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MetroMPG 11-28-2007 09:20 AM

Expanding, sprayable foam as an aero construction / shaping material
Had a thought recently after reading about the shaped foam board construction of an aerodynamic prototype which was used to make a mold. ... Ever since I made the Kardboard Kammback, I've been thinking about foam/fiberglass construction approaches.

I went up to the Home Depot and picked up an $8 pressurized can of high expansion urethane foam this evening.

"Cured foam dries rigid and can be shaped, sanded, trimmed, painted or stained."


I'm going to try this stuff out making a couple of things on the car:

- shaped deflectors ahead of all 4 wheels/tires

- radiused transition at the aft edge of the front wheel arches (I've got lots of space between the arch and the tire to do this)

- possibly also boat-tailing behind the front wheels/tires

- this might make a good grill block plug material - custom fit! - though I've already got a "bra" on that does that job.

Tin foil taped/pressed in the area that will serve as the base of the object being created makes a decent non-reactive "release agent", and will protect the paint.

It'll be a bit of an experiment to see how well it adheres to vertical & upside-down surfaces, and whether I need to approach it in layers, or do it all at once.

Then of course the big issue will be discovering how nice or not it is to shape/smooth & what techniques work best.

bennelson 01-10-2008 10:22 PM

Expanding foam is a neat-o material.

It is also IMPOSSIBLE to remove from my favorite t-shirt.

Be carefully using it. Wear disposable clothing...

MetroMPG 01-10-2008 10:24 PM

Oh, I've got lots of disposable clothing since I started messing around with flooded lead acid batteries. (Formerly good clothing, I don't have to add. :P)

Silveredwings 01-10-2008 10:25 PM

It comes off your hands if you use emery paper.

Coyote X 01-10-2008 10:41 PM

use cardboard and masking tape to reinforce it if you make a large part. Just make some narrow 1/2" strips of cardboard or paper internally to help it hold it's shape better. Once you get it formed you can sand it carefully then cover it with a layer of lightweight fiberglass body filler then paint it.

I have patched body panels on cars like that and it actually works really well. You can use a high speed sanding wheel to shape the foam a bit lower than you want it then cover it with fiberglass body filler. Smooth the filler like any other body panel then you can paint it and it is perfect looking.

Also it is nearly impossible to keep the nozzle clear after you use it so once you start using it, make all the parts you want. It will probably be clogged up next time you go to use the can :D

AndrewJ 01-10-2008 11:31 PM


Originally Posted by Silveredwings (Post 4872)
It comes off your hands if you use emery paper.

You'll know you got most of it off when the bleeding starts.

Why does such an otherwise wonderful product have to be such a P.I.T.A.?

PaleMelanesian 01-11-2008 12:44 PM

Acetone cuts through it before it cures.

Great idea! I'll have to look into that.

trebuchet03 01-11-2008 01:04 PM

I've used "great stuff" for making foam parts and such.... Not sure what type of foam it is off hand - but you can't make too thick a layer otherwise the innards won't cure...

sonicsqwirl 01-11-2008 01:07 PM

you can make thicker layers.. .just have to poke holes so the air can get to the inners... i've embedded straws from McDonalds in mine till it cured... then just trim the straw ends off

metroschultz 01-12-2008 01:13 AM

whoa i got this stuff lyin around
I bought some of this recently and was using it to fill empty spaces in the Geo. It takes several cans to fill the spare tire hole. I wanted to keep the back as flat as possible, so's I can still use it to haul engines and stuff from the boneyard to home.
I still be the neighborhood fixer-upper doncha know.
BTW. A Metro 1.0 will actually haul (in one load) : :eek:
2 300+# men
1 400+# 5.0 V8 fully dressed
1 250 # 727 Torque Flight Transmission
1 ??? # front grill for early full size Bronco

But you don't get home as fast as you went. S.

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