it looks like it depends on the chemical composition of the nylon lifter, for the melting point
from wiki:
The 428 °F (220 °C) melting point of nylon 6 is lower than the 509 °F (265 °C) melting point of nylon 66.
Nylon 6/6 has a continuous service temperature of 223⁰ C (121⁰ F)
both are definitely lower than the 660 °F that many people run their soldering irons at and since you really only want to soften it up, you'll probably go down to something like 200 - 240 °F
like MikeO stated, I would "walk" up the temperature on your soldering iron on a spare piece to see at what temperature the nylon starts to be affected by the heat
