Comment by cwkoss | Hacker News Reader

Comment by cwkoss | Hacker News Reader

"Then, in the "reaction" portion of the process, the researchers burn away the hydrogel portion of the structure in a furnace that reaches 700 to 1100 degrees Celsius, depending on the material. Because the melting point of all metals is higher than the combustion temperature of the hydrogel, the metal remains intact.

The heat not only removes the hydrogel, it also causes the overall structure to shrink as the hydrogel burns off, resulting in an even tinier metal structure. With this process, in addition to pure metals, the team can 3-D print metal alloys and multicomponent metallic systems, with feature sizes around 40 microns, or less than half the width of a human hair."

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How much does it shrink? Does the shape deform as it shrinks? I would imagine certain geometries wouldn't work because the outsides would shrink faster than the inside, which could break/bend some features.

Seems like awesome tech, but I suspect there are a number of limitations to this technique which the article does not discuss.

There's a hint at the limitations in the article:

While developing the process, the team produced 3-D printed structures made from copper, nickel, silver, and various metal alloys.

Copper, nickel, and silver are all relatively "noble" metals, easily reduced to the metallic state from their salts or oxides. I'm guessing that this technique does not work with more reactive metals like aluminum, titanium, or magnesium. It may not even work with iron.

EDIT: the full article in Nature is open access and has more details: https://www.nature.com/articles/s41586-022-05433-2

Calcination in air converts the metal-salt-swollen hydrogels to metal oxides, and subsequent reduction in forming gas (95% N2, 5% H2) yields metal or alloy replicas of the designed architecture.

So this process is limited to elements that can be reduced from the oxide to the metal by gaseous hydrogen. Iron could work but aluminum, titanium, magnesium, and other reactive metals would not.

The upper temperature there (1100 C) is above the boiling point of magnesium at 1 bar, so magnesium fails just from vaporizing.

This feels like a refinement on the existing practice of 3D printing metallic powder mixed with a low-temp meltable binding agent, then sintering the resulting object in a kiln to burn off the binder and generate the final part. As far as I know, this also has shrinkage that must be accounted for, as well as a particularly rough finish. That's not that big of a deal if there's subsequent machining steps, but perhaps this direct precipitation of metal out of solution improves upon the precision or tolerances.

There are both selective laser sintering and selective laser melting that do not require a kiln step and therefore the final size is the final size. Both processes can produce some amazing metal designs that would not otherwise be possible with a typical metal+binder extrusion process. I've talked to some vendors who have shown me some really cool waveguide and cavity filter designs they've made using either SLM or SLS. There are also some methods using lasers to effectively remelt the surfaces to smooth them out to get rid of the grainy texture that's typical for any sintered material. In aerospace, size and weight are very limited so bolting a bunch of off the shelf waveguide parts together can get messy, being able to get a really compact design that fits exactly can really help with miniaturization.

At those temps I believe many metals will also end up being heat treated. And it sounds like the time will be dependent on how much hydrogel is present, so there will be at least a lower bound on what heat treatment you need apply. I wonder how that works with the shrinking, along with lots of other properties you may want of the end product

Metal deforming during fabrication is basically standard everywhere. You design for it.

For reference, 1mm = 1000 microns.

Why bring this up? Because human hair width varies between fine to coarse samples quite a bit.

And why do people still refer to microns? Why not just be consistent and use micro-metre, um?

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