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Old Jan 31, 2013, 12:01 AM
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United States, OR, Lostine
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Question
How much is too much?

I have have a section of a 3" fiberglass tube I am laying up that needs to have a 1/4" thick wall. In a situation like this is it better to use many layers of light fabric or fewer heavy ones? Should this much epoxy resin be laid at one time or in stages? I am not having to contend with any weight or cosmetic issues on this part.

thanks,
Bruce
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Old Jan 31, 2013, 02:20 AM
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A fewer heavy layers is usually stronger. Doing it all at once is also usually stronger. The 1/4 thickness could be a problem. Depends on the epoxy, temperature, and how fast you can work.
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Old Feb 01, 2013, 03:21 AM
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Why so thick? Can you use a core?

I'd always thought multiple layers was better than single heavy ones?
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Old Feb 01, 2013, 10:47 PM
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Multiple lighter layers with varying orientation IS stronger than one or two heavy layers (assuming equal thickness).
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Old Feb 02, 2013, 04:12 PM
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Thanks for the feedback guys.

I have another question related to thick layups. I understand there is a point where too much resin left in a mixing cup for too long will have an exothermic reaction. Can this happen to a part if too much thickness is laid up at one time? And, is it likely the part's shape would distort in the process. Lastly, are there any rules of thumb I could follow to avoid ever having the issue.

Thanks again
Bruce
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Old Feb 02, 2013, 05:52 PM
Life begins at transition
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Yes, easily. Use a resin with a long working time (hours, not minutes). Most datasheets I've read use a 100g mass for it's temperature/cure information, which is quite a large blob.

If it's on a mould or a form, it probably won't distort. That faster it cures (hotter = faster), the more it will shrink though. That can definately make for some problems removing it from the mandrel.

What drives the 1/4" requirement? Is it structural, or does it need to fit something else? Two thinner skins with a core could easily be made to 6mm, without any danger of overheating
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Old Feb 02, 2013, 07:47 PM
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I've done some pretty thick layups with epoxy and not had a problem. Polyester is different story. It typically cures hotter and faster.
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Old Feb 02, 2013, 10:29 PM
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I am generating the 1/4 inch thickness at a point where I am fairing two tube diameters together and also trying to create a slip joint. This particular project is ground based, and thus isn't weight critical. The additional strength for the slip joint will probably come in handy.
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Old Feb 02, 2013, 10:36 PM
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Sounds like a great candidate for winding. You can get a roll of 1" fiberglass tape and start winding on your mandrel at 45 degree bias. Get your thickness slightly more then what you want and then wrap with Teflon tape. After a 12 hour cure you can post cure at elevated temp then machine the OD while still on the mandrel
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Old Feb 03, 2013, 12:33 PM
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First, Thanks again to everyone for the input. All in all, I feel like things are going well with my project. I have pulled a few parts from my mandrel now. I am still using Vasaline and tape at this point but I'm working up the courage to go to wax and PVA then to wax only.

So here's my question of the day. The slip joints I am going to make will likely receive some pretty good side loads, And at this point I am planning on using braided sleeves to lay them up. What materials should I use. From what I see on this forum it is common to use Kevlar fabrics on female parts like wing tubes, and carbon for male parts like wing rods. Let me know of you need more info.

Thanks again
Bruce
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Last edited by CarPainter; Feb 03, 2013 at 12:35 PM. Reason: I can't spel.
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