Fig. 5. A comparison of stress development in 0.25-in. thick shells shows significantly higher stress occurred in the un-aged patterns.
performed above the glass transi- tion temperature for at least 24 hours will effectively prevent shell cracking when using foam patterns. According to the authors, longer aging times increased shrinkage, especially for times less than 24 hours. After approximately 20 hours, the shrinkage for a pattern aged at 212F (100C) subsided as it neared the completion of its transformation from an amorphous structure to a crystalline structure (Fig. 3). When the aging temperature was
above the glass transition temperature, the amount of shrinkage increased from 0 to 0.5%, which supported the idea that the aging mechanism is the foam’s transformation from amor- phous to crystalline. Te researchers determined the activation energy for aging from the shrinkage. Te study showed that during pat-
tern removal, the strain in the pattern consisted of aging, thermal and elastic strain. Te strain in the shell consisted of thermal and elastic strain. Foam aging induced a negative strain or pattern shrinkage. Tis negative strain reduced the overall pattern expansion in the shell, thus lowering the stress developed in the shell during pattern removal (Fig. 4-5). Te researchers applied this con- cept to a shell cracking model to show aging’s effect on the stress developed in the shell during pattern removal. Specific cases were experimentally verified in the lab shell cracking test (Fig. 6). Experimental results were consistent with the predicted crack formation from the model (Table 1). Aging prevented cracking of the 0.15- in. (3.8-mm) thick shells and lowered stress development in the 0.25-in. (6.4-mm) shells.
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August 2012 MODERN CASTING | 43
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