Received question about sculpting: spray foam from a can better than sheets?

[MetroMPG]

Someone PM'd me this question. I have very littl experience shaping foam, and I know others have more, so I'm posting it here for the group:

Found your post from awhile back, contemplating use of spray foam from a can like Home Depot sells.

How'd it work out? Easy to work with? Dimensionally stable? Smooth and paintable?

I want to sculpt a car nose out of foam, trying to decide which foam, and whether to start with a big chunk and carve down, or use a form and build up.

I've read that polyurethane foam (Great Stuff, etc.) from a spray can is not as good as sheets of polyurethane already cured.

Any thoughts based on your experience would be much appreciated.

Thanks!

[TomO]

From personal experience on the front bumper of my car using Great Stuff: I would've liked to use sheet foam since it was cured already and can be shaped just as easily as cured Great Stuff.

I waited 24 hours after applying the Great stuff so it would fully cure (as per the instructions on the can) but the foam still expanded more after I had started working on it, causing some unwanted cracking.

For my summer bumper project I'm going to use sheet foam since it won't expand on me.

[Otto]

Originally Posted by TomO

From personal experience on the front bumper of my car using Great Stuff: I would've liked to use sheet foam since it was cured already and can be shaped just as easily as cured Great Stuff.

I waited 24 hours after applying the Great stuff so it would fully cure (as per the instructions on the can) but the foam still expanded more after I had started working on it, causing some unwanted cracking.

For my summer bumper project I'm going to use sheet foam since it won't expand on me.

Any idea what Great Stuff will NOT stick to?

I may use the inside surface of my old steel header panel as a female mold, spray Great Stuff in and let it expand/cure completely, then pull out. That way, I get the exact contour of the panel, at least the portion of the panel with the contour I want.

Also, if I knew what chemical barrier (PVA is used to keep epoxy from sticking to fiberglass molds, so something like that would work) would keep Great Stuff from sticking, I could cover the outside of my fenders or whatever with it, for an instant and exact female mold* of the part, then use that for fiberglass layup to make duplicates.

So, again, any idea what Great Stuff will NOT stick to?


*Might be cheaper if we had a good chemical barrier to apply it to the original part, take it to a foam contractor insulating a house, and have him spray it. That would take him about 5 seconds and the small amount of foam would be insignificant to his house insulation job.

[Frank Lee]

I've used poured foam... consistency not as good as sheet but otherwise it was OK... IMHO if I can make sheet stock work I'd prefer to do it that way. Sanding/forming foam creates such a MASSIVE MESS.

[TomO]

Originally Posted by Frank Lee

Sanding/forming foam creates such a MASSIVE MESS.

True, but it usually vacuums up well with a shop vac.

Originally Posted by Otto

Any idea what Great Stuff will NOT stick to?

I haven't found anything that it DOESN'T stick to yet, but I'm no foam expert.

[dremd]

It will not stick to mould release wax.
I've casted with both spray foam + 2 part pour foam before.
I use Partial paste #2 wax, EPIC, NOTHING sticks to it on a smoothish surface.

[JasonG]

I've heard that painters plastic works as a release media.
A lot of websites have info on plug molding for fiberglass shells. Not what we want though.
This guy Using Fiberglass/Foam Sandwich Composite builds some interesting projects. Don't cry, the aero potential of his builds gets ruined by style scoops and flares. Like the attached flow killing lump on the XR3.
Anyhow........ be cafeful of your foam-resin matching. No bondo layer = much less weight, and cost of course.

[Otto]

Originally Posted by Frank Lee

I've used poured foam... consistency not as good as sheet but otherwise it was OK... IMHO if I can make sheet stock work I'd prefer to do it that way. Sanding/forming foam creates such a MASSIVE MESS.

Frank, having worked with sheet urethane foam (I assume common insulation with foil cover as sold at Home Depot, etc..), do you know if the stuff is thermo-formable?

In other words, suppose you had 1/2" thick sheet urethane foam, peeled the foil off, put on a cookie sheet and into an oven at maybe 250 degrees for long enough for the heat to fully penetrate, then took the foam and pressed/vacuumed it against a compound curve shape like a basketball, would it drape and conform, keeping a permanent spherical shape as it cooled?

Or, would it just go back to flat stock shape?

[3-Wheeler]

Originally Posted by Otto

....Any idea what Great Stuff will NOT stick to?....

Yes.

.....Saran Wrap.....

Only precaution is to use minimal amount over the foam, otherwise it will not setup properly.

The expanding foam needs contact with the air to cure, and wrapping it tightly with any kind of wrap will prevent air contact with the foam.

Jim.

[Otto]

Originally Posted by 3-Wheeler

Yes.

.....Saran Wrap.....

Only precaution is to use minimal amount over the foam, otherwise it will not setup properly.

The expanding foam needs contact with the air to cure, and wrapping it tightly with any kind of wrap will prevent air contact with the foam.

Jim.

Thanks, Jim, I've wondered about that, or Reynolds Wrap, etc..

Using such, I could wrap the stock nose/bumper cover on the car, spray urethane foam on it, and the foam would conform to the nose shape but not stick due to the Saran Wrap acting as a prophylactic. The foam over the Saran Wrap could expand and cure properly. Once cured, it could be carved, sanded, or sculpted to shape. Then, the carved foam piece could be lifted off the nose without sticking.

Since it is d