The Role of Capillarity in Shell Mold DewaxingJudy Mackenzie - Purdue Doctoral Candidate
Ceramic shell mold cracking and the associated defects have beleaguered the investment casting industry for many years. Good manufacturing practices can prevent many instances of shell cracking, but the cracking of the fine inside layer of the ceramic mold during dewaxing is a particular problem due to the 18% thermal expansion that occurs in most parrafin waxes as they melt.
This rapid expansion occurs right at the surface of the wax model and develops tensile stress in the adjacent ceramic shell (which is relatively weak in the grean state) and it easily develops cracks.
To date this has been primarily a study of how various theories in capillary motion and heat transfer occuring in this situation might help lead to a solution.
If "no wetting" of the ceramic were to occur, there would most certainly be cracking, but it is found that in fact parrafin easily wets the ceramics of the shell, so there is hope that a certain amount of "infiltration" during the melting process could accomodate for the expansion of the wax itself.
The heat transfer of the situation (particulary for steam autoclave dewaxing), is that initial heating of the ceramic occurs by convection with some condensation being a complicating factor. Heat travels inward to the wax by a complex conduction, and once the wax is reached, the rate of heat transfer becomes dramatically slower. Most damage to a shell occurs within the first second or so of wax surface melting.
Among the models investigated is an equation by Washburn, combining LaPlace and Poiseuilles equations to estimate the rate of wax transfer into the inner surface of the shell. Unfortunately, this model predicts an "infinitely fast" transfer right at the boundry, (as capillary depth becomes less than capillary diameter) which is quite contrary to actual observation.
Some further modeling and reinvestigation of the material properties will be required to reconcile the observed behavior, and to make progress in mitigating the effect.