Effect of ring length and diameter on effective radial setting expansion.

It was previously demonstrated that flaring of MOD inlay castings was inversely proportional to the diameters of casting rings. The purpose of this study was (1) to determine if radial setting expansion of investment was primarily responsible for the inverse proportion and (2) to characterize further the fluid mechanics of setting expansion by determination of setting expansion in casting rings of different lengths and diameters. Eight sets of ten MOD inlay wax patterns with radiopaque markers at the axiogingival and axiopulpal junctions were individually invested in plastic casting rings in two groups: (A) 28 mm diameter and 30 mm, 32 mm, 34 mm, and 42 mm long; and (B) 28 mm, 40 mm, 51 mm, and 60 mm diameter and 34 mm long. All specimens were imaged (by xeroradiography) at specific intervals during the setting reaction; dimensions were measured on the images at each interval. In casting rings of different lengths, no significant differences (p greater than 0.05) were found in total effective setting expansion. Because setting expansion in the 28-mm-diameter rings was from 1.3% to 1.5% and diminished proportionately in rings of increasing diameter, setting expansion of investment appears to be the primary determinant of mold expansion. In casting rings of different diameters, significant differences (p less than 0.001) were found in total expansion in an apparent inverse proportion (p less than 0.001). Changes in dimension at intervals were strongly associated with time, and with time combined with diameter (p less than 0.001).