I have experienced poly threads melting. The iron had been dropped a few times, and I didn't realize that the heat setting had gotten broken so it was full heat all the time (the iron never stopped heating - it got really hot!). Not only did I melt thread, I had serious shrinkage from poly cotton fabric too. The iron got so hot it eventually melted through the cord at the base and shorted out, so I know it was getting way hotter than it was suppose to ever get.
During the malfunction time, it really put a great crease in cotton fabric with very little effort.