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課本Phillips_ Science of Dental Materials 12e P203-204

VARIABLES THAT AFFECT HYGROSCOPIC EXPANSION

    The magnitude of the hygroscopic setting expansion of a dental investment is generally proportional to the silica content of the investment, other factors being equal. The finer the particle size of the silica, the greater is the hygroscopic expansion. In general, α-hemihydrate is apt to produce a greater hygroscopic expansion in the presence of silica than is the β-hemihydrate, particularly when the expansion is unrestricted.

    A dental investment should have enough...

課本Phillips_ Science of Dental Materials 12e P203-204

VARIABLES THAT AFFECT HYGROSCOPIC EXPANSION

    The magnitude of the hygroscopic setting expansion of a dental investment is generally proportional to the silica content of the investment, other factors being equal. The finer the particle size of the silica, the greater is the hygroscopic expansion. In general, α-hemihydrate is apt to produce a greater hygroscopic expansion in the presence of silica than is the β-hemihydrate, particularly when the expansion is unrestricted.

    A dental investment should have enough hemihydrate binder with the silica to provide sufficient strength after hygroscopic expansion. Otherwise shrinkage occurs during the subsequent drying of the set investment. At least 15% of binder is necessary to prevent drying shrinkage.

    With most investments, as the mixing time is reduced, the hygroscopic expansion is decreased. This factor is also important in the control of the effective setting expansion.

   The older the investment, the lower is its hygroscopic expansion. Consequently the amount of investment purchased at one time should be limited. The greatest amount of hygroscopic setting expansion is observed if the immersion takes place before the initial set. The longer the immersion of the investment in the water bath is delayed beyond the time of the initial set of the investment, the lower is the hygroscopic expansion.