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Old 07-24-2014, 06:03 PM   #1
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1983 31' Airstream310
Iowa City , Iowa
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Plastic Welding 310 MH Black Tank Outlet

After my dump valve assembly was assaulted by a piece of highway debris, I had a cracked valve adapter, a bent valve and leaks around a crack in the black tank outlet (that had already been there but apparently was worsened by the impact.)

One issue is that the old, and the new adapter stretch that crack; another is that you can't glue polyethylene.

Looking over the web I found out about techniques for welding poly, for example this from Urethane Supply:

Cracked agricultural water tank repair with a plastic welder

I now have a plastic hot air welder and am practicing with it. Sure would appreciate input from anyone with similar experience repairing poly tanks!

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So far I drilled a tiny hole at end of the crack and began cleaning the area. And now I'm practicing on milk jugs with the hot air welder...
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Old 07-24-2014, 07:15 PM   #2
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ouch. I've done a fair share of ABS heat welding, and it's pretty easy, plus I get to use a wide variety of solvents.

If this were my trailer, I would try cutting some poly off the mounting flange or somewhere else just to make sure you had an exact chemical mix, unless you know the milk jugs would do it. If the milk jugs will stick to the other with heat, then I would cut the thinnest strip I could around and around a milk jug. Then, wrap it around putting as much tension as possible. Then, I would slip the steel strap on the widest area, melt the plastic. That will keep tension from opening it back up again while the material cools. Not sure if I conveyed that properly....
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Old 07-24-2014, 08:45 PM   #3
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I am by no means an expert but I had a similar crack where the toilet flange screws into the top of the tank .

Cleaning is critical , sand inside and out and even the edges of the crack .

Harvest the rod plastic from somewhere on the tank if at all possible . I didn't have much luck with milk jugs .

Not sure how the black valve half adheres to the tank . Caulking ? Make sure it fits properly and doesn't force the crack open when inserted .

Temporarily clamp the outlet , top to bottom , to close the crack .

Plastic weld the crack .

Re-install the round clamp , I would use two if there is room , to keep pressure off the repair .

Remove the temporary clamp .

Practice , practice , practice . Good luck
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Old 07-30-2014, 09:26 PM   #4
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Good Advice, Thanks!

Make sure the materials are compatible, make it very clean, and learn how to do it on scrap!

Thanks for your encouragement; after I get some practice I'll post again!
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Old 08-02-2014, 07:54 PM   #5
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1983 31' Airstream310
Iowa City , Iowa
Join Date: Aug 2006
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Keeping Fingers Crossed on 'not recommended' Fix.

OK, I cleaned the crack, drilled a little hole to stop its spread, and ground the edges into a "V" shape.

Bought a pump sprayer of very similar thickness and practiced, using milk jug strips. Worked well if I would have ever become proficient with it...

Tried this: Made an almost identical crack in the sprayer, used an 80 watt glue gun to fill it. Encouraged by the result, I have done the same with the black tank.

Used double clamps on the adapters that slip into the tank outlets, tightened them, rotated them and tightened them again. Did not need to use a sealant there, so if my experiment fails I will be able to disassemble the gate assembly without breaking or sawing anything.

The adapter almost completely covers the crack, with just a tiny exposure where the hole was drilled, and the entire setup is well supported, with no routine stresses on the area of the crack, so far as I can tell.

I will try to learn how to weld plastic, still, whether forced to or not, because I have cracked ABS banana wraps by my rear bumpers...

Thank you for your helpful input on this issue; I promise to do another post if/when the leak recurs!
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