Reducing casting defects: a basic green sand control program.
This article introduces the basic tests used for controlling a green sand system, how they are run and how they relate to sand control. There are four basic variables that the foundrymen must monitor--aside from system engineering controls--to effectively control green sand. These include the addition of water, bond, new sand and carbonaceous material.
Shown in Fig. 1, the basic sand tests commonly used to control green sand systems include moisture, compactibility, density and specimen weight, permeability, green strength, AFS/25 micron clay, AFS grain fineness, methylene blue clay and loss on ignition (LOI).
The daily/green properties tests should be run hourly, or as often as practical on a daily basis. The methylene blue test is listed with the weekly/structural properties tests because it is a structural property of the sand, but it should be run as often as needed to keep the bond addition in line. The rest of the weekly/structural properties tests can be run weekly, unless the variations in the process indicate the need for a greater testing frequency.
Adjustments made to the sand system based on test data must be made with cycle time of the sand considered. If cycle time is not considered, more variation can be introduced by over-correcting.
The basic tests relate to control of the four basic variables. Moisture and compactibility are the key tests for controlling water additions. Green strength, methylene blue and AFS or 25 micron clay provide important information relating to control of the bond. Density, specimen weight, permeability and grain fineness relate to new sand additions. LOI is used for controlling the carbonaceous material. Sand temperature tests should also be included because hot sand often contributes to sand-related problems.
Optional tests that are useful and also recommended include:
* green deformation;
* dry compressive strength;
* splitting strength;
* cone jolt toughness;
* wet tensile strength;
Green Sand Sampling
Sampling is an important consideration before testing. Molding sand samples for routine testing should ideally be taken at the molding machine. When evaluating the effectiveness of the sand system control, samples can be taken at various points, such as at the muller, molding machine, shakeout, and before and after cooling.
When a molding sand sample is taken for testing, it should not be carried in an open container. Sealed containers with lids are recommended. The molding sand must not be packed into the holding containers, but should be of the same loose consistency as found at the point of sampling.
Riddling the molding sand through a coarse screen into the container before testing helps to improve repeatability of results. In this way, large core butts, tramp metal and other foreign matter are removed. Again, the container must have a lid, with the sample covered at all times.
If sand temperature tests are taken, a suitable thermometer should be inserted in the container at the time of sampling. The temperature can be read as soon as the thermometer stabilizes. The daily green properties tests should then be performed immediately after the sand temperature reading. The weekly structural properties tests, which use a dried sand sample, can be conducted after the green properties tests.
The moisture test should be run first. This test is simply a quantitative measure of the amount of water in the sand. This test is run by placing a sample of the molding sand in a pan and drying it with a forced air dryer at 220-230F. Within this temperature range, constant weight can be reached in about five minutes, without loss of volatile organic material such as seacoal. The weight loss upon drying is used to calculate the percent moisture.
The next test that should be run is compactibility--a measure of bulk density that can be used to control the temper of the sand. The higher the compactibility, the wetter the sand. The lower the compactibility, the dryer the sand.
It is very important to understand the relationship between moisture and compactibility. Moisture must be adjusted to control compactibility. Since the amount of moisture-absorbing materials (such as clay and carbonaceous) in the sand vary, the moisture must be varied to produce the target compactibility. The higher the clay and carbons content of the sand, the higher the amount of moisture required to produce a given compactibility. Thus, moisture cannot be used as a control because the moisture requirement varies.
Compactibility is the key control variable. It must be controlled to ensure consistent compaction at the molding station. This can be accomplished on a real-time basis in production with sand system controls such as compactibility controllers that sample sand from the mixer and automatically adjust the water controls to achieve the required compactibility level.
In the laboratory compactibility test, a specimen tube and cup pedestal are placed underneath a funnel stand. Sand is riddled through a coarse sieve on top of the funnel, until the specimen tube is overflowing. The excess sand is then struck from the top of the tube using a strike off blade.
The filled specimen tube is then positioned on a standard sand rammer and the sample is rammed three times. The compactibility reading, or the volumetric percentage decrease in the height of the sand, is then taken by sighting the top of the plunger rod on the compactibility scale.
If the sand is dry, the bond can't hold the sand grains together and the grains tend to erode away from the mold surface. The sand's compactibility must be high enough to avoid dry sand molding and casting problems such as cuts and washes, friable broken edges, hard to lift pockets, copedowns, crushes, penetration, burn-on and erosion scabbing.
If the sand is wet, it resists compaction, tends to deform, develops excessive hot strength and contains free water that can cause gas defects. The sand's compactibility must be low enough to prevent wet sand problems such as over-sized castings, shrinks, blows, pinholes, supervoids on vertical faces, poor finish/rough surfaces, expansion defects, gas, difficult shakeout and high ramming resistance.
Improper control of compactibility is the leading cause of green sand casting defects, since water affects every sand property except the fineness of the base aggregate. Figure 3 lists sand problems resulting from low and high compactibility.
Density and Specimen Weight
Before proceeding with the other tests that require a standard test specimen, the specimen weight must be determined. Compacted density can be determined simultaneously.
The specimen weight is the weight required to produce an AFS standard specimen. The standard AFS specimen is a cylindrical specimen that is 2 in. in diameter and 2 in. high, after three rams with the standard sand rammer. An indicator, which mounts to the top of the sand rammer, can be used for this determination.
When using a density indicator, a 165-gram sample of sand is weighed and rammed with three rams from the standard sand rammer. A lever arm is then flipped into position and the compacted density of the sand and the specimen weight is read from the scale.
The specimen weight varies as the sand composition changes, so it must be determined each time a sample of sand is taken for testing. It is important to record the specimen weight because the weight provides useful information regarding changes in sand composition.
If the specimen weight increases, this indicates that the sand's silica content has increased, since silica is the heaviest component of the sand. If it decreases, either the additives have increased or there is an increase in the amount of dead clay and ash accumulating in the sand. In this way, it can be used as a guide for determining the need for new sand additions.
AFS permeability is the rate at which 2000 cc of air passes through an AFS standard specimen with a head pressure of 10 cm of water. Permeability can be measured with an electric or drum type permmeter.
Gases are produced in a mold from the heat of the molten metal. The water in the mold produces steam and the carbonaceous materials in the sand produce other gases. There must be a provision to vent these gases from the mold as they are produced or else gas defects will result. Permeability provides an important relative measure of the sand's venting characteristics.
Gases vent readily through sands with high permeability. Many factors affect sand permeability, but compaction and grain fineness are the major variables. The higher the density to which the sand is compacted, the lower the permeability because the sand grains are forced tightly together, leaving smaller voids between the grains through which air can pass.
The grain fineness and distribution of the base sand is another important factor. The finer the sand, the lower the permeability--again because the voids between the sand grains are smaller.
The degree of mulling also influences permeability because it affects the distribution of the clay and additives on the sand grain. Usually, the higher the degree of mulling, the higher the permeability of the sand.
Low permeability produces a smoother casting surface finish because the voids between the sand grains are smaller. Low permeability, however, increases the likelihood of problems with blows, pinholes and other gas-related defects. Low-permeability sands also can produce expansion defects if the permeability is low as a result of high packing density of the sand grains.
Green Compressive Strength
AFS standard green compressive strength tests are typically used to control the strength characteristics of the sand. In this test, an AFS standard specimen is loaded in compression and the maximum load to failure is recorded. Green strength is commonly used to relate to molding and handling properties of the sand. Dry strength also is used sometimes to complement the green strength data by relating to the sand's shakeout properties of the sand.
As moisture increases, green strength reaches a maximum and then drops as the sand becomes overtempered. Dry strength, however, continues to rise as moisture increases.
If green compressive strength is low, the sand will have good flowability and the cost to maintain the system will be lower. If too low, however, broken molds and poor draws may become a problem. Low green strength indicates low clay content and/or poor mulling.
If green compressive strength is too high, the molds will be stronger, but difficult shakeout, poor casting dimensions, poor flowability and high ramming resistance are likely problems. Also, the cost to maintain the system will be higher due to use of excessive bond.
Dry Compressive Strength
Dry strength tests, which are run similarly to green strength but on specimens dried at 220-230F, are sometimes used to indicate the sand's shakeout characteristics as well as its properties when the sand is dried by the molten metal's heat.
If dry compressive strength is low, shakeout will be enhanced. But if it's too low, loose friable edges, cuts and washes, burn-in, inclusions and erosion will be problems as the flow of the metal erodes the sand away.
If dry compressive strength is high, the molds will be stronger with less of an erosion tendency, but difficult to shake out. Cracks and hot tears may be a problem if the mold geometry is restrictive and the mold does not readily collapse when the metal begins to shrink upon solidification. A large loss of return sand also will cause the system to be more expensive to operate because good sand is carried out on the casting at shakeout, instead of being recirculated in the sand system. In turn, this also will cause the system to become brittle, since the sand will have less cumulative mulling.
Another optional green properties test is green deformation. One way to measure deformation would be to stop loading at various intervals during a green strength test, remove the specimen and measure it with a gage type micrometer. However, excessive handling of the specimen can lead to error.
Instead, green strength can be run simultaneously with green deformation using an accessory. This accessory measures the deformation of the sand as it is loaded in the green strength test, without additional handling of the specimen.
Green deformation is a measurement of the plasticity of the sand. A certain amount of deformation is needed for pattern stripping and reduction of expansion, but too much deformation will cause mold wall movement, swells and oversized castings.
Splitting Strength Test
Compressive strength tests have conventionally been used to control the strength characteristics of molding sands. However, tensile and shear properties of molding sands are actually more critical because they are a weaker characteristic.
Also, in the foundry, molding sands rarely fail due to compressive stresses. Most mold failures, such as in pattern stripping, are actually tensile or shear failures.
Shear strength tests are sometimes used to complement green strength data, but the splitting strength test offers a more repeatable alternative. In the splitting strength test, an AFS standard specimen is loaded across its diameter. When the specimen fails, it actually fails in tension.
Splitting and shear strength tests relate to the degree of mulling. The ratio of shear and splitting strength to green strength increases as the degree of mulling increases.
Beyond the Basics: Tests for Specific Sand Problems
The tests discussed so far are the conventional tests that have been used to control foundry sands. While sand control programs are best kept to a minimum number of tests, sometimes problems are encountered that require additional information.
For example, one common condition in U.S. foundry sands is sand brittleness. Sand brittleness can be caused by an excessive influx of core or new sand, or by poor moisture clay relationships due to poor or insufficient mulling.
When brittle sands are tested using the conventional tests, their properties can sometimes appear adequate. The conventional tests do not always indicate when there are sand problems. The tests that have proven to be indicative of sand brittleness are friability and cone jolt toughness tests.
The wet tensile test measures the ratio and condition of the clay. All three tests are performed immediately after the basic green properties test.
Friability is a measure of the abrasion resistance of a sand. A friable sand is a sand that is not able to withstand the erosive flow of the molten metal. It will lose sand grains to the moving stream, and will be subject to producing erosion and inclusion defects.
In the friability test, two standard AFS 2-in. specimens are required. The test normally is performed immediately after specimen preparation, but specimens can be tested after various air drying intervals.
The two specimens are placed side by side in a rotary screen and rotated for one minute. As the specimens rotate, the sand abraded from the surface is collected in a pan.
The weight loss in grams, divided by the original starting weight (or twice the specimen weight) and then multiplied by 100 produces the percent friability.
Molding sands can become very friable if there is too high an influx of core sand or new sand and bond. New bond requires several passes through the mixer before its properties are developed.
As might be expected, friability is inversely related to compactibility. The lower the compactibility, the higher the friability. Some molding sands, depending upon their composition and moisture/clay relationships, are extremely moisture sensitive. A small drop in compactibility, or a brief air drying period, will produce a large increase in friability. Friability studies can be performed to determine the effect of line downtime on the mold before pouring.
Work of the AFS Green Sand Test Committee (4-D) suggests that a friability level of under 10% is generally satisfactory. If friability is over 10%, the sand will be subject to producing erosion- and inclusion-type defects.
Friability Level 0-10%: adequate Over 10%: erosion, inclusions
Cone Jolt Toughness Test
Friability measures surface brittleness of the sand. The cone jolt toughness test measures the sand's bulk brittleness, and is related to difficulty in pulling deep pockets in a pattern and broken molds.
To perform this test, a special cup pedestal and top plate with indentation cones are used. These are used to form locating recesses in the top and bottom of the specimens.
The specimen is then placed on a movable platform and a cone-shaped weight is set on top of the specimen. When the unit is activated, a cam raises and drops the specimen platform 30 thousandths of an inch (0.030 in.). The specimen is subjected to a jolting action while under the weight of the cone. The number of jolts before specimen failure is recorded as cone jolt toughness.
As a general role, if the cone jolt toughness level is under 40 jolts, the sand is brittle. Difficulty in pulling deep pockets in a pattern and broken molds would be expected. A level of 40-50 jolts is borderline. More than 50 is adequate.
Cone Jolt Level
0-40 Jolts: brittle sand, difficulty in pulling deep pockets, broken molds
40-50 Jolts: borderline
More than 50 Jolts: adequate
Wet Tensile Test
In this test, a wet layer is formed in a sand specimen to simulate the wet layer formed in the mold from the heat of the molten metal. The strength of the wet layer is then measured. The wet layer is critical because it is the weakest layer in the mold. Failure of the wet layer produces expansion defects such as scabs, buckles and rattails. The wet tensile test can also be used to monitor the condition of the clay and the ratio of western and southern bentonite.
In running the test, the wet tensile specimen is rammed with an aluminum lift-off ring in the cup pedestal. After ramming, the specimen is inverted so that the lift-off ring is at the top and the end of the specimen is exposed inside of the ring. When the specimen is pushed into test position on the wet tensile unit, a heating plate at 600F automatically lowers onto the exposed end of the specimen. When the specimen is heated, the moisture in the sand is driven back and condenses, creating a saturated wet condensation zone called the wet layer.
After the preselected heating time interval, a tensile load is applied to the ring by raising it with a pneumatically operated fork, until the specimen falls through the wet layer, and the wet tensile strength is read from the gauge. The heating time is set to produce an even break, through the wet layer. This is determined by running at the time which produces minimum strength, usually 15 seconds.
If wet tensile strength is too low, scabbing, rattailing and buckles may be a problem, since these defects are caused by weak wet layers that form in the mold. This test also relates to bond formulation. A chart on wet tensile strength versus bentonite ratio in earlier research (AFS Transactions 89-63) shows that the higher the percentage of western bentonite in the system, the higher the wet tensile strength. The higher the percentage of southern bentonite, the lower the wet tensile strength.
The ratio of the clay in a sand system is not always the ratio in which it is added to the system. Southern bentonite burns out faster than western bentonite. Also, salts and condensates from organic materials such as seacoal and core binders can cause the properties of bentonites to degrade, lowering wet tensile strength. Thus, wet tensile strength can be used to monitor the ratio and condition of the clay in the system.
Low Compactibility (dry sand) causes:
* Cuts and Washes
* Friable Broken Edges
* Hard to Lift Pockets
* Cope Downs
* Burn on
* Erosion Scabbing
High Compactibility (wet sand) causes:
* Oversize Castings
* Supervoids on Vertical Faces
* Poor Finish/Rough Surface
* Expansion Defects
* Difficult Shakeout
* High Ramming Resistance
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|Title Annotation:||includes related article; part 1|
|Author:||Krysiak, Mary Beth|
|Date:||Apr 1, 1994|
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