed in a heated build chamber, with approximately 60% of material strength across layers. In terms of optical properties, high transparency was observed and complex caustic patterns were created with LED light sources depending on sample geometry.

3D Printing with Glass Liquid Ink

A team at Karlsruhe Institute of Technology in Germany reported in ‘Nature’ on a way to print 3D objects made of pure glass using conventional 3D printing equipment.

The new system is based upon the creation of a ‘liquid glass’ ink. This glass ink consists of a glass nanocomposite, with glass nanoparticles suspended in a photocurable prepolymer. Once 3D printed the glass is moved to an oven for heat treatment, which cures the glass and burns off the extraneous materials in the composite. The result is an object made of pure, transparent fused silica glass, and can be printed with features on the micrometer scale.

The printed fused silica glass is non-porous, with an optical transparency comparable to commercial fused silica glass, and a smooth surface, with an irregularity of only a few nanometers. The process has been enhanced further by doping the glass ink with metal salts which allows colored glass objects to be printed. 

3D Printing Glass Objects at Room Temperature

In April 2017 a team at the Lawrence Livermore National Laboratory (LLNL) 3D-printed transparent glass at room temperature, a world first. This method uses silica inks, which are 3D printed and thermally processed to produce optically transparent glass structures with sub-millimeter resolution features.

The inks contain silica powder suspended in a liquid and are printed using direct ink writing. The custom inks are formed from concentrated suspensions of glass particles with highly controlled flow properties so they can be printed at room temperature. Printed structures are then dried and sintered at temperatures well below the silica melting point to form amorphous, solid, transparent glass structures, which would have previously required conventional glass fabrication.

Following printing, the object is opaque, however once the printed object undergoes a thermal treatment, the material densifies and all evidence of printing vanishes. The heat treatment ensures optical uniformity and transparency. The final step is an optical quality polish which allows the glass to obtain optical uniformity.

This method is particularly advantageous for optical manufacturers as it allows the refractive index of the glass to be manipulated. This means glass with varying refractive indices can be made into a single flat optic rather than a complex shaped optic. Flat optics are far easier to polish and refine than traditional curved optics, and it is hoped that this method will make optical components more affordable.

Expected Trends in 3D Printing with Glass

The 3D printing of glass is certainly going to expand the capabilities of optical engineers, and it is also expected to impact the world of microfluidic devices. Glass is an excellent material for microfluidics due to its optical transparency, chemical resistance, and its facility towards chemical surface modifications. 

Technical Glass from Mo-Sci

Mo-Sci Corporation provide expertise and quality in precision glass technology. They can provide a range of specialty glasses to custom requirements or even work alongside other companies in the development of glass materials for ground-breaking applications. Mo-Sci Corporation in collaboration with Northwestern University is currently developing a 3D printing method for complex-shaped soda-lime, borosilicate, sealing, and other glass components for high temperature applications. 

References

  1. Kate Cummins, The rise of additive manufacturing, The Engineer, May 2010, Retrieved December 2017
  2. Frederik Kotz et al., Three-dimensional printing of transparent fused silica glass, Nature (2017). DOI: 10.1038/nature22061
  3. John Klein et al., Additive Manufacturing of Optically Transparent Glass, 3D Printing and Additive Manufacturing, Volume 2, Number 3, 2015
  4. D. T. Nguyen, et al., 3D-Printed Transparent Glass, Adv. Mater. 2017, 29, 1701181.

 

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