A cool iron setting is generally around 275 degrees Fahrenheit, as opposed to the hottest setting which is usually 445 F. Synthetic or delicate fabrics such as acrylic and nylon should be ironed on a low setting to avoid damaging the material.
Other fabrics that should be ironed on a low setting include acetate and silk. To turn the iron to a low setting, check the front of the iron to find the temperature knob. If the knob has a number scale to indicate temperature settings, turn it to 1 or 2 for the coolest temperatures. Some irons also indicate what type of fabric is suitable for each setting.