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Water dissolvable mold core material

Freemasm

Well Known Member
Novel approach or doomed to failure? The scoop to air box I/F is less than aerodynamically smooth. I’ve got one of the older legacy designs. Didn’t want to fill with that much dry micro. Trying to fill with foam wasn’t going well. After looking for a dissolvable mold core material, I experimented with cheap-a$$ drywall joint compound.

Known bad = Yes it shrinks. Yes it takes time to dry thick fills. Yes it cracks.

Potential good =. It’s cheap. Available most anywhere. Cleans/dissolves with water. Sand-able or smooths with water and sponge, finger, etc.

I filled a 1” pipe and let it dry. It “hydroblasted” out with a siphon gun and water. An occasional gouged hole speeds things up as you’d imagine. Have the deep parts rough filled and will soon smooth and glass it in. Hopefully it works or it may be a complete waste of time. I’ll pass along. Hopefully another cheap method for dealing with similar issues in the future.
 

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Never used drywall mud as a mold material (seems appropriate for some options - thanks for the tip), but I have used almost everything else under the sun and one "go to" tip is to use clear packing tape as the mold release. I do lots of one off male molds in foam and I used to go to great lengths to prep the surface, wax, polish, etc - no more. Use tape and once you dig out the mold material, the tape peels right out and leaves a nice finish. Maybe not "ready to paint" on an exterior panel, but good enough for something like the inside of a scoop.
 
3M Heavy Packing Tape

Never used drywall mud as a mold material (seems appropriate for some options - thanks for the tip), but I have used almost everything else under the sun and one "go to" tip is to use clear packing tape as the mold release. I do lots of one off male molds in foam and I used to go to great lengths to prep the surface, wax, polish, etc - no more. Use tape and once you dig out the mold material, the tape peels right out and leaves a nice finish. Maybe not "ready to paint" on an exterior panel, but good enough for something like the inside of a scoop.

3M Heavy Packing Tape is my preference as well. I use it for mold release and to protect from sticking. Glass parts pop right off.
 
In past (non airplane) projects, I've made a mold out of pink insulating foam board and then just dissolved it with acetone.

Water would sure be cheaper and have fewer health concerns than acetone, so why not give it a try if you want? The proof is obviously in the result, but foam sure is easy to sand to shape...
 
So far…

@ Michael. Thanks for the packing tape tip. The surface finish won’t be exposed but the resin popped right off like you said.

@ Terry. Tried a couple of types of foam. The surface to be glassed is smaller than the associate backside cavity. Any pressure deforms the thin, slender rear cross piece. Nothing wanted to stay in place by itself and was trying to avoid the glue gun or similar. A two part, pour type foam would have been awesome but everyone suggested to stay away. I had a post in Build Techniques about possible commercial foam product applications but got only warnings versus suggestions.

Core removal is working but a little slower than I’d hoped. The fill cavity “passage” from sides to front is pretty narrow. Still do-able with current method but I may try to speed it up with a soak or maybe drill a couple of small water injection/weeps holes.

Not saying this is a best practice but it could have plenty of applications. Learning fiberglass and how much I don’t like it.
 

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