Objectives: Aim of the study was to analyze the mold filling capacity and the dimensional accuracy of a spinel-based investment for titanium castings.
Methods: Expansion of the investment in dependence of the preheating temperature was measured in a dilatometer. The degree of transformation of MgO and Al2O3 to spinel (MgAl2O4) was evaluated by means of X-ray powder diffraction. Mold filling capacity was assessed by casting a grid and calculating the percentage of completed segments. Dimensional accuracy was analyzed by casting a hollow cylinder and measuring the difference between the inner diameter of the resin pattern and the resulting titanium casting.
Results: Spinel formation starts at 819 degrees C. Diffraction patterns prove the formation of spinel from MgO and Al2O3. The amount of spinel increases with increasing preheating temperature. The final expansion of the investment at the end of the preheating cycle at 450 degrees C shows a linear correlation to the maximum preheating temperature. The degree of mold filling is reciprocal to the preheating temperature. The dimensional accuracy shows a linear correlation to the amount of spinel. Best dimensional accuracy was obtained at about 900 degrees C. After a preheating temperature of 884 degrees C, as recommended by the manufacturer, the cast specimens showed a slightly lower inner diameter as compared to the resin patterns.
Significance: The results suggest that with the spinel investment analyzed an excellent accuracy of titanium castings may be obtained.