Additives.

Part 2 of this practical series explores the basics of proper use - and misuse - of clay and other green sand additives.

While the first installment of this series (November '95) focused on different sands used in green sand molding, this article describes the proper use (and misuse) of green sand additives - those items added to fix certain problems to help you achieve the most from your green sand system.

Table 1. Advantages and Disadvantages of Bentonite Clays

Sodium (Western) Bentonite

Advantages

1. good green strengths with high hot strengths

2. highest green and hot deformation

3. more durable than calcium bentonite

4. best clay with hot sand

5. highest wet tensile - best draw

6. toughest - most deformation

7. holds suspension in slurry operations

Disadvantages

1. least flowable of clays

2. highest water requirement

3. most difficult to mix (will ball in mulling if muller isn't in good shape or if too much water is added).

4. lumpy shakeout - high dry strength

5. may hang up in hoppers

Calcium (Southern) Bentonite

Advantages

1. strongest bonding clay, highest green strength

2. most flowable - rams or squeezes denser

3. mulls in faster

4. operates at lowest moisture

5. lowest dry and hot strengths

6. easiest shakeout, best collapsibility

7. less broken or cracked castings

Disadvantages

1. most brittle - poorest draw

2. most moisture sensitive and poorest with hot sand

3. least durable

While each additive was probably introduced for a specific purpose - to fix a problem - it often created new problems and a new "fixer" had to be added. By the time the new fixers were added, the sand mixture looked like my wife's stew, consisting of everything left in the kitchen.

I once asked a foundryman why he added iron oxide to the mix, and was told, "Charlie added it and Charlie died and there ain't nobody messing around with Charlie's mix." The oxide used at that foundry will stay forever.

To illustrate the common misuse of additives, say a foundry adds cereal to its sand system to increase its green deformation while adding silica flour to decrease metal penetration. These two additives may actually negate each other's beneficial characteristics. If an additive is needed, use it correctly and at the proper levels. A little may be good, but a lot isn't always better.

When the additive is no longer needed because of improvements in mulling, patterns, hot sand, casting configuration, etc., take it out of the sand system. Again, the best rule of thumb is that if you don't need to add something, don't. Use the KIS system: Keep It Simple.

This article examines each of the additives commonly associated with green sand systems.

Clay

Because the clay is used to bond or hold the mold together, it is necessary to select the proper clay and control its level in the system. The three types of clay used in the foundry industry are sodium (western) bentonite, calcium (southern) bentonite and fireclay (kaolinite).

Each of these clays has specific properties that give advantages for special casting applications.

Table 2. Pounds of Clay Required Per Ton of Iron Poured

Clay                               lb Needed

Sodium (Western) Bentonite           13.3
Calcium (Southern) Bentonite         20.0
Fireclay                             22.9

(based on 40 min. shakeout time and 2:1 sand to metal ratio)

Sodium Bentonite - Sodium bentonite provides a high hot strength and is used in steel and iron casting, where the metal temperatures and pressures require this higher hot strength characteristic.

This clay has the capability to hold large quantities of water and can swell 14-21 times its size when exposed to sufficient moisture. This water retention phenomenon helps reduce scab-related defects caused by the evaporation of moisture from the mold/metal interface and subsequent re-condensation away from the casting surface.

Normally, this bentonite isn't used by aluminum foundries due to its high hot and dry strength, and because the effective temperature of destruction for this clay is well above the pouring temperature of the molten aluminum. The removal of the sand from the casting would require high mechanical energy and could damage the aluminum castings.

Calcium Bentonite - Calcium bentonite is similar to sodium bentonite but due to changes in chemistry, it is classified as a non-swelling bentonite and provides much lower hot strength. This lower hot strength provides excellent shakeout properties, and this is the clay of choice for most nonferrous operations. Calcium bentonite provides a slightly higher green strength than sodium bentonite and requires less mulling energy to evenly distribute and develop the clay coating. Table 1 compares the pros/cons of both types of bentonite clays.

Fireclay - Fireclay is different from the bentonite clays. The clay particles are much larger and it requires a significantly higher amount of fireclay to provide the green strength of the bentonites.

To obtain the same green strength as 5% of either sodium or calcium bentonites, 12% fireclay is required. At the 12% addition level, the fireclay provides hot strengths similar to sodium bentonite. Combinations of the fireclay and sodium bentonite provide hot strengths higher than either of the individual clays. Table 2 shows how much of each of the three types of clays is needed per ton of iron poured.

Table 3. Physical Properties of Individual Clay Types

Clay Type                     Sodium      Calcium     Fireclay
                              Bentonite   Bentonite

% Clay                           5.0         5.0        12.0
% Moisture                       2.5         2.5         3.0
Green Compression               11.2        12.4        10.2
Dry Compression                 101           69        71.5
Hot Compression at:
1000F (537.7C)                  110           55          75
1500F (815.5C)                  210          103         145
1850F (1010C)                   520          150         170
2000F (1093.3C)                 345           72         510
2500F (1371.1C)                   8            4          27

When any of these clays and water are added to the sand at the muller, the development of the clay is time- and energy-dependent. The bentonite particles are in the form of a deck of cards, with the water being absorbed between the "cards" or platelets [ILLUSTRATION FOR FIGURE 1 OMITTED].

As the water enters the spaces between the platelets, the clay is softened and can be distributed on the sand's surface. Since the clay platelets are small (sodium bentonite - 0.2 [[micro]meter]; calcium bentonite - 0.5 [[micro]meter]; fireclay - 20 [[micro]meter]), the small openings between the platelets require a great deal of time to fully absorb the water and soften the clay for distribution (for reference, a human hair is 80-100 [[micro]meter] in diameter).

Table 3 illustrates the physical properties developed by additions of 5% bentonite or 12% fireclay to a silica sand system.

Blending the clays helps obtain properties that the individual clays can't provide. Table 4 shows the properties obtained when blending the clays. The total clay content has been adjusted to reflect a typical 12% addition level for the fireclay and a 5% addition level for the calcium and sodium bentonites.

Excess additions of clay require an increase in the moisture addition to activate these clays. If the water and clay levels are both increased, the heavy molding sand becomes difficult to feed through hoppers and be compacted into molds. This heavy, wet sand resists compaction and may cause mold wall movement, oversized castings, a rough wire wool appearance on the casting surface, penetration, difficult shakeout and hot tears in the castings.

If the clay is increased but the water remains at its previous level, there won't be sufficient water to develop the clay present, and the sand will appear dry and friable. This type of molding sand results in cuts, washes, sand inclusions and premature shakeout (run-outs).

Cereals

Certain additives accomplish specific changes in sand properties during sand preparation, molding, shakeout and the production of sound castings. One of these is cereal.

The types of cereal used in foundry sands can come from many different grains. Some of the types of cereals used for foundry molding sands are corn flour, milo flour, wheat flour and rye flour.

Cereals are generally added to increase the plasticity or deformation of the sand. Together with water, the cereal (such as wheat flour) makes a paste and provides a plasticity that reduces the sand's brittleness, often enabling the foundrymen to draw deep pockets that couldn't be drawn otherwise. Since the cereal forms a paste (like Elmer's Glue) when it is allowed to dry, this paste aids in reducing the friable edges on the molds and the associated increase of sand inclusions in the castings.

When cereal is exposed to water and then heat, the soluble starch and sugars are carried by the moisture to the mold surface. There, the water evaporates, leaving the starch and sugar as a bonding agent on the surface of the mold.

This deposition of the starch and sugars helps hold the sand grains together, decreasing the friability of the mold surface. In "hot sand" molding systems, cereals can help reduce the loss of moisture and avoid sand inclusions caused by the dislodging of the grains due to moisture loss at the mold's surface.

If the quality of the patterns is poor and they have vertical walls, back drafts or gouges on their surfaces, adding cereal provides plasticity and aids in drawing the molds from the patterns.

What happens if I add too much cereal? Because cereal is an organic material, it decomposes when exposed to the heat of the metal and will generate the gases associated with this decomposition. Excessive amounts of cereal can produce pinholes, gas blows, misruns, cold shuts and other gas related defects.

Cereal absorbs water quickly and can take water away from the clay, thus reducing the bond strength developed from the clay. If a small portion of cereal is placed in a microscope and water is added, you can see the cereal rapidly absorbs water and expands, similar to popcorn when exposed to heat. This quick absorption ties up water that may be needed for the clay's softening and distribution on the sand's surface. If excess cereal is present and additional water is added, the bonding - caused by the combination of the cereal and clay - may result in difficult shakeout and casting removal.

Cellulose

Cellulose additives, such as wood flour, oat hulls, rice hulls and ground nutshells, tie up excessive moisture in the sand. They are generally added to enhance the flowability of the sand during the mold fill sequence, and to aid in promoting a better breakdown of the sand during the shakeout cycle.

The particles of wood fiber hold the sand groins apart during the compaction phase and are thermally degraded by the casting during pouring and solidification. The reduction in size of the wood flour, when exposed to the metal temperatures, allows the sand grains room to expand and fill the newly created voids. This degradation of the wood flour promotes a better shakeout and the removal of the sand from the castings.

What happens if I add too much cellulose? Cellulose are fractured or ground organic products with a porous or open structure. These products quickly absorb moisture and give the sand a greater flowability by inhibiting a portion of the moisture present from reacting with and being absorbed by the clay platelets. Because the cellulose products don't bond the sand grains, and the porous structure surrenders its moisture through evaporation into the air, this lower water-to-clay ratio can result in dry, friable molds.

All organic additives decompose when exposed to the temperatures present during metal pouring, and the gases, if not adequately vented, may result in gas-related defects. Excessive additions of cellulose can cause premature shakeout, cuts, washes, sand inclusions, mold wall movement and oversized castings. Since these products don't form a starch or sugar, they don't contribute to the plasticity of the sand and don't form the "glue" like that of cereals.

Table 4. Hot Compression Strength

Blend                 1000F     1500F     1850F     2000F     2500F

75% S.B./25% C.B.       70       185       395       227        3
50% S.B./50% C.B.       67       185       280       155        3
25% S.B./75% C.B.       60       110       150       105        2
75% S.B./25% F.C.      215       350       575       545       10
50% S.B./50% F.C.      190       300       535       470       11
25% S.B./75% F.C.      270       350       775       910       24
75% C.B./25% F.C.       76       140       255       210        8
50% C.B./50% F.C.      100       110       265       325       21
25% C.B./75% F.C.      120       155       240       400       18

S.B. = sodium bentonite; C.B. = calcium bentonite; F.C. = fireclay.

All clays were adjusted to provide a typical proportion of each of
the respective clays (i.e., the 50% S.B./50% F.C. mixture would
contain 2.5% S.B. and 6.0% F.C. for a total clay addition of 8.5%.)

- from CMI "Principals of Sand Control."

Iron Oxide

Iron oxide isn't normally added directly to the green sand system, but is generally introduced by the decomposition of cores containing the oxide additions.

When iron oxide is directly added to the green sand, it is usually done to introduce fines to provide a higher degree of mold rigidity by increasing the grain-to-grain contact. The lower melting temperature of the iron oxide can provide a thermal cushion to the sand to reduce expansion-type defects. This thermal expansion is only beneficial if the pouring temperature of the metal is higher than the melting point of the oxide.

What happens if I add too much oxide? Iron oxides are a finely ground inorganic material that fill the voids between the sand grains, reducing the permeability of the sand. This reduction in permeability may result in pin-holes, blows and other gas-related defects. The fine grains of the oxide increase the contact points between the sand grains, and can provide a higher compression strength. This higher value might cause the foundryman to reduce the clay addition, which may result in erosion-type defects.

The fineness of the oxide provides a large surface area, which ties up moisture and reduces the water available for absorption by the clay. The increase in the compression strength caused by increasing the contact points between the sand grains, the filling of the voids between the grains and the reduction in moisture available for the clay provides a brittle sand that is difficult to draw from the pattern pockets.

Excessive levels of oxides lower the refractoriness of the sand and can result in burn-on with the sand fused to the surface of the casting. This defect is usually first noticed on the ingate areas and in the heavy sections of the castings.

Polymers and Chemicals

Polymers are a fairly new addition to the molding sand formulations. These additives are used to lower the surface tension of the water and to make the clay platelets easier to wet and soften. The basic categories of polymers and chemicals are soda ash (sodium carbonate), wetting agents, chemical modifiers and organic polymers.

Introducing hydrocarbons (hydraulic oil and parting agents) and residual core sands often produces sand and clay particles that don't readily absorb water. Polymers are often used in foundries having problems with hot return sand and those with insufficient mulling and sand cooling capacities.

Additions of chemicals like soda ash are made to increase or control the pH of the sand. The excessive use of cereals or the introduction of residual acids from the acid catalyzed chemical binders can lower the pH of a green sand system, thus depressing the performance of the bentonite.

What happens if I use too much soda ash or polymers? Because these additives are introduced to activate clay, which is usually latent or dormant, the foundryman observes an increase in the muller amperage and the green compressive strength, and the sand becomes stiff and resists compaction. The normal response is to reduce the clay level and bring the system back to its typical performance limits.

The increased strength and mulling efficiency are the result of the use of clay that is present but hasn't been wetted and developed as a bonding clay. This clay provides a cushion for normal variations, like the temperature or moisture level in the return sand. By reducing this available (latent) clay, the sand is much more sensitive to these variations.

Defects like wet layer scabs may be observed, or the sand may become stiff, and this resistance to the molding compaction may result in a rough casting finish and erosion. The molding sand may become brittle, causing broken molds or the inability to draw deep pockets, and sand sticking to the pattern.

Seacoal and Other Carbons

Seacoal is used in iron foundries to provide a better peel or finish to the castings. There are three theories as to how seacoal works in metalcasting.

The first concept is that the seacoal decomposes as the metal enters the mold and forms a reducing gas interface between the mold and the metal. The second theory is that the seacoal forms a lustrous carbon coating on the mold surface, filling the voids between the grains to enhance the casting finish. The third is that the seacoal expands in the reducing atmosphere and fills the voids in the sand, stopping penetration by extruding the carbon into the casting area and forming a carbon interface between the mold and the metal.

Often, combinations of combustible agents are used to provide the properties required. Combustible additives may be used individually or as a combination of these products.

The most common types of combustible agents used in the foundry are seacoal, asphalt, gilsonite, lignite (causticized) and petroleum distillates.

What happens if I use excessive amounts of these combustible agents? Because the materials are added for a reducing atmosphere and to provide carbon, they generate volumes of smoke and gas. If the mold isn't properly vented, this gas can result in porosity, misruns, cold shuts or cope lift.

Excessive carbon in the mold cavity can displace the metal and produce a defect called kish (a mottled surface finish generally found on the cope surface of the casting). The castings may appear blue, especially in thin sections or flash areas.

Excessive quantities of asphalt or gilsonite can waterproof the sand and the clay particles, inhibiting the development of the sand/clay bond. Excessive amounts of the fine seacoal and the other carbon additives may fill the voids between the sand grains, lowering the sand's permeability while increasing the gases that must be vented.

To help the foundrymen control the levels of these additives, several companies preblend the molding sand additives in proportion to the thermal degradation they will incur during the casting process.

These mixtures are generally based on the properties the foundry needs to make the molds and its analyses of the sand returning to the muller for reuse. If changes in the sand-to-metal ratio, cooling times, core weights or other process changes take place, the foundry must advise the preblended additive manufacturer so they can make any required adjustments in the blend.

Water

Water is the most abused of all of the additives. Because water is cheap and doesn't require physical effort to put it in the muller, it is often the least controlled additive.

Water affects nearly every physical property that must be controlled to produce good molds and castings. Excessive additions of water expand the clay and resist compaction, often leading to penetration-type defects. Excessive water additions lower the green compressive strength and increase the dry and hot compressive values. The clays and other additives need water to be distributed on the sand surface, but too much or too little water can be a disaster.

If, in your bare feet, you were to walk along a river bank in the clay area by the water's edge, you would sink into the clay and it would squash between your toes (low green strength). If you took this wet clay and baked it, in an oven, it would take on the characteristics of adobe (high dry and hot strength).

Now, step back from the water's edge to an area that has the same clay, but is drier. The clay is hard and resists your weight (high green strength). If you were to bake this clay it would be very friable and form very weak bricks (low dry and hot strength).

Green sand systems with low water content often incur cut/wash and erosion-type defects with sand inclusions. Green sand molds with excessive moisture may cause mold wall movement, penetration, misruns and gas defects.

RELATED ARTICLE: Green Sand Additives Checklist

* Clay

Added to bond sand together.

Types: sodium bentonite, calcium bentonite, fireclay.

At low levels will cause broken molds, cuts and washes, poor draws, burn-on, expansion defects. Too much causes poor shakeout, poor casting dimensions, poor flowability, mold voids.

* Cereals

Added to control sand expansion, increase green deformation and lower friability.

Types: corn flour, milo flour, wheat flour, rye flour.

Too much causes resisted compaction, increased stickiness, poor shakeout, gas-related defects, metal penetration, change in pH.

* Cellulose

Added to control sand expansion, increase mold collapsibility, improve shakeout, improve friability.

Types: wood flour, oat hulls, rice hulls, ground nutshells.

Too much causes brittle and friable molds, metal penetration, erosion, cuts and washes, gas-related defects.

* Iron Oxide

Added directly to introduce fines to provide a higher degree of mold rigidity by increasing grain-to-grain contact.

Too much causes burn-on defects caused by reduced sand refractoriness.

* Polymers and Chemicals

Added to lower the surface tension of the water and make the clay platelets easier to wet and soften.

Types: soda ash, wetting agents, chemical modifiers, organic polymers.

Too much causes wet layer scabs, resisted molding compaction, broken molds, inability to draw deep pockets from pattern.

* Carbons

Added to obtain peel, control expansion, decrease fusion between sand grains.

Types: seacoal, asphalt, gilsonite, lignite, petroleum distillates.

Too much causes: gas-related defects, kish defects, inhibited sand/clay bond, reduced sand permeability.

* Water

Added to activate the clay and prepare it for distribution on the sand; achieve benefits of other additives

Too much causes: oxidation, gas, lack of flowability, difficult shakeout