On Mon, 11 Apr 2011 09:53:05 -0700, Tim Wescott
wrote:

On 04/10/2011 03:19 PM, Brian Lawson wrote:
On Sat, 09 Apr 2011 13:47:57 -0700, Tim
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.

Interesting thought. Probably beyond the sophistication I was aiming
for (mostly hand tools, with bits & bobs turned on the lathe).

And I think blow molding is for styrofoam beads, which isn't what Depron
is. I'm not sure that I've seen styrofoam beads that are small enough
for a wheel that's 3/16" thick and 3/4" diameter.

The only blow molding I'm familiar with is for extremely high
production of containers like milk bottles, shampoo bottles, etc. A
parison is extruded, clamped in the mold, then the parison is blown
against the mold with air. Usually a bunch of molds are arranged on a
wheel or other contraption, and open and close around the parisons
automatically. Spits out containers at a high rate. There may be a
lab analog of this process for testing, don't know.

Foam sheet is turned into cups in thermoforming machines. Large
plastic sheets are shaped into various objects via vacuum forming.
There are hobbyist versions of that process. Other processes include
rotomolding (large tanks), injection molding (solid parts), injection
blow molding, injection stretch blow molding, etc.

Pete Keillor

Pete Keillor