Subject: Speaking of Poyurethane

[RonHorowitz <RonHorowitz@domain.elided>]

Hi, Stefano!

>Is it easy to get the liquid

Yes.

>and make some simple molds?

A little bit more difficult, but yes.

>. Anyone have any info on where to get the stuff and
>are there different types; the kinds used for suspension parts vs. other
>applications?

Yes, yes, yes, and yes.

I looked into this some years ago, so the facts and numbers may have
changed a bit, but probably not by toooo much. Here are the negatives.

The small quantity kits that you can buy tend not to work very well for
functional parts like suspension bushes and the like. Unlike things like
two part epoxies, the resins aren't very tolerant of improper mixture
ratios, incomplete mixing, curing temperature and all of that. I suppose
you want to make sure you have fresh resin, too, since these typically have
expiration dates. I'm not a polymer chemist, and I haven't really kept up
with things, so I can't really say what all the issues are these days. With
a few tricks, you can get static mixers to work OK for mix ratios up to
about one to three. I'm told higher ratios can get a little tricky.
Temperature control should be self-evident, but if you don't have ready
access to a Tenny or Blue M or whatever, it may be easier imagined than
achieved.

If you have access to a lathe, tooling is relatively simple, but may
require a few iterations to get right if you're new at it. Depending on the
design, you may have to draft and or polish the molds to get the bushes to
pop out when they're done even using mold release. Remember that if you try
pushing a compliant part out of a mold, you have a "Chinese handcuff"
situation, so it's generally better to pull than push.

On the other hand, tooling for bushings is relatively inexpensive. A better
approach is probably just to have them made. Tooling was quoted at around
$500 per bush. The down side for applications that won't sell many parts is
that you can't amortise the tooling over a large enough production run to
make it economic. Likewise, minimum production quantities are quoted in
volume of polyurethane rather than quantity of parts. I think the minimum
volume was quoted as two gallons or maybe it was five quarts, I don't
remember. Anyway, to a limited point, the more parts you run, the cheaper
it gets, but you may be sitting on a lot of unsaleable parts for a long
time if you over-estimate the market. The present worth of the future cash
flows may be less than expected when you account for the interest lost over
a number of years. Eventually, the present worth of future cash flows
becomes negative, which is why slow moving parts inventories get scrapped
out or just thrown into a dumpster.

Just one last point about making products for sale. There's one hell of a
big difference between desire and demand. What people want and what people
will actually pry their squeeky checkbooks open for are _totally_ different
things. That aside, all of your vaguest acquaintances will expect you to
sell parts to them at a loss, rather than a profit. Unless you're a hell of
a salesman, or unless you're just doing it all for a lark, you'll likely be
considered a "bad guy" unless you're prepared to let people take advantage
of you.

Hope that helps!

RON
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