I Need a Plastic Welding Lesson

[Clarke Echols]

One problem to watch out for:

The filler rod ***MUST*** be chemically identical to the material being welded.  An easy way to ensure this is to cut a narrow strip off of the edge of the panel you are welding (assuming you have a large sheet you're cutting down to the desired size).  Use that strip as a filler rod.  It'll be square but it still works.

Air is 70% nitrogen and about 20% oxygen.  Getting pure nitrogen gets rid of the oxygen.  When the plastic is hot, it gets closer to the "flash point" where full combustion can occur.  Even below flash point, it still oxidizes much more rapidly than at lower temperatures; hence the browning.  Nitrogen is not an inert gas, but it tends to not react with nearly as many other substances, so it makes for a cleaner, stronger weld because it doesn't react with the elements making up the polyethylene, polypropylene, etc. (mostly carbon, hydrogen, and maybe oxygen, depending on the substance.  Other elements or compounds may be included in the polymer panel in order to produce the desired physical characteristics such as strength, pliability, etc.  These compounds can create problems in welding, so that's why you are able to "guarantee" a compatbile filler by cutting a piece off of the parent panel.

Clarke

[tekebird]

When I built my new tfresh tank....I used a liquid plastic weld.  Brand name Weld-it I think......comes in different thicknesses.

tnk is two peices.....bottom and all sides are one with the sides   folded up.....clamp and put the weld-it on....good as gold....no burning or plastic.   

[gumpy]

While everything Clarke said is probably true, the primary reason to use nitrogen is because it's dry. Compressed air has moisture in it which condenses as it leaves the weld gun and coats the welding rod and base material, and makes it more difficult to fuse the two together.

[gumpy]

Gumpy I have a question you said (due to the way the upper tanks were plumbed).
They broke.
What happened? Because rigid piping?
Do you need a flex joint? Expansion joint ECT?
Just curious.

I have a smaller tank on each side of the bay, mounted in the area between the tunnel and the door. I initially connected them to the main tank on the floor using 1/2" PEX. The idea was that as the lower tank filled, it would displace air up through the upper tanks and out their vents. When the lower tank was full, water would then fill the upper tanks through the connection lines. When they were full, water would run out the vent tubes and I'd shut it off. There wasn't enough difference in height to be a problem with pressure.

And it did work, for the most part, but the problem was that the 1/2" PEX was just too small, and trying to push water through those lines with city water pressure caused the top of the main tank to bulge with the pressure. I had seen this, and usually throttled down the fill when it started filling the upper tanks. This time, though, I was busy doing something else. When I looked in on it, I saw the top of the main tank really bulging, and reached for the shutoff valve, but before I could get it, the top seam blew out in the middle of the tank. Just popped it, and squirted water all over.  I replumbed the upper tanks so I could still use them separate from the lower tank and we continued on our trip. I've now changed the plumbing permanently so the upper tanks are filled separately from the lower tank. Now, I use the water in the lower tank first, till it gets down about half way. Then I open the valve and let the upper tanks drain into the lower tank. I just have to remember to shut the valve when the upper tanks are empty, or my pump draws air from their lines. Not quite as convenient as before, but easier than trying to change the connection lines to 3/4", since I'd have to remove my black tank drain lines to get the upper tanks out.