a valve for controlling the flow of a gas therethrough which requires no machining in order to attain an effective seal between the valve seating surface and a rotatable plug element . the valve seat surface remains as cast while the plug consists of an overmolded plastic . the overmolding defines an array of pliable sealing ridges . the use of overmolding facilitates the use of cost - saving plastic materials , and obviates the need for expensive machining of the cooperating surfaces .