• Dry compression strength. • Methylene blue clay content. In addition, the following tests should be done on a weekly basis: • AFS or 25 micron clay content. • Screen analysis. • Total combustibles (LOI). • Volatiles at 900F (482C). • Available bond. • Working bond. • Muller efficiency. Tis article will review the basic
green sand tests, what the tests results tell you about your green sand and why they are necessary to prevent common casting surface defects. (Tests are referenced by the test number and protocol as defined in the AFS Mold and Core Test Handbook).
Everyday Tests Compactability of Molding Sand
Mixtures (Rammer Method)—AFS test 2220-00-S Te purpose of this test is to deter- mine the percentage decrease in the height of a loose mass of sand under the influence of compaction (i.e., how resistant is it to squeeze and compac- tion?). Compactability is probably the most common green sand test and is usually performed by a 3-Ram com- pactability tester. Also there are inline automated testing systems such as the “Hartley” sand tester used by many metalcasters. Te compactability test tells you how wet or dry the green sand is and helps control the most common green sand defects. It is directly related to the performance of the green sand in the molding operation and reflects the degree of temper of the green sand. It indicates how a fixed volume of green sand will react to a fixed energy input (such as mulling or molding). Metalcasters want to select a com- pactability level high enough to avoid cuts and washes, friable broken edges, hard-to-lift pockets, cope downs, crushes, penetration, burn on and erosion scabbing. Yet, compactability must be low enough to avoid oversized castings (due to mold wall movement), shrinks, blows, pin holes, super voids, poor finish, expansion defects, gas and rough surfaces, shakeout problems and high ramming resistance. Moisture Determination (Forced
Regular sand testing ensures your green sand system is producing quality molds consistently.
Hot Air Method)—AFS 2218-00S Tis test is used to determine the
percentage of moisture in the molding sand. Moisture in molding sand affects the plasticity of the clay bond, which controls most sand-related defects. Moisture content of a green sand molding system is not an arbitrary number. It must be maintained within a narrow range. Te moisture content is affected by the hydration of the binder composite, coating of the sand particles and muller efficiency with regard to working and available bond. Moisture affects all other green sand properties and is the most abused ingredient in green sand. Te two major factors that affect
moisture requirement are the type and amount of clay and the type and amount of additives in the green sand mix. Excess moisture will produce an
oxidizing atmosphere in the mold, promote excess gas evolution and lower permeability, cause high dry and hot strength, reduce mold hardness and result in poor flowability of the sand. Insufficient water produces dry fri-
able green sand that is difficult to mold. Te specimen weight in the Forced Hot Air Method is an indicator of the consistency of the green sand and the
presence of oolitic materials, referred to as dead clay or ash, which is diluted by new sand additions. It also indicates changes in sand distribution. Te 2 x 2-in. specimen weight called
for by the test should be recorded at or near a predetermined compactabil- ity. A variation in the specimen weight indicates green sand density changes are taking place in the system. When the specimen weight drops,
it indicates a build-up of oolitic mate- rial. Tis build-up can lead to burn-in, burn-on and penetration defects. A trend of lower specimen weight indi- cates not enough new sand is entering the mix to dilute the oolitic material. Compression Strength (Green
and Dried)—AFS 5202-00S Te green and dried test determines
the compression strength of an AFS 2 x 2-in. specimen. Green compres- sion strength indicates the maximum compression stress the sand mixture is capable of sustaining and is used to control the rate of clay addition to the green sand system. Te degree of mulling, sand to
metal ratio, clay content, compactabil- ity range and additive type affect green compression strength. Low green compression strength
December 2014 MODERN CASTING | 27
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