If it's black, why do they call it green sand?
Learning how to use the tests for green sand control is just as critical to success as understanding the proper use of sand and additives.
Like a doctor examining ill patients, it's difficult for a foundryman to diagnose green sand problems without some history of the subject's healthy state. Thus, the benefits of regular checkups apply the same to green sand as they do to a trip to the doctor's office.
While the first two parts of this series described the relationships of all the key elements in a green sand system - sands, clays, specialty additives and water - testing and process control also must be well understood for the success of every green sand operation.
Many foundries submit samples to suppliers' labs and ask, "What's wrong with our sand?" These labs often respond, "What was the sand like before you encountered the problem and what changed in the system?" Many foundrymen don't know what their sand properties are when things are running well because they're too busy making castings. The system was obviously in control, so why test?
The best time to test sand is when the molds and castings are good. This way, you have a benchmark to determine the desired properties, which helps find the source of any changes.
Tests must relate to the problem your foundry is having, however. For example, the AFS Grain Fineness (GFN), or sand sieve test, shows the distribution of the base sand as supplied to the foundry, but it doesn't reflect the particle size and distribution of the return sand.
If your foundry has a sand lab, regular testing practices should be established and followed. Individual samples - rather than composites - should be tested, since metal enters one mold, not an average of the molds. Foundry personnel should be trained in defect causes to establish appropriate responses to eliminate the defect's source. Use your supplier's lab to support your own analyses.
This article describes the daily and weekly tests that should be conducted, and what the results may indicate. For guidelines in conducting these tests, consult the AFS Mold & Core Test Handbook.
By testing the green sands in your system regularly, your foundry can reduce the day to day variations associated with changes in casting sizes and sand to metal ratios.
To effectively monitor the green sand system and compensate for day to day changes in the sand, several tests must be performed daily - or more frequently - when the data indicates significant changes in the properties.
The daily analyses should be plotted to provide the technician with a method of comparing previous tests with those just completed. Changes are often minor and gradual, and therefore may go undetected. Always compare the current results with several previous tests to ensure the process remains in control and subtle trends are detected.
Compactibility - Changes in compactibility tests indicate the clay/water ratio is incorrect, the muller isn't developing the bond or the system is encountering hot return sand.
Moisture Content - Changes in the moisture content may indicate changing clay levels. For muller systems with automated controls, it can also mean the return sand is too hot, or that water is still entering the muller during the final stages of the mulling cycle and it isn't being distributed.
Green Compressive Strength - Changes in the green compressive strength are generally related to changes in: moisture, mulling efficiency, clay content or sand temperature. It may also show a sand sample that has been contaminated by hydraulic oil or some other undesirable element entering the sand system. Fig. 1 shows a worker conducting the test.
Last month's article on system additives described the effect of baking a green sand sample with a high water content. The high water content allowed uniform clay distribution across the sand surface, and allowed the clay platelets to expand and absorb water deeply into their crystal structure. When the sand sample is baked and moisture is driven out, the sample provides a high dry or baked strength.
Molds made with high moisture-content sands develop the same high hot and dry strength, resulting in a difficult shakeout and lumps of sand carried over the shakeout deck. These lumps must either be crushed and reintroduced into the return sand, or landfilled.
The smaller particles of the mold that pass through the shakeout deck are returned to the muller for recoating with new clay. These agglomerated grains, which are significantly coarser than the as purchased base sand particles, provide the performance properties of the molding sand and affect the casting finish. If the foundry loses control of its water addition and incurs a difficult shakeout, a rough surface finish results. This problem will continue until the sand system is turned over several times and the agglomerated grains are purged from the system.
Methylene Blue Clay Content - The Methylene Blue Clay Content test indicates the amount of available clay that can be developed if provided sufficient water and mulling energy. Because this test analyzes a 5-g sample (assumed to be typical of a 5000 lb muller load), if the sample contains a 0.5-g piece of metal flash or a 0.5 g core butt, the analyses indicate that the clay content is low by 10%.
Conversely, if the sample contains a clay ball, analyses may show excessive clay is present in the system. Always recheck any analyses that vary from normal values, and make all changes in the bond or premix addition levels gradually.
Sand Temperature - Sand temperature dramatically impacts all the sand properties. Hot return sand drives the temper water away from the heat source (the return sand and clay) and inhibits the clay's ability to absorb moisture between the platelets and soften the clay for distribution on the sand's surface.
Frequently, the technician obtains the sand sample and may be unable to test it for several hours. When the sand is finally tested, all its typical properties are normal and the system appears in control.
This phenomenon occurs because the moisture in the sample is sealed in the sample container and, as the sand cools, it's absorbed into the clay. In the foundry, the sand is transported on open belts and the moisture is lost to the atmosphere, providing a drier sand at the molding station. It is vital to determine the temperature of the sand returning to the muller, at the discharge from the muller and at the point of molding. Samples should be obtained at the molding station - not just at the muller.
Table 1. Sieve Analyses Comparison at a Midwest Iron Foundry.
Sieve Washed Unwashed Sand Sand
12 0.02 0.25 20 0.02 0.48 30 0.13 1.81 40 5.47 23.29 50 33.02 44.64 70 29.13 20.57 100 19.06 7.09 140 7.77 1.50 200 1.50 0.14 270 3.89 0.21 PAN TRACE 0.20 TOTAL 100.01 100.8 AFS GFN 60.43 42.61
Permeability - Sand permeability measures its ability for the gases formed during casting to pass between the sand grains and exit the mold, rather than becoming entrained in the casting. Lower permeability values indicate a higher degree of compaction or excessive fines filling the voids between the sand grains. This condition may result in longer pour times, misruns in thin sections, porosity or gas-related defects.
High permeability values indicate excessive voids between the grains due to a resistance to compaction, excessively coarse sand, a lack of fines or clay balls. This may produce weak molds, ran-outs and penetration defects.
Specimen Weight - Weighing the sand in a standard 2 x 2 in. specimen provides insightful data on the sand's composition. Increased specimen weight shows the sand's silica content has increased, while lower weights show that additives have increased or that dead clay and ash have accumulated.
Other tests also must be conducted less frequently, perhaps weekly.
AFS Grain Fineness - It's good practice to obtain green sand samples, dry the sand and perform a sieve analysis on the dried, unwashed sand [ILLUSTRATION FOR FIGURE 2 OMITTED]. After completing the dry sand sieve analysis, conduct the AFS clay wash and sieve analyses on the same sand sample. Comparing the two particle size distributions and their AFS numbers is useful to indicate the level of agglomeration the sand is developing.
An example of washed vs. unwashed sieve analyses from a Midwest iron foundry is illustrated in Table 1. The foundry wouldn't consider using a 42 AFS GFN sand for its critical casting applications, but due to a high moisture content, the muller is distributing the new clay additions over a 42 AFS average particle size aggregate.
A comparison of the sieve analyses reveals that the agglomeration of the particles forms clusters of grains that have a particle size 18 AFS GFN points coarser than the newly purchased sand. Compared with individual new sand grains, the structure of these clusters may inhibit sand compaction and result in more brittle bonding. These clusters may result in metal penetration between the grains and sand inclusions, since the agglomerated grains break apart when contacted by molten metal.
The distributions in Table 1 reveal the weight retained on the 270 sieve is greater than that retained on the 200 sieve. This indicates that: the dust collectors are returning material into the sand system; a fine aggregate (like silica flour) is being added via core processes; or, more likely, that the 200 sieve has a small tear in the sieve or a break in its solder. Looking for the possible root causes of changes in the sand analyses aids the technician in learning how individual properties relate to the sand system and casting quality.
AFS Clay Content - The AFS Clay Content measures the cleanliness of the sand system. This test determines the quantity of fines, dead or burned clay, available clay, soluble materials (cereals, starches, polymers) and specialty additives (fireclay) not detected by other testing procedures. High AFS clay values increase the moisture requirement to satisfy the additional surface areas these fines provide.
Excessive AFS clay levels indicate the presence of fine particles (less than 20 microns) and will reduce the permeability of the mold. Increases in the AFS clay may reveal the dust collector system isn't functioning properly or that the foundry isn't adding a sufficient quantity of new sand to the return sand system.
Low AFS clay level values slightly higher than the totals of Methylene Blue Clay and Loss on Ignition (LOI) indicate the system is too clean and that the system sand is prone to moisture loss. The edges of the molds may become friable, with the grains easily dislodged by the metal flow. This condition is usually related to excessive new sand additions or the production of heavily cored castings.
Loss on Ignition/Volatiles - The LOI test indicates the quantity of organic material (combustibles) present in the sand, and the volatiles test measures the freshness, or volatility, of those carbons.
Much of the organic material being measured is deliberately added to improve casting or molding properties. In gray iron, for instance, seacoal provides a better peel between the sand and the casting, resulting in a better surface finish. In a reducing atmosphere (low oxygen level) the seacoal doesn't burn, but expands between the sand grains and stops the metal from penetrating in the mold. Other organic materials may not be desirable, so an increase in the combustible content may indicate a problem in the foundry (such as hydraulic oil leaks).
Changes in the combustible content should be investigated immediately. While low combustible levels may not provide the peel or casting finish desired, a high combustible level may result in misruns, or gas related defects.
With today's emphasis on control and documentation of the system's properties, many foundries have installed automated testing mechanisms.
These systems provide a computer printout of the batch number, moisture content of the return sand, temperature of the return sand, muller amperage, compactibility, green compressive strength and moisture of the mulled sand.
Interpreting the Results
With modem molding machines running at 400 molds per hour, sand testing must keep pace with the sand use. The ability to generate large quantities of information may give good statistical values, but understanding the relationship of the numbers to mold and casting quality - and then determining corrective actions - will make the difference between a low and high scrap rate.
If your foundry doesn't have automatic testing and documentation equipment, a charting system helps determine desired sand properties, acceptable ranges, property trends and their relationship to casting quality. Charts can be designed to record data of specific interest to your operation [ILLUSTRATION FOR FIGURE 3 OMITTED].
When considering the sources of defects, remember: "Sand is stupid, it isn't smart enough to put the bad sand in the same area on each mold." If the defect has a repeating pattern or if it shows up in the same area on each casting, the problem isn't entirely sand related. Some condition in the gating, pattern design, or molding practice needs to be addressed. The changes in the sand properties may be sufficient to increase these types of defects, but the real correction should be to fix the source and to control the sand.
Green sand molding is an old and established process, but changes - in mulling, molding and automated testing equipment, as well as different additives - make it one of the "newest." How the green sand system works is up to us - we need to control the raw materials and plant processes, and test regularly to minimize variations to obtain the best performance.
* Moisture Content;
* Green Compressive Strength;
* Methylene Blue Clay;
* Sand Temperature;
* 2 x 2 in. Specimen Weight.
* AFS Grain Fineness;
* AFS Clay Content;
* Loss on Ignition/Volatiles.
These tests help ensure that your green sand system is within control
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|Title Annotation:||Testing and Process Control, part|
|Author:||Hoyt, Daryl F.|
|Date:||Feb 1, 1996|
|Previous Article:||Diecasting '96: a status report.|
|Next Article:||Escaping the heat of casting defects through sand cooling.|