Some factors affecting the compressive strength of the gypsum-bonded investment

ABSTRACT

Gypsum-bonded investments have been widely used in dentistry for reproducing precious and semi precious cast restorations. Meanwhile, the casting of high fusing non-precious alloys such as nickel- chromium and cobalt-chromium alloys is usually performed with phosphate-bonded investments. However, some gypsum-bonded investments are used for casting nickel-chromium alloys with a relatively low fusing temperature. The main disadvantage of the phosphate- bonded investments is the reaction with the molten alloys during casting resulted in reaction products that are difficult to remove and take much effort in the finishing of the cast restorations. A sample size of 105 gypsum-bonded investment blocks [5 cm diameter and 5 cm in length] was employed in this study. The first 6 groups were mixed mechanically under vacuum while the last one was mixed by hand. After mixing, the molds were filled with the different investment mixes using the vibrator for 5 seconds, then the surface of each mold was covered by a flat smooth glass slide in order to standardize the length of each sample as well as to assure that, the base and the top surface are parallel to each others. Results revealed that, the compressive strength values decreased with the increases of burnout temperature at different treatments. Furthermore, vacuum mixing increase the compressive strength value when compared to hand mixing. Samples obtained by proper mixing under vacuum with low W/P ratio provided the maximum compressive strength values at 900 degree F. burnout temperature. On the other hand, the maximum strength values were obtained at 1100 degree F. burnout temperature when samples were prepared with paper water/ power ratio and short mixing time under vacuum. Vacuum performed before, during and after mixing resulted in the maximum compressive strength values at 1350 degree F. High W/P ratio reduced the compressive strength of the gypsum-bonded investment at various burnout temperatures