Calculating your grain fineness number.
The size of the sand grains in your molding sand mixture is a critical element in the quality of the castings you produce. The optimal grain fineness number (GFN) in a sand system is determined by the type of metal poured, pouring temperatures, casting product mix (heavy vs. light castings) and required surface finish. Once that optimal fineness level is determined, maintaining a consistent grain structure becomes a critical factor in the quality of the final castings. Sand that is too fine (higher GFN) can create low permeability and result in gas defects. Sand that is too coarse (lower GFN) can create high permeability and lead to problems with metal penetration, rough surface finish, burn-in and burn-on.
The AFS Grain Fineness Number (AFS-GFN) is one means of measuring the grain fineness of a sand system. GFN is a measure of the average size of the particles (or grains) in a sand sample. AFS-GFN gives the metalcasting facility a means to verify its molding sand is staying within specification for the castings being produced and avoid conditions that could lead to potential casting problems.
Technical Information and Practice
The grain fineness of molding sand is measured using a test called sieve analysis, which is performed as follows:
1. A representative sample of the sand is dried and weighed, then passed through a series of progressively finer sieves (screens) while they are agitated and tapped for a 15-minute test cycle.
2. The sand retained on each sieve (grains that are too large to pass through) is then weighed and recorded.
3. The weight retained on each sieve is divided by the total sample weight to arrive at the percent retained on each screen.
4. The percentage of sand retained is then multiplied by a factor, or multiplier, for each particular screen (Table 1). The factors reflect the fact that the sand retained on a particular sieve (e.g. 50 mesh) is not all 50 mesh in size, but rather smaller than 40 mesh (i.e. it passed through a 40 mesh screen) and larger than 50 mesh (it won't pass through 50 mesh screen). The result should be rounded to one decimal place.
5. The individual screen values then are added together to get the AFS-GFN of the sand, representing an average grain fineness (Table 1).
This number is the weighted mathematical average of the particle size for that sand sample. Many metalcasting facilities have developed computer spread sheets to perform these calculations, limiting the potential for human error.
By itself, GFN does not identify if the sand will be a good molding material or produce the qualities needed in a particular metalcasting sand system. Because GFN represents an average fineness, sands with very different grain size distribution may have similar GFN numbers. So, the distribution of sand grains on the screens is another critical factor in effective sand molding. The distribution refers to the quantity of sand retained on each individual sieve, rather than the average of all sand retained on all sieves.
Metalcasting facilities typically want to avoid an excessive amount of either coarse (6-30 mesh) or fine (270-PAN) materials and should favor a bell-curve type distribution. Table 2 shows a comparison of two sands that have the same AFS-GFN but different distributions. Sand A is a 5-sieve sand because 10% or more of the sample was retained on each of five adjacent sieves (40, 50, 70, 100 and 140). Sand B is a 3-sieve sand because 10% or more was retained on each of only three adjacent sieves (50, 70 and 100). Many metalcasting facilities would prefer the 5-sieve sand because the wider distribution generally helps to minimize expansion defects. The standard for AFS-GFN and sand distribution is usually determined by the individual metalcasting facility for its particular sand system and depends on factors like casting quality requirements, molding process used and alloys poured.
Note: U.S screens are manufactured using inches as the measurement for the screen openings (openings per linear inch), as designated in ASTM E-11. Some screen manufacturers in Europe and Asia may have metric screen size openings. AFS measurements using metric screens will not compare directly to U.S.-based screen measurements.
Table 1. Mathematical Factors for Calculation of AFS-GFN (sample size of 78.4 g) ASTM E-11 Weight Retained Percent Sieve Size on Sieve (g) Retained Multiplier Product * 6 Mesh 0 0 0.03 0 12 Mesh 0 0 0.05 0 20 Mesh 0 0 0.1 0 30 Mesh 0.7 0.9 0.2 0.18 40 Mesh 3.9 4.9 0.3 1.47 50 Mesh 19.4 24.7 0.4 9.88 70 Mesh 37.3 47.6 0.5 23.8 100 Mesh 16.3 20.8 0.7 14.56 140 Mesh 0.8 1 1 1 200 Mesh 0 0 1.4 0 270 Mesh 0 0 2 0 Pan 0 0 3 0 TOTAL 78.4 100 -- 50.89 ** * Product is percent retained times multiplier ** AFS GFN=50.9 (sum of all products rounded to one decimal) Table 2. Distribution of Two Sand Samples Sieve Sand A Sand B Percent Retained Percent Retained 20 0 0 30 1 0 40 24 1 50 22 24 70 16 41 100 17 24 140 14 7 200 4 2 270 1 0 Pan 0 1 AFS-GFN 57.X 57.X
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|Title Annotation:||CAST TIP--Technical Information and Practice|
|Date:||Dec 1, 2009|
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