An apparatus for molding externally ribbed and corrugated thermoplastic pipes, including a pipe die for extruding a parison of a thermoplastic material; moldblock halves circulating in endless paths and cooperating with each other to form a travelling mold; and a means withstanding the hydraulic pressure...http://www.google.es/patents/US4681526?utm_source=gb-gplus-sharePatente US4681526 - Apparatus for molding thermoplastic pipes
