| During the years since the cobalt - chromium casting alloys became available for cast removable partial denture ( RPD) restorations, they have continued to increase in popularity.High - fusing casting alloys, referred to as base metal, include Ni - Cr and Co - Cr alloys, have a high cast shrinkage. That may result a bad fit denture. The denture will take seat with difficulty. It must cost a lot of clinical time to adjust the frameworks to seat in the perfect position. Therefore, compensation of this shrinkage became very important in the procedure of making a cast denture. Usually, we use the expansion of investment to compensate it.Therefore the purpose of this study was to find the best combination of investment materials, method, and the design of feed direction, which can produce the best castings.Material and methodsThe factors of this study included: two type of invest materials, investing with ring and without ring, and two designs of feed direction, direction and indirection1. Mold fabricationThe mold with two parts was made of epoxy resin, the under part imitate the maxillary edentulous.2. Duplicating the refractory castsThe under mold was duplicated with agar impression material to produce 32 refractory casts. 16 refractory casts used X - 20 phosphate - bonded investment materials, 16 refractory casts used Glivest - II phosphate -bonded investmentmateria . The x - 20 and Glivest - II phosphate - bonded investment materials were accurately proportioned according to the manufacturers instructions and mixed with its special liquid. The mix was vacuum spatulated 30s before it was poured into the impression. The refractory material set for 1 hour. The casts were carefully removed from the molds and dried in an oven for 2 hours. The casts were dipped in beeswax to obtain a smooth surface3. Wax pattern fabricationWax was melted in a wax melting pot and poured into the upper mold, before the wax became hard, matching the upper mold and the refractory cast with firm even finger pressure. The 16 refractory casts which used X - - 20 phosphate - bonded investment materials divided into two groups, one group was feed direction and the other was indirection. 16 refractory casts which used Glivest ?II phosphate ?bonded investment material also divided into two groups, one group was feed direction and the other was indirection. Each wax pattern was painted with SOFU wax cleaner and dried with air.4. Investing the wax patternThese wax pattern which used different design of feed direction divided into two groups,one group with ring,one group without ring. Each refractory casts was invested within 1 hour,and used the same one poured the refractory casts, And then left to beach set for 24 hours.5. Wax pattern elimination and castingThe invested patterns were placed in a thermostatically controlled furnace at room temperature was then gradually raised to 3001 in 1 hour, then to 9001 in another hour . The temperature was maintained at 900X1 for 30 min. An high frequency centrifugal casting machine ( Tian Jin, JN -AII) was used for casting. The Co - Cr alloy was cast into the mold cavity.6. Casting separationThe recovered castings were sandblasted to remove investment. The sprues were cut and all external nodules or fins were removed.7. MeasurementThe frameworks were completely seated in the under parts of the epoxy resin mold. Then measured the gap between the frameworks and under part.8. Statistical analysisData were analyzed using the statistical package SPSS for Windows. The effect of all researching factors on the fit of RPD frameworks was evaluated.Result1. The fit of castings of invest materials and with ring or without are significant different, and their interaction is significant. The difference between the groups investing with metal rings or not is significant ( P < 0.01).2. The fits of use Gilvest - II investment invested by without rings is best, and the fit of Gilvest - II investment group was better than other groups.3. I... |
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