"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.
--
Ed Huntress
* AFAIK, just about anything with wheels and a minimum weight calls out 7
grams -- 14 grams for outdoor.
--
Tim Wescott
Wescott Design Services
http://www.wescottdesign.com
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