Dissolving Foam


Last winter, with a storm howling and rattling the hangar doors, I decided to make an intake duct for our Tundra project. The male mold was carved from laminated slabs of blue insulation foam, then wrapped in 2-inch-wide vinyl electrical bundling tape to give it a smooth, non-porous surface. Next, I waxed it with an old can of car paste wax I keep around as a mold release.

After a couple of layers of glass had been applied and allowed to cure, it was time to remove the core. Carving it with a knife is one alternative, but a far quicker and simpler trick is to get out a little acetone and pour it into the foam core. Heck, the entertainment value of this technique is worth it, all by itself. The acetone dissolves away the foam like Alien blood eating through the Nostromo’s decking material (old sci-fi movie reference). The glue holding the layers of foam together doesn’t react with the acetone, so you end up with some interesting planes of dissolution (not to be confused with the plains of desolation). With the foam rapidly disappearing, it was a simple matter to grab an edge of the vinyl tape and pull it away from the fiberglass—the paste wax acting as a perfect parting agent.

After the fiberglass has cured, acetone is used to dissolve the male mold, which is made from blue insulation foam.

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As the foam begins to disappear, it should be easy to grab an edge of the vinyl tape and pull it away from the fiberglass.

Carefully peeling from both ends of the intake tube, I was able to contain the gooey foam residue inside the tape cocoon for easy disposal. The fiberglass came out clean and dry, ready for a fit check in the cowling…but that’s for another day. Oh, and if I ever decide to build a foam-core, moldless composite airplane, remind me to keep the acetone far, far away from the workbench!

Carefully peeling from both ends of the intake tube makes it possible to contain the gooey mess inside the tape for easy disposal.

The completed intake tube with the tape removed. The fiberglass came out clean and dry, ready for a fit test in the cowling.

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Paul Dye
Paul Dye, KITPLANES® Editor at Large, retired as a Lead Flight Director for NASA’s Human Space Flight program, with 40 years of aerospace experience on everything from Cubs to the Space Shuttle. An avid homebuilder, he began flying and working on airplanes as a teen, and has experience with a wide range of construction techniques and materials. He flies an RV-8 that he built, an RV-3 that he built with his pilot wife, as well as a Dream Tundra they completed. Currently, they are building a Xenos motorglider. A commercially licensed pilot, he has logged over 5000 hours in many different types of aircraft and is an A&P, EAA Tech Counselor and Flight Advisor, as well as a member of the Homebuilder’s Council. He consults and collaborates in aerospace operations and flight-testing projects across the country.


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