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Foam-core Toroids

While there is nothing that looks as nice as a professionally spun toroid,
consider the agony you would feel if and when it fell off the top of your
coil and became permanently dented.  I think most of us that have made
toroids out of corrugated aluminum duct have eventually had them dented from
these sorts of mishaps.

I recently tried an experiment to improve upon the durability of home-made
toroids.  I attempted to fill my 6" x 22" Al duct toroid with a polyurethane
foam.  This stuff cures to a fairly rigid high density material.  The stuff
I used is called "Great Stuff", at Home Depot, for $3.97 a 12 oz can (not to
be confused with a latex-based product also sold there).  

I drained two cans of the stuff and it appears that another 1-2 cans are
still needed to completely fill it.  I would recommend that anyone
attempting this have more cans than necessary on hand so that it can be
filled in one operation, rather than on successive days, with a portion of
the filling hardened, then filling more.  I was initially concerned that the
foam wouldn't cure being in an air-tight container.  The can said that to
ensure curing in enclosed spaces, to sprinkle some water, so I squirted a
couple oz. of water into the toroid prior to foam injection.  It turns out
that there is no problem with the foam curing, it hardened just fine.

The resulting foam core toroid is now more dent-resistant than a non-filled
one, but the density of the Great Stuff foam is still less than what I would
like it to be to render the toroid totally dent-proof.

What I would REALLY like to find is a vendor who can supply a nice smooth
styrofoam toroid, ready for me to cover with aluminum tape.  I think the
density and light weight of this construction would make it very resistant
to denting, and hopefully such a foam toroid could be bought for far less
than a spun metal toroid.

I'm interested only in sizes larger than my 6" x 22" so life preserver cores
don't help.  Any thoughts on whether such a styrofoam core could be
custom-made for a nominal fee?

Regards, Gary Lau
Waltham, MA USA