I did not see the first thread show up in history, but it was on using a moderately hot iron to do plastic welding type repair.
So that works for the most part, but what do you do when it still will not hold.
It actually helps with the sewing repair.
Big caution: do not make your stitches to close together and stager them, other wise you will make a perforation line and the seam will just rip along the dotted line.
(I am not a seamstress so I do not know the name of the stitches. Please bear with me.)
Turn the pants inside out, so the seam is more accessible.
I take the first stitch in the elastic band and tie a knot.
Then Line up the edges of the and about half way up the seam and about 1/4 of an inch push the needle though the plastic and bring the needle and thread up over the edge and back to the side you started. Move down about 1/4 of an inch and repeat the process (just attempt to not go in a straight line with the stitches). Keep repeating the process until you reach the other end of the seam and tie a not in the elastic.
Now turn the pants over so the needle and thread are at the same "hand side" as you started with.
Now fold the seam in half. Then start the sewing process over again.
Do the same 1/4 inch stitch placement. It will be a little harder to sew because you are going though 4 layers of plastic now. Go from one elastic band to the other and tie off the thread.
Turn the pants inside right and there should be a nice sealed seam.
This is a fix and not a strong repair. I am just attempting to get the "bang for my buck".
In the one case the seams just could not take any "pressure" from when I sat down and would come undone. I messaged to the company about this issue and got Zero Response. The other ones I have had two out of 6 pairs do this un-weld thing.
This will not work on waist banned or leg ban separation.
The first pair I did this with lasted a couple of months with reasonable usage.
This is what worked for me, and I hope it will help somebody else get a few more wearing's out of $18 plastic pants.