Fabrication and Evaluation of New Investment for Dental Application

Investment materials have been used to produce molds for casting of Inlays, Onlays, crowns and bridges. In this study, gypsum and investments routinely used in Iran, were evaluated and characterized, then α hemi hydrate calcium sulfate and a new dental investment with a gypsum-micro silica binder were fabricated. The purpose of this study was to produce and evaluate a new gypsum investment product in Iran. Using this production, it could be not necessary to import this material from other countries.Methods and Materials: Yellow, well-mixed, orthodontic gypsums and casting investments routinely used in Iran, were evaluated and characterized with scanning electron microscopy elemental analysis (SEM) and XRD phase analysis. The morphology of these materials also was evaluated. Gypsum mineral (CaSO4.2H2O) was fired at 3 atmosphere autoclave and dental stone produced, and evaluated. New dental investment was fabricated with fine silica, Alumina, Silicon Carbide, Micro Silica and Polypropylene. Cold crushing strength (CCS) and setting expansion tests were performed. Casting was done at new molds. Phase changing of investments was tested at high temperatures with x- ray diffraction phase analysis. Compressive strength of some samples was tested after firing. Using above techniques showed that yellow and well-mixed gypsum are dental stones, orthodontic gypsum is high strength stone and investments have phosphate bonds. New investment has efficiency at 1000°C, and more. There isnt any decomposition in gypsums at these temperatures. Silicon Carbide and polypropylene increase CCS. Compressive Strength of these new investments increases 7 times after firing. Micro silica is used as a refractory and binder. Micro silica is very fine and has a large specific surface. So it is a good binder and it leads a smooth surface castings. Silicon Carbide increases investment strength and thermal resistance. Poly propylene increases spaces for escaping of gases during casting. New gypsum- micro silica investment can be used at temperatures more than 1000°‍‍‍C. There are other materials which were proposed before to increase thermal resistance of gypsum investments, but they are more expensive than micro silica based products.