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.
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)
Zep