Effect of Spruing on Occurrence of Porosity in One-piece Bridge of Ag-Pd-Au Alloy

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  • 金銀パラジウムブリッジにおけるスプルーイングと鋳巣の関係

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Abstract

Ag-Pd-Au alloy is commonly used in dental practice. Although a number of casting porosities are often observed on the polished surface of the one-piece cast bridge of this alloy, no definite casting methods have yet been established for eliminating porosities. In this study, the effects of spruing were investigated on the aspects of internal porosities in the one-piece cast bridge of Ag-Pd-Au alloy. A centrifugal machine was used for casting. The cast specimens were sectioned and examined by an image analyzer connected to a metallurgical microscope. The occurrence of porosities was found to depend on the distribution of the centrifugal casting force and the solidification rate of the cast alloy. They tended to concentrate at the portions where the centrifugal force was less exerted and solidification began and/or finished last in the casting. When one sprue was attached to the pontic, the solidification of the whole cast alloy proceeded so rapidly that small porosities were widely dispersed all over the bridge. The use of a reservoir could not reduce the porosities but retarded the solidification of the pontic resulting in their concentration near the occlusal surface of the pontic. When two sprues were attached, one to each of the two crowns, marked concentration of porosities was found at the joint regions between the pontic and crowns. It is probably because of the hot spot formation at those regions by the passage of a large quantity of molten alloy into the pontic as well as the smaller centrifugal pressure. When three sprues were attached, one to the pontic and to each of the two crowns, the porosities were concentrated to the pontic and joint regions with general increase in the percentage of their occupation in the whole casting. The application of a runner-bar to the two -or three- sprue method was not so effective for reducing porosities as might have been expected. The concentrated porosities appeared to be united with each other to form larger and irregular voids, while the dispersed porosities were generally smaller and roundish.

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