I've tried a heat gun, a soldering iron (pulling that out kind of defeats the purpose of these things) and a lighter. I too find it next to impossible to get the solder to melt before the tube itself is destroyed. I thought this would be an easier alternative to soldering and using shrink tubing. It is not. It takes longer and the end result is iffy at best. The tube shrank, but did the solder melt? I can't tell. I pull the wires and they come apart. I have to strip and try again. Yea! The solder melted, but the tube did too. Strip and try again.This is a great idea that should speed up soldering projects, but it doesn't. I found that it takes longer to use one of these that it takes to solder a couple of wires and using regular shrink tubing. I can only think that it needs solder that has a lower melting point so the plastic tube isn't ruined before the solder melts. It does work, but not nearly as well as it should. What should be a simple process is more time consuming than other methods that should take longer. I've stuck with a box of them so I'll continue to use them until they're gone, but I won't be buying more. I'm sticking with what works. Back to soldering the wires and using regular shrink tubing.
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