Hi Lavs,
Yes, fine grained sand or plaster of paris - they get some investment-cast parts out of the mold that are close enough that they don’t need machining, even for such close-tolerance parts as revolver cylinders. It’s a tremendous savings in machining costs. The Ruger firearms people here in the states were a leader in this: http://www.ruger.com/Casting/index.html
Now, to the original question…
If you shaped your EPS blank and covered it with a skin, then poured in gasoline or similar to dissolve the styrene foam…ahm, I had a boat, still do, that had styrofoam floatation blocks under the deck. Well, some gasoline got loose down there, turned the styrene foam into something like Marshmallow Fluff that had dried, or maybe chewing gum that was stuck to the underside of a theater seat. Globs of it still show up on the boat now and then, hardened on the outside and a little bit flexible. It doesn’t necessarily liquefy into something that pours out, you see.
The way they get the foam out of molds for metal is to pour molten metal on it, stuff at least 1000°F, which probably wouldn’t work out all that well in this case. That or bake it ( as they do with the plaster molds) in a very hot oven, which also vaporises the styrofoam.
Using a solvent to get the foam out also has a couple other problems: first off, to get enough in there to do a decent job will require shooting it in under pressure, which will likely cause some problems like blowing the skin off.
And then there’s the effects of that much solvent on the wood. From the method you’re describing, it’d be difficult or impossible to glass the underside of the balsa and of course your longitudinal framing stringers. Putting the solvents to it under pressure - wayull, the thing is that balsa would take up the solvents quite well indeed - balsa will take up more than it’s weight of water at atmospheric pressure, imagine what it’d do with unleaded under pressure. You wind up with a hollow, gasoline saturated balsa skinned board that might literally explode on a hot day if somebody struck a match nearby, or a spark of any kind including static electricity. The fiberglass and epoxy outermost layer might act to hold in pressure until it failed, catastrophicly, and then you’d get shrapnel with sharp edges flying, mostly upwards 'cos if it’s sitting on top of the water it’ll go towards air. This, by the way, is why fireworks and pipe bombs are wrapped in something fairly strong, to enhance the explosion.
Imagine if you were sitting on it at the time.
Ouch…
All is not lost, though. Lets say you take your EPS blank, shape it and then glass it lightly. You could then vaccum a skin to the top, glass and remove it, do the same to the bottom, glass the insides of the halves, fit the two together with as much or as little internal framing as you wanted and there you’d have it.
I’m suggesting glassing the EPS so it’d have less tendancy to warp under the pressures of vaccum bagging, but if you used heavier, stiffer foam you could maybe dispense with that, which would also allow you to fiddle with the shape some on subsequent boards from the same basic male mold. Cut across it and add a midsection to lengthen the thing, or have a removable section so you could make it shorter, lots of possibilities -
hope that’s of use
doc…