DIYbanter - Silicone/urethane rubber help needed

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Silicone/urethane rubber help needed

I need to fabricate some rubber parts for a custom air intake system
for an automotive application. The function of the rubber parts is to
connect various pieces of steel tubing together with an air tight seal
and still allow some flexing. The parts also need to be able to take a
hose clamp without tearing. I'm thinking a shore A hardess of about 40
should be adequate. It's a non-production, hobbyist application.
Temperatures will unlikely reach over 350F. The end result will be
something similar to a coupler used for turbo/intercooler plumbing, but
that's not my intended use.
There are lots of sites out there that offer products for MAKING
silicone/urethane molds, but very few talk about products for CASTING
the product. Those that do, have not returned my e-mails. Maybe
because I don't want to buy 55 gal. drums of their product. Can anyone
offer some help on who might carry a product that will satisfy my
requirments? Thanks for any ideas.

-Jim

On 14 Feb 2005 10:22:36 -0800, "Jim" wrote:

I need to fabricate some rubber parts for a custom air intake system
for an automotive application. The function of the rubber parts is to
connect various pieces of steel tubing together with an air tight seal
and still allow some flexing. The parts also need to be able to take a
hose clamp without tearing. I'm thinking a shore A hardess of about 40
should be adequate. It's a non-production, hobbyist application.
Temperatures will unlikely reach over 350F. The end result will be
something similar to a coupler used for turbo/intercooler plumbing, but
that's not my intended use.
There are lots of sites out there that offer products for MAKING
silicone/urethane molds, but very few talk about products for CASTING
the product. Those that do, have not returned my e-mails. Maybe
because I don't want to buy 55 gal. drums of their product. Can anyone
offer some help on who might carry a product that will satisfy my
requirments? Thanks for any ideas.

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.

http://www.iasco-tesco.com/

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.

Jim wrote:
I need to fabricate some rubber parts for a custom air intake system
for an automotive application. The function of the rubber parts is to
snip
There are lots of sites out there that offer products for MAKING
silicone/urethane molds, but very few talk about products for CASTING
the product. Those that do, have not returned my e-mails. Maybe

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.)

There are lots of sites out there that offer products for MAKING
silicone/urethane molds, but very few talk about products for CASTING
the product. Those that do, have not returned my e-mails. Maybe
because I don't want to buy 55 gal. drums of their product. Can
anyone
offer some help on who might carry a product that will satisfy my
requirments? Thanks for any ideas.

www.smoothon.com 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. www.mcmaster.com
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

I'm not sure if I'm reading correctly but are you talking about the 100%
silicone caulk a the hardware store? If so could you describe the basic
technique and proportions?
Thanks
Karl

"Ian Stirling" wrote in message
...
Jim wrote:
I need to fabricate some rubber parts for a custom air intake system
for an automotive application. The function of the rubber parts is to
snip
There are lots of sites out there that offer products for MAKING
silicone/urethane molds, but very few talk about products for CASTING
the product. Those that do, have not returned my e-mails. Maybe

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.)

Karl Vorwerk wrote:
I'm not sure if I'm reading correctly but are you talking about the 100%
silicone caulk a the hardware store? If so could you describe the basic
technique and proportions?

snip quoted message incorrectly placed at bottom

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.

Thanks
Somewhere along the line I'm sure this will come in handy.
Thanks
Karl

"Ian Stirling" wrote in message
...
Karl Vorwerk wrote:
I'm not sure if I'm reading correctly but are you talking about the 100%
silicone caulk a the hardware store? If so could you describe the basic
technique and proportions?

snip quoted message incorrectly placed at bottom

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.