| During the years since the cobalt - chromium casting alloys be-came 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 re-sult 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 patients' mouth. 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 combina-tion of investment materials, method, and the design of sprucing , which can produce the best castings.Material and methodsThe factors of this study included; three type of invest materials, investing wax patterns ring and ringless and two designs of sprucing, top sprucing and inverted sprucing.1. Mold fabricationThe mold with two parts was made of epoxy resin. The under part imitate the maxillary edentulous. The upper one is female of the un-der.2. Duplicating the refractory castsThe under mold was duplicated with duplicating material to pro-duce 60 identical refractory casts. The three type 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 molds. 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 surface.3. 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 60 refractory casts divided into two groups. One group was top sprued and the other was inverted sprued. Each wax pattern was painted with SOFU wax cleaner and dried with air.4. Investing the wax patternEach refractory casts was invested within 1 hour, and use 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 oven at room temperature was then gradually raised to 300?C in 1 hour , then to 900?C in another hour. The temperature was maintained at 900?C for 15 min. An high frecmency centrifugal casting machine (Tianjin, JN - A ) was used for casting. The Co - Cr alloy was cast in-to 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 two parts of the ep-oxy resin mold. Then measured the gap between the upper and under part.8. Statistical analysisData were analyzed using the statistical package SPSS for Win-dows. The effect of all researching factors on the fit of RPD frame-works was evaluated.Result1. The effect of invest materials and methods on the fit of castingsare significant different, and their interaction is significant. The difference between the groups investing with metal rings or not is sig-nificant (P <0.01). In this study we dont find difference between top sprucing and inverted sprucing( P < 0.05 ).2. The fits of use Heravest investment invested ring and ringless were not significantly different , ever though the discrepancy of fit of the ringless was consistently higher. But the fit of Heravest investment group was better than other groups.DiscussionThe design of wax pattern in this study was imitate the posterior palatal plate RPD framework. The complex shape decided various fac-tors that may influence the fit of castings. Addition to the factors such as investment materials and special liqui... |
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