In injection molding, the function of a non-return valve is to stop the flow of melted resin back into the screw channels while the screw is moving forward during injection. Valves are manufactured from steels or alloys that are best suited to the processing environment in which they will be used. Various tool steels that are heat-treated and/or nitrided, nickel alloys, chrome plating and special coatings are all options from which to choose.