Threaded inserts in plastic are not used too often. Unless the material is plastic and may not be able to take the weight of metal inserts. Using metal inserts may crack the plastic material. In such scenarios plastic inserts may be used. Thermal insertion is used to press the insert against the material https://www.tappexinserts.com/. Sometimes, there could be heat produced due to friction of threaded insert in the material when thermal insertion is used, this may cause melting of the threaded insert.