Electric lamp or space discharge component or device manufacturi – Process – Spark plug or spark gap making
Reexamination Certificate
2002-05-01
2004-06-22
Ramsey, Kenneth J. (Department: 2879)
Electric lamp or space discharge component or device manufacturi
Process
Spark plug or spark gap making
C501S014000, C501S021000
Reexamination Certificate
active
06752677
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method of producing a spark plug.
BACKGROUND OF THE INVENTION
A spark plug used to ignition of an internal engine of such as an automobile generally comprises an insulator made of alumina based ceramics disposed inside of a metal shell to which an earth electrode is fixed, and a center electrode disposed within the insulator. The insulator is axially projected out of a rear opening side of the metal shell, and has a terminal metal fixture disposed in the interior of the projected portion, which terminal metal fixture is connected to the center electrode through a resistor or a conductive glass seal layer which is formed in a glass sealing procedure. Thus, by supplying a high voltage through the terminal metal fixture, a spark is produced over a gap between the earth electrode and the center electrode.
However, under some combined conditions that, for example, temperature of the spark plug is high and environmental moisture becomes high, even a high voltage supplied fails to cause the spark over the gap, but instead, a so-called flashover occurs, that is, a discharge occurs between the terminal metal fixture and the metal shell as running around the projected portion of the insulator. Therefore, almost all of generally used spark plugs have a glaze layer formed on the surface thereof for preventing the flashover phenomenon. On the other hand, the glaze layer also serves to smoothen the surface of the insulator, prevent contamination or increase chemical and mechanical strength.
In case of glazing over spark plugs which are attached to engines, the glaze temperature becomes higher than that of a normally insulating porcelain. Further, in recent years, being accompanied with high performance of engines, ignition voltage supplied to the spark plug has been high, and the glaze has been demanded to have an insulating property endurable against severe environments.
On the other hand, in general as to automobile engines, a rubber cap is utilized to attach the spark plug to the electric system of engines. In order to heighten the flashover resistance, adhesion between the insulator and the interior of the rubber cap is important. In this case, it is required that the glaze layer to be formed on the surface of the insulator is generally free of unevenness and is smooth to the utmost.
For forming the glaze layer on the surface of the insulator, the following procedures are generally adopted. At first, glaze materials are mixed to be a predetermined composition, and then the composition is melted by use of a crucible, and is rapidly cooled for solidification to be a vitreous glaze composition. The glaze composition is finely pulverized by means of a ball mill, and is finally reduced into a glaze slurry of glaze particles suspended in a water solvent. The glaze slurry is coated to the surface of the insulator by way of spraying, spin coating or dipping, and is dried, followed by a glaze baking treatment.
Herein, in case of obtaining a glaze layer of little unevenness in thickness, it is significant that the glaze slurry has fluidity. The slurry of good fluidity spreads well by coating over the surface of the insulator and easily brings about the glaze layer of smooth and uniform thickness. On the other hand, a slurry of bad fluidity will be biased on the surface of the insulator and will result in a coat of uneven thickness.
SUMMARY OF THE INVENTION
The glaze for spark plugs contains SiO
2
(silicon oxide) as an indispensable component for forming a vitreous skeleton, and for properly lowering the glaze baking temperature so as to enhance the fluidity while melting, the glaze generally comprises a silicate glass mixed with various sub-components such as boric acid (B
2
O
3
), alkaline metal components (Li
2
O, Na
2
O, or K
2
O), or alkaline earth metal components (MgO, CaO, SrO, or BaO). Among the components, SiO
2
itself has scarcely problem because of low solubility to a water. However, some of the mixed sub-components may be of comparatively high solubility. (hereinafter referred to as called “water soluble glaze component”) The representative ones are alkaline metal oxides (Li
2
O, Na
2
O, K
2
O), or boric acid (B
2
O
3
).
In case of using the glaze of a high containing rate of these components, when wet-pulverizing the glaze with the water solvent, or storing it under a state of the water solvent slurry, the water soluble solvent glaze components gradually elute into the solvent. This elution gives changes to a rheology of the slurry suspending the glaze particles, so that viscosity of the slurry increases as a time passes, and the fluidity of the slurry is spoiled to easily make the above mentioned problem the uneven thickness in the coating. For example, even at an elution level which is almost a matter of no problem under a condition of the glaze layer after baking tho glaze, but if the slurry is reduced into a fine powder (average diameter is, for example, around 5 to 45 &mgr;m) which can be suspended as the slurry, the influence of elution is especially difficult to be neglected.
In case of the alumina insulator for spark plugs, a lead silicate vitreous glaze was conventionally used where the silicate glass was mixed with a comparatively much amount of PbO to lower the softening point. However, with the global concern of environment protection having grown up in the recent years, the glaze containing Pb has progressively come to be out of consideration. For example, in the automobile industry using many spark plugs, considering adverse influences to environment by waste spark plugs, an investigation has been progressed to abolish use of spark plugs using Pb containing glaze in a future. However, when planning leadless spark plugs, it is unavoidable to use an increased amount of alkaline metal component or boric acid component for the purpose of restraining the rise of melting point which is caused by elimination of the lead component. As a result, the viscosity of slurry inevitably increases as the time-passing as a problem particular in consideration of the leadless glaze.
The problem of the invention is to effectively avoid the uneven glaze coat on the surface of the insulator owing to tho viscosity rising in the glaze slurry, and in turn it is an object of the invention to provide a method of easily producing a spark plug excellent in flashover resistance.
The invention has been provided to solve the problems as mentioned above, and relates to
a method of producing a spark plug, wherein an insulator of alumina based ceramics is disposed between a center electrode and a metal shell, and a glaze layer as a main of oxide is formed to cover at least part of the insulator, characterized by the steps of
preparing glaze particles to be coated, respective surfaces of which are covered with coating layers for suppressing (restraining) elution of water soluble components contained in the glaze,
preparing a glaze slurry by suspending the glaze particles to be coated in a water solvent,
piling the glaze particles for forming a glaze particle piled layer by coating the glaze slurry to the surface of the insulator, and
baking the glaze particle piled layer to the surface of the insulator by heating the insulator to form the glaze layer.
According to the invention, prior to suspending the glaze particle in the water solvent to make a slurry, the surface of glaze particle was covered with a coating layer for suppressing elution of the water soluble components contained in the glaze, thereby to obtain a glaze particle to be coated which is used to prepare a slurry, so that the water soluble components in the glaze (for example, B, F, Li, K, Na) do not easily elute into the water solvent in the slurry, and the inconvenience that the slurry viscosity rises as the time-passing may be effectively restrained. As a result, when coating the glaze slurry to the insulator, a proper coat thickness may be easily obtained, and dispersion in the coated thickness is little. Thus, the proper thickness of the glaze layer formed by the glaze-baking an
Nishikawa Kenichi
Sugimoto Makoto
Morgan & Lewis & Bockius, LLP
NGK Spark Plug Co. Ltd.
Ramsey Kenneth J.
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