The effect of powder-liquid ratio and the mixing method on the surface hardness of phosphate-bonded investment materials.

In this experimental study the surface-hardness changes of three phosphate-bonded investments were observed in relation to: a) the ratio of liquid-powder, b) the way of the mixing, in 3 hours and 24 hours after mixing. The Vickers hardness test was used for testing the surface hardness of the specimens (ISOMA SA, Bienne, Swiss, hardness testing apparatus). This study indicated that the surface hardness of the specimens increased after mechanical mixing, and also that it was higher in 24 hours than in 3 hours. Also, as more water was included in the mixing liquid, as lower became the surface hardness of the materials. Finally, it was observed that one material appeared to have the greatest fluctuations of surface hardness: Maruvest MI.