I have tested both my YLI nylon monofilament thread and my poly thread by placing a strand on my ironing board and ironing with my hottest iron setting (which on my Rowenta is very hot!). Neither melted. In fact, I could not perceive any change in the threads even after leaving my hot iron on them for a minute at a time. I think a problem with melting thread is a rarity these days. Manufacturing advances are probably the reason.