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but also can result in poor casting peel, poor fi nish and poor shakeout. High combus- tibles and vola- tiles cause lower expansion but can result in pinholes, smoke, blows, brittle sand, higher cost and higher moisture requirements. Available Bond Available bond


indicates the moisture- absorbing materials in the sand system, including live, latent and dead clay and additives. T e value is derived by relating green compression to moisture using a prepared graph, slide rule or the calcula- tion (0.105 x GCS) + (1.316 x MOIST). T e live clay actively bonds the


sand, and the latent clay can be acti- vated with further energy input. T e dead clay does not add to green tensile


Expansion scabs can be caused by high compactability, low perme- ability or high loss on ignition.


or green splitting strength, but does absorb moisture. Working Bond T e working bond percent indi-


cates the amount of clay that actually is producing bond strength in the sand mix. Working bond (or eff ec- tive clay) is derived by relating green compression to compactability using


a prepared graph, slide rule or the calculation (15.29 x GCS) / (132.1 - COMP). Higher working bond


indicates more effi cient use of the clay. Large variation indicates variation in the clay


additions or in the eff ectiveness of the mulling. Mulling Effi ciency


T e higher percent mulling


effi ciency, the greater the clay utiliza- tion and the lower the clay content required. T e working bond value divided by the available bond value (and multiplied by 100) gives a percent mulling effi ciency reading. Segregation in transport, loss or buildup of fi nes due to a lack of dust collection, bond quality, temperature and muller or mixer condition all may aff ect mulling effi ciency.


T is article is based on a presentation originally given at the AFS 2014 Sand Casting Conference.


December 2014 MODERN CASTING | 29


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