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  #1  
Old 09-24-2022, 11:05 AM
Freemasm Freemasm is offline
 
Join Date: Nov 2017
Location: Orlando
Posts: 896
Default Water dissolvable mold core material

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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  #2  
Old 09-24-2022, 12:00 PM
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Toobuilder Toobuilder is offline
 
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Default

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.
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WARNING! Incorrect design and/or fabrication of aircraft and/or components may result in injury or death. Information presented in this post is based on my own experience - Reader has sole responsibility for determining accuracy or suitability for use.

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  #3  
Old 09-25-2022, 04:26 PM
Aluminum Aluminum is offline
 
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Location: San Jose, CA
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Default

Perhaps 3D-printing a mold out of water-soluble PVA could be accurate and more repeatable?

https://www.amazon.com/eSUN-Soluble-.../dp/B08RRVXW45
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  #4  
Old 09-25-2022, 05:00 PM
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wirejock wirejock is offline
 
Join Date: Oct 2005
Location: Estes Park, CO
Posts: 5,708
Default 3M Heavy Packing Tape

Quote:
Originally Posted by Toobuilder View Post
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.
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I cannot be, nor will I be, held responsible if you try to do the same things I do and it does not work and/or causes you loss, injury, or even death in the process.
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  #5  
Old 09-26-2022, 02:35 PM
Desert Rat Desert Rat is offline
 
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Location: Wichita KS
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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...
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  #6  
Old 09-26-2022, 08:51 PM
Freemasm Freemasm is offline
 
Join Date: Nov 2017
Location: Orlando
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Default 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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Last edited by Freemasm : 09-26-2022 at 09:17 PM.
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