provides good flowability but can result in broken molds and poor draws. Indicators of low strength are low clay content, dry sand and poor mulling. High green compression strength
means stronger molds but difficulty in shakeout, poor casting dimensions, poor flowability, high ramming resis- tance and higher cost. High clay con- tent is an indicator of high strength. Te dry compression strength test
determines the maximum compression load a dry sand is capable of sustain- ing. It indicates the resistance of the mold to stresses during pouring and casting cooling and the ease of shake- out. Te higher the dry compression strength, the greater the number of hard lumps present at shakeout. An increase in moisture content,
the type and amount of clay, rammed mold density and excessive moisture significantly affect dry compres- sion strength. Low dry compression strength means easy shakeout by loose friable sand, cuts and washes, burn- in, inclusions and erosion. High dry compression strength leads to stronger molds, but difficulty in shakeout, loss of return sand, cracks and hot tears. Permeability (Standard 2 x 2 in.
Test Specimen)—AFS 5224-00-S Permeability is a test of the venting
characteristics of a rammed green sand mold. Important factors in regulating the degree of permeability include sand grain size, shape, distribution and type, binder composite quantity and the density to which the green sand has been rammed, and moisture content. Low permeability will
lead to a smooth cast- ing surface finish, but also could cause blows, pinholes and expansion defects. High permeability reduces gas pressure but can result in mechanical penetration and a rough surface finish. Methylene Blue Clay
Test (Molding Sand)— AFS 2211-00-S Tis test measures the amount of live clay
present in a sample of molding sand. Te test determines the amount of exchangeable ions present in the active clay by adsorption of the methy- lene blue dye. Clay that still has ion exchange capability will contribute to green, dry and hot strength properties of the green sand. Te methylene blue clay value varies depending on the type of clay in the binder composite.
Weekly Tests
AFS or 25 Micron Clay Content In this test, the percentage of clay and other particles that settle at a rate of less than one in. per minute of water is determined, which indicates the amount of fines and water-absorbing material in the green sand system. Any particle that does not settle
through 5 in. of water in five minutes may contain active clay, dead clay, silt seacoal, cellulose, cereal, ash, fines and any other materials that float in water. Only active clay gives bonding capac- ity to the green sand system. Sieve Analysis (Particle Size
Determination of Green Sand)— AFS 1105-00-S Grain Fineness Number (AFS
GFN, Calculation)—AFS 11-6-00-S Te purpose of the sieve anaylsis test is to determine the particle size distribution and estimate the average sieve size of green sand using standard testing sieves. Calculating the grain
fineness number gives an estimate of the average sieve size of a sand sample. Te distribution of the green sand
has a bearing on the physical proper- ties that can be developed by the sand system. Te distribution influences the amount of bond required and the surface finish of the castings. Te screen, or sieve, analysis should
be run on the washed system sand and the dried system sand. A comparison of the dried sand screen analysis and the washed screen analysis shows how much agglomeration is taking place in the green sand system. Loss on Ignition (LOI)—AFS 5100-00-S Loss on ignition measures the
weight change of a sample, consisting of weight losses and weight gains, when a sample is fired at 1,800F (982C). Tis includes weight loss due to volatiza- tion of organics, removal of chemically bound water, dissocation of inorganic compounds (with one or more com- ponents given off as a gas), and weight gain due to oxidation reactions. Loss on ignition determines the total amount of combustible material in the green sand. Te green sand sam- ple is fired at 1,800F until it reaches a constant weight. Te quantity of gas forming materials in the green sand will affect casting results. A high LOI may produce gas
defects such as pinholes, blows and scabbing. In steel castings, a high LOI could lead to carbon pickup on the casting surface. A low LOI can lead to poor casting peel and a rough casting surface finish. Volatile Material at 900F (482C)—AFS 2213- 00-S
Tis method is used
Low permeability, high loss on ignition, and high compactability in your green sand can lead to blows and pinhole shrinkage.
28 | MODERN CASTING December 2014
to determine the amount of material in the system sand or additives that will volatize at a temperature of 900F (482C). Results from this and the LOI test are used hand in hand. Low combustibles (as deter- mined by the LOI test) and volatiles lead to lower cost and less moisture required
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