Plastic Welding

Quick question to which I don't really expect a solid yes-or-no answer, as its a bit specific.

I have a fill neck like the top one in the above picture. It wouldn't fit under the hood (engine swap problems), so I cut it in half, removed a 1.5" section, and JBWeld-ed it back together. Held up for a little while, but wound up breaking at the seam. Tried again, same deal. Tried JBWeld with a layer of fiberglass cloth and resin on the outside. Broke again.

Is this a plastic that could be "welded" with a small propane torch? Get the two pieces next to each other, apply torch, use small mixing rod to stir the resulting weld puddle?

The alternative is to buy the filler neck shown on the bottom in the picture and skip the cutting and pasting entirely. I can do this pretty cheaply, but would have to get back down to the scrapyard and hope that they have what I'm looking for on the day I'm there. I've already got the cut-up pieces of the original, and want to make it work if possible.

Thoughts?

Do-able depending on what kind of plastic it is, but I'd use a soldering iron with a plastic welding tip.

I'd epoxy it. Not JB Weld.

Epoxy would definitely hold plastic together better but I dunno if it would stand up to gasoline better...

Um... JB Weld is epoxy with metal solids mixed in, so...

You would first have to determine if it is a plastic that can be welded (thermosoftening plastic).

If you have a plan B, just try to weld it, but like Tom Suddard, said don't try with a propane torch, because all you will do is burn the plastic. If you use a soldering iron, make sure you ventilate very well. It cools off the plastic faster, making it easier, and it will keep the toxic fumes away from your face.

In reply to GameboyRMH:

No gas, just oil. Sorry I didn't make that distinction.

Dave, if I were going to epoxy it, what would you suggest I do for prep work? Do I need to bevel the edges where they meet, or...? Haven't worked with epoxy before.

Tom, I'll try the soldering iron when I actually get the thing apart, but I think I've tried it already to no avail... may be a problem with the plastic, may be a problem with the heat level. I'll try it again.

It's probably a problem with the plastic, I wouldn't think they would use thermosoftening plastic for an engine part.

To epoxy it, scuff it up real well with 80 grit sandpaper and bevel the edges slightly.

What's the OD of the pipes? Could you use a PVC pipe connector to join the two pieces with said epoxy?

Depending on the kind of plastic, maybe this: http://www.frantone.com/designwritings/design_writings4.html#welder

Question for JohnInKansas: did you use the new crappy 'quick' JB weld, or the older good 'slow' stuff?

AFAIK, most underhood plastic is a fiber filled Nylon variant (for the temp).
So welding ought to work pretty well, adhesives, not so much.

In reply to erohslc:

I'm not sure. I bought it within the last year. Came in two small tubes, mix it and go.

I would rough up the surfaces to give the epoxy good purchase and use slow-setting epoxy.

I may try a torch/soldering iron just to see if it would work. If not, what am I looking for in a slow-set epoxy?

Loctite used to make some that took a while to set. Can't find it now.

That piece looks easy to replicate with metal. Is plastic necessary?

Wonder if you could sleeve the inside of the plastic tube with a thin metal pipe, then epoxy metal pipe into filler tube and put top tube into metal part.

If you cut it in a smooth section, use a short chunk of fuel filler hose and clamps.

In reply to pilotbraden:

No, plastic's not necessary. Fabbing a custom filler is also an option, though I'd rather not.

In reply to Kenny_McCormick:

I'd rather not do that either, but it would work. I'm vain, what can I say.

Those are probably made of mineral-filled nylon which is essentially impossible to adhere to.

Beyond that, the area that's being adhered is very small - just the diameter x the tube wall thickness.

I'd suggest a length of hose and 2 screw hose clamps.

We faced this problem on our robotics team this year. After some trial (and a lot of error) we found the best way was to take an identical plastic and melt it over the "weld" spot with a heat gun, you might be able to use a soldering iron but we didn't try that. You have to heat up the added material "sodder" considerably before heating the rest of it otherwise, you will distort the plastic.

I customized the filler shown at the top of the photo above. I used regular JB Weld; it's been 8 or 9 years, but as I recall I ground down the O.D. of one side and the I.D. of the other and slip-fit it together. Given that I'm somewhat OCD, I probably cleaned the hell out of the surfaces prior to the JB Weld. The joint has held ever since.

As Tommy said, welding with a soldering iron is far superior. If you've got a variac to regulate it down, it's better still. With the likes of a torch, or even the hot air guns, I burn plastic. With a soldering iron, I can melt without burning, and can stir the plastics to help join them. It's still not 100% for me, and it's ugly, but it's the best I can get.

And as Tommy said, to join them with an adhesive, it's going to be a mechanical bond. So you need the surfaces extremely roughened for the adhesive to have something to grasp. I use a cheese grater file to get the basic shape. I then use the roughest grit paper I have, preferably in the 20-30 range. I then follow up with the cheese grater, but only jabbing it, to make deep pockets, lots of them. And with the ethanol in the fuels today, I'd still expect it to fail.

I'd be far more inclined to cut the two ends I needed, and join them with rubber hose and clamps.

Depending on the size, you may be able to get away with a plumbers clamp used to joint pieces of pipe and fittings together. The stainless steel ones work great on exhaust systems. Minus the rubber sleeve of course.

Depending on the size, you may be able to get away with a plumbers clamp used to joint pieces of pipe and fittings together. The stainless steel ones work great on exhaust systems. Minus the rubber sleeve of course.