Do your additives add up? Picking the right additives will make your green sand recipe a can't-miss dish.

Green sand additives can be beneficial and economical, but miracle ingredients don't work as a replacement for poor metalcasting practice or engineering limitations. In order to achieve the best results, your additives must be used properly.

For instance, selecting the most cost effective pre-blend formula for your metalcasting facility will depend on more than the additive cost. The most cost effective pre-blend may be higher in initial price but result in a dramatic bond reduction in the molding sand to offset the cost.

Although green sand molding is one of the oldest metalcasting processes, the additives used have varied significantly over the years. Product families for these materials include:

* cereal/starches

* polymers

* treated organic materials with inorganic salts

* treated organic materials with polymers

* inorganic salts

* organic additives

* cellulose materials.

To determine the right blend of green sand additives, a metalcaster must consider the clay type, metal poured, rate of addition and the traditional product with which the materials will be blended (Table 1).

Cereal/Starches

Cereal and starches are popular additives for most metal types (steel, iron, aluminum and copper-base) because of their ability to improve the surface characteristics of molding sand for excellent casting finishes.

These green sand products work well with western (sodium) and southern (calcium) bentonite and their blends and improve mold squeezability and reduce brittleness. In steel casting applications, a blend of western bentonite and heavy weight cereal is most common, usually at 5% or less based on the weight of the bond. In aluminum casting, a blend of southern bentonite and starches is most common, usually at 2% or less, based on the weight of the bond. Iron and copper-base metalcasting facilities will blend starches and cereal with both clay types, usually at 2% or less.

The cost of cereal and starch additives is considered medium to high, relative to other products.

Polymers

Polymers, or liquid additives, have increased in popularity in the past 10 years. Their chemistries vary by manufacturer, but all are utilized at the same application rate according to the designed performance. Polymers are added to green sand to improve the wettability of molding sand, reduce bond and improve the prepared sand mulling. Usually, they are added to green sand at a rate of 2% or less, based on the weight of the bond addition.

Iron and copper-base metalcasting facilities often use this family of products with a majority of western bentonite for maximum performance. However, polymers can be used in iron and copper-base facilities with blended clay additions. Aluminum casting facilities use polymers with southern bentonite predominately.

A cost comparison usually ranks this product family in a medium price category. A potential cost increase can be nullified by the bond reduction accomplished with the application of these materials.

Treated Organic Materials With Inorganic Salts

The primary product in treated organic materials with inorganic salts is a causticized lignite. This product has been used in the metalcasting industry since the 1970s, but its popularity has increased greatly in recent years. Benefits include:

* reduction of bond requirement

* reduction in emissions during pouring, cooling and shakeout 

* improved flowability of the molding sand

* reduction of required seacoal in green sand.

This material, which is applied in iron and copper-base facilities only, favors the application of western bentonite and is used at a 10% application rate. Its cost usually rates in the medium price category, but a potential cost increase often is nullified by the bond reduction. In addition, since the manufacturers treat the product with soda ash, adding soda ash to the blend is not needed.

Treated Organic Material With Polymers

Treated organic material with polymers is a blended technology of polymers and treated organic materials with inorganic salts. The advantage of incorporating the two product families can be seen in the resulting sand properties, which include bond reduction, source of volatile materials (which improve metal flow), seacoal reduction and improved mulling and wettability.

The base raw materials in this application can include gilsonite, lignite, seacoal, the polymers and/or a blend of the materials. The primary drawback to this family is cost. As an index, these materials have a tendency to be more expensive with minimal gain in bond reduction. They are not compatible with steel or aluminum metalcasting applications but can be used in iron and copper-base facilities at 7% or less.

Treated organic materials with polymers are compatible with all clay types, and metalcasters use them most often with products that are blends of western and southern bentonites.

Inorganic Salts

Some of the most popular materials added to green sand molding systems are inorganic salts, which are used to balance the pH of molding sand, reduce sand agglomerations and reduce variations in water that is added to the molding sand. The primary material used in this application is soda ash.

Inorganic salts can be used with all clay types at a typical addition rate of 0.5%, although they can be added at up to 1%. Their cost is relatively low.

Nearly all types of metalcasting facilities can use inorganic salts, but it is predominantly used in iron and copper-base facilities.

Research circles currently are reviewing these materials, which have not been studied since the 1940s.

Cellulose Materials

Cellulose materials have been used longer than any other additive in green sand molding. They were used in naturally bonded sand systems before today's modern synthetic molding sands. Cellulose materials improve the flowability of molding sand and shakeout, reduce expansion defects and do not negatively impact the emission characteristic of the molding sand during pouring, cooling and shakeout.

Cellulose materials can be used with all clay types and blends. They are predominately used in copper-base and iron casting facilities and very rarely seen in aluminum or steel facilities. Because of the relatively low cost, cellulose materials have been added to molding sand at levels of 5% or less.

The source of a cellulose material does not matter. Products that have been used successfully in the metalcasting industry include ground corn cob flour, ground wood flour, ground oat hulls and ground peanut hulls. The primary criteria for selecting cellulose material are sizing and availability.

Return Core Sand

Until recently, return core sand found in green sand was considered a contaminant because it was either not intended for use in the green sand system or did not belong there. However, the current understanding is that return core sand can be intentionally incorporated into the sand system as an additive to be combined with existing molding sand. Many metalcasters use return core sand as all or part of the new sand addition to the molding sand, so it must be controlled like any other green sand additive.

The amount of core sand added has an impact on the quantity and emission characteristics of molding sand in the system. As a quality assurance tool, monitor the quantity either through the weight of the core sand set in the mold (subtract core butts) or the quantity of core sand produced (minus the broken cores not set in the molds). This becomes a tool similar to one used with bond addition, which can be calculated based on the weight of metal poured. In order to balance the quantity of sand being added, the metalcaster can vary the new sand addition in order to have a consistent quantity of sand.

For More Information

"Would Your Additives Pass an Emissions Test?" G. Crandell, C. Glowacki, S. Knight, L. Stahl and V. LaFay, MODERN CASTING, October 2008, p. 30.

RELATED ARTICLE: Path to an optional blend.

A decision tree is useful in choosing the optimal green sand additives for your operation. To use the tree, start at the beginning of the diagram. Look for the properties you are trying to achieve and the corresponding additives that will result in those properties.

Following are a few examples:

* If an iron casting application called for a reduction in the required amount of bond and emissions at pouring, cooling and shakeout, then the answer would be to add treated organic materials with inorganic salts.

* If an aluminum casting application required a reduction in friability and brittleness, then the answer would be to add a cereal or starch.

* If a copper-base casting application required a reduction of emissions at pouring, cooling and shakeout, then the answer would be to replace some of the seacoal with treated organic material with inorganic salts.

* If a steel casting application called for a reduction in friability and brittleness, then the answer would be to add or change the quantity of cereal or starch.

[ILLUSTRATION OMITTED]

Vic LaFay, Hill and Griffith Co., Cincinnati

Vic LaFay is the vice president of research and technical development for Hill and Griffith Co., Cincinnati.

Table 1. Comparison of Green Sand Additives

Product                                Sand
Family                              Properties

Cereal/Starches              improved squeezability,
                               reduced brittleness

Polymer or                   improved wettability of
liquid additives                molding sand, bond
                           reduction, improved mulling

Treated organic              bond reduction, emission
materials/Inorganic            reduction, improved
salt                      flowability, seacoal reduction

Treated organic       source of volatiles, lustrous carbons,
materials/Polymer       etc., seacoal reduction, improved
or liquid additive      wettability of molding sand, bond
                           reduction, improved mulling

Inorganic                  balanced pH in molding sand,
salt                       sand agglomeration reduction

Organic additive           source of volaties, lustrous
                                  carbons, etc.

Cellulose                    improved flowability and
                               shakeout, expansion
                                 defect reduction

Product                   Preferred        Typical Addition Rate in
Family                    Clay Type        Pre-blend (based on the
                                           weight of the preblend)

Cereal/Starches          western and                2% or
                      southern bentonite             less

Polymer or                 western                  2% or
liquid additives          bentonite                  less

Treated organic            western             10% based on the
materials/Inorganic       bentonite         weight of the western
salt                                              bentonite

Treated organic          western and                7% or
materials/Polymer     southern bentonite             less
or liquid additive

Inorganic                western and                1% or
salt                  southern bentonite             less

Organic additive         western and         seacoal-34% or less;
                      southern bentonite    gilsonite-10% or less

Cellulose                western and              5% or less
                      southern bentonite     (depending on alloy)

Product                 Blended       Cost as
Family                Capability     an Index

Cereal/Starches           all       medium/high

Polymer or            not usually     medium
liquid additives        with #3

Treated organic       not usually     medium
materials/Inorganic     with #2
salt

Treated organic       not usually      high
materials/Polymer      with #2,3
or liquid additive

Inorganic             not usually       low
salt                    with #3

Organic additive          all           low

Cellulose             not usually       low
                      with #1, 3