Research On The Investment For Dental Crown And Bridge Casted By Pure Titanium

Dental crown and fixed bridge could be fabricated by wax mold casting with pure titanium which has excellent biocompatibility, physical and mechanical properties. Perspective could be expected. But because of high chemical reactivity and easily contaminated surface, it is a problem that how to select the appropriate refrectory which is inert to molten titanium and adequate to compensate the shrinkage of the casting process. In the earlier research a type of vacuum-centrifugal-pressure dental titanium casting machine had been constructed. The purpose of this study was to develop a special investment for precise titanium crown and bridge. Firstly, technique parameters of different binder and refractory material matching and performance of casting molds were compared. Final matching result for binder and refractory was determined. High quality of pure titanium castings could be achieved according to the result. The perfect proportion of used binder and refractory was farther confirmed. According to this proportion room temperature strength, high temperature strength and residual strength were observed. Thermal expansion curves of the molds were plotted. The procedure of burning-off was established. Problem of reactivity between refractory and molten titanium was emphatically solved. The accuracy ofstandard crown was measured. Component and thickness of the reaction layer was analyzed. The tensile strength, yield strength and elongation of the castings were measured. Biological safety evaluation was done for the investment before clinic. Practical casting technique program was simulated by modified master cast of patient. The marginal fit of crown and fixed bridge was measured. The results were indicated as below:1. Based on current investing technique, effects of venting or not, amount of inner investment layer and casting temperature on macroscopical quality and accuracy of pure titanium castings were severally researched. An improved integrated investment scheme was put forward. The fit of titanium crown fabricated by improved technique and current technique was compared. The accuracy was advanced by improved technique. The mean value of occlusal inner fit for control group was 177um, 104 m for testing group. The mean value of marginal fit for control group was 151 m, 88 m for testing group.2. Plate-shape,castings were made according to 3 powder/water ratios using ZrO2 investment and binder A or B. Temperature was slowly elevated in burn oven. When temperature was 900 C surface of mold of binder A group scaled with a low strength. Mold cavity appeared alveolate structure. Casting could not be continued. Slurry of binder B group with powder/water ratios 7.5 : 1 had good dispersing and fluidity. When temperature was 900 C high strength was acquired. Mold cavity was smooth. The castings were sound with gray yellow appearance. The value of PH of binder was 10.01, dynamic viscosity 2.9~3mPa-S, specific gravity 1.35g/cm3. These values had no significant difference in 5 days. The value of PH of casing mold was 9.54. Lg( 34 ) orthogonal design was done. Powder/water ratio 7.5 :1 , binder/hardening agent ratio 200 : 1, powder/refractory ratio 7 : 20, mixing time 30s~60s were the best situation. Matching group is No. 5. For group 5fluidity of slurry was 212.5mm, initial set time 604s, final set time 1350s. Room temperature strength, high temperature strength and residual strength of the mold were 1.49MPa, 3.84MPa, 4.88MPa. Three-peak grain size distribution curve was plotted. The self-developed investment was named FUS-invest.3 . Tough sheet-shape a-crystalloid was observed in metallographic structure of the casting. ZrO2, SiO2 and Mg2TiO4 were found through XRD (X-ray Diffraction) and these elements were proven by SEM (Scanning Electron Microscope) . The thickness of reaction layer was about 85-105um by analyse of metallography and microhardness. The values of tensile strength, yield strength, elongation and elastic module were 668.4MPa, 569.3MPa, 4.5% and 123.5GPa. The structure of tenac...