Electric lamp and discharge devices – Spark plugs – Particular electrode structure or spacing
Reexamination Certificate
2000-08-16
2004-06-15
Patel, Nimeshkumar D. (Department: 2879)
Electric lamp and discharge devices
Spark plugs
Particular electrode structure or spacing
C313S142000
Reexamination Certificate
active
06750597
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a spark plug for use in an internal combustion engine and to a method for manufacturing the same.
2. Description of the Related Art
A typical spark plug used for ignition in a gasoline internal combustion engine for automobile use, includes a center electrode, an insulator disposed around the center electrode, a metallic shell disposed around the insulator, and a ground electrode joined to the metallic shell so as to form a spark discharge gap in cooperation with the center electrode. The spark plug is mounted on the cylinder head of the engine by means of threads formed on the outer circumferential surface of the metallic shell. Generally, the metallic shell is made of an iron-based material, such as carbon steel, and in many cases is plated with zinc for corrosion protection. Further, in order to improve anticorrosive performance, the surface of the zinc plating layer is subjected to chromate treatment.
For plating the metallic shell with zinc, so-called barrel plating is effective in terms of productivity. However, if the ground electrode is joined to the metallic shell through, for example, resistance welding after the barrel plating process is performed, a poor weld is likely to result, due to the presence of the zinc plating layer (or chromate layer). Also, since the zinc plating layer is destroyed at the weld zone, corrosion resistance is impaired. Generally, therefore, the ground electrode is joined to the metallic shell, and subsequently the resultant metallic shell assembly is plated with zinc through barrel plating and, as needed, subjected to subsequent chromate treatment.
Manufactured in accordance with the above-mentioned method, the surface of the ground electrode welded to the metallic shell is also covered with the zinc plating layer and in some cases with an additional chromate layer. If the insulating chromate layer covers a portion of the ground electrode which is to define a spark gap, discharge performance will be adversely effected. In order to improve resistance to spark-induced consumption, it has been determined that a chip containing a predominant amount of a high melting point metal, such as Pt or Ir, is welded to both the ground electrode and the center electrode at respective positions corresponding to the spark gap so as to form spark portions. However, when the chip of a high melting point metal is welded to the ground electrode, the existing zinc plating layer, and in some cases additional chromate layer, may cause a poor weld, potentially causing separation of the chip from the ground electrode.
In order to avoid the above-mentioned problem, a base metal of a free-end portion of the ground electrode is exposed without being covered with the zinc plating layer, so as to weld the chip of a high melting point metal to the exposed base metal. Conventionally, in order to maintain a free-end portion of the ground electrode in a bare state, a rubber tube, for example, is fitted to the free-end portion so as to prevent contact with a plating solution, thereby preventing formation of the zinc plating layer thereon. Because of difficulty in fitting the rubber tube to the free-end portion and difficulty in automating the fitting process, this method is of very poor efficiency. Also, when the metallic shell assembly is handled for fitting the rubber tube to a free-end portion of the ground electrode, oil and dirt are likely to adhere to the free-end portion. Such adhering oil and dirt impair welding of the chip, potentially causing separation of the chip from the ground electrode.
SUMMARY OF THE INVENTION
The present invention provides a method for manufacturing a spark plug including a center electrode; an insulator disposed around the center electrode; a cylindrical metallic shell disposed around the insulator; a ground electrode disposed in opposition to the center electrode so as to form a spark discharge gap therebetween; and a chip of a high melting point metal welded to at least the ground electrode at a position corresponding to the spark discharge gap so as to form a noble metal spark portion having a discharge face. The method comprises the steps of preparing a metallic shell assembly through joining of a root-end portion of the ground electrode to an open end portion of the metallic shell; forming a zinc-based plating layer containing a predominant amount of zinc on a surface of the metallic shell assembly; removing the zinc-based plating layer from a free-end portion of the ground electrode of the metallic shell assembly covered with the zinc-based plating layer; and welding the chip of a high melting point metal to the free-end portion of the ground electrode from which the zinc-based plating layer has been removed.
As described above, the method of the present invention does not involve a conventional masking process; i.e., a step of masking the free-end portion of the ground electrode with a rubber tube in order to prevent formation of a plating layer thereon. Thus, the invented method can manufacture at high efficiency a spark plug having a chip of a high melting point metal, serving as a spark portion, welded to the ground electrode. Also, the invented method avoids adhesion of dirt and oil which would otherwise result from employment of the masking process. Through execution of the step of removing the zinc-based plating layer, the surface of the base metal of the ground electrode is appropriately eroded to thereby be cleaned. As a result, weld strength of the chip is enhanced, thereby avoiding separation of the chip from the ground electrode.
A physical process, such as ion etching or shotblasting, may be employed in the step of removing the zinc-based plating layer. However, preferably, the ground electrode covered with the zinc-based plating layer is immersed in a remover so as to chemically remove the zinc-based plating layer therefrom. This chemical process enhances the efficiency of the removal. Also, the etching effect of the remover brings about cleaning of the surface of the base metal of the ground electrode.
Preferably, in order to chemically remove the zinc-based plating layer from the ground electrode covered with the zinc-based plating layer, the covered ground electrode is immersed in an acid remover so as to electrolessly remove the zinc-based plating layer therefrom. Preferably, the acid remover contains at least any one of nitric acid, hydrochloric acid, sulfuric acid, and an organic acid. This acid remover exhibits excellent performance in removing the zinc-based plating layer and thus can be used effectively in the present invention. Particularly, a mixture of nitric acid and hydrochloric acid is preferred as the acid remover. This mixture not only exhibits excellent performance in removing the zinc-based plating layer, but also is less likely to impair or discolor the surface of the base metal of the ground electrode, when the base material is an Ni-based metal (for example, an Ni-based heat resistant alloy) or an Fe-based heat resistant alloy. The base metal exposed through removal of the zinc-based plating layer exhibits good weldability to the chip of a high melting point metal. Preferably, in order to remove the zinc-based plating layer from a free-end portion of the ground electrode, the ground electrode is immersed in the remover such that a predetermined length of the root-end portion is exposed above the surface of the remover while the remaining free-end portion is submerged in the remover, thereby removing the zinc-based plating layer from the submerged free-end portion. Through mere adjustment of the depth of immersion of the ground electrode in the remover, the zinc-based plating layer can be removed from a predetermined length of the free-end portion.
Preferably, when the spark plug is configured such that a free-end portion of the ground electrode is bent so as to form the spark discharge gap in cooperation with an end portion of the center electrode, the zinc-based plating layer is removed from the free-end portion
Brinks Hofer Gilson & Lione
Guharay Karabi
NGK Spark Plug Co. Ltd.
Patel Nimeshkumar D.
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