I purchased the Amazon brand and the plastic tends to start melting before the solder. I tried a mini butane torch as well as a heat gun with the same result. I would recommend a better quality item in the future. The idea is good, though.
The shrink tube shrunk before the solder melted on these but I just kept sweeping back and forth and turning the wire and the solder melted nicely without the plastic melting at all.
