METHOD FOR PRODUCING A COMPOSITE CENTER ELECTRODE

摘要

An improved method for producing a composite center electrode for a spark plug is disclosed. The method comprises the steps of forming from a corrosion-resistant metal a cup having a closed end, walls extending upwardly from the closed end to an open end and a cavity extending concentrically therein, and then forming a composite billet having closed and filled ends by inserting into the open end of the cup a billet of a metal having a high thermal conductivity and fitting tightly within the cup walls. The method also includes extruding the composite billet into an electrode blank having an upper headed portion and a lower portion of reduced diameter extending longitudinally therefrom. The improvement comprises the steps wherein the positions and the relative sizes of the billet and of the cup are controlled so that the former fits tightly within the cup walls adjacent the closed cup end, but terminating short of the open cup end so that the cup walls extend thereabove at the open cup end, and the cup walls adjacent the open cup end are rolled radially inwardly to form the composite billet. The method includes the additional improvement wherein the closed end of the composite billet is first inserted into a die having a stepped bore including an extrusion orifice of reduced diameter equal to that of a desired electrode head, and is then forced through the extrusion orifice to form an elongated composite billet. The elongated billet is inserted into a second die having a stepped bore including an extrusion orifice of reduced diameter, wherein all except an upper headed portion is forced through the extrusion orifice to form the composite center electrode.