Polyester won't melt at a steam setting...I use it a lot. Nylon will melt...rayon may also...but since I don't use rayons, I can only say that rayon fabric has a low melt temp...so I'd test before using it. Polyester is a very stron thread, and will hold up to cotton thread...I use a cotton setting w/steam to press my seams/blocks...have not had any problem in over 40 years. (did I just jinx myself? LOL)