Promote Homogeneity for Optimal Green Sand Conditioning.

Appropriate clay/water ratios, homogenous clay hydration, well-dispersed additives and adequate aeration are important to realizing a properly conditioned green sand system.

There is more to green sand control than just having all system constituents pres-ent in the proper percentages. One can pour a gallon of water onto a dry heap of sand and correctly state that the resulting combination is now at 3% moisture, but this doesn't take into account how well the sand has been conditioned. Conditioning refers to how well the ingredients have been mixed, how evenly the clay has been hydrated and "smeared" onto the sand grains, and how "fluffy" and moldable the sand actually is.

Well-conditioned prepared sand mainly is a function of the hydration state and temperature of the return sand provided to the muller. Regular maintenance and calibration of mullers, coolers, aerators and compactibility units is critical to create proactive control, instead of just "putting out fires." The muller is the heart of the green sand system, and its settings and sequence of additions have great bearing on how well the sand is processed. In some cases, chemical surfactants and soda ash are needed to provide increased clay dispersion rates. By providing well-conditioned green sand to molders, foundries will enjoy better mold properties and reduced casting defects. By understanding the variables involved, a sand technologist can communicate sand system needs to upper management so that logical decisions can be made and profits maximized. Faster production rates at the expense of ill-prepared sand will result only in higher defect levels and broken molds.

Clay/Water Ratio

One of the most critical parameters in assessing how well a green sand mixture has been conditioned is the methylene blue (MB) clay/water ratio. This ratio affects how well the green sand forms mold features, the severity of defects due to excessive moisture and the amount of sand inclusions. Specifically, this ratio means the difference between a mix that is "too sticky" and one that is not strong enough to hold together during molding.

If the moisture level is too high, moisture-related defects will occur. Clay/water ratios below 2 almost always result in excess penetration, sand inclusions and scabbing. Many foundries experience high moisture problems at 2.2 or below. Low MB clay/water ratios can be caused by dry or hot return sand, insufficient mulling time (high sand demand), high sodium (western)/calcium (southern) bentonite ratios, poor equipment maintenance or excessive moisture sponge levels (dead material that competes with clay for water).

High MB clay/water ratios also can cause problems when they soar somewhere above 3.5, but this upper level seems less well-defined than the lower limit--it really depends on the foundry. Friability, erosion and sand inclusions all are symptoms of excessively high clay/water ratios.

Return Sand Temperature and Moisture Effects

Green sand conditioning begins well before the muller. If the return sand system does not adequately cool and hydrate the hot, dry shakeout sand, then even the best mulling will have a difficult time creating somewhat uniform clay hydration. Hot return sand is a problem because the water spends most of the limited mulling time evaporating or boiling away rather than penetrating into the clay. For best results, well-conditioned return sand below 120F (49C) should be provided to the muller.

Low return sand moistures are more of a problem than hot return sand. It is much harder to hydrate dry clay ([tilde]0.35 conductivity) compared to moist clay ([tilde]1.0 conductivity). Water added at the muller will be drawn first into the clay with the highest moisture level. In other words, if you place dry clay particulate into a clay paste and then add water, you will end up with a dry clay particulate within a clay slurry. Similarly, if the clay in the return sand is bone-dry and the bond added at the muller is at 8% moisture, then the water added at the muller will preferentially hydrate the bond added at the muller. This situation is schematically represented in Fig. 1. If, however, the moisture percentage in the return sand clay is somewhat similar to the moisture level in the bond added at the muller, then fairly uniform hydration will take place.

The premix consumption should be higher when bone-dry return sand is involved because the return sand clay would make little contribution to the overall "glue" strength during the limited mulling cycle. Instead, the clay in the return sand will remain as dry granules or dry coatings on sand grains. This means that extra bond must be added in order to achieve the target compactabiity given the same moisture and mulling time. With continued cycling through the system, layer upon layer of clay will build up on the return sand grains since it never is washed off.

Many sand technologists have confronted the question, "Why don't we add our bond in the form of a slurry?" Moist clay tends to grab and hold water faster than dry clay. Normally, the clay in the return sand is at a similar moisture level to the bond added at the muller so that all clay is being hydrated at the same rate (Fig. 2). When slurry is added in lieu of water and bond, there should be a tendency to leave more of the return sand clay unhydrated and again build up layers of clay coating on the return sand grains. Further, if water is not being added, then there is no wash cycle for batch mullers.

There is another perhaps more important reason for keeping the return sand moistures high. By keeping the moisture level in the return sand high, less water will be needed at the muller to hit the compactibility target. Then the muller water addition will be much more accurate since it will be "tweaking" the moisture level up to the desired target, instead of taking the clay from a bone-dry state up to the desired moisture level. In general, the moisture in the return sand should be as close as possible to the prepared sand moisture without causing plugging of any return sand equipment.

Material Effects

"Moisture sponges" are lightweight, "dead" materials that compete with clay for water. They generally can be defined as AFS clay minus MB clay (or 25 micron minus MB clay)--that is, all the fine material minus all the fine live material equals all the fine dead material. Because moisture sponges compete with clay for moisture, high moisture sponges will result in a low clay/water ratio (or high water). Sand technologists have noted direct correlations between high moisture sponges and pinholes, scabbing, erosion, sand inclusions, penetration and even mold "explosions."

As with all green sand constituents, moisture sponge levels are determined by the balance between dead material generation, removal and readdition rates. Dead material generation rate is dictated by sodium/calcium bentonite ratio (thermal durability), percent MB clay, percent seacoal, and sand-to-metal ratios. Removal rate is determined by dust collector air flow rates and core plus new sand addition rates. Readdition of these materials not only increases the overall level of moisture sponges, but also is notorious for causing substantial variation in this material percentage.

While high moisture sponges force higher moisture levels to obtain compactibility, low moisture sponges increase both variation in compactibility of the prepared sand and compactibility loss to the molding machines. They reduce compactibility variation by absorbing water addition errors and by absorbing the water first and then slowly releasing it into the surrounding clay. Compactibility loss between the muller and the machines is reduced since the moisture sponges hold onto the water less strongly than the clay, thereby releasing it when evaporation occurs. Therefore, the "glue" doesn't change its water content--the moisture sponges do.

Surfactants have proven their value in some foundries by increasing the rate of clay dispersion so that compactibility targets are reached more quickly. This is especially true when hard water, hot return sand, high sodium bentonite or short mulling times are involved. These soaps and/or polymers (depending on the brand) allow the water to disperse the clay faster by lowering the surface tension of the water and through forced ionic separation of the clay platelets. Soda ash also is added for this effect and because it makes available soluble sodium ions for the clay base exchange. Realize that adding too much soda ash will cause the pH to rise so high that the beneficial effects may be lost.

Many foundries push the calcium bentonite levels higher when business is good and all machines begin demanding green sand. This is because calcium bentonite develops strength faster than sodium, allowing for shorter total mulling times to reach compactibility targets.

However, faster clay activation is not always a plus. Low sodium/calcium ratios, black water, surfactants and/or slurry additions all cause the green sand to reach its compactibility target much faster than normal, but there is less mulling time so that other additives are not as well dispersed. With shorter mulling times, there also is less scrubbing action to hydrate dry return sand coatings or to remove the detrimental lustrous carbon coatings for the partially burnt core sand grains. The longer mulling time associated with sodium bentonite is one reason that increasing the sodium bentonite percentage seems to combat high core sand input.

Equipment Considerations

Mulling equipment problems also can play a major role in poor sand conditioning. The purpose of the muller is not just to blend the ingredients, but also to force clay hydration and smear the clay around the sand grains. Worn or improperly positioned plows reduce the effectiveness of the mulling since the sand is not thrown in front of the wheels. In a vertical-wheeled muller, excessive distance between the wheel and floor lowers the smearing pressure of the clay onto the grains. Dust collection from these mullers should be minimal or at least engaged toward the end of the cycle so that massive amounts of bond and fines aren't drawn off. A regular maintenance program of inspection and replacement is critical to provide a good mull.

Many set points on batch mullers are critical to a good mull. The muller cycle should start with a wash cycle that adds as much water as possible into the muller (without leakage) and allows for several seconds of movement so that wheels and plows are washed clean. Without this cycle, the wheels and plows will build up the compacted green sand, thereby reducing their effectiveness. Water addition rate frequently is the rate limiting factor when it comes to mulling time. The faster you add the water, the faster the clay becomes hydrated and the better your green sand conditioning will be.

The conditioning of green sand within continuous mullers is directly proportional to how long the sand is held within the muller. "Kicking the flow rate up to meet the demand" is employed frequently at the expense of proper sand conditioning. The continuous muller door, when properly functioning, will float--not slam open or closed. Its settings will dictate bed height within the muller and, hence, the amount of conditioning the green sand receives.

Batch-type automatic green sand testing and control units have been a great benefit to foundries, but they may work better in some foundries than others because they are at the mercy of moisture sponge variation. They are not equipped to measure moisture sponge levels and try to compensate by using an adjustment factor based on previous batch information. This can help, but it is like driving while looking in the rearview mirror. There can be a real problem if there are abrupt changes in return sand moisture sponge levels. The key to making these units run well is to reduce return sand variation in moisture sponge levels, temperature and moisture levels. In this way, they become fine water adjustment units so that better green sand conditioning will result.

Aerators are great for taking lumpy or precompacted green sand and ripping it apart with whirling teeth so that it is fluffy and more moldable. In many foundries however, worn-out aerator blades go unnoticed since the unit is encased and the wear is gradual over time. Worn-out blades mean lumps go in and lumps come out.

Coolers usually are a blessing but can turn into a curse if not properly maintained and adjusted. Maintenance crews will sometimes "adjust" them so that they pull out all the fines and/or the sand comes out bone-dry. Many times, they are undersized, especially when several molding lines are added after the initial system installation. Again, the purpose of the cooler should be to create return sand with low, stable temperature and high, stable moisture.

Water addition units may have several nozzles each set at different flow rates. For example, one unit may have nozzle "A" set at 10 gal/mm, nozzle "B" at 5 gal/min, and nozzles "C" through "E" set at I gal/min. If the sand entering this unit is somewhat warm, the unit may be asked to deliver 7 gal/min and will activate nozzles B, C and D. If the entering sand becomes hotter, the unit may be asked to deliver 12 gal/min and activate nozzles A, C and D. What if nozzle A is clogged? The unit will deliver 7 gal/min to the warm sand and 2 gal/min to the hot sand, causing massive variations in the return sand at the muller. Since these are closed units, they should be regularly opened, inspected and calibrated to ensure that they are delivering the correct amount of water.

The purpose of return and prepared sand silos is to increase the total volume of sand within the system, thereby minimizing the effect of system changes, such as sand/metal ratio and core sand input rate. "Rat-tailing" occurs when the silo storage capacity is severely reduced due to buildup on the sidewalls. This reduces the total residence time of the system so that the sand is forced through the system much more rapidly, and, therefore, more rapid changes take place with respect to moisture, temperature, moisture sponges and core sand percentages. The problem mainly is caused by condensation on the inside of the silo from warm, moist green sand in contact with cold silo walls. The best way to counteract rat-tailing is to cool the sand as soon as possible after shakeout and to insulate the silo walls.

Prepared sand silos and large surge hoppers have an "additional" function in that some tempering takes place after mulling. During this "layover time," the wet clay will surrender some of its water to the dry clay, resulting in a slightly more homogenous water distribution. While the sand is not nearly as hot as the sand in the return silo, it has a greater water content, so that rat-tailing also is a consideration, along with bridging and precompaction. Bridging and precompaction occur when freshly prepared green sand falls from a height and compacts. This precompaction means that the sand no longer is flowable, and in severe cases, the mass has enough strength to hold itself in place and block the flow of sand altogether. Minimizing drop distance and parabolic bin shapes usually remedy this problem.

Well-conditioned prepared sand is a function of return sand, equipment and materials in the system. While the MB clay/water ratio speaks volumes about the condition of the green sand, the homogeneity of the clay hydration perhaps is even more important. The sand technologist should not expect the muller and compactibility controllers to create well-conditioned prepared sand from hot, dry return sand. Regular maintenance and calibration of all sand equipment is critical to avoid problems before they start.