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Further experiment with expanding polyurethane foam

I wrote the other day about trying to injection mold using expanding polyurethane foam in a bid to make a really small, light capsule for a high-altitude balloon project.

Today, I got round to freezing the test capsule. The thermal performance was awful (but I wasn't surprised how thin the walls of the test capsule were). Here's the sealed capsule (with a simple piece of duct tape around the middle):

But first I noticed that after a week of sitting around the capsule had changed shape. One of the flat ends had changed and that half of the capsule had deformed slightly. This change happened about 48 hours after I made it.

Next I placed the sealed capsule in the freezer and measured the internal temperature at 15 minute intervals. The freezer is at -18C. Here's the precipitous chart of the internal temperature drop. The internal temperature dropped from 25C to -18C in 60 minutes.

So that particular capsule isn't going to work. Given how easy it is to cut this material I think the best capsule will be a sphere with hollowed out spaces for the electronics (perhaps with a flattened top for the GPS antenna).

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