Nov.11, 2014 | By Alec

3D printing in metal is, in some ways, the caviar of the additive manufacturing menu. Not only is it capable of achieving extremely high levels of detail, it is also extremely durable, long-lasting and suitable for countless industrial and medical applications. There's just one little obstacle barring the way for private usage: it's terribly expensive. Fortunately, researchers from the University of California have developed a new 3D printing technology that could make desktop metal 3D printers a reality in the near future.

Current metal 3D printing technology is confined to industrial level 3D printers, and tend to use one of either three production methods: electron beam, inkjet and the most popular: laser sintering. Selective Laser Sintering technology (or SLS 3D printing) relies on a high powered, very accurate laser to graft metal powders together into solid objects. While very accurate, you definitely pay the price for the results. A typical laboratory model costs anywhere from $100,000 to a million dollars, and is therefore way out of the league of all small businesses and hobbyists.

And that's why this study is so intriguing. Recently published in the journal 3D printing and Additive Manufacturing, it has been authored by Payman Torabi, Matthew Petros and Behrokh Khoshnevis, all from the University of California.

They have developed a very peculiar application of inkjet technology that is capable of 3D printing metal, called Selective Inhibition Sintering, or SIS. As they explained in their paper, 'Traditional research in powder sintering has mainly focused on enhancement of the sintering process. In contrast, the SIS process […] is based upon the retardation of sintering.'

The metal-based selective inhibition sintering (SIS) process

While SLS 3D printers produce layer upon layer of metal powders that are lasered into the desired shape, the concept behind the SIS process revolves around preventing selected regions from sintering. You see, rather than having a very expensive laser to sinter, this process fuses particles together an external sintering oven. Key to the production process is therefore treating all the regions that shouldn't be sintered with an anti-sintering agent. 'In this sense, the SIS process can be considered an inverse to traditional metal AM processes.'

Where FDM printers have an extruder and SLS printers have a laser, the SIS technology has a commercial piezoelectric printhead that deposits a liquid chemical solution around the periphery of the part for each layer. Just like a regular SLS printer, layers are produced one by one, but in this case all the irrelevant areas are sprayed. 'The inhibitor deposited at the part' s boundary decomposes into hard particles that impede the sintering process. The particles in this region are prevented from fusing, allowing for removal of inhibited boundary sections and revealing of the completed part.'

SIS-metal machine

Block diagram of the machine

While slightly complicated to get your head around, the researchers behind SIS technology think its best seen as encasing your desired object into a mold. While your object hardens in the furnace, the excess material doesn't.

Of course, the inhibitor they use is key to its success. This test relied on an inhibitor solution of sucrose (C 12 H 22 O 11 ), which was dissolved in water. They had tested other inhibitors, like ceramic salts and carbohydrates, but found that this particular solution was worked best with the bronze alloy filament that was used to test their SIS printing technology.

Not only does the lack of a laser move this printer into the range of desktop, commercially affordable 3D printers (less than $5000), the technology as a whole also comes with a number of other advantages. For one, the process is much faster as only the boundary of the part is treated rather than the whole part. Furthermore, it can use powders that are completely uncontaminated by any sort of binder or residue. And finally, support issues for your printing project are easily solved, being encased in support.

And they have already successfully produced a number of bronze objects using their SIS 3D printer, like the wrench seen above. Just like a regular printing process, CAD-developed STL files were printed using a layer thickness of 120 μm. The only real difference can be seen in the sliced image generations; as this printer only sprays inhibitor on the outer periphery of the object, sliced images can look a bit strange.

The scientists behind this innovative printing technology warn for overenthusiasm, as a lot more research will need to be done before this technology is ready for market. 'While the proof of concept for a high - resolution and affordable metal alloy 3D printer has been established, there is much room for improvement. Software and hardware upgrades are necessary to improve the robustness of the process. In addition, part strength and porosity have not been characterized for the finished bronze parts. Shrinkage and surface quality of parts may be improved upon as well.'

While this means it could take a few years before SIS 3D printing technology becomes available to a wider audience, it does mean that a quality revolution for the 3D printing community is definitely on its way.

Posted in 3D Printing Technology


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BDub wrote at 5/3/2016 8:01:35 PM:

With powder being an expensive medium, only somewhat mitigated by its reuse, this process seems wasteful as the inhibitor sprayed onto the supporting powder would be unusable afterwards. Unless there is some unmentioned method of reprocessing the medium for later use, that seems like either a glaring oversight to not mention it, or an Achilles' Heel to the whole thing.

Nick wrote at 4/2/2015 11:23:04 PM:

This is cool, but what about the possibility of people printing guns?

AMnerd wrote at 11/20/2014 10:58:34 AM:

Thanks for the clarification Joel, you are very helpful

Joel wrote at 11/13/2014 2:06:09 AM:

And Layerwise appears to use laser melting. *Curses* I normally know what I'm talking about on this subject, I swear....

Joel wrote at 11/13/2014 1:45:34 AM:

Aaaand - I just stuck my foot in my mouth about Exone. *Shakes head* Glass houses man.... *meep* So, correction: Contra LayerWise, Exone's 3d metal processes appear to involve either 3d sandcasting - which suffers regarding supplies, process, and potential minimum knowledge base compared to SIS ( and 'pure' 3d printing in general, and infiltration casting( THAT would be where the binder comes in.... ), which while closer to the advantages and flexibility of straight 3d printing, still carries many of the process difficulties of casting, plus the additional requirements in using a binder to infiltrate. Both are still very different processes from Selected inhibited Sintering, and the latter potentially far less expensive and inconvenient.

Joel wrote at 11/12/2014 9:29:06 PM:

Brian, you are flat wrong. As the paper itself explicitly spells out, SIS builds an inhibited shell around loose powder, which is then sintered in bulk and the unsintered casing removed, where as the Exone and Layerwise processes use a binder to build up the 'green' part, itself, which is then fired in a kiln - a somewhat more tempermental process I would imagine. In other words, they are two COMPLETELY DIFFERENT processes. AMnerd, you are technically correct, but wrong in implication. Selective Layer Deposition existed long before the first commercialized Rep Rap derivatives came to market. Selective Layer Sintering is only just on the brink of entering the prosumer/consumer market after two and a half decades of commercialization. Even the experimental work arc welding being pursued at Michigan Tech and, most promisingly, by the recently public Weld3d has already been fully proven as a commercial process in a collaborative effort with Britain's Cranfield University - under the moniker of Wire and Arc Additive Manufacture, or WAAM - involving the suxccessful near-net shape print and finishing of a Titanium wing spar in a single piece, followed by it's installaton and flight in a British Tornado. Yes, this group has researched this process for a decade. And now they believe it's reached a level of maturity that is almost ready for commercialization at the consumer level, as demonstrated by the machine and prints described and photographed in their paper( and on the university department's website along with more part photographs, all in full and living color instead of the black and white of the paper and next to a penny for size and detail comparison, and which I highly recommend everyone visit. ). And that is both awesome and potentially historic. ( For the record, a bit of googling reveals that suitable kilns at the low end aren't that unobtainably priced, either. ) Don't be so quick to knock something exciting without the full context! :-)

AMnerd wrote at 11/12/2014 11:35:35 AM:

Yeah it's not incredibly new: SIS – a new SFF method based on powder sintering Type: Research paper Behrokh Khoshnevis, Bahram Asiabanpour, Mehdi Mojdeh, Kurt Palmer Rapid Prototyping Journal, Volume: 9 Issue: 1, 2003

Brian wrote at 11/12/2014 8:07:19 AM:

It looks nothing new. Exone has already this type of techique:

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