Expanding Foam for Negative Mold?

barbquebutt

Active Member
Anyone ever used expanding foam for mold making?

I want to reproduce an old vacuformed Christmas decoration that has seen better days, so I'd to make a buck from it to form a new one, but I don't want to put anything dense/heavy like clay or plaster on or in it for risk that it might misshape or crack the original. I was thinking that expanding foam would shape well around the front without putting pressure on it, and then I could pull the original, coat the mold with some release agent and pour some plaster to make a positive for vacuforming.

The foam mold would pretty much only have to survive long enough for me to pull a positive from it.

So is it crazy enough to work? Or does anyone have any better ideas?
 
You mean PU foam?
If you're worried about cracking your item with the weight of silicone - the expanding foam would crumble it for shure! This stuff builds up surprisingly much force when expanding in chemical reaction... don't do it!

Alginate is the way to go!
 
I second that. To get a decent positive, you'd have to use a self skinning foam. One that forms a smooth surface, instead of open holes.

Those require pressure in the mold.
 
Do you have a picture of what you are trying to make a mold of?
Your idea might work depending on the shape of the original piece, and the quality you expect of the resulting foam mold. It'll likely lose detail.

Most of the foams I have used are urethane foams, and they stick tenaciously to whatever they are poured on. But! I have wrapped parts in Saran Wrap (or similar) to create a barrier, and then poured foam over them. I was trying to make a cradle for the objects, rather than a mold. This method could make a rough mold - there would be some wrinkles from the Saran Wrap, and fine details might not show up. If you are comfortable with sculpting your plaster casting to add details and smooth the wrinkle ridges, it could work.
 
Good points in the thread...

Another thing to consider is polyfoam gives off tremendous heat during its curing process. Other than it wanting to tenaciously grab the surface of the vacuum form.. it could very well deform it as well.

I would pack it as best you can with paper or something if you don't want to use clay, and make an alginate and plaster bandage mold.
 
You taking about one of those old vac formed wall decorations or santa or somesuch?

You could bed it face down into a box of sand for support and fill the back with plaster. I doubt the heat would be a issue unless it's a deep one.

Sent from my HTC One_M8 using Tapatalk
 
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