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Old Nov 09, 2012, 11:48 AM
ONEHANDRC
gallfer's Avatar
United States, WI, Genoa City
Joined Dec 2007
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Question
What foam for polyester resin?

Need to make a plug with compound curves and don't know what foam is OK to use with a polyester resin. Want to use poly res since I have it, will go to epoxy res if needed. Trying to fabricate a pod to hold a lipo on a Polecat sailplane. NO room for much stuff in the fuselage! Thanks!
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Old Nov 09, 2012, 01:37 PM
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wyowindworks's Avatar
Cody, WY
Joined Nov 2007
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This would work:

http://www.fibreglast.com/product/Po...heets_440/Foam

or

http://www.fibreglast.com/product/6-...heets-448/Foam

It's probably cheaper to buy some epoxy and use polystyrene insulation foam.
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Last edited by wyowindworks; Nov 09, 2012 at 01:47 PM.
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Old Nov 09, 2012, 06:29 PM
Registered User
United States, OH, Ashtabula
Joined Jan 2009
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Foam

Hello,

If you go to Home Depot they carry a brown Urethane foam for home insulation. 4x8 sheets and it usually has a silver backing on it. It peels off easly. Our Home depot has it several different thickness. The only problem is nothing will melt it so you have to dig it out if you want to remove it.

I built a full size car body with it and used Poly for the resin. Oh, and another thing DO NOT hot wire it..It sands really nice.

So remember Brown, not blue or pink or white...

Rick
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Old Nov 09, 2012, 07:18 PM
ONEHANDRC
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United States, WI, Genoa City
Joined Dec 2007
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The digging part doesn't sound good, need a smooth surface. I have made cowl plugs from balsa but was trying to avoid the task of building up blocks then doing lots of sanding.

Gino
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Old Nov 13, 2012, 08:17 PM
ONEHANDRC
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United States, WI, Genoa City
Joined Dec 2007
603 Posts
Plug

Here's a couple pics of my plug. Sanded smooth down to 1500 grit sandpaper. 5 coats of spray polyurethane followed by 4 coats of partall wax.

Can I make my mold out of it now? Should I use a spray parting compound first?
I have made a few parts this way before, but it's always been a struggle.
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Old Nov 13, 2012, 08:19 PM
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Cody, WY
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I've never tried to release off of polyurethane. It may react with the layup resin. Spraying PVA over the wax would block any reactivity issues.
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Old Nov 13, 2012, 08:24 PM
ONEHANDRC
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United States, WI, Genoa City
Joined Dec 2007
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I have some PVA, Will do that first. Thought the wax would block any migration of resin.
Thank You!

Gino
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Old Nov 13, 2012, 08:36 PM
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Cody, WY
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Wax provides slip but is unable to block any chemical reactivity.

Read this: http://www.rexco-usa.com/why-molds-stick/
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Old Nov 13, 2012, 08:41 PM
ONEHANDRC
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United States, WI, Genoa City
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Very informative and easy to understand. Another 'Thank You'!
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Old Nov 14, 2012, 10:46 AM
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Elk Grove, CA
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Quote:
Originally Posted by wyowindworks View Post
Wax provides slip but is unable to block any chemical reactivity.

Read this: http://www.rexco-usa.com/why-molds-stick/
In part, the article reads, " PVA functions not just as a mold release but more importantly as a surface reactant which accelerates creation of an inert molding surface."

Then why can't you just coat the mold surface with PVA, let it react with the mold surface and wash it off? It might take more than the two gelcoat cycles mentioned, but it would still be faster and cheaper.

Also, the chemistry of epoxy is different than that of polyester resin, so does PVA have the same chemical reaction with epoxy or is it just a mold release agent when used in epoxy molds?

FC
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Old Nov 14, 2012, 12:54 PM
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Quote:
Originally Posted by FoamCrusher View Post
Also, the chemistry of epoxy is different than that of polyester resin, so does PVA have the same chemical reaction with epoxy or is it just a mold release agent when used in epoxy molds?
The advantage to performing a light layup over the PVA during breaking is to reveal the quality of the release and seasoning progression.

I don't think that PVA will make an epoxy mold surface inert. It will block the reactivity between the mold surface and the layup resin.


When working with epoxy surface coats it's important to:

1) Let the mold cure two weeks and/or use gentle heat to cure the mold. This will maximize the molecular linking within the resin thus reducing the reactivity.

2) Accurately and thoroughly mix your surface coat resin.
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Old Nov 15, 2012, 08:15 PM
ONEHANDRC
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United States, WI, Genoa City
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Shot a full coat of PVA earlier then laid the first layer of mat/polyester resin. Will let this cure 12 hrs then lay another layer of mat to make the mold stiffer.
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Old Nov 15, 2012, 08:18 PM
ONEHANDRC
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United States, WI, Genoa City
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Forgot the pic!
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Old Nov 15, 2012, 08:20 PM
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Cody, WY
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You have a nice looking turd.

How are you going to get the mold off the plug. It looks locked on.
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Old Nov 15, 2012, 08:46 PM
ONEHANDRC
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United States, WI, Genoa City
Joined Dec 2007
603 Posts
The mat is laid over the top only. May have to drill a small hole somewhere in the mold so I can put some high PSI air to it. And yes, it is a nice turd. Some here in WI might think it was a funny looking sausage
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