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The Limits of Epoxy

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The Limits of Epoxy

Unread postAuthor: Jimmy K » Thu Mar 03, 2011 9:53 pm

I am getting ready to epoxy some fittings into my valves for my upcoming cannon and started thinking: what are the limits of epoxy? My cannon's max pressure will be around 100 psi and I don't want fittings to become unsecured while charging or discharging the cannon, naturally.

So is it pressure rated for a certain thickness or for the nature of the material being bonded? To what degree can I trust this stuff?
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Unread postAuthor: Gun Freak » Thu Mar 03, 2011 9:55 pm

JSR uses the stuff up to 800 psi... you'll be fine at 100 :wink:
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Unread postAuthor: Crna Legija » Thu Mar 03, 2011 10:03 pm

what type of fittings are you using, is it slip on like the ones you can solder for copper or just using epoxy as a sealant for threaded fittings

if you using slip on ones, make sure you roughed up both sides really good, if you doing correctly 300psi should be very safe

some brands clam 3000psi ratings but would be in different circumstances with ideal conditions
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Unread postAuthor: Jimmy K » Thu Mar 03, 2011 10:13 pm

I'm using 3/8" male pipe fittings, so the epoxy would seal the threaded fittings as you said. It sounds like I underestimated the capability of epoxy.
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Unread postAuthor: Gun Freak » Thu Mar 03, 2011 10:29 pm

Why use epoxy to seal threaded fittings... thats what teflon tape is for... :wink:
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Unread postAuthor: Crna Legija » Thu Mar 03, 2011 10:31 pm

i would use PTFE tape then, no need for epoxy but i would trust that stuff to at least 500psi
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Unread postAuthor: Jimmy K » Thu Mar 03, 2011 10:54 pm

The 3/8" pipe nipple is going to be epoxied in to an opening without threads that cannot be tapped, not another fitting. Sorry for the confusion.
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Unread postAuthor: Gaderelguitarist » Thu Mar 03, 2011 10:56 pm

Make sure to rough up the contacting face that has no threads. JSR uses a broken drill bit sometimes IIRC. I generally use very coarse sandpaper or a rasp.
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Unread postAuthor: jackssmirkingrevenge » Thu Mar 03, 2011 11:10 pm

This is my general advice on the subject.

It basically boils down to the quality of the epoxy, preparation of the surfaces and design of the join so it is adequately supported.

My favourite example with my recent hybrid experience of good and bad use of epoxy:

Fail at 8x

Win at 14x and counting
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Unread postAuthor: Fnord » Sat Mar 05, 2011 2:27 pm

There are two major limits to epoxy:
1. Tensile strength
2. Adhesive strength.

To overcome the first, you need to make sure there are no large forces torquing the connection in unusual ways. Don't expose large areas of it to gas pressure without some other material supporting it from behind, and avoid using it for connecting long pipes where leverage applied may cause it to break.

Number two can only be accounted for by giving it enough surface area to cling to and cleaning off any dirt and oil. Roughing up or grooving the surface helps.
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Unread postAuthor: FighterAce » Sat Mar 05, 2011 4:37 pm

Quick question.... I have a part of my chamber thats constantly leaking no matter how much PTFE I put on it. The threads are not deformed... I have no problem with permenantly bonding the threads with epoxy.
Any ideas how high I can take that chamber partialy sealed with epoxy?

Threads are 3/4 NPT, max pressure I intend to use is 40-50 bar.
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Unread postAuthor: Gun Freak » Sat Mar 05, 2011 5:23 pm

FighterAce wrote:Any ideas how high I can take that chamber partialy sealed with epoxy?

However high you feel comfortable with. If you intended to use 40-50 bar, then go for it. The epoxy will be fine as long as it is good quality and mixed well.
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Unread postAuthor: FighterAce » Sat Mar 05, 2011 5:40 pm

Question is what is quality.... I have some 5min Soudal's epoxy... cost me about $8 for 24ml. On the side among some other stuff it says " number average molecular weight <=700"
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Unread postAuthor: Gun Freak » Sat Mar 05, 2011 6:01 pm

I think you'd still be fine as long as you mix it good enough to seal the threads... The strength of the epoxy is not the problem, rather the adhesive property. If it doesn't stick good then it will still leak. Remember, teflon tape doesn't make the joint any stronger. Maybe if you roughen the threads a bit it would probably help.
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Unread postAuthor: saefroch » Sat Mar 05, 2011 6:42 pm

If you're sealing the threads with epoxy, I would think that the epoxy would be the last part to fail, considering how little of it will be exposed to any of the chamber pressure, and how much surface area is available to adhere to the threads.
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