Perhaps the most profound and over looked variable that a foundry has is inconsistent slurry mixing. A surprising number of foundries are not as consistent in their slurry preparation as they should be. Most foundries consider the introduction of “consistent” slurry to their process a constant and that variation from batch to batch is not significant. However, in this author’s experience, entrapped air, degree of particulate wetting and slurry temperature do vary significantly from batch to batch or shift to shift- even in the best of foundries. As a result, it is not unusual for slurry operators to make adjustments to their dip tank after a recent batch addition based upon the assumption that their addition was stable and fully creamed-in. This can lead to unacceptable swings in the solids ratio and ultimately shell performance as the operator unknowingly chases a moving target. This is particularly important in robotic shelling applications.

There are a number of mixing techniques available to the industry, each with a different mixing efficiency: mixing in a rotating tank, side mounted prop mixing, side mounted prop mixing in a rotating tank, high shear mixing, and vacuum high shear mixing. Minco has encouraged foundries to use high shear mixing because of its ability to provide fully wet in slurries quickly with less labor and time. However, this paper will describe the benefits of vacuum high shear mixing which has created a new definition of “consistent” as applies to investment casting slurries.

Through use of a 2.5 gal vacuum high shear mixers (and verified on a 50 gal mixer) this report documents how vacuum high shear mixing can be used to provide ready to use, fully wetted-in slurries, immediately after mixing. This contrasts markedly with conventional rotating tank or propeller mixing which requires days to stabilize (or weeks if surfactants are not used).

Shell properties are markedly similar once slurry viscosity and formulas are adjusted to the vacuum mixing process. Such a formula adjustment can yield a 6-9% savings in colloidal/latex. Lack of formula change with vacuum mixing however is demonstrated to result in thinner slurries and shell properties to match. Floor space can be opened up by elimination of slurry “cream-in” tanks which are no longer needed with vacuum high shear mixing. No change in refractory particle size was noted with increased mixing times or between mixing techniques.