High-quality synthetic hair extensions can be straightened using a flat iron no hotter than 320 degrees Fahrenheit. A flat iron heated to hotter than 320 F may cause the extensions to melt.
Those straightening synthetic hair for the first time might benefit from slightly wetting the extensions before applying the heat from a flat iron. This offers extra protection from the heat. Unlike human hair, it is important to straighten synthetic hair slowly and a detangling spray is recommended. Always section the hair and work from the bottom up.
Cheap or inexpensive synthetic hair should not be straightened. Synthetic hair extensions and wigs should never be pulled and will damage more easily than human hair extensions.