The ones meant for plastic extrusion are normally rated to about 350C, as most extruders will operate at around 150-300C depending on the particular machine. The leads however are often only rated to 200C, as they are going to be outside the heater blocks and this is a cheaper insulation to make in a flexible form. Somw will operate to 600C, but then you will find the leads are made from ceramic beads threaded on the wires.