Putting green sand to the test.
Swedish diplomat Dag Hammarskjold once said, "Never look down to test the ground before taking your next step: only he who keeps his eye fixed on the far horizon will find his right road." But according to two metal-casting specialists, if you don't bother to test your sands, you'll end up surrounded by defective castings.
Two areas to explore for improving sand properties and casting quality are on-line mold testing and sand system control, both of which were addressed at the AFS International Green Sand Molding Conference in San Antonio, Texas, March 5-6.
On-Line Mold Testing
Matt Granlund, Foundry Systems Control, Winfield, Illinois, is a firm believer in on-line mold testing of green sand. While many foundries depend completely on lab testing, he said it is adequate only for monitoring the uniformity of the prepared sand.
Under laboratory testing conditions, a sand can have acceptable three-ram or squeeze compaction properties. But if poorly compacted due to process variables, it may produce molds on the production line that will produce casting scrap. According to Granlund, this is why you can't afford to rely on lab testing alone.
"It is important to complement the lab sand tests with on-line mold testing results," he said. "We don't pour metal against squeezed or three-ram laboratory specimens, but we do pour metal into molds. Mold quality determines our success."
Granlund noted that several handheld tools are available for measuring compaction of green sand molds:
Mold Hardness Tester--Best used on softer molds under 90 mold hardness, this instrument easily measures the hardness to which the sand is compacted. On "B" model testers, a large radius ball penetrator presses into the mold surface under a spring load of 980 grams, providing uniform readings of 0-100, with a "0" reading offering no resistance and a "100" mold allowing no penetrator movement.
For denser molds, the "C" model tester gives more meaningful readings. The penetrator foot is smaller and the spring load is 1500 grams at full scale. Readings are graduated from 65-100, spreading out the high readings for greater sensitivity. However, these testers can be used only on flat surfaces for the tester to be positioned correctly.
Mold Strength Tester--This unit measures mold strength, which is the product of the bond strength and ramming energy used. The curve of mold hardness versus compaction flattens out above 92 hardness. In this range, mold strength is more sensitive. Mold cuts, washes and swells largely are controlled by mold face strength.
Using a thin round penetrator foot, psi mold strength data can be taken anywhere into the mold surface under stiff spring load. This unit is better suited for testing high density molds. More durable and accurate electronic, digital readout versions are available.
Mold Permmeter--This new electric instrument can be taken to the molding floor and an attachment allows permeability measurement without damaging the mold cavity. Mold permeability can be reproduced in the squeezed or three-ram lab specimen by varying the ramming energy in the laboratory. This unit is portable, but requires an electric cord and flat surface over 1-1/8 in.
Mold Quality Indicator--The mold quality indicator (MQI) measures the density of a compacted surface by its resistance to air flow under pressure. Using a padded contact head instead of a penetrator foot, this battery-operated, digital tester doesn't damage the mold cavity.
Rather than providing a measure of the bond strength, this indicator relates more directly to mold density. MQI is more sensitive to mold permeability, thus improving casting finish quality. The MQI is applicable to both green sand and nobake molds and cores.
"Producing consistent sand properties and making sure sand is compacted well is the first step in producing quality castings," Granlund said. "By monitoring mold compaction with on-line mold test tools, poor mold properties can be detected before substandard or scrap castings are produced."
Sand System Control
Speaking about sand tests and casting quality, Alan Volkmar, Brillion Iron Works, Brillion, Wisconsin, said his firm established a critical group of sand testing areas for green sand systems to help maintain consistent sand properties. "With the correct properties, we make quality castings," he said. "We achieve continuous improvement by improving the consistency of the sand."
Volkmar considers basic sand tests a requirement for sand system control. The most important area for testing is compactibility. Compactibility tests assign a percentage number to the degree of wetness in the sand, which is practically independent of sand composition. As sand gets wetter, it compresses further under compaction forces. The temper point of molding sand is 45% compactibility. For high-density, high-production molding machines, Volkmar suggests using sands on the dry side of temper, such as 36-42%.
He said if sand is too dry it can lead to molding problems such as stickers, tear-ups and castings with poor finish or erosion defects. If too wet, penetration and/or gas defects can occur.
The other areas for basic testing are green compression strength and moisture. According to Volkmar, neither test has any direct effect on the molds or castings except when at extreme levels. These two tests, however, do provide valuable numbers that relate to bond values, including working bond, available bond and mixing effectiveness.
Working bond is the relationship between green compression strength and compactibility. This term indicates the amount of bond that actively holds the sand together. For a given system, a minimum working bond level is necessary to make good molds and castings. If it is too high, casting defects such as penetration, blows and poor finish occur. Volkmar said he uses the working bond test to control the clay additions of sand.
Available bond is the relationship between green compression and moisture that indicates the amount of active, latent and inactive bond in the system. Brillion conducts this test to monitor overall system bond levels. In addition, while many foundries use the methylene blue test to control clay levels, available bond tests are useful because they include all bonding influences.
Mixing effectiveness is the working bond divided by the available bond. While some foundrymen believe greater mixing effectiveness leads to better sand system control or efficiency, Volkmar finds this isn't necessarily true. "We find the optimum range is 55-65%," he said. "Over 65%, the ability to control compactibility becomes difficult because 0.01% of moisture can cause a 3-4% change in compactibility."
Advanced Level Tests
Second- and third-level sand tests become complementary additions as you progress in your steps of continuous improvement. Volkmar said his firm runs some second-level tests, including wet tensile strength ones. "Experience has convinced us that maintaining a certain minimum value for each sand system prevents expansion-type sand defects," he said.
Other areas examined with second-level tests include splitting strength, muller sand temperatures and sample weight tests. Splitting strength relates directly to green tensile strength and thus has a direct influence on the sands' ability to be drawn from pockets in the pattern. This test is necessary only when experiencing problems with pockets.
When muller sand charge temperature fluctuations exist, it usually means there's a failure in the sand cooling equipment and indicates a need to adjust the muller water addition.
Third-level tests help monitor the overall condition of the sand system and include weekly tests, such as AFS clay, washed/unwashed screen distribution, LOI and volatiles. Other third-level tests, such as friability, cone jolt toughness, sand micrometer and dilatometer, provide foundries with additional insight into their sand systems, Volkmar said.
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|Title Annotation:||part 1; on-line mold testing for foundry sand|
|Author:||Lessiter, Michael J.|
|Date:||Jun 1, 1992|
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