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TESTING 1-2-3


Preventing Cracks in Large Investment Shells


The ability to predict shell crack formation could facilitate the use of plastic foam patterns to produce large investment castings. A MODERN CASTING STAFF REPORT


improve dimensional tolerances and casting surface quality. How- ever, plastic foam promotes crack formation in investment casting shells during pattern removal using standard firing procedures. Researchers at the Missouri Univ.


U 1


sing rigid plastic foam instead of the typical wax material for large investment cast- ing patterns can


of Science and Technology, Rolla, Mo., set out to accurately predict the occurrence of shell cracking during pattern removal by examin- ing the aging strain of the pattern. The team, including Wesley Ever- hart, Simon Lekakh, Von Richards, Jeff Smith, Haifeng Li, K. Chan- drashekhara, Hongfang Zhao and Paul Nam, used ASTM standard tests and independently developed experimental methods combined with finite element modeling to pre- dict stress development in the shell. The model takes into consideration the thermal properties of the pattern and the shell materials to determine the heat transfer to establish a ther- mal gradient within the materials. In their study, the researchers


investigated the ability to combine the known thermal gradient with mechanical properties to determine the thermal expansion stresses in the shell during firing.


Question How does pattern aging affect


a shell’s tendency to produce crack formations?


40 | MODERN CASTING August 2012


Background Te investment casting


process generally is used to produce small, thin-walled castings with high detail.


Traditionally, the process entails using a wax pattern (although sometimes polymeric foam is used) dipped into a ceramic slurry. Stucco is applied to the wet slurry coating to form layers of what eventually becomes the pattern’s ceramic shell. In cases where larger patterns are


required, wax often proves too weak to hold the shape due to the higher weight. Polymeric foams, such as expanded polystyrene (EPS), have been considered for large pattern production, but the buoyancy of EPS causes problems when the pattern is initially dipped into the slurry. EPS foam and wax patterns also show some dimensional change when they


ADDING IT ALL UP Breaking down the industry’s latest research papers is as easy as 1-2-3:


“Foam Pattern Aging and Its Effect on Crack Formation in Investment Casting Ceramic Shells” W.A. Everhart, S.N. Lekakh, V.L. Richards, J.D. Smith, H. Li, K. Chandrashekhara, H. Zhao and P.S. Nam, Missouri Univ. of Science and Technology, Rolla, Mo.


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Background—Plastic foam has been considered as a wax replacement in large investment casting patterns for added strength, but the material is prone to forming cracks in the ceramic shells. Procedure—Researchers tested for shell cracking on polyurethane foam patterns to produce a finite element model to predict crack formation during pattern removal. Results and Conclusions—To effectively prevent shell cracking, the researchers recommend aging above the glass transition temperature (140-213F [60C-100C]) for at least 24 hours.


are cooled after production. Polyurethane foams are stronger, higher density foams that can be made into complicated shapes with high sur- face quality and dimensional accuracy. However, they have high coefficients of thermal expansion and high decom- position temperatures, which can cause the pattern to expand and break the shell when it’s being removed. Te idea of using aging to prevent


shell cracking during pattern removal is based on the change in pattern dimen- sions over time after the shell was built. Te polymeric foam aging mechanism relates to the development of crystal- linity. Preliminary aging of polymeric foam inside the shell before heating may increase the ordered domain and crystallinity of the pattern during fir- ing, leading to volume reduction and possibly reducing the overall thermal expansion of the polyurethane pattern.


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