Plastic welding

[tbburg]

Anybody have any experience with plastic welding?

I'm using a Harbor Freight air welder kit. plastic-welding-kit-with-air-motor-and-temperature-adjustment

I got an itch the other day and decided to fab up a cold air intake for the barbie-jeep. It had a Spectra kit on it when I got it, but I threw that away pretty quick. 'Left a hole in the fire wall which I've been wondering what to do with, and I'm thinking about a tube back from the stock air box up to the firewall penetration, then route it inside the cowl above the hvac duct and out through the right side of the cowl. Add an air intake cap and I have cold air, which can double as a snorkel by adding an extension tube.

With this in mind, I bought a length of Sch.40 PVC electrical conduit to fool around with. The thought was plastic tube wouldn't transfer heat in the engine compartment, and the conduit has stabilizers to increase heat resistance. Anyway, I started fooling around making fittings and bends, and quickly found the conduit is pretty hard to weld. 'Probably something to do with the stabilizer additives. Adjusting an air welder cool enough to not burn the surface and it's too cool to melt the joint. I've managed to do a few pretty ugly joints, but I was hoping for better results. I made some adjustments to the air horn on the factory air cleaner box, which is made from polypropylene, and that came out pretty good, so I don't think it's the welder.

Anyone with experience with an air welder out there can give me some tips? Is this a problem specific to conduit or is it PVC in general?

 

[time2getdirty]

I bet its the PVC thats giving you the problem. Why not use pvc cemment or glue?

 

[Fred]

Polypro is all I ever welded. You do have to have the right plastic welding rods to go with the welder. I see HF sells PVC welding rods, is that what you are using? You can't just melt pieces together, it takes the proper welding rod, just like metal.

 

[tbburg]

I bet its the PVC thats giving you the problem. Why not use pvc cemment or glue?

PVC conduit doesn't use normal fittings. The piping is made to snake wiring through, and a 90deg. corner fittings for the pipe size I'm using(2") is about 18" long. There's not a lot of room under the hood for that stuff, so I thought I'd try my hand at making angled fittings by sectioning straight pipe. That means a lot of butt-welds, which is where I ran into the problem.
I suppose I could give up and use white pipe and sweep fittings, unfortunately I have this stubborn streak about projects I get started on,..

You do have to have the right plastic welding rods to go with the welder. I see HF sells PVC welding rods, is that what you are using? You can't just melt pieces together, it takes the proper welding rod, just like metal.

Yeah, I'm using PVC rod(the HF rod), or trying to anyway. Like I said, I'm running into a charring problem when I get the joint hot enough for the rod to start to stick.

 

[Fred]

I wonder if something is adsorbed on the surface that is causing the charring?
This wiki bit seems to indicate it can't be welded:

The heat stability of PVC is very poor, when the temperature reaches 140 °C PVC starts to decompose. Its melting temperature is 160 °C. The linear expansion coefficient of the PVC is small and has flame retardancy, the oxidation index is up to 45 or more. Therefore, the addition of a heat stabilizer during the process is necessary in order to ensure the product's properties.