| PrefaceHigh -fusing casting alloys, referred to as base metal, were introduced to the dental profession as more economic alternatives for dental restorations. Many dental restorations contain dental alloys castings. Now Ni - Cr alloy and Co - Cr alloy are wildly used in dental base metal. Dental casting restorations should fit the abutment teeth accurately for preventing caries and enhancing retention. The shrinkage of base metal alloy is greater, that can affect the fit of the denture framework. The denture will be seated with difficulty. It cost a lot of clinical time to adjust the frameworks to seat on the abutment suitably. Different base metal alloy has the different melting point and linear coefficient expansion. The shrinkage of the castings is the major factor that can affect the fit of dentures. The expansion of investment can compensates this shrinkage.The purpose of this study was to find the best combination of alloy, investing method, and height of crown which can produce the best castings.Material and methods1. Mold fabricationThe mold which has two parts was made by epoxy resin. The inside diameter of the lower part is 14.90mm and axial height is respec-lively 4mm, 6mm, 8mm. The upper part is cylinder, with diameter 12.90mm and axial height 3mm, 5mm, 7mm. Two small caves were made in cylinders bottom to be the marking point of measure.2. Wax pattern fabricationWax was softened by spirit lamp and put into the mold. The upper and lower parts of mold match completely by pressing. And then took the wax patterns out and removed the fins by sharp razor. An 2mm wax sprue was attached at a 45 - degree angle to the edge of oc-clusal surface. The wax patterns are 12. 90mm internal diameter and respective 4mm, 6mm, 8mm axial height and 1mm thickness. Seventy - two wax patterns were made in each condition.3. Investing the wax patternEach wax pattern was invested within 1 hour. Six wax patterns were invested in each casting rings. Two wax patterns of each height were attached to 5mm wax sprues. The diameter and height of the metal rings were the same as those of the paper rings. Invested 18 molds with each type of rings. Each wax pattern was painted with wax cleaner(SOFU,JAPAN) and dried with air. The X -20 Chrome investment was used according to manufacture recommended: 200 gram of powder were mixed with 26 ml water. Investments were hand mixed for 10 seconds, followed by 30 seconds mechanical spatulation at 1725r/min under vacuum. The wax patterns were invested and allowed to bench set for 24 hours before burnout.4. Wax pattern elimination and castingThe invested patterns were placed in a thermostatically controlled furnace at room temperature. The temperature was then gradually raised to 300℃ at a constant rate of 6℃/min, then to 900℃ at 9℃/ min. The temperature was maintained at 900℃ for 15 min. A highfrequency centrifugal casting machine ( Tianjin, JN - AII ) was used. Three type alloys, which are Co - Cr alloy (Co 63. 0% , Cr28. 5% ) (shanghai) , VeraBond ( Ni 77. 95% , Cr 12. 60% ) ( Aalbadent corp. ,USA)and RUBY SOFT TYPE (Ni 73. 8% ,Cr5. 1% ) , (JAPAN) , were cast at the temperature according to the manufacture's instructions.5. Casting separationAll castings were cooled to room temperature before removal from the casting ring. The castings were sandblasted at 0. 30Mpa to remove the residual investment and the oxidized layer. All sprues were cut with an ultrathin abrasive disk. All castings were divided into 18 groups according to the alloys, investing methods and the height of crowns and put into boxes.6. MeasurementThe inside diameter of castings were measured by digital caliper. Both of the vertical and horizontal internal diameter were measured and calculate the average.7. Statistical analysisData were analyzed using the statistical package SPSS for Windows. The effect of all researching factors on casting shrinkage was e-valuated.Result1. Differences between groups casted with the three type of alloy were sign... |
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