What do the numbers mean? Find out which sand tests you should be doing on a regular basis and what aspects of your sand system the tests are measuring that will help you produce good castings.
Metalcasters have two primary reasons for green sand testing:
* To check the consistency of the prepared green sand.
* To determine if the green sand has the physical and chemical properties to produce good castings.
Poor quality sand can lead to a number of casting-related defects. To ensure the properties necessary to avoid casting defects and produce the quality you desire consistently throughout the entire green sand system, metalcasting facilities need to test their green sand daily for:
* Specimen weight.
* Green compression strength.
* 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.
This 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).
Compactability of Molding Sand Mixtures (Rammer Method)--AFS test 2220-00-S
The purpose of this test is to determine 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 compaction?). Compactability is probably the most common green sand test and is usually performed by a 3-Ram compactability tester. Also there are inline automated testing systems such as the "Hartley" sand tester used by many metalcasters. The 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 compactability 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 Hot Air Method)--AFS 2218-00S
This 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. The 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.
The 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 friable green sand that is difficult to mold.
The 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.
The 2 x 2-in. specimen weight called for by the test should be recorded at or near a predetermined compactability. 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 material. This build-up can lead to burn-in, burn-on and penetration defects. A trend of lower specimen weight indicates not enough new sand is entering the mix to dilute the oolitic material.
Compression Strength (Green and Dried)--AFS 5202-00S
The green and dried test determines the compression strength of an AFS 2 x 2-in. specimen. Green compression 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.
The degree of mulling, sand to metal ratio, clay content, compactability range and additive type affect green compression strength.
Low green compression strength 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 resistance and higher cost. High clay content is an indicator of high strength.
The dry compression strength test determines the maximum compression load a dry sand is capable of sustaining. It indicates the resistance of the mold to stresses during pouring and casting cooling and the ease of shakeout. The 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 compression 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 casting 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)--AFS2211-00-S
This test measures the amount of live clay present in a sample of molding sand. The test determines the amount of exchangeable ions present in the active clay by adsorption of the methylene blue dye. Clay that still has ion exchange capability will contribute to green, dry and hot strength properties of the green sand. The methylene blue clay value varies depending on the type of clay in the binder composite.
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 capacity 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
The 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.
The distribution of the green sand has a bearing on the physical properties that can be developed by the sand system. The distribution influences the amount of bond required and the surface finish of the castings.
The 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).This includes weight loss due to volatization of organics, removal of chemically bound water, dissocation of inorganic compounds (with one or more components 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. The green sand sample is fired at 1,800F until it reaches a constant weight. The 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 221300-S
This method is used 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 determined by the LOI test) and volatiles lead to lower cost and less moisture required but also can result in poor casting peel, poor finish and poor shakeout.
High combustibles and volatiles cause lower expansion but can result in pinholes, smoke, blows, brittle sand, higher cost and higher moisture requirements.
Available bond indicates the moisture-absorbing materials in the sand system, including live, latent and dead clay and additives. The value is derived by relating green compression to moisture using a prepared graph, slide rule or the calculation (0.105 x GCS) + (1.316 x MOIST).
The live clay actively bonds the sand, and the latent clay can be activated with further energy input. The dead clay does not add to green tensile or green splitting strength, but does absorb moisture.
The working bond percent indicates the amount of clay that actually is producing bond strength in the sand mix. Working bond (or effective 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 efficient use of the clay. Large variation indicates variation in the clay additions or in the effectiveness of the mulling.
Mulling Efficiency The higher percent mulling efficiency, the greater the clay utilization and the lower the clay content required. The working bond value divided by the available bond value (and multiplied by 100) gives a percent mulling efficiency reading.
Segregation in transport, loss or buildup of fines due to a lack of dust collection, bond quality, temperature and muller or mixer condition all may affect mulling efficiency.
This article is based on a presentation originally given at the AFS 2014 Sand Casting Conference.
SCOTT HONEYMAN, CARPENTER BROTHERS, INC., MINNEAPOLIS
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|Date:||Dec 1, 2014|
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