Testing Molding Sand for Compactability
By: George DiSylvestro, DiSylvestro Videography Service

Green sand molding is a process that combines the advantages of versatility, productivity and low cost for the production of quality castings of any metal that is castable. Because of the advent of high density molding, using higher pressure automated molding machines, improved casting dimensions have been obtained.

A simple test that has replaced the "hand feel test" is the compactability test. Compacted mold uniformity is a vital factor in achieving near-net-shape casting production. The test can reduce mold-wall movement that could be a prime cause of apparent shrinkage. The test is reported in percentage and establishes a relationship between molding sand compaction characteristics independent of its composition. Compactability simply indicates the degree of temper or relative wetness of the molding sand mixture. It provides a percentage number that can be related in quality control an/or computer control programs. It is recommended that the compactability test be performed at the mulling or mixing station for quick control response of the governing factors and decision making.

The compactability test determines the percentage decrease in height of a loose mass of sand under the influence of compaction. The compactability molding values are directly related to the performance of a molding sand mixture. If controlled by some of the major conditions that affect the test, the use of this could yield excellent casting finish and reduce cleaning.

Figure 1 Compactability Equipment

Under constant conditions, a riddled sample of tempered molding sand is compacted and the percentage decrease in height is determined. The test closely simulates the actual filing of a flask with molding sand and compaction by a molding machine. It is independent of the specific gravity of the sand, and is therefore superior to the bulk density test for measuring degree of temper or determining the water requirements of a sand mixture. Figure 1 indicates the equipment required to perform the compactability test.

There are several guidelines to remember when testing for compactability. These should be strictly followed, as failure to do so can affect the results of the test.

• Always lubricate the specimen tube before testing.

• Be sure the strike off blade is held perpendicular to the top of the specimen tube. If not, it can pre-compact sand slightly.

• When transporting or placing the sample tube into the rammer, do not jar it as this could also pre-compact the sand.

• Lower the rammer onto the specimen tube carefully.

• When ramming the specimen do not go extremely fast or slow as this can affect the degree of ramming. A guide here is to always pause at the 4 o’clock position between rams.

• Do not use compactability specimens in other tests calling for 2" x 2" specimens.

• There are several charts and scales available from vendors which have proven a useful aid in compactability testing. Contact your supplier for information on these.

• Moisture content

• Clay content

• Carbonaceous material

• Inert fines (water absorbing)

• Length of mulling time

Control of compactability can produce a uniformly dense mold. A firm mold can yield a near-net-shape casting with an excellent surface finish as is shown in Figure 4.

Figure 2  Expansion Scabs	Figure 3  Hard Mold Penetration	Figure 4
Figure 5  Mechanical penetration	Figure 6 Internal Shrink

• Improved casting dimensions

• Better casting finish

• Gas-blow-pinholes

• Expansion problems

• Difficult shakeout

• Hard mold penetration

• Brittle mold surfaces

Figure 2 indicates a problem that can be encountered with high compactability molding sand. In this case the compacted sand is too dense and the result is an expansion type defect termed a scab. Another result of high compactability is shown in Figure 3. A very highly flowable sand has produced a very dense mold resulting in "hard mold penetration".

• Friable mold edges

• Crushes - inclusions

• Hard to lift pockets

• Mechanical penetration

• Apparent shrinkage

• Cuts and washes

• Cope drops

• Oversize castings

• Rough surfaces

Figure 5 shows a result of low compactability. A stiff molding sand that resisted compaction produced open voids in the mold and metal penetration occurred. Shown in Figure 6 is an example of mold dilation due to a soft mold which caused an oversized casting and metal shrinkage.

SUGGESTED READING

For those who are interested in furthering their knowledge of this subject it is recommended that they obtain copies of the following:

• AFS Publication, AFS Mold and Core Handbook, 1989 ed.

• F. Hoffman, H. Dietert, A. Graham, Compactability Testing, A New Approach in Sand Research, AFS Transactions 1969.

• A Volkmar, A Basic System Sand Control Procedure, AFS Transactions 1973.

• C. Wenninger, A. Volkmar, A New Control Tool: A Graph for Evaluating Effectiveness of Available Bentonite Within Foundry Sand Systems, AFS Transactions 1970.

• A. Volkmar, System Sand Control by Compression vs. Compactability Testing, AFS Transactions 1970.

• J. Herivel, Compactability for Production Control, H. W. Dietert Company.

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