| Dental casting restorations have been applied widely since Tag-gart used " casting by losing wax" firstly in dental restoration in 1907. Most of dental fixed prosthesis, including inlays, crowns, dowels and fix bridges prosthesis etc, many of removable partial dentures and parts of base plate of full dentures are completed with the method of " casting by losing wax" . But the volume of an alloy will reduce when it get cooler in dental, casting technology, and the shrinkage of base metal casting will reach around 2% . The irregular shrinkage of base metal can cause the distortion of castings, even effect their accuracy. Now the researches which domestic and oversea scholars study investment materials mostly focused on how to utilize the expansion of investment materials in order to fill up the shrinkage of castings.However, the dilatability of investment materials expressed as an actinomorphic picture from their centers. If using a ring, it can limit the horizontal expansion of investment materials without limitation of vertical expansion, and leads to the poor - proportion between them. In order to obtain the accordant vertical and horizontal expansion of investment materials, a general method is that liners were used in a ring to cushion the horizontal expansion and to reduce the limitation ofring. Other experts suggested that a ringless investment method should be used in order to eliminate the limitation of ring. A wax pattern was invested with siliceous rubber, soft plastic or paper rings without metal rings. Molds to obtain free and uniformity temperature expansion did not limit the expansion of investment materials during burnout. But now it has not been reported from domestic or oversea literatures if the moulds without metal rings can sustain the impact of molted alloy with high temperature and produces previous size castings not breaking to pieces.In this experiment, four sorts casting investment materials, which are used commonly in clinical high - melting point alloys casting process, will be measured about their compressive strength to confirm the compressive strength of them and to compare one with another.Experimental Materials1. Heravest Speed Phosphate - bonded Investments: mixed with liquid ( Heraeus - Shanghai Dental Ltd, China).2. GIL VEST IIS Phosphate - bonded Investments: Mixed with liquid ( BK Giulinli Chemie Germany) .3.X -20 Chrome Investment: mixed with liquid ( Whip - Mix Corporation USA. ).4. X - 20 Chrome Investment; mixed with water (Whip - Mix Corporation USA. ).Experimental MethodThe 12 models were made with each of 4 sorts investment materi-als respectively, total 48. Each was mixed mechanically under vacuum and room temperature according strictly to manufacturer's instruction. Then the mixed investment material was poured into a PVC casting ring, which was wall thickness 1 mm, outer diameter 47mm and high 60mm. The casting ring would be removed when the investment material had become solidification completely. The achieved model was a cylinder and dried naturally for 24 hours. They were placed into cold oven and raised to 300℃ slowly, then remained 300℃ for 30 minutes. They eventually raised to 900℃ from 300℃ in two 2 hours and then remained for 15 minutes. They had being pressed by a universal material experiment machine until smashed. The overall stress was recorded.Statistical analysis: All measured dates were analyzed and tested with ANOVA and t - test of SPSS 10. 0 software by a Legend PHI computer.Result1. The graph of the compressive strength of 4 sorts investment materials shows that the compressive strength of the X - 20 Chrome Investment mixed with liquid is highest, whereas that of the X - 20 Chrome Investment models mixed with water is lowest.2. There is significantly difference between the groups of 4 sorts investment materials (P <0. 05) ; the group of the X -20 Chrome Investment mixed with liquid is significantly difference to other groups ( P<0. 05) , moreover there is no significantly differenc...
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