AMS Spring 2023

Emerging Metal 3D Printing Technology from CalTech: Hydrogel Infusion

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CalTech has been doing interesting work in 3D printing for a number of years now, especially Professor Julia Greer and the Greer Group. The team works in hierarchical, nano, battery and other new materials, as well as processes for 3D printing. One novel area that the team have been doing exploring over the years is related to hydrogel derived materials.

3D Printing Metal Parts with Hydrogels

By and large, these processes use vat photopolymerization to produce structures using hydrogels. These structures are then filled with metal or ceramic precursors which are then turned into the final metal component through a reaction. In some cases, the team has worked in metal oxides, namely zinc oxide, but also on manufacturing electrodes. Typically, the approach of the researchers could be a very fruitful one because it can rely on standard vat polymerization processes, such as digital light processing (DLP), stereolithography (SLA) and masked SLA (mSLA) using standard printers and materials.

Now, in a paper in Nature, titled “Additive manufacturing of micro-architected metals via hydrogel infusion,” written by Max A Saccone and Rebecca A Gallivan, we learn of a new refined approach to metal printing. The team is using vat photopolymerization, specifically DLP, to 3D print hydrogels before filling the structure with metal precursors and calcinating them, a thermal process that, when applied here, results in metal parts. The team says that it has made objects with a size of around 40 µm with very high hardness. Furthermore, the researchers believe that materials could be tuned and even make multilateral parts.

The tested lattice structures are made with beams of around 50 μm, resulting in hard parts with feature sizes of 100 μm. The researchers have worked with nickel, silver and copper, as well as tungsten niobium, a refractory metal combination that is difficult to shape. They were also able to blend multiple materials, such as copper and cobalt. A unique feature of the work is that, starting from the hydrogel step, it’s possible to put different salts in different areas, which can then be simultaneously calcined. A curious detail is that hardness seems to be between 47% and 15% higher than anticipated, which could be something to be exploited further. Generally, there is a need to control calcination, which could be a bit of an impediment to industrializing this for many geometries. Shrinkage is about 60%, which could also limit geometries and process control.

Indirect Metal 3D Printing for Small Parts

Through the use of standard chemistries, there is no need for new machines or settings, and costs are kept low. This could lead to this technology being adopted quite rapidly by service bureaus because they have the requisite equipment and knowledge of part of the process. It’s also easy to see how companies like Arkema or Stratasys’s materials unit could get behind this approach. A lot of parties could band together to industrialize this. Furthermore, through the use of vat photopolymerization they can ride the wave of this technology in producing accurate, tiny details and features. The resulting parts are small and accurate. Perhaps for creating networks of tiny channels, this technology could be a very high performing one indeed.

Similar work is being done with direct write 3D printing and two photon photopolymerization. Another approach results in vat polymerized parts that are carbonized, making for good conductors. On the whole here, the details related to hardness, refractories, and thin beam lattices could be unique.

Now, there are other technologies out there. HoloAM, MetShape, Incus and Lithoz use what I refer to as “slurry SLA”, in which a resin is preloaded with metal particles and then printed using vat photopolymerization systems. Parts are then debound and sintered. Of course, this requires numerous steps and the part sizes for slurry SLA are not large. Sintering exposes results in issues with shrinkage, as well. However, for certain geometries, slurry SLA makes detailed parts with great internal surface texture (because the resin is the support and washes out). For slightly larger parts, there’s binder jet, which can in and of itself be a highly productive and inexpensive technology. One might also consider lost wax casting together with vat photopolymerization. For tiny parts, one would probably not consider material extrusion or powder bed fusion. But, there are a few micro machining and printing technologies that may be competitive here.

 

All of these technologies are battling for a potentially very competitive space, where billions of optimized sub-centimeter to sub-micron components could make a difference in manufacturing, electronics, medicine and industrial use. The volumes attainable for the company that cracks this could be astounding. MOSFET-like devices, heat sinks, electronics components, antenna, batteries, sensors and more could all lead to millions or perhaps billions of parts.

For that to happen someone has to be able to make the right geometry at the right specification for the right price. When compared to alternative technologies, matrix-like structures, filters, TPMS fields, meshes and applications like flexible batteries might be best 3D printed with this technology. A small, yet strong lattice twould be an excellent application for this technology. Now, of course, it would need to be industrialized first and would require new equipment. This is still very early days yet, but could be a very fruitful approach.

Top image, Max Saccone.

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