CHARACTERISATION OF PHOSPHATE-BONDED INVESTMENTS FOR PLATINUM CASTING
Keywords:
Platinum casting, phosphate-bonded investment, lost-wax casting, investment casting, jewelryAbstract
Platinum jewelry is manufactured by investment casting process. Phosphate-bonded investment is used to make a mold for platinum to withstand a high melting temperature above 1,700°C. The investment powder composes of quartz to provide thermal expansion to compensate the subsequent contraction of the alloy. Two types of commercial phosphate-bonded investment materials for platinum casting with different particle size distribution were investigated. The microstructural analysis and thermal behavior, chemical composition and crystal structure of the investments were analyzed. The investment composed mainly of SiO2 as refractory material with aluminium phosphate-based binder. During the burnout step, silica experienced α-quartz inversion resulting in change of the interplanar spacing and crystallite size for the investment after firing. The fineness of the investment powder related directly to surface quality of the as-cast platinum jewelry in term of surface smoothness and casting defects.
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