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Topic: Shrinking Expanding Foam?  (Read 926 times)
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RKEM
« on: August 23, 2012 09:15:58 AM »

Here's a puzzle / problem I've been running into. These two skulls (paint job excepted) used to be the same. They were molded with paper mach using a skeleton I have. When I cut them off the skeleton, they are capable of holding their shape (like the one on the left).

So with the one on the right, I made several cuts to extract the resin skull and I closed everything with more paper mach. Then I filled with with "great stuff", and put way too much so the excess dripped out from the hole I had left in the neck area. I let it cure on the counter for a night and in the morning more had come out and was now hard. The skull looked completely full and like the one on the left in the picture.

Then after a day or so, I noticed that it (and a jawbone I had done in the same manner) started to shrink and shrivel, basically the expanding foam on the inside appeared to have been shriveling. I'd say the skull on the right lost at least 25% of its mass through this.


Now I've found a use for the shrunken skull (or more precisely my significant other did) but I'd like to understand what went wrong and how I can get an inexpensive filler for props and such that won't shrivel and be fairly light (I know of various foam compounds but they seem rather expensive).

Should I have left a larger opening while the foam cured? Maybe do smaller amounts rather than filling it entirely in one shot? I heard that somehow water or humidity helps ... maybe run the shower and let it cure in the bathroom? Has anybody else experience something similar? Are there brands I should avoid? Maybe go for the "minimal expansion" filling foams?

Thanks
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Blacksmith
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« Reply #1 on: August 27, 2012 07:36:02 AM »

Read the instructions on the can, not for gaps larger than 2-3" I bet.
The likely reason is that the foam in the center didn't expand and cure like the stuff on the outside. The easy fix is to do it in steps, get the outside of the skull done first, but do a THIN layer first. Let that one cure and then do a slightly thicker layer, etc etc etc.
You can also use Styrofoam blocks and spheres (look in the floral section of your craft store) for the center core of the skull.

Good luck.
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edelC
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« Reply #2 on: December 04, 2012 10:56:33 AM »

I suspect what happened is this, the layer of cured foam on the outside acted as an insulation, which allowed the uncured foam on the inside to stay soft for longer, this provided a way for the entrained gas to escape (as it wasn't 'set' into the foam bubbles) this caused the foam to locally collapse as it effective 'over cured' ..you can see this sometimes with honeycomb bars, that have a darker centre with bigger bubbles.

the solution, do a layer on the outside, allow to cure... then use some kind of filler if possible (makes it cheaper too) wadded up paper could work and fill in the gaps/bottom with another layer of foam....or just do it in stages, allow the first layer to cure, before the second one...

(it is so nice when my profession intersects with crafting..lol..I am a plastics engineer)
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steiconi
« Reply #3 on: December 05, 2012 10:36:29 AM »

I'd be inclined to pack the skull with something cheap and environmentally friendly, like scrap paper.  If you want it stiffer, try dipping the paper in something to stiffen it--maybe glue or plaster of paris--before stuffing.

I actually really like the wrinkled skull; much more menacing than the white one.

And the collapsed part of "honeycomb bars, that have a darker centre with bigger bubbles" are the best!
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