Fig. 5. Each dot on this standard deviation chart represents the average of 30 methylene blue clay tests.


• Individual screens. • Permeability. When performing the permeability


test, it is important to keep in mind that while sand size is the biggest influencer, other variables can affect permeability, such as moisture and clay levels. Grede-St. Cloud runs a permeability


test every time it runs lab green sand tests—every two or three hours. Te cast- ing facility conducts the other four size


distribution tests, which are more labor- intensive, daily and plots the results on a run chart. Sand casting operations that are performing these tests less frequently than weekly are opening themselves up to missing a problem. It’s also important to build a history of normal ranges to be able to determine when something is not operating regularly. Table 1 shows results from two dif- ferent values used to define sand size,


GFN and surface area. GFN tends to emphasize coarser grains where surface area emphasizes fine grains. So, if a shop is more susceptible to having problems with sand becoming too fine, the surface area numbers should be used. If a shop trends to coarse sand, it should pay closer attention to the GFN results. Vendors deliver new sand to a set


specification, but it can range from the high or low end of that specification.


36 | MODERN CASTING March 2015


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