The Effect of Longitudinal Restrictive Force on Hygroscopic Expansion

¹ Ransom & Randolph Co., Toledo, Ohio

The force apparently prevents the water from entering the investment, and it is highly possible that the excess of hygroscopic expansion over normal setting expansion is completely independent of the setting reaction of calcium sulfate hemihydrate. The setting reaction might be considered a chemical reaction, while the hygroscopic effect may be the result of a physical reaction, such as absorption. Possibly, chemically pure calcium sulfate hemihydrate would show no hygroscopic expansion, and the small amounts obtained from hydrocal, density, or plaster of paris may be due to impurities. It is difficult to obtain pure calcium sulfate hemihydrate, as it usually contains some anhydrite and some dehydrate as well as minor foreign impurities. (Note: An investment for chrome castings exhibits hygroscopic expansion. This investment contains no calcium sulfate, and the setting reaction depends on an oxide-phosphate reaction. An investment of this type with a normal setting expansion of 0.15 per cent may show hygroscopic expansion of 0.85 per cent.)

Longitudinal force exerted on one investment during setting indicated a marked decrease in the longitudinal hygroscopic expansion, especially in the lower ranges below 100 gm/cm², but it takes more than 1 atmosphere to reduce the hygroscopic expansion approximately to the value of the setting expansion. Apparently, hygroscopic expansion has adequate strength to move invested wax patterns. Force has a much smaller effect on the normal setting expansion.

Using the wire method,⁵ a definite reduction in hygroscopic setting expansion was exhibited in a vertical direction, although not nearly so pronounced as in the horizontal direction, using a balanced dial gauge. The wire method always indicated a greater percentage expansion than that of the balanced dial gauge, possibly because of the restrictive effect of the friction of the investment on the asbestos liners. By extrapolation of the curve for hygroscopic expansion (rectangular trough) to the termination of the curve hygroscopic expansion (wire method), this force might be as little as 10 gm/cm² or less.