How To Create Hermetically Sealed Electrical Connections

September 23, 2020 by Lewin Day 25 Comments

[Eric Strebel] is no stranger to pressure and vaccum tanks, regularly using them for all manner of resin casting jobs for his product design business. However, sometimes it becomes necessary to run equipment within a pressure tank, such as for rotomoulding or other similar jobs. In order to get power into a tank under pressure, [Eric] built a special plug with a hermetic seal to do the job. (Video, embedded below.)

The build starts with a large metal plug which screws into the pressure vessel, into which a square recess is machined. For the electrical passthrough, [Eric] selected GX-16 aviation connectors, in this case packing six conductors. The connectors are hooked up back-to-back through the hole in the metal tank plug, using bare copper wire. This is to avoid insulation on wires acting as a channel for gases to pass through. With the connectors wired up and an acrylic disc in place to stop overflow, the metal plug is filled with resin to create the hermetic seal.

Results are good, with the connectors functioning electrically and the resin acting as a perfect seal. There’s a small risk of short circuit with the exposed copper conductors, but [Eric] is exploring some easy solutions to avoid issues. We’ve seen his work before, too – like this great discussion on cardboard as a design tool. Video after the break.

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How To Make Bisected Pine Cones Look Great, Step-by-Step

February 10, 2019 by Donald Papp 8 Comments

[Black Beard Projects] sealed some pine cones in colored resin, then cut them in half and polished them up. The results look great, but what’s really good about this project is that it clearly demonstrates the necessary steps and techniques from beginning to end. He even employs some homemade equipment, to boot.

Briefly, the process is to first bake the pine cones to remove any moisture. Then they get coated in a heat-activated resin for stabilizing, which is a process that infuses and pre-seals the pine cones for better casting results. The prepped pine cones go into molds, clear resin is mixed with coloring and poured in. The resin cures inside a pressure chamber, which helps ensure that it gets into every nook and cranny while also causing any small air bubbles introduced during mixing and pouring to shrink so small that they can’t really be seen. After that is cutting, then sanding and polishing. It’s an excellent overview of the entire process.

The video (which is embedded below) also has an outstanding depth of information in the details section. Not only is there an overview of the process and links to related information, but there’s a complete time-coded index to every action taken in the entire video. Now that’s some attention to detail.

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3D Printing Compressed Air Tanks

June 2, 2016 by Brian Benchoff 107 Comments

Using PVC pipe as a pressure vessel for compressed air can be a fun and enjoyable hobby. It’s safe, too: while there are are reports of PVC pipe being the cause of accidents, these accidents include a black powder potato gun[1], and welding too close to a PVC pipe containing compressed air[2]. Compressed air stored in a PVC pipe is never a proximal cause in any accident, and the OSHA’s Fatality and Catastrophe Investigation Summaries bear this out; there was no industrial or occupational accident recorded in these summaries where a pressure vessel made out of PVC was the cause of any injury or death[3].

Although PVC pipe can be a perfectly safe, effective, and cheap pressure vessel for hobby applications, it’s not always the best choice. A group of students in Renens, Switzerland are building autonomous robots for the Eurobot competition, and this year’s robot uses pneumatics. That means compressed air, and that means a pressure vessel. Since just about everything else on this robot is 3D printed, they asked the obvious question. Is it possible to 3D print a tank for compressed air?

The tank for this robot would only be used up to about 4 bar (400kPa), and after a few quick calculations, the team discovered the wall thickness – even in a pressure vessel with corners – would be pretty low. The first prototype, a 40mm cube with 20% infill and a hole drilled in the side, held 6.5 bar (650kPa) for an hour. This success didn’t last, though: he second prototype, a 65x40x80mm rectangular prism printed without as much infill, exploded at 5.5bar (550kPa).

The third time’s the charm, and with filleted ribs inside the tank, the third prototype was able to hold pressure up to 6.5 bar. Of course no 3D print is perfect, and the third prototype did leak, but a bit of acrylic spray paint applied to the outer surfaces held the air in.

While it’s not as fun, easy, cheap, rewarding, or safe as using PVC pipe as a pressure vessel, the team did manage to build a 3D printed pressure vessel with a custom shape. You can’t do that very easily with round pipe. And 3D printing opens up all manner of internal structure to experiment with. We’d like to see this developed even further!

Sources: [1], [2], [3]