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Explaining


Shrinkage in Molds and Cores


A series of tests show how each variable in chemically bonded molds affects shrinkage. KATHLEEN LOWE AND RALPH SHOWMAN, ASK CHEMICALS, DUBLIN, OHIO


molds and cores will either expand or contract a small amount following the initial cure, most of this evidence is anecdotal and lacks the support of published data. Earlier work has shown that


W


phenolic urethane coldbox (PUCB) cores shrink around 0.1% during the two days following coremaking.


hile it is com- mon knowledge chemically bonded


To expand on this study, researchers recently used a controlled environment and PUCB binders as the standard to further characterize how diff erent sands, binders and process conditions aff ect dimensional changes of cores and molds. While dimensional changes may


be smaller following the initial cure and during storage and handling may be small, they can be signifi cant for applications with tight dimensional


tolerances. T e data can be used during casting design to account for these small but potentially important changes and may indicate the need for process- specifi c testing for critical applications.


Binder Type Matters T e most signifi cant factor aff ect-


ing shrinkage is the solvent used in the binder, specifi cally the evaporation of the solvent. Vapor pressure of solvents affects


34 | MODERN CASTING October 2011


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