reducing casting defects - a basic green sand control program.doc

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Reducing casting defects: a basic green sand control program.

By Krysiak, Mary Beth

Publication: Modern Casting

Date: Friday, April 1 1994 You are viewing page 1

The first of this two-part series looks at the tests that need to be performed daily to control

the green properties of rooming sands.

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

You are viewing page 2




are run and how they relate to sand control. There are four basic

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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.








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;

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* dry compressive strength;

* splitting strength;

* friability;

* cone jolt toughness;

* wet tensile strength;

* volatiles.

Green Sand Sampling






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over-correcting.











* friability;



* volatiles.

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

4

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.

Moisture Testing

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.






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over-correcting.











* friability;


6




* volatiles.

Green Sand Sampling












at all times.






Moisture Testing






moisture.

Compactibility Test

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.

7

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.






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over-correcting.











* friability;

9






* volatiles.

Green Sand Sampling












at all times.






Moisture Testing






moisture.

Compactibility Test

10























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.

11

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.






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over-correcting.











* friability;



* volatiles.

Green Sand Sampling





13








at all times.






Moisture Testing






moisture.

Compactibility Test












14































15

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.

Permeability Test

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.






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over-correcting.











17





* friability;



* volatiles.

Green Sand Sampling












at all times.






Moisture Testing






moisture.

Compactibility Test

18
































19

















composition.





additions.

Permeability Test









20

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.






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over-correcting.









22








* friability;



* volatiles.

Green Sand Sampling












at all times.






Moisture Testing





23


moisture.

Compactibility Test






























24



















composition.





additions.

Permeability Test







25







air can pass.



smaller.








of the sand grains.








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.

26

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.






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over-correcting.











* friability;



28



* volatiles.

Green Sand Sampling












at all times.






Moisture Testing






moisture.

Compactibility Test




29

































30













composition.





additions.

Permeability Test













air can pass.

31



smaller.








of the sand grains.




















32











Green Deformation

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.

33






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over-correcting.

34


















* friability;



* volatiles.

Green Sand Sampling












at all times.




35



Moisture Testing






moisture.

Compactibility Test























36


























composition.





additions.

37

Permeability Test













air can pass.



smaller.








of the sand grains.








38























Green Deformation








39













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 Test

40

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.






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41










over-correcting.











* friability;



* volatiles.

Green Sand Sampling












at all times.

42






Moisture Testing






moisture.

Compactibility Test



















43






























composition.

44





additions.

Permeability Test













air can pass.



smaller.








of the sand grains.


45





























Green Deformation



46


















fails in tension.












47





Friability Test




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


48





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over-correcting.

49


















* friability;



* volatiles.

Green Sand Sampling












at all times.




50



Moisture Testing






moisture.

Compactibility Test























51


























composition.





additions.

52

Permeability Test













air can pass.



smaller.








of the sand grains.








53























Green Deformation








54













fails in tension.
















Friability Test

55













are developed.






before pouring.






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.

56

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






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over-correcting.











* friability;



* volatiles.

Green Sand Sampling

58












at all times.






Moisture Testing






moisture.

Compactibility Test








59































60











composition.





additions.

Permeability Test













air can pass.

61



smaller.








of the sand grains.




















62











Green Deformation




















fails in tension.

63
















Friability Test













are developed.

64






before pouring.








and broken molds.











Cone Jolt Level




Wet Tensile Test

65

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.






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over-correcting.









67








* friability;



* volatiles.

Green Sand Sampling












at all times.






Moisture Testing





68


moisture.

Compactibility Test






























69



















composition.





additions.

Permeability Test







70







air can pass.



smaller.








of the sand grains.













71


















Green Deformation












72









fails in tension.
















Friability Test




73










are developed.






before pouring.








and broken molds.








74




Cone Jolt Level




Wet Tensile Test






bentonite.







layer.










75



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

* Crushes

* Penetration

* Burn on

* Erosion Scabbing

High Compactibility (wet sand) causes:

* Oversize Castings






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over-correcting.











77





* friability;



* volatiles.

Green Sand Sampling












at all times.






Moisture Testing






moisture.

Compactibility Test

78
































79

















composition.





additions.

Permeability Test









80





air can pass.



smaller.








of the sand grains.
















81















Green Deformation

















82




fails in tension.
















Friability Test











83



are developed.






before pouring.








and broken molds.











Cone Jolt Level



84


Wet Tensile Test






bentonite.







layer.












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:

85

* Cuts and Washes

* Friable Broken Edges

* Hard to Lift Pockets

* Cope Downs

* Crushes

* Penetration

* Burn on

* Erosion Scabbing

High Compactibility (wet sand) causes:

* Oversize Castings

* Shrinks

* Blows

* Pinholes

* Supervoids on Vertical Faces

* Poor Finish/Rough Surface

* Expansion Defects

* Gas

* Difficult Shakeout

* High Ramming Resistance

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reducing casting defects - a basic green sand control program.doc

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