Additives improve sand molding.

A change in the method of making green sand molds occurred when foundries began producing harder molds, a process that continues to evolve. Harder molds mean that foundries must better understand the materials that make a green sand mold.

Bonding Clay

In most cases, fireclay, western or southern bentonite are used to bond sand molding mixtures, each serving a specific function in a mold.

Western bentonite received its name from the locations where it is mined--Wyoming, South Dakota and Montana. It is used to develop a lower green compression strength, and higher dry compression and hot compression strengths.

Another commonly used material is southern bentonite, found in Alabama and Mississippi. It is used to develop higher green compression strengths and lower dry and hot compression strengths.

Fireclay is used to a lesser degree than bentonites. At low concentrations, it is a clay supplement. Fireclay has a greater moisture latitude than bentonite/sand mixtures, longer dryout times and increased hot compression strength. However, a drawback is its low methylene blue uptake caused by a buildup in a sand system.

To gain the maximum benefit from bonding clays preblends of these materials are widely used. Preblends offer improved molding sand consistency when incorporated into a green sand system.

Carbon Additives

Carbon additives, like seacoal, provide a reducing atmosphere that limits the reaction between the molten metal and the molding sand. Seacoal is a ground bituminous coal that offers balanced chemical properties. Many supplemental carbon sources exist, usually containing a higher level of volatile carbons that increase casting peel. One of the rules of thumb applied to these supplements is: for every 1% of seacoal supplements used in a preblend, 2% of seacoal can be replaced.

When seacoal supplements are used, the sulfur content should be monitored. In ductile iron operations, the sulfur content of the molding sand must be monitored because seacoal supplements tend to have a higher sulfur content than coal.

Inorganic Additives

To balance green sand's chemical and physical properties, the application of an inorganic material such as soda ash (sodium carbonate) is beneficial TABULAR DATA OMITTED but must be controlled at low levels. The recommended level in a batch of molding sand at the muller is 0.0025-0.0050%, a difficult level to maintain. Adding soda ash to a preblend is recommended at about. 0.25-0.50%.

Soda ash will balance the pH of the molding sand, control variations in the water quality at the muller and reduce the encapsulation of additives and fines with the sand grains.

Cellulose

Cellulose is a fibrous, organic material derived from ground corn cobs, wood flour, ground peanut hulls and ground oat hulls and are all suitable materials for foundry use. Cellulose acts as a cushioning material in green sand molding and burns out rapidly. It also is used as a water absorbent to decrease the sensitivity of temper water additions.

Cellulose is widely used because it controls sand expansion, increases mold collapsibility, reduces hot tears and cracks, improves shakeout and enhances flowability.

Cereals and Starches

Cereals and starches are extensively used in steel foundries and to a lesser degree in iron and nonferrous foundries. They are derived from corn and potatoes. Many varieties of cereals and starches are available, but their selection is a matter of their cold water solubility. This chemical property will determine the water-absorbing characteristic at the muller and, ultimately, its effect on high-pressure molding.

Cereals and starches generally increase the green deformation of the sand mixtures, making it easier to draw a difficult pattern. An excellent measure of this characteristic is the cone jolt toughness tester. They also broaden the moisture range of green sand mixtures and increase the dry compression strength of molding sand.

Because foundries are being asked to push their sand systems farther than imagined a few years ago, they must reduce sand-related scrap, place as many castings as possible in a mold and increase the dimensional accuracy of the castings. Sand additives are a key element to the success of these goals.