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Vacuum forming multiple moulds
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drcrash
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PostPosted: Thu Nov 08, 2007 10:52 am    Post subject: Reply with quote

ANH trooper wrote:
Do you think coating the pink foam with something like polyester resin (very thin coat) or maybe even varnishing it just to make it hard enough so you can get a decent pull to take a plaster mould from it?


Polyester resin will eat polystyrene foam. There's a lot of styrene monomer in polyester resin, and it's a solvent for styrene polymer.

A lot of RC model guys seal foam with epoxy (which will not eat the foam) before applying polyester body filler.

I'm not sure that's the best thing, because polyester resin will eat epoxy to some extent, too, so you may get a weak layer or layer interface there. (If you do put polyester resin over epoxy, it's best if the polyester resin is as fully cured as possible... warming it for a few hours and letting it rest for a few days is a good idea to get the epoxy curing reaction to run as close to completion as possible.)

I suspect there's some plasticky kind of thing (paint or sealer or primer) that you can just spray on foam that would make a good sealant before applying polyester, but I don't know what it would be; I'm not clear on what eats what.
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jegner
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PostPosted: Thu Nov 08, 2007 11:06 am    Post subject: Reply with quote

http://sculpturalarts.com/foamcoating.htm is a material just for this sort of thing.

Check it out.

Jim
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drcrash
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PostPosted: Thu Nov 08, 2007 11:15 am    Post subject: webbing & multiple molds & simple plug assist Reply with quote

Webbing happens where the plastic is stretched too much and doesn't contract enough when you suck it down, so the extra plastic folds over on itself instead of just shrinking onto the mold.

For basic background on webbing, see this thread on rcuniverse:

http://www.rcuniverse.com/forum/m_5086453/anchors_5086453/mpage_1/key_/anchor/tm.htm

For webbing between multiple molds (not really covered in that thread), the problem is usually a humped piece of plastic stretched between the two molds, making a sort of arched tunnel. When you apply the vacuum, it may suck inward faster than it sucks down to the platen, so that the arched tunnel collapses to a sharp ridge and then to a web.

A common industrial technique for avoiding webbing between male molds is to have a set of sticks that are lowered onto the plastic between the molds, pushing the plastic down between them, before the vacuum is applied. That ensures that the plastic will suck down to the platen there before it pulls inward enough to make a web. In effect, you're partitioning the plastic area with frames around each mold, or pieces of frame between molds where it counts. Those frame parts don't need to push the plastic all the way down, or make a seal---they just have to stretch the plastic down enough that it will pull downward to the platen before it pulls inward onto itself.

(I don't know a standard name for this technique, but it's the drape forming equivalent of plug assist. Plug assist prestretches the plastic down into female molds before sucking it outward against the mold. This prestretches the plastic down around male molds before sucking it inward against the molds.)

If you just have one place where webbing is a problem, after arranging your molds as cleverly as you can, you might be able to do this with your gloved hand, or a dowel or something. Lower the plastic over the molds, then push it down with your hand where you want to prevent a web from forming, and apply the vacuum.

So for a simple example, if you're just making two pieces side by side, just push the plastic down between, so that the plastic is stretched down on both sides of both molds pretty much the same way it would be if each was framed by a half-sized frame.

For repeatability, you might want to set things up so that you can quickly clamp a dowel or something across your clamping frame in a particular way, just before pulling it over the molds. (A pain to do in real-time, especially singlehanded, I know.) That would let you partition your frame into two sub frames, which wouldn't have to be the same size or even rectilinear.
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drcrash
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PostPosted: Thu Nov 08, 2007 11:23 am    Post subject: Reply with quote

jegner wrote:
http://sculpturalarts.com/foamcoating.htm is a material just for this sort of thing.

Check it out.

Jim


Do you know if Sculpt or Coat can take the heat of vacuum forming, or if you can put polyester body filler (Bondo) over it without dissolving it?

Some theater and puppet people use elastomeric roof patch from Home Depot as a poor man's Sculpt or Coat. (You can thin it a little with water.) I don't know if it can handle heat or polyester, either, though. If so, that might be pretty cool.
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