Silicone/urethane rubber help needed

Vinyl plastisol might work. It's a liquid that you dip a heated (350F) metal mandrel into. The material forms on the mandrel until the mandrel cools or you pull it out of the stuff. Build of up to

1/8" isn't hard to achieve. If you want more build, use a mandrel with an internal heater to keep its temperature up. There's probably a limit to this, I haven't explored it.

You then bake it at 350-400 for half an hour or so and then strip it off the mandrel. Those football-shaped squeeze pounches for pocket change were made with it. I've made sparkplug boots with it. For a part of very convoluted shape, as a bellows with deep corrugations, you might need to make your mandrel so it can be disassembled for extraction, but the stuff does stretch some.

It's available in small qty from Industrial Arts Supply (IASCO) in Minneapolis. Its natural color is an insipid milky white like a condom, but IASCO has a few dyes including black and red. You mix the dye with the liquid before use.

You might not find the stuff on their website, but they have it.

Some of the two-part RTV's can handle up to 650F, but I don't know if they're tough enough for your application. Those are probably the mold-making materials you mentioned.

Take a large candle, or a big slab of wax, and carve out the mold. I've then had good results with using cheap acetoxy cure (smells like acetic acid) silicone, with added water+alkali, to cure rapidly (5 mins) Depending on what you make it with (bicarb of soda will give a foaming effect, finely divided chalk won't.)

What a coincidence!

Check out my website at

I used 80 shore polyurethane and stretchable polyester fabric. The elbow is in my car now and sees 20 lb of boost. I got my polyurethane locally and polyester fabric at local fabric store. I believe the you can order polyurethane from

Regards,

Boris Mohar

Got Knock? - see: Viatrack Printed Circuit Designs (among other things)

has a decent set of how-to articles online that show how to make a mold and how to cast using the mold. They also sell rtv and polyurethanes but I don't know if they have any that go to 350F. I do know that when I called with a quick question they cheerfully passed me to a guy who knew the answer. I think you will want more like shore

70A for air filter plumbing, but that's just my guess. sells a set of test pieces about 1" x 2" x 1/8" covering shore 40A to 70D (with some gaps) that is pretty cheap.

-- Regards, Carl Ijames carl.ijames at verizon.net

Yes.

Acetoxy cure silicone (the one that smells like vinegar when setting sets by absorbing moisture from the atmosphere, and losing acetic acid. If you add an alkali solution to the silicone, and mix it thouroughly (and rapidly, it begins to set quite fast) then this supplies the water, and removes the acetic acid.

If you use bicarbonate of soda as the alkali, then it reacts with the acetic acid to also produce CO2, which produces a limited foaming effect. If you use another alkali that does not contain a carbonate, then you generally don't get foaming.

IIRC. For a 300ml tube, it was about 5ml of saturated bicarbonate of soda solution.

Check out

. We use their stuff where I work.