On Sat, 09 Apr 2011 13:47:57 -0700, Tim Wescott
wrote:

SNIP
Because the process is to put a flat piece of foam in the mold, clamp
it, then chuck it in an oven to heat. The foam* softens and expands to
meet the edges of the mold. Then you take it out of the oven and let it
cool, and voile! A part (or a mess -- it's best to enter any sort of
casting process with your eyes open).
SNIP

I've never done it, but it reads like what you want to do would be
achieved with blow molding. You could even put "tread" on the tire
then.

Take care. Good Luck.

Brian Lawson,
Bothwell, Ontario.