The present invention relates to a method for manufacturing a head assembly comprising a tube head and a metal barrier insert, said method comprising steps of: providing a barrier insert comprising an intermediate metal layer being a disc having an inner opening, defining an upper surface, a lower surface, an inner edge and an outer edge, an upper layer made of plastic material covering said upper surface of the intermediate metal layer, and a lower layer made of plastic material covering said lower surface of the intermediate metal layer, heating at least one of said upper and lower layers, so that said upper or lower layer melts and the plastic material flows and covers at least the inner edge or the outer edge of the intermediate metal layer, attaching said barrier insert to the inner side of the shoulder of a plastic tube head.