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Old Feb 12, 2013, 11:08 PM
Which one's my plane?
Propbuster1948's Avatar
United States, ID, Caldwell
Joined Oct 2009
171 Posts
Question
Polyester resin vs. epoxy for molds

I'm building a half round plug using two part finish epoxy over basswood. The plug is finished and I have the parting agents (5 coats of wax and three coats of PVA) on the plug. I've been looking for an answer about using polyester resin over the plug instead of epoxy. I checked with a few local hobbyists, who have worked with plugs and molds, and getting answers like, "yes it works fine, no, it didn't work for me and I've never tried it, let me know how it works."
Any idea's? This is an engine nacelle for a ducted fan unit... 22 inches long by 4 inches at the widest point. I will be making it in two halves and supporting the nacelle with lite ply which will also support the ducted fans. It also has two tapers with the widest part being at 7 inches from the front if that makes any difference.

Rosie Summerplace

Feb 2013
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Old Feb 12, 2013, 11:50 PM
Just fly it!
wyowindworks's Avatar
Cody, WY
Joined Nov 2007
6,915 Posts
In my opinion it comes down to your expectations for the mold. Polyester is able to meet the expectations of some people while it's unable to meet the expectations of others.

The pluses to polyester:
1) It's cheap
2) It cures quickly

The minuses to polyester:
1) It can get quite hot when doing thick laminations for molds. This often requires the laminating to be done in stages.
2) It has a higher degree of shrinkage than epoxy. This can lead to higher degrees of fabric print-thru in the mold surface and lower degrees of stability (molds ability to hold it's shape).
3) It smells bad.

When making molds it's best to use an isophthalic polyester resin.
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Old Feb 13, 2013, 10:33 AM
Which one's my plane?
Propbuster1948's Avatar
United States, ID, Caldwell
Joined Oct 2009
171 Posts
Thank you WWW. I will stick with epoxy then. I'd rather spend a bit more than have to deal with 'print through' as you call it. There's been too much time into building the plug to mess things up now.

Rosie Summerplace
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Old Feb 13, 2013, 11:00 PM
Entropy is happening!
Jim.Thompson's Avatar
Bellingen NSW Australia
Joined Aug 2008
6,497 Posts
Vinylester.

Rosie,

I have not used polyester, but I have used Vinlyester which has some of the disadvantages of the polyester.
I have done 5 fuse moulds with Vinylester and every one stuck to the plug.
Main problem with polyester and vinylester is the heat build up. It "cooks" the paint (where paint is used) on the plug.
A serviceable mould can be produced with poly/vinylester resins I am assured. However, I have not managed to make one without a stick up.

Epoxy only for me in future for moulds and parts............

Anything that works well and without problems is cheap, regardless of the price!

Jim.
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Old Feb 14, 2013, 12:23 AM
Which one's my plane?
Propbuster1948's Avatar
United States, ID, Caldwell
Joined Oct 2009
171 Posts
Jim:
Thank you for your reply. I've been getting the same response from a car audio forum where they use epoxy instead of polyester for those reasons and others. One person had his final mold blister as it ate into the paint under the parting agents. I am told that weather imparts its problems in polyester like fish eying because of humidity. I'm lucky I live in a fairly environmentally stable part of the country.

Rosie
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Old Feb 14, 2013, 10:35 AM
Composites guy
North OC, Ca.
Joined Jun 2005
1,176 Posts
Try to use the slowest curing agents in the epoxy tooling products to prevent exothermic heat build up. Slow tool cures also resist shrinkage and dimensional changes better than a faster cure. Very slow cures are used when vacuum bagging a mold.

I have one old tool system with an 8 hour GEL time. Cures in 24 hours= good for very big/thick tools( like 2" thick).
Make sure there is a perimeter flange to maintain concentricity of the tool and a good parting plan flange.

Scott
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Old Feb 15, 2013, 12:23 AM
Which one's my plane?
Propbuster1948's Avatar
United States, ID, Caldwell
Joined Oct 2009
171 Posts
Quote:
Originally Posted by sarmoby View Post
Try to use the slowest curing agents in the epoxy tooling products to prevent exothermic heat build up. Slow tool cures also resist shrinkage and dimensional changes better than a faster cure. Very slow cures are used when vacuum bagging a mold.

I have one old tool system with an 8 hour GEL time. Cures in 24 hours= good for very big/thick tools( like 2" thick).
Make sure there is a perimeter flange to maintain concentrically of the tool and a good parting plan flange.

Scott
Thank you Scott. Every bit of information is good to know.

Rosie
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Old Feb 15, 2013, 12:29 AM
Entropy is happening!
Jim.Thompson's Avatar
Bellingen NSW Australia
Joined Aug 2008
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I second what Sarmoby says about resin.
Don't be tempted to start with inferior materials. Using poor quality resin is like trying to learn to ride a bike with one hand tied behind your back!
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Old Feb 16, 2013, 12:31 AM
Composites guy
North OC, Ca.
Joined Jun 2005
1,176 Posts
The is a muphys law of tools:

The "one-off" tool made cheaply ends up making the most wanted plane which leads to
Trying to push the tool beyond what the materials can provide.

Good tools are a work of art.
Tooling makes the part.
Scott
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Old Feb 17, 2013, 12:47 PM
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Joined Feb 2004
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I am about to make an epoxy mold. I am wondering what others are using as a gel coat? I normally use epoxy /microfibre mix tinted with concrete tinting powder. However I have recently been told that colloidal silica is superior to microfibres. The main issue I have had with mixing my own gel coat is small bubbles getting mixed in and marring the mold surface.
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Old Feb 17, 2013, 02:54 PM
Entropy is happening!
Jim.Thompson's Avatar
Bellingen NSW Australia
Joined Aug 2008
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Quote:
Originally Posted by Wetafly View Post
I am about to make an epoxy mold. I am wondering what others are using as a gel coat? I normally use epoxy /microfibre mix tinted with concrete tinting powder. However I have recently been told that colloidal silica is superior to microfibres. The main issue I have had with mixing my own gel coat is small bubbles getting mixed in and marring the mold surface.
I use a tooling coat described by WWWorks (Adam) here somewhere. Do a search, if you can't find it I will have a look later.
Anyway, it consists of mixing flocked cotton/colloidal silica (Wests 404 in the US & 403 in Aus.) and graphite. I guess the proportions, but the optimum weight ratios are described by him.
He also uses heat (gas torch) to pop the bubbles. I don't due to fear of overdoing it and compromising the release system. Also, I have not had problems with bubbles anyway. Perhaps just leaving it sit for a few minutes after mixing does it? I'm not sure about that. Adam has a video somewhere of this part of the operation too.

This mix makes an excellent tooling/surface coat.

Jim.
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Last edited by Jim.Thompson; Feb 17, 2013 at 08:26 PM. Reason: correction of information.
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Old Feb 17, 2013, 03:20 PM
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Cheers, thanks Jim. Yes I did the search and found the ratio. I will do a test on the flame system. I am using Zyvax release which is high temp.
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Old Feb 17, 2013, 03:32 PM
Entropy is happening!
Jim.Thompson's Avatar
Bellingen NSW Australia
Joined Aug 2008
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Quote:
Originally Posted by Wetafly View Post
Cheers, thanks Jim. Yes I did the search and found the ratio. I will do a test on the flame system. I am using Zyvax release which is high temp.
That's good Weta,

Would you like to post the link up to share? I will bookmark it for future reference as this question comes up from time to time. Also, it will remind me of the correct or optimum mixing ratio.
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Old Feb 17, 2013, 05:17 PM
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Here is the thread.
http://www.rcgroups.com/forums/showt...epoxi+gel+coat
And another
http://www.rcgroups.com/forums/showthread.php?t=1830789
Here is the ratio.

The ratios that I use are 30% West System 404 and 7% West Systems 423 by weight. Find the total weight of your mixed resin and add these % by weight.

And another.
I add 27% West Systems 404 high density filler (Sodium Metasilicate, a little fumed silica, and a little quartz) and 9% graphite powder.

This last one is more recent.
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Last edited by Wetafly; Feb 17, 2013 at 05:22 PM.
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