Ed Huntress wrote:
"Tim Wescott" wrote in message
...
On 04/11/2011 10:14 AM, Spehro Pefhany wrote:
On Mon, 11 Apr 2011 09:53:05 -0700, Tim
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.

Blow molding is what is used to make plastic bottles. Basically a
thick plastic goober* is extruded vertically (the plastic has to be
of a type that has a viscosity high enough that it doesn't drool
off the nozzle). The mold clamps around the parison (sealing off
the bottom end) and then it's inflated with compressed air through
the top. After it cools a bit, a nice sharp knife called a moil**
slices off a bit at the top (usually the top of the neck where the
closure goes), the mold opens and the bottle falls out. The tab at
the bottom is stripped off automatically or manually.

I don't know how you'd do this without a rather complex machine- but
the molds are relatively easy because the pressures are very low by
molding standards.


* It's called a "parison", a term borrowed from a French term
"paraison", relating to glass blowing.

** not joking, that's what it's called-- at least it's not spelled
"mohel".

Ah -- that wouldn't work, and once again I'm confused in my
terminology. The application is very weight-sensitive: indoor rubber
power competition rules call out a seven gram minimum weight for
this sort of plane*, and that minimum is _very_ hard to build down
to. I've got a pair of wheels that look injection molded, but have
about the same wall width as most of the blow molded things I've
seen. They weigh in at 600mg for the pair, while my wheels weigh in
at less than 100mg each.


As others have said, blow molding is a process carried out with solid
resins in a semi-liquid state. I've never heard of anything that
small being blow molded, nor have I heard of doing it with styrene
beads, but technology marches on and I mostly don't.


So what do you call molding with styrofoam beads?


Molding polystyrene beads. g (Styrofoam is a trademark name for EPF
-- expanded polystyrene foam, and it's close-cell material. Molding
the beads produces a porous material.)

Molding polystyrene beads is done by filling a mold with them and then
injecting steam or hot air. I don't think you want to go there.


* AFAIK, just about anything with wheels and a minimum weight calls
out 7 grams -- 14 grams for outdoor.

--

Tim Wescott

what about rotational molding? that's usually done with powder, and the
wheels would be hollow when done.