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PRACTICAL APPLICATION TESTS Casting Precision This test is a measure of how an accurate dental casting can be produced using the lost wax casting technique. The metal shrinkage after solidification must compliment the expansion of the wax and investment in order to produce the desired results. The correct (accurate) fit of a three unit casting over the original model is used to verify the casting precision of the alloy Metal/Ceramic Bond Tests In this test, 1, 3 and 6 unit castings are subjected to porcelain application per the manufacturerÕs instructions. Following this procedure, the castings are examined for cracking and the integrity of the metal to ceramic bond. Mechanical separation of the porcelain from the metal reveals the metal/ceramic bond characteristics. Polishability This is an arbitrary measure of the luster one can achieve by polishing tin the laboratory within a reasonable amount of time for a particular alloy. Soldering Tests The strength of a solder joint performed in the dental laboratory can only be implied from the handling characteristics of the soldered unit. This empirical determination depends on successful splinting, investing, heating, fluxing and soldering operations applied per the manufacturerÕs instructions. The solder bond strength of and alloy is determined in the metallurgical laboratory using a cast alloy tensile specimen per ASTM E8, ISO-1562 and gauge length and soldered together with a separation of 1 mm (the thickness of a business card). The soldered bar is then pulled to failure on the tensile test machine. Bond strength values in excess of 340 N/mm2, which is equivalent to the strength of a Type III dental gold alloy, are adequate and acceptable for all practical applications. Porosity Evaluation The alloy to be evaluated is cast into a crown configuration. After examining for gross defects and inclusions, the crow is sectioned, mounted, polished and examined under the metallurgical microscope to determine the size and distribution of any resulting porosity. |
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