Factors That Affect Compactability and Consistency in Green Sand

     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 pressure, high density, automated molding machines, improved casting dimensions are obtained consistently.
     A simple test that has replaced the “hand feel test” is the compactability test. Compacted mold uniformity is a vital factor in achieving net shape casting production. Correct interpretation of this test can help reduce mold wall movement that could be a prime cause of apparent shrinkage. The test is reported as a percentage and indicates the relationship among molding sand compaction characteristics, composition, sand preparation and transportation through the mold cycle.
     The compactability simply indicates the degree of temper or relative wetness of a molding sand mixture. It provides a percentage number that can be used in auditing sand consistency for quality control and automation. The compactability test determines the percentage decrease in height of a loose mass of sand under the influence of controlled compaction. The compactability molding values are directly related to the performance of a molding sand mixture. With control of some of the major conditions that affect the test, it’s use can yield excellent casting finish and reduce cleaning cost.

Essentials affecting compactability
In the past only a few factors were researched and reported as important factors to control.

However, in the past 30 years since the introduction of the test, extensive production experience has been gained along with research studies. The quantitative measurement of the process allowed foundry personnel to respect compactability as a meaningful tool to achieve consistency in green sand molding production.
This report was developed to better define and further share knowledge and refinement of these and other factors with a much greater detailed explanation. The importance of controlling compactability and consistency in molding sand can be summarized as follows:

High compactability could result in -

Improved dimensions

Gas/blow/pinholes

Better casting finish

Brittle mold surfaces

Expansion problems

Difficult shakeout

Hard mold penetration

Low compactability could result in -

Friable edges
Crushes - inclusions
Hard to lift pockets
Mechanical penetration
Apparent shrinkage
Cuts and washes
Cope drops
Oversize castings
Rough surfaces

* Western (sodium type)
Sodium bentonite when wetted has a pronounced effect on the compactability of molding sand. This is due to their physical and chemical characteristics. The following factors are most conducive to changes in compactability and the amount can vary with the composition and physical properties of the prepared molding sand.

* Southern (calcium type)
Calcium bentonite mixes easier than sodium and has a very minimum of swell and viscosity compared to sodium. Calcium bentonite contributes more green compression strength at lower moisture levels. Since green compression strength affects compactability, the amount of calcium bentonite used will impact compactability.

* Cereals and Starches
The following is a list of the most popular semi-bonding additives used that affect compactability in order of bonding strength. They all contribute to green compression strength similar to bentonite.

Of all of these, dextrine and corn flour are very effective in developing sand toughness and green deformation. They rate as high as sodium bentonite in affecting the change in compactability at much lower additions. They are most preferred in casting steel and heavy weight or thick section castings. The high toughness that these additions is develop, render the measuring of compactabiltiy impracticable. The inconsistency of compaction is due to the skin formation and fast drying during handling. The variation in compactability is also influenced by the higher affinity for water absorption during mulling. Adding water to the sand in the muller first, improves the efficiency and consistency of cereal starches and bentonite. If this practice is not followed, the development of clay balls can occur, especially with an increase in fines. Understanding these conditions, reproducibility of compactability readings becomes difficult. The existence of clay balls in molding sand causes non-uniform mold compaction.

* Cellulose additives
Non-bonding filler additives mostly used to accommodate sand expansion, such as wood flour, ground corncobs, ground oat hulls, and similar ground cellulose fibers, have an opposite effect on compactability. They absorb moisture, and increase compaction by reducing the resistance developed by the bonding agents used. Cellulose additives contribute to more drying and effect continual change in compactability, reading, especially true with higher return sand temperatures and time. These additives contribute to brittleness and require careful auditing and control to prevent sand inclusions.

* Variations in water
There are four conditions that have a major effect on compactability, that are caused by water. Use of (1) hard water, (2) water temperature, (3) reused water from other sources, (4) water from wet dust collectors, “black water.”

(1) Effect of hard water - The condition of the water and its source can influence the development of bonding agents and their characteristics in use. Use of hard water reduces the activation of sodium bentonite. The use of electrolytes and additives that enhance bentonite development or deactivate its toughness (mostly acidic) can be used but are least recommended. They are very difficult to test for and to control the amount used and retained.

(2) Water temperature - The temperature of the water has an effect on the development of the bentonite and starches during mixing. Use of cold water at below 45F makes bentonite activation more difficult. As mixing time increases the internal friction heats the sand. More energy is consumed and mulling time should be increased.

(3) Recycled water - Recycled water from cooling furnaces etc., is usually warm and poses no problems of significance. The water quality should be monitored for PH. If the water becomes acidic, it has a major effect on deflocculating sodium bentonite. These variations affect compactability readings.

(4) Black Water - Black water is recycled water from wet dust collectors and is recycled for environmental reasons. It usually contains a percentage of the bonding additions plus fines. The problem in its use, is the assurance of consistency. This must rely on the dust collection system and its maintenance. The problem that accelerates is the variation of fines put back in the molding system unknowingly. Although we have not discussed the molding sand fineness as related to compactability, a major deviation can occur as fines develop. The addition of water described can provide the catalyst to develop the cohesiveness of the clay bond and additives. Thus it has the greatest influence on the development of the compactability reading at the same MB clay level. Likewise, as moisture evaporates during the transporting, the compactability level is constantly changing. This condition is greatly accelerated with hot returned sand and the cooling of the sand prior to mold compaction.

The controlling of moisture and or additions of bond are key variables that are used for automatic compactability control equipment. The equipment effectiveness is based on monitoring return sand temperature fluctuations. By consistent adjustments made for returned sand, these automatic controllers can produce consistent compactability.

* Mulling and mixing
Compactability increases steadily as calibrated mixing and mulling energy is applied to a sand mixture. Since the return sand temperature is an important factor, (based on mold/metal weight fluctuations and shake out time) the evaporation of water is the main reason for the variation of compactability. This is important enough to repeat. There is constant change in molding sand mixtures from the reduction of sand temperature during transport and handling from shakeout, mulling and return to the molding machine. In some cases with high intensity mixing or mulling, the friction causes heat. In other cases with high intensity mixers the excess energy and increased time can actually create fatigue in the bentonite, which renders the sand more brittle and friable. This lowers the compactabilty reading. This condition is detrimental and can affect casting quality by an increase in sand inclusions. This is due to a mold surface dryness. Aerators can be used to improve the consistency of compactability. The sequence of additions to the muller if not consistent, can affect the compactability readings. If the formulas are added as a premixture, the variability is reduced.

*Conclusions
All conditions that contribute to the development of green compression strength and air set strength affect the variability of compactability readings. All of the raw materials used in the green sand formulation have some effect on the variability of compactability. The following are the largest contributors in order of most influence:

In the light of repeated successes foundry men have adopted, improved, computerized, and automated the compactability-testing concept. They have proven its effectiveness for stabilizing and controlling automated green sand systems, with improved production rates.
It is very important to know and specify the raw materials being used, control the sand composition, temperature and processing methods. These all in synergism, control compactability and ultimate consistency of green sand molding. The success has been confirmed in the field where over 90% of green sand foundries have made it a production test necessity.