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Sand Plays a Part in Shrinkage Te sand used to make a mold


or core will have an impact on its shrinkage. Initially, it was thought these effects were related to core density differences caused by the sand. However, little correlation exists between density and shrink- age. Testing showed sand size, shape and distribution do affect shrinkage, while core sand density does not affect shrinkage. When chemically bonded molds


Within 24 hours, sand cores will shrink, depending on the sand or binder used in the process.


high, rapid shrinkage. Te curing mechanism of furan nobake binders involves a chemical reaction known as condensation, and more specifically dehydration, during which water is formed as a reaction byproduct. Te cured binder has greater density than the uncured binder. Reduced shrink- age of PUCB with extended benchlife also can be explained by the solvents evaporating from the mixed sand prior to the core being blown. Te standard PUCB binder tested used a moderate to low volatility solvent, which may have slowed the shrinkage under the different condi- tions. Te 24-hour result may not reflect the maximum shrinkage that would occur over extended times. Te heat-cured inorganic binder


showed rapid out-of-box shrink- age, but this was likely caused by the thermal contraction of the sand rather than changes in the binder. Other systems that saw elevated temperatures as part of the process also were stable after heating. For organic binders, this was likely due to the rapid loss of solvents at higher temperatures. Unexpectedly, shrinkage


decreased with higher PUCB binder levels. The extended time study showed the higher binder level eventually reached the same shrink-


36 | MODERN CASTING October 2011


Fig. 3. The graph shows the shrinkage produced with sands of different grain fineness numbers (GFN). The sand with the highest GFN had the most shrinkage.


age level. With more time, it may have surpassed the shrinkage of the cores with the lower binder levels. A possible explanation of the slower shrinkage rate may relate to resin bridges. With more binder, resin bridges would be larger in diameter and have a lower surface area-to- volume ratio. This lower ratio may delay the loss of solvents and the resulting shrinkage rate.


and cores are produced, the sand is first mixed with the binder to coat the surfaces of the sand grains. Ide- ally, all the sand grains have a thin, uniform layer of coating. When the coated sand is blown into a corebox or compacted in a flask, the sand grains come into close proximity and the surface films of the binder contact each other to create a resin bridge. Te resin bridges separate the sand grains with a layer of liquid. When the resin cures, the sand is bonded together. If the resin layer shrinks or contracts, the sand grain will be pulled closer together and the entire mold or core will shrink (Fig. 2). Chemically bonded molds and


cores are 95-99% sand. During the next test to further examine the core shrinkage phenomenon, all test cores were made using standardized condi- tions and methods, except for the


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