Ok, looks like gas tanks are made out of High-density polyethylene HDPE . I found if you use Polypropylene PP, it wont stick. The Harbor Freight welder comes with PP. But HDPE is used on a lot of stuff, including bleach and detergent bottles. I'll be practicing tomorrow on a PW50 tank with the HDPE pieces I cut from a detergent bottle.

You'll want to match roto molded hdpe for welding or blow molded hdpe for welding. Different processes, similar plastic to start with but different melting points and mollecular structure for the final product.

Those detergent bottles will be be blow molded, it's cheaper for large scale production and stronger/more flexible, but has less uniform thickness. Higher melting temperature for the final product.

While most gas tanks are blow molded, Rotopax tanks as an example are rotomolded. The actual molds are cheaper, but it's a higher per piece production cost as it's significantly slower production time per item, yet it allows for a more uniform thickness and as a result better rigidity. Lower temperature melting point with the final product.

I've never looked into which method is used for motorcycle tanks. Having been around both types of molding with kayaks I'd say an acerbis tank I owned had rotomolded traits, stiff, even thickness, some less smooth spots where it wasn't cooked perfectly. They are not doing the numbers to offset the blown mold cost, maybe kawasaki was. Not sure how you'd find out except in my experience the low setting on your run of the mill heat gun can fully melt rotomolded plastic but not blow molded.

Good info darinm, I ordered some HDPE welding strips.
Thanks again Darin

I got some PE plastic strips and welded the tank, seems to stick, time will tell. I covered with plastic epoxy to make it look better. When its good and dry ill paint the tank. I ran my finger inside the tank and felt where the weld oozed through.
I know....its ugly. I pressurized the tank, checked for soap bubbles and the pressure held until i released, about 15min.

Sounds like you got good penetration without overheating it, hope it holds up well!

Thanks Darin for all the info. Yeah hope it holds, last resort buy a tank,

I'm impressed.