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Old 12-17-2007, 12:21 PM   #1
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Repairing plastic tanks

I've got a crack in my fresh water tank in the Safari and a toilet fitting torn half-way loose in the Sovereign. So repairing them is the next "big effort." Here's a few threads that have addressed this problem in the past:

http://www.airforums.com/forums/f443...eak-16489.html

http://www.airforums.com/forums/f444...1-a-35719.html

http://www.airforums.com/forums/f444...nks-34408.html

The choice of methods, so far, are plastic welding or Scotch Weld 8005. The $30 hot air welding tool available from Harbor Freight is familiar to most, so no photo is necessary (IMHO). Here's the Scotch Weld system, about $100 for the tool, nozzles, 10:1 injector, and two tubes of glue:

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I labeled the rod as "suspect" because I can't get it to glue or weld to the sheet material. Both items are labeled LDPE. As you can see in this photo, the rod material (which had been cut on a bandsaw to provide a flat face) pulled away from the sheet in the weld area and in both glue areas:

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So I thought about it and decided that either the rod was bad material or the fact that I clamped it down pretty tight might have pushed all the active glue solvents out from underneath the rod, preventing any gluing action. One piece of evidence that either of these might be true is that the squeezed-out glue was stuck pretty hard to the sheet.

New experiment--try to glue some pieces of sheet to itself. One piece to be clamped down tight and the other one just placed with momentary finger pressure. I also tried to weld a piece of sheet to the sheet, but once again no luck at all. The two glued pieces are TIGHT. Neither Bedfords nor I could break them off with hand pressure. I haven't taken any extraordinary effort to see what it would take to break them off.

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Bedfords (Joe) has had good luck welding this material, but we don't know why our results are different. He left some of his material with me to experiment with (a $2 cookie sheet--why did I order the industrial stuff for $40?). I'll test weld his material and report those results.

The next big task is to test the sheet material on the tanks. The cold weather here may delay that for some time...

Zep
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Old 12-17-2007, 12:52 PM   #2
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How difficult is it to replace the tank? I have been told that repairs, glue, weld etc. are not very permanent. They said the oils in the plastic continue to leach causing it to fail. This may only be for roto mold tanks? I don't think tanks are vary expensive restively speaking. I have dealt with a company I think is called Tank Depot in Ft.Lauderdale Fl. water tanks, plastic tanks, septic tanks
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Old 12-17-2007, 06:54 PM   #3
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Phil

From my past experience manufacturing rotocast tanks a proper weld will last a very long time. I've seen repaired tanks 10+ years later that were still functioning just fine. Tank manufacturers hate that as they don't make any money when someone repairs a tank. It does take some patience and practice to get it right, and always test before installing - surprisingly enough a common error.

One caveat though is the location of the repair. In some situations it happens to be in an area where there's a lot of stress or pressure, either in or out, right where the repair is needed. If this is the case you should rethink how the tank is supported/protected to eliminate the pressure, rub area or exposure.

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Old 12-17-2007, 07:11 PM   #4
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Zep & co.

Plastic welding is sort of an art, but one that can be mastered in an hour or so of practice. When done correctly, it is just as strong as the material itself....just like a properly completed metal weld.

BTW, I have the HF welding unit. It is a bold knock-off of a $450 American made unit. Having used both, I can truly say that the HF unit, while not having the expansive selection of welding tips that the US unit has, does a proper job in skilled hands.

That said, you should cut some PVC pipe to practice on. Like any weld, the seam should fit as tight as physically possible. Tack the pieces together so the hold on their own and cut a shallow channel at the weld seam with the welder's fine tip.

Next, you need a welding rod of the same material. THIS IS KEY TO SUCCESS!! You heat the seam and rod to the melting point and as you apply the rod to the seam, you will see the plastic flow from the rod and the base material will 'juice' or just slightly melt. This process is very necessary for a solid weld.

When the item is cool, it should not be able to be broken apart and in fact will break away in a different spot from the weld if done propery. After a few tries with the PVC (or ABS for that matter) practice material, your welds should be fluid and uniform and you are ready for prime time!!!

Again, selection of the proper welding rod is very important to the process. If you are not sure if the materials are alike, do a burn test. The rod and base material should react identically as to smell, smoke and burn characteristics if the are indeed the same.

The flow setting on the welder is equally important and should be vaied during the test and practice sessions to obtain the optimal air flow (and the associated heat setting)

Hope this helps a little. (no goggles required!)
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Old 12-17-2007, 07:35 PM   #5
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I went with low tech, a soldering iron and a milk jug to fix a black tank. AFAIK, it is still leak-free several years later.
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Old 12-17-2007, 08:44 PM   #6
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Milk jugs are High density polyethylene. Some tanks are also, but not most of them. Check the tank material first before you go out looking for the proper filler rod. Proper welding requires proper grooving or grinding before you start so the weld will have proper penetration, just like metal welding.
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Old 12-18-2007, 05:21 AM   #7
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Quote:
Originally Posted by lewster
...you need a welding rod of the same material. THIS IS KEY TO SUCCESS!! You heat the seam and rod to the melting point and as you apply the rod to the seam, you will see the plastic flow from the rod and the base material will 'juice' or just slightly melt. This process is very necessary for a solid weld.
...
Again, selection of the proper welding rod is very important to the process. If you are not sure if the materials are alike, do a burn test. The rod and base material should react identically as to smell, smoke and burn characteristics if the are indeed the same.

The flow setting on the welder is equally important and should be vaied during the test and practice sessions to obtain the optimal air flow (and the associated heat setting) ...
Lew, the obvious sometimes escapes me! A simultaneous burn test! Wow, now I don't have to figure out if it smells like a "candle" or is "acrid." Thanks.

Yes, the proper rod is important, that's why I can't figure out why the "suspect" rod won't weld. The sheet and rod are both supposed to be LDPE. Even a piece of the sheet won't weld to itself. A plastics house here told me that it's extremely difficult to get LDPE to weld. Maybe this is an example of that.

Here's a shot of the HF welder. You can see the router speed control in the background. I don't have a too hot problem, so the controller is handy because it has a convenient on-off switch. The heat (per the expert mentioned above) is properly applied between the rod and the groove.

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I've got photos of successful welding [yesterday] of the HDPE, polypropylene, and the cutting board that Bedfords left me. I used rod and sheet for the HDPE and polypropylene test, and a strip cut from the cutting board for that test. The welds are pretty ugly, but at least they demonstrate good adhesion. I don't think the welder is hot enough--the welding is slow--but I've reduced the air pressure to 3-1/2 psi to get it as hot as I dare, which is getting pretty close to burning out the welder. Here's just one example of the polypropylene weld--the others are similar.

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Old 12-18-2007, 06:11 AM   #8
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Looks like you're on the right track .

I forgot to mention that the air pressure thru the welder is key to controlling the heat, which in turn is key to a proper weld.
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Old 12-19-2007, 09:08 AM   #9
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I took another stab at welding the LDPE. No luck. Looked good (see photo, not pretty, but looked like the other successful welds), but the rod peeled right out of the groove I had cut in the sheet. This second effort used a different, smaller diameter LDPE rod. My only advice is that even if the weld looks good, the only proof is an aggressive pull test.

I am ready to concede that LDPE may be near impossible to weld, at least for us amateurs. Thank goodness it seems to glue fine with the Scotch Weld. You can see that the two pieces that were previously glued on are still there. The longer piece, behind the rod, was an attemp to weld a piece of the sheet to the sheet, to eliminate any possibility that the rod was different material. It came off even easier than the rod.

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Before I go glue up the black tank, I only have one remaining doubt. Telling the difference between LDPE and HDPE is not easy. Generally, all the samples I've handled tell me that HDPE is stiffer than LDPE and has a hard, slick surface feel, rather than the supple and oily feel of LDPE. I know this isn't foolproof (to me, polypropylene feels and lot like LDPE). One thing I'm going to do for sure is the simultaneous smell test to make sure it's polyethylene. If I had to pick right now, I'd still say the tank is LDPE (1972 Black Tank--the fitting looks to have been spun in).

Attachment 47435 (photo of fitting in a different thread)

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Old 12-19-2007, 01:49 PM   #10
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High density Polyethylene has a density of 0.95 - 0.96 grams /cc. (same as SG. Low density Polyethylene has a density of 0.916 -0.919. You can also use a melting stage (which is a little hot plate with a themometer attached. LDPE melts at 215 F. HDPE melts at 250F. Your problem with the LDPE rotocast tanks may be that they were using crosslinked LDPE which does not melt. You really can not weld that stuff.

An interesting way of accurately measuring SG on a small sample is to make up solutions of alcohol and water. Alchol has a lower density that water. What % ehtyl alchol = what density can be determine from googling it or using a hydrometer. If the part floats it is less dense less that the test solution. If it sinks it is more dense. If you use ethyl alcohol, you might celebrate by drinking the test solution.
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Old 12-19-2007, 02:18 PM   #11
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Okay, so I gotta "go there"....

If it's just the fresh water tank and you don't need a special compound shape, seems like a new tank might end up being cheaper, faster and a better fix. So far I see about $130 in tools that aren't doing the trick. A new 30 gallon standard tank is usually <$100. They only get expensive when you go custom ~

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Old 12-19-2007, 02:34 PM   #12
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Ditto to what Shari said. There comes a point where.....................

Mind you, any project that includes new tools is always a good thing - for a while . It sure does help justify things with the DW.


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Old 12-19-2007, 06:58 PM   #13
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Quote:
Originally Posted by InsideOut
Okay, so I gotta "go there"....

If it's just the fresh water tank and you don't need a special compound shape, seems like a new tank might end up being cheaper, faster and a better fix. So far I see about $130 in tools that aren't doing the trick. A new 30 gallon standard tank is usually <$100. They only get expensive when you go custom ~

Shari
Shari, ask your better half if there is ever, or ever has been, an inadequate excuse for buying a new tool!

Plus, I am in no mood to take the double dump valve apart, the rest of the belly under the tanks, and then get the black tank (in the Sovereign) out. I'd much rather putz around for a month and figure out if I can do this the lazy guy way!

The fresh tank (in the Safari) will be easy in comparison. It's just a straight 4" crack and is totally accessable from above through an existing cutout in the floor.

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Old 12-19-2007, 07:46 PM   #14
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Here is a link to my water tank repair -

http://www.airforums.com/forums/f443...are-15409.html

Two and a half years later, up to Alaska and back, the tank does not leak. I had all the tools (Soldering iron) and materials. Since my tank is an in floor the replacement cost was somewhere around $350 plus shipping I decided to try the reapair.

I'd go for repairing with a plastic welder or your soldering iron over repalcement. If it does not work, you are in the same place you started. My 36 year old tank is still flexible and does not exhibit signs of becoming brittle.
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