A thinner mix of a gypsum-bonded investment will:
(A) Produce a smoother casting (B) Decrease setting expansion (C) Increase setting expansion (D) Increase thermal expansion
(B) GYPSUM-BONDED INVESTMENTS
The α-hemihydrate form of gypsum is generally the binder for investments used in casting gold-containing alloys with melting ranges below 1000 °C. When this material is heated at temperatures sufficiently high to completely dehydrate the investment and to ensure complete castings, it shrinks considerably and occasionally fractures.
A mixture of silica and calcinated gypsum powder (calcium sulfate hemihydrate, CaSO4• 1/2 H2O) results in setting expansion greater than that of the gypsum product used alone. The silica particles probably interfere with the intermeshing and interlocking of the crystals as they form. Thus, the thrust of the crystals is outward during growth, and they increase expansion.
Thinner mix of a gypsum-bonded investment will decrease setting expansion.