Duplicate Boring

Yes. The other codes have meaning, but they aren't relevant here.

How hot does it have to be?

That's very cool. Do you know who Bob Hatschek is? (Or have you heard of the Hatschek Hook?) He held some free-flight glider records at one time, and he got me interested in indoor rubber. I made some with film-covered wings about 20 years ago but it was only a passing interest.

The Hatschek Hook, which was a towline release hook for competition gliders, and which Bob sold worldwide, is the only product I ever knew of that was made exclusively on a Unimat.

Bob was an editor at _American Machinist_ in its glory days.

Damned if I know. I _do_ know that if I bake it for 30 minutes at 150 degrees it forms, but doesn't seem to do so as well. If I bake it for 30 minutes at 200, it works better. I should try 250 -- I expect I'll melt the plastic, but that'll be a learning experience, right?

Yeah, it sounds like it's the right time for some experiments. And if the right temperature turns out to be 212 F, you may have a couple of problems solved at once.

SNIP

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.

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

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.

EPS (Expanded Poly Styrene) molding.

I looked into setting up a line once, and it was too much hassle- required a boiler to heat the beads etc., but I think this process might actually be possible on a table top using your aluminum molds. Worth looking into. Alternatively maybe some kind of reaction molding (RIM) that leaves the wheels full of bubbles but with a tough skin. Getting the materials in sensible quantities for you could be a challenge.

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.

Molding polystyrene beads. (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.

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

That's for things like industrial drums and plastic kayaks. I don't think you'd have much luck in such small sizes.

If it were me, the first thing I'd try is pressing the wheels out of Styrofoam (closed-cell) or beadboard (open-cell) with hot mold halves. It doesn't take a lot of heat to make polystyrene pliable. You'd have maximum density at the wheel tread, and a bit of skin on the outside. You probably could get away with a low-density polystyrene foam.

Correction, to avoid confusion: Beadboard, and other products made from compacted polystyrene beads (such as coffee cups), are *not* open-cell. The finished product usually is somewhat porous, but the beads themselves are closed-cell.

Styrofoam is extruded, close-cell foam, and it's not made from compacted beads.

Also BTW: you might need some mold release on those mold halves if you're going to trying heating them and pressing the wheels out of sheet. PVA (polyvinyl alcohol, available at boat-repair supply places and craft shops) might be the right thing. It's used as the primary release agent for polyester resin, which is loaded with styrene monomer. But considering that the pressed sheet won't be liquefied, vaseline petroleum jelly

You may be looking for the word "Extruded".

Kevin Gallimore

Styrofoam is indeed extruded, but the generic definition for EPF is "expanded." Extruded foam was expanded in the process of making it.

You'll see both definitions for EPF in the industry literature.

Right now I'm cold-pressing, and then heating. I'm not sure if I want to go to the trouble of hot-pressing -- but, maybe it's the way to go. Hmm. More complications...

(And there's no sign of the product sticking to the mold).

I think vacuum forming might give you something useful for relatively light weight. Heat up a sheet of something formable clamped in a frame via IR (think toaster oven), pull it out, place it over two male half molds, suck down. Cut out halves and glue together when cool.

On the other hand, it probably won't be as light as some rims bent up and glued out of graphite prepreg tape, with graphite spokes and graphite tube hub (maybe cure in the same toaster oven). If your target complete model weight is 7 grams, anything molded will be pretty heavy by comparison.

Pete Keillor

It might be "complex," but it doesn't necessarily have to be "big" - Many years ago, there was an amusement park in Excelsior, MN, with a way kewl wood coaster, fun house, arcade, and all that other fun amusement park stuff that's illegal today because it's too dangerous for our oh-so- fragile little darlings. One of the things in the arcade was a box no bigger than a jukebox, that for a quarter would blow-mold a little plastic unicorn shelf knick-knack right before your very eyes. It would close the mold, shoot the goo, run cooling water through the mold, open the mold, and scrape the little unicorn off the platen down into the pick-up bin.

I'd think the only "complex" part would be figuring out the right temperatures and pressures for the molding process itself - the rest is just sequencing. :-)

I once made a process sequencer with a stack of garolite cams that I cut on the rotary cable, and a bank of industrial-grade microswitches. :-)

Cheers! Rich

I dunno, but do they have that in an aerosol form, like that expanding urethane stuff?

Good Luck! Rich