flash fire ovens. This wax is composed of pattern and stick wax, with a small addition to maintain strength and fitness. None of the filler is removed during processing. The high loading of this


thermally conductive filler al- lows for short injection cycles while producing flat sticks with no sink, which is important when using an automatic pat- tern assembly machine. During dewaxing, the wax


expands upon heating, and this expansion is constrained by the shell. The pressure builds rapidly until either the mold cracks or the wax begins draining or diffus- ing into the shell. The two basic theories on the mode of pressure relief during de- waxing include wax drainage through the gating system or vents and wax diffusion into the shell. Ide- ally, the gating system will melt prior to the pattern, providing a path for pressure relief, which can be aided with strategically placed vents and stick materials. The results of a trial to deter-


mine the extent to which stick wax affects pattern cracking showed that a moderate reduction in pattern cracking occurred when the stick was completely removed prior to dewaxing.


Formulating Better Slurries Wax diffusion into an investment


casting shell occurs when the viscos- ity of the wax is low enough to be forced into the shell’s pores. The point at which this occurs depends on the pressure within the shell, shell permeabil- ity and wax viscosity. Increasing shell perme- ability allows the wax to diffuse into the shell at a lower pressure, reducing the severity of mold cracking. The permeability of the shell is a direct result of the particle size distribu- tion of the primary and backup slurries. The standard prime


MODERN CASTING / August 2010


mesh fused silica. The facility compared the


high permeability slurry with its standard prime slurry and found minimal improvement. Pine Tree Castings moved


on to test its standard backup slurry to a high permeability slurry containing organic fi- bers. The tests revealed that a high permeability slurry, paired with a high permeabil- ity backup slurry, was almost 400% more permeable than the existing system (Fig. 7-8). Further tests comparing the


Fig. 8. High permeability shells produce visible wax weep-through in the early stages of dewaxing (left) compared to low permeabilty shells (right).


slurry Pine Tree Castings used was 100% 325 mesh zircon flour. Two or three layers were necessary on some parts in order to properly build fine detail. The permeability of the prime layers determine the sensitivity to mold cracking and casting fillout. Prime slurries can be formulated for high permeability. Low mass slurry and stucco refractories can be used to improve casting fillout. Pine Tree Castings created a high-


permeability slurry by blending fine 325 mesh zircon with course 200 mesh zircon. A low viscosity, high refractory second prime slurry was developed by blending 325 mesh zircon with 200 mesh zircon and 170


standard system with a high permability slurry containing inorganic fibers revealed a 200% improvement in perme- ability. This slurry was used in three additional trials and in each case outperformed the standard slurry.


Transferring Heat Energy In flash fire dewaxing, the heatup


rate of the wax pattern is in part governed by the rate of energy trans- fer through the shell. To promote rapid melting, this energy must be transferred as quickly as possible. Materials with high thermal diffusiv- ity will transfer heat faster than those with lower diffusivity. Replacing zir- con refractories with fused silica or alumina-silicate should improve heat transfer through the shell. Pine Tree Castings ran a trial to de-


termine if the thermal characteristics of the prime slurry layers impacted dewaxing. The results confirmed the belief that thermal diffusivity had a large impact on dewax per- formance. Increasing the thermal diffusivity of the shell reduced the dewax crack sever- ity (Fig. 9).


MC About the Author


Mark Oles is process engineer- ing group leader for Pine Tree Castings Inc., Newport, N.H.


For More Information


Fig. 9. Fused silica shell refractory has high diffusivity and transfers heat faster than zircon, which results in reduced crack severity caused by dewaxing.


“Improving Investment Cast- ing Mold Permeability Using Graphite Particles,” D. Kline, S. Lekakh, V. Richards, 2010 Proceedings of the 114th


(10-087). 31


Metalcasting Congress


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