[David Olsen]

I’m still working on my first project, so don’t take this as advice, just ideas. The simplest thing to do would be to have the TPU side inside and just seal the top and bottom together around the inner and outer of the donut – but that would give you an obvious seam sticking out when inflated, and the pressure would be trying to pull the seams apart. I assume the problem you are imagining is you seal a loop of fabric around the outside of your donut, but when you complete the loop it won’t stick to itself?

Kind of a cop out, but you can buy a sheet of thin fabric with TPU both sides like this. Since it has TPU both sides you can use it almost like glue, to fill in the gap where you need to complete the loop. That’s what I’m planning on doing in a spot where I have several pieces of fabric coming together, and it seemed OK in my prototyping.

Otherwise if you want to stick to single sided, you could fold back one side of the loop and seal the other side on top of it. This will create a small tab that sticks out the side of the donut. Seems a likely spot for leaks though. I would try the join with a few strips of scrap fabric first to see if it looked reasonable. If that doesn’t do it, you could fold back both sides of the loop and seal a patch of fabric over the whole area.

Or, wacky idea… Have the TPU inside for the top half and TPU outside for the bottom half, so you can join them directly to each other. I expect this would be difficult, especially around the inner circle where it is a tight turn.

Thinking about the big picture, keep in mind that this fabric doesn’t stretch like a rubber inner tube. I expect the outside edge of a donut constructed that way would end up very wrinkly when inflated.

[David Olsen]

I’m a beginner (just practicing welds at this point) so take this all for what it’s worth (nothing) but I picked up the Aviation Art sealing iron from Stewart Systems. Looks like they are selling it for use on full size aircraft, so a bit more money than a model aircraft iron, but still pretty reasonable to me. I think it’s basically a U.S. version of a Prolux sealing iron.

Positives:

• Precise temperature control (+-5C). They are very confident in their calibration, which should take some guesswork out of things, and the box even explains that the center point will be 20-30 degrees hotter than the measurement point. I think this is one case where “do not rely on your iron’s temperature scale” does not apply.
• Sturdy – you can push down with a reasonable amount of force while heating.

Negatives:

• Big – as has been mentioned elsewhere, it’s wider than you really want. The sides of the iron are usable but awkwardly angled upwards. I’ve mostly been using the slight upward slope at the front tip of the iron, but it’s easy to catch and curl other parts of the fabric with the back while doing that, so it might be a good idea to tape things down before you start.

I think I’ve settled on 210 celsius for the 210D nylon. I tried as low as 200C, and it would seal (slowly) but when I tore it apart all the TPU stayed on one side – I think only one side really melted. At 210C I seem to get a nice strong weld, it kind of frays the nylon on both sides if I can manage to get it pulled apart. I think higher temperatures would work (it maxes out at 220) but I felt rushed when I tried it – I want to go slowly and carefully, at least for now.

I’ll have to see how manageable it is in tight corners, but irons seem hard to find so I hope this proves a reasonable option.

[Author]

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