Melting and flattening is the quickest way to quickly secure a paracord knot, and it works decently well. But there are a lot of different companies making paracord, and some of the stuff is just awful, it wears quick and therefore can come unknotted easily. Extra strength can be added by flowing a bunch of liquid superglue into it. It's not to glue the cords together, it stiffens and strengthens the cord making it unlikely to work its way loose. For stuff that is going to be permanent and weight-bearing or hard use, I take a short piece of nano cord or heavy duty nylon thread (same stuff they sew backpacks with) and pierce through both working ends of the paracord. Wrap 3-5 whips around the working ends, pierce in a different place, and tie off. I've never had a whipped paracord knot fail.