Previous experiments have shown that values for the linear setting expansion of investments do not correlated with the resultant effective setting expansion and subsequent distortion of wax patterns. As a better understanding of the fluid mechanics of setting expansion will lead to scientific methods of lessening distortion of castings, the quality of dental care will be improved. This project and future research will lead, hopefully, to unification of discrepant setting expansion values previously determined by other methods. The long term goals are to develop mathematical models for setting expansion of investments and for dental die stones. A method has been created to observe distortion of wax patterns while the investment is setting. Xeroradiographic images may be made and the amount of bending of MOD wax patterns measured (as MOD wax patterns are the most sensitive to distortion compared to other geometric forms). The apparatus consists of a conventional oral radiation generator of 90 KVP, a cone to which is affixed a stage on which rests a Xeroradiographic photoreceptor in an adjustable position, a grid that ensures reproducible placement of plastic casting rings, and a dental Xeroradiographic unit that produces images similar to black and white photographs. The cone, stage, photoreceptor, and plastic rings are enclosed in a lead-lined box that will absorb all scattered radiation. Exposures are made of the wax patterns immediately after investing in a gypsum-bonded cristobalite investment and at five minute intervals for one hour. The distances between the axial proximal line angles are measured with a traveling microscope and plotted graphically versus time. Early runs have indicated that an initial expansion of the wax pattern may be followed by a contraction, which is contrary to current views of the fluid mechanics of setting expansion. It is expected that by varying ring diameters and number and position of wax patterns, that specific quantitative information relating mass and flow characteristics of gypsum-bonded investments can be derived. The same method may be used for studying phosphate-bonded investments, and although not yet tested, may be of use in examining die stone expansion.
| Davis, D R; Watters, J L; Nguyen, J H et al. (1993) Effect of denture wax-pattern position on radial setting expansion. Clin Mater 12:83-7 |
| Davis, D R; Nguyen, J H; Grey, B L (1992) Ring volume/ring liner ratio and effective setting expansion. Int J Prosthodont 5:403-8 |
| Davis, D R; Kawashima, S S; Nguyen, J H (1990) Effect of ring length and diameter on effective radial setting expansion. Dent Mater 6:56-9 |
| Davis, D R (1989) Effect of investment mass on distortion of MOD wax patterns. Dent Mater 5:93-6 |
