Hackaday Prize Entry: The Strength Of 3D Printed Parts

[Sam Barrett] is doing something that is sorely needed. He’s doing real materials research on FDM parts.

There’s nothing wrong with the rough experiments like hanging a 1 L bottle of water from the end of a rectangular test print to compare strengths. We also have our rules-of-thumb, like expecting the print to perform at 30% of injection molded strength. But these experiments are primitive and the guidelines are based on hearsay. Like early metallurgy or engineering; 3D printing is full of made-up stuff.

What [Sam] has done here is really amazing. He’s produced a model of a printed ABS part and experimentally verified it to behave close enough to the real thing. He’s also set a method for testing and proposed a new set of questions. If it couldn’t be better, he also included his full research notebook. Make sure to read the FDMProperties-report (PDF) in the files section of Hackaday.io.

Sam finally answered a question we've had of what it looks like when the printer over extrudes.
Sam finally answered a question we’ve had of what it looks like when the printer over extrudes.

If research like this is being done elsewhere, it’s either internal to a large 3D printer manufacturer, or it’s behind a paywall so thorough only the Russians can help a regular peasant get through to them. Anyone with access to a materials testing lab can continue the work (looking at you every single engineering student who reads this site) and begin to help everyone achieve an understanding of 3D printed parts that could lead to some really cool stuff one day.

Materials to Know: Medium Density Fiberboard

MDF is the cheapest and flattest wood you can buy at local hardware stores. It’s uniform in thickness, and easy to work with. It’s no wonder that it shows up in a lot of projects. MDF stands for Medium Density Fiberboard. It’s made by pressing materials together along with some steam, typically wood, fibers and glue. This bonds the fibers very tightly. Sometimes MDF is constructed much like plywood. Thinner layers of MDF will be made. Then those layers will be laminated together under glue and steam.The laminated MDF is not as good as the monolithic kind. It tends to tear and break out along the layers, but it’s hard to tell which kind you will get.

Proper way to attatch a fastener to MDF.
Proper way to attatch a fastener to MDF.

MDF is great, but it has a few properties to watch for. First, MDF is very weak in bending and tension. It has a Modulus of Elasticity that’s about half of plywood. Due to its structure, short interlocking fibers bound together by glue and pressure, it doesn’t take a lot to cause a crack, and then, quickly, a break. If you’d like to test this, take a sheet of MDF, cut it with a knife, flip it over, and hit the sheet right behind your cut. Chances are the MDF will split surprisingly easily right at that point.

Because of the way MDF is constructed, fasteners tend to pull out of it easily. This means that you must always make sure a fastener that sees dynamic loads (say a bearing mount) goes through the MDF to the other side into a washer and bolt. MDF also tends to compress locally after a time, so even with a washer and bolt it is possible that you will see some ovaling of the holes. If you’re going to use screws, make sure they don’t experience a lot of force, also choose ones with very large threads instead of a finer pitch. Lastly, always use a pilot hole in MDF. Any particle board can split in alarming ways. For example, if you just drive a screw into MDF, it may appear to go well at first. Then it will suddenly jump back against you. This happened because the screw is compressing the fibers in front of it, causing an upward force. The only thing pressing against that force is the top layer of laminate contacting the threads. The screw then jumps out, tearing the top layer of particle board apart.

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Boeing’s New Microlattice, Now the Lightest “Metal” Ever

Mr McGuire: I just want to say one word to you. Just one word.
Benjamin: Yes, sir.
Mr McGuire: Are you listening?
Benjamin: Yes, I am.
Mr McGuire: Plastics.

You may recognize the above dialog from the movie “The Graduate” starring a young [Dustin Hoffman], whose character is getting advice about what line of work he should get into after university. Maybe Mr McGuire’s advice should have been “Microlattice.”

If you take a step back for a moment and survey the state of materials, you’ll see that not much has changed in the last 50 years. We’re still building homes out of dead trees, and most cars are still made out of iron(although that is starting to change.) It’s only been just recently has there been advances in batteries technology – and that only came about with the force of a trillion-dollar mobile phone industry behind it. So we’re excited by any new advance we see, and Boeing’s new “Microlattice” tickles our fancy.

Boeing isn’t giving away the recipe just yet, but here is what we know: it’s 99.99% empty space, making it extremely light. It’s so light, that if you drop it, it floats to the ground. It’s also compressible, giving it the ability to absorb energy and spring back (you can see it in action in the after the break.) It’s made by creating a sacrificial skeletal structure the shape of the final lattice, then coating that template with nickel-phosphorus alloy. The temporary inner structure is then etched away, leaving a “microlattice” of tiny interconnected hollow rods with wall thickness of about 100 nanometers. Of course it doesn’t take a rocket surgeon to figure out why Boeing is interested in such materials, they are eye it as an extremely lightweight building material for planes and spacecraft.

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Making aerogel at home

[Ben] outdid himself. He successfully made monoliths of silica aerogel in his garage. Aerogel, the light-weight solid that has been referred to as ‘hard air’ is really freaking expensive especially in non-granulated form.

The techniques behind producing aerogels have been on the Internet for a fairly long time. A few uncommon chemicals and a supercritical drying chamber are required for production, meaning it takes a lot of know-how to make hard air at home. Somehow, [Ben] got ahold of some tetramethoxysilane, the hard to come by ingredient and made a supercritical drying chamber out of pipe fittings and liquid Carbon Dioxide.

In the end, [Ben] was able to make a few small pieces of aerogel. The size of his pieces were constrained by his “mold” (actually a syringe) and the size of his drying chamber. It’s very possible [Ben] could build a larger supercritical drying chamber and make larger pieces of aerogel that would be sold commercially for hundreds of dollars.

Check out the very informative walkthrough of [Ben]’s process after the break. It’s 10 minutes long and makes for a great lunch break video.

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