On Apr 11, 2:26*pm, Tim Wescott wrote:
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.
So what do you call molding with styrofoam beads?
* AFAIK, just about anything with wheels and a minimum weight calls out
7 grams -- 14 grams for outdoor.
--
Tim Wescott
Wescott Design Serviceshttp://www.wescottdesign.com
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"Applied Control Theory for Embedded Systems" was written for you.
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I seem to remember seeing something about expanded polystyrene molding
in a microwave oven. If that's possible, then you could make your
molds out of wood, which would be a whole lot easier.
Another option might be vacuum formed plastic over a wood
pattern.Shouldn't be too hard to cobble up a small vacuum forming rig.
You could even buy a Mattel Vacuform - There are plenty of people
still selling sheets for it.