| Base metal ( Ni - Cr alloy and Co - Cr alloy) was used in the dental profession, such as framework of removable partial denture ( RPD) , artificial crown and fixed partial dentures ( FPD ) . These alloys increase strength of denture, have higher melting temperature, and are cheaper. The shrinkage of casting is the major factor affecting the fit of casting. So the expansion of investment should compensate for the shrinkage of base alloy. The phosphate investment has adequacy expansion to compensate for casting shrinkage. The phosphate investment is used in clinic.The purpose of this study was to find the different investment, the different investing method, and the different crown height affecting the shrinkage of base metal.Material and methods1. The mold fabricationThe molds are made by epoxy resin. Mojd is two parts. The lower parts are 14.9mm in diameter and 4mm, 6mm, 8mm in height. The upper parts are cylinders, which are 12. 90mm in diameter and 3mm, 5mm, 7mm in height. Two small caves were made in cylinders bottom to be the marking point of measure.2. Wax pattern fabricationWax was softed by spirit lamp at room temperature and put into the mold. The upper and lower parts of mold were completely seatedby pressing. Then the two parts of mold were separated and the superabundance was taken away by sharp razor. The wax lines of 2mm diameter, 10mm length are secondary sprues. The wax patterns are 12. 90mm in diameter and respective 4mm, 6mm, 8mm in height and in 1mm thickness. Each condition is 96 wax patterns. Wax patterns were then cleaned with a surfactant. Before investing wax patterns were stored within 1 hours in room temperature.3. Investing the wax patternWax patterns were divided 24 groups according to different factors. There were!2 specimens in each group. Patterns were invested with four different investment; X - 20 Chrome water, X -20 Chrome liquid, GILVESTII S, Herasvest, the ratio of power and liquid following the manufacturers instructions. Six wax patterns were invested in each casting ring. Two wax patterns of same height were fixed on master sprue (D =5mm). The casting rings were metal rings and paper rings, which are the same diameter and height. Investment was hand mixed for 10s, followed by 30s mechanical mixing under vacuum at a rate of 1725 rpm, with the combination unit. The wax patterns were invested with shaking lightly and allowed to bench set. Recommended bench set times were 24 hours.4. Wax pattern elimination and castingThe invested patterns were placed in a thermostatically controlled furnace at room temperature, then gradually raised to 300C using heat rate of 6 C/min. Hold for 30 minutes, then to 900C at a heat rate of 10C/min. The temperature was maintained at 900 C for 15 mins. A high frequency centrifugal casting machine (JN - A II Tian-Jin,) was used for base metal casting. The castings were sandblasted (Sablomat, ITALY) to remove investment and the oxidized layer by0.30MPa pressing. The sprues were cut with carborundum disks. Then the castings were numbered and put into boxes.5. MeasurementThe inner diameters of castings were measured with digital caliper (0.01mm, E-PK15B, Nakarnura Mfg Co, Japan).6. Statistical analysisAnalysis of variance ( ANOVA) was calculated for all data by SPSS software.ResultDifferences between groups with the four types of investment were significant( P < 0. 05 ). There are significant difference between the groups investing with ring and ringless ( P <0. 05). Differences between the groups investing with different height were not significant. ( P > 0. 05 ) . The expansion of GILVESTII S and Herasvest with ring-less is larger than others.DiscussionThe linear shrinkage of the base metal alloy is about 2%. Its essential to achieve compensation for the shrinkage of the solidifying alloy by investment expansion for the accuracy of the casting crown. Several important considerations for using phosphate - bonded investment with base metal alloy include adequacy of investme... |
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