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Green Sand Metalcasting Foundry News

"S" - Glossary of Foundry Additives (Including Southern (calcium) Bentonite)

Posted by Hill and Griffith Company on Jul 25, 2018 3:32:50 PM

S - Foundry Additives Glossary

This oil is not as popular as other plant oils used in core oil formulas. Safflower is a plant native to the East Indies, Asia, and Egypt. The oil is pressed from the seeds of the plant. It is used sparingly in foundry core oils.

The common name for sodium chloride is "salt." The mineral name is "halite." Its chemical formula is (NaCl). It is used in the foundry to harden core and mold coatings so as to give a glaze to mold and core surfaces. Salt is used to glaze refractories such as ladles, linings, and as an addition to clays and other clay-like minerals to lessen their viscosity when used with water slurries. The Moh's hardness of salt is very low, only 2.5. Its specific gravity is 2.1 to 2.6. It has a melting point of 1472° F. (800 ° C.). Expendable cores are made from salt brines for use as cores in casting light metal, such as aluminum. Wonderful results have been obtained with the use of salt cores. Salt brines are used in the quenching of metals. Salt has a world-wide use, but it is not commonly used in sand mixtures of the foundry.

Sand is a specific particle size of grains, rocks, or minerals. Sand is a size, not a mineral. It is formed in nature by winds, water action, and other elements of the weather. Sand composes much of the earth's surface. The most useful and commercial sand found for use in the foundry is silica, but other materials such as zircon, chromite, olivine, silicon carbide, or ceramic materials may be called "sand," if they are of that "size." There are more sand foundries in the United States than those casting by any other process. Metal cast into sand molds comprises 96% of the total foundry industry. Die casting into metal molds represents about 3%. All other methods account for the balance of 1%. Silica sand is perhaps the best known sand used in the foundry for making molds, cores and refractories, but there are many minerals used as "foundry sand." (Also See: CHAMOTTE-CHROMITE-FOUNDRY SAND-HEVI-SAND­MOLDING SAND-NATURALLY BONDED SAND-OLIVINE -SILICA-SILICA SAND-SILICON CARBIDE-SYNTHETIC SAND-TENNESSEE SAND-ZIRCON SAND.)


Banding sand obtained its name as an "abrasive sand" which is used for band grinding tool handles. Banding sand is used for the grinding of plate glass as well as for scouring abrasives. The foundry uses fine banding sands, not for grinding purposes, but rather for their fine grain size, so as to give better detail and finish to castings pro­duced. Banding sands are the finer grades prepared in the washing, grading and screening operations. Much banding sand is sold from the Ottawa, Illinois district to the foundry industry.

Some building sand is used in the foundry industry, particularly those foundries located near rivers, lakes or sand dunes. Building Sand is usually too impure for use by the foundryman. Any variety of sand fine enough can generally meet the specifications of a building sand. Earlier specifications meant that the sand grains were to be sharp or angular, but these specifications have now been erased and any size or shape is generally suitable for other than foundry purposes. It should be clean, but Building Sand normally contains too much clay, loam, organic material, and mineral ingredients such as shells, lime and insects to be acceptable universally by the foundry industry where best casting results are sought. Nevertheless, some Building Sand is used by certain foundries where casting specifications are not too stringent and where facings cover the Building Sand mixtures used.

Silica sand is employed in glass making. It is a washed, screened and graded silica sand of a higher commercial purity. Although the nomenclature of this sand indicates it is used solely for glass manu­facture, the foundry industry consumes many thousands of tons in the making of various castings.

Is a compounded, blended, or formulated sand with selected in­dividual materials such as clay, carbons, cellulose, and water. When mulled with temper water, it produces a molding sand mixture that gives proper molding properties which is then used to make foundry molds or cores. The principal ingredients are a base sand, a clay bond, carbon, if desired, perhaps a small amount of cellulose for cer­tain purposes, such as cereal or wood flour for specific properties.

A fish oil, or marine oil, is produced by pressing sardines and obtaining the oil. Sardine oil was used in many commercial core oils, but less is used today.

The use of "coal dust" in molding sand originated in England. It was later introduced to the North American foundrymen as "sea­coal." One particular English or Welsh coal seam appeared to be the most satisfactory for foundry purposes and became the standard with which other coals were compared. Some authors believe that since the coal seam was near the Welsh coast and the mines were deep under the ground, extending under the sea, the term "seacoal" emphasized both its origin and its sea transportation to the U.S.A. However, the author has pursued the term "seacoal" for many years and believes that it originated from Lord Dudley's nomenclature in his patent applications in 1620. Lord Dudley applied for a patent in behalf of his son, "Dud" Dudley, for "melting oron ewre (ore) and of making the same into caste workes or barrs with "SEA-COLES" or "pit-coles" in furnaces with bellowes. " These "sea-coles" were described as being low in ash, sulphur and impurities for making the metal. The foundryman thought the same coal should be used in making sand molds. After a long and tiring search by the author, an old English file produced this answer after many years of investigation.

Seacoal (coal dust) is a highly volatile bituminous coal, ground to various degrees of fineness by pulverizing mills which is then graded by screening or by air classification methods. It is mixed and mulled with molding sand in various proportions. When it, or the products of its destructive distillation come into direct contact with molten iron, seacoal seems to improve the casting finish. Where molds are cast which contain seacoal, the sand peels more freely from the castings and the cleaning of the castings are more easily accomplished. Seacoal is one of the least expensive materials used in molding sand to give these results. Seacoal has been termed a "facing" in the foundry as a result of this property.

The effect of seacoal is so well known that it is accepted as a requirement in the pr3oduction of gray iron, ductile (nodular or S.G.) iron and malleable castings. The many advantages it offers have perpetuated its foundry use. 

Several principal factors must be considered in the selection of seacoal (coal dust). The Volatile Combustible Matter, Fixed Carbon, Ash Content, Sulphur, and Moisture Content of the coal source measures its quality. Cost and reliability of the producer are the most important factors when purchasing the coal. Other factors to be considered in the selection of a specific grade of seacoal are: 1) The grain fineness of the base sand, coupled with its sand grain distribution. 2) Type and amount of bonding material used. 3) Permeability of the molding sand desired. 4) Weight of the casting to be produced. 5) Desired surface finish of the casting. 6) System of gating the pattern. 7) Metal pouring temperature and length of pouring time. 8) Density of the rammed sand mass around the casting. 9) Length of time the molds stand after pouring. 10) Type and temperature of the metal poured.

There are many additional variables to be considered, as seacoal affects most of the properties of the molding sand. Investigators claim that seacoal principally acts as a reducing agent, thus it prevents sand from adhering, or burning-on to the casting. Others dispute this claim.

Functions of Seacoal as a Sand Additive
Coal and coke in the molding sand burn, thus consuming the oxygen in the mold cavity to provide a reducing atmosphere as the mold is initially poured. Heat is absorbed by the mold from the molten metal. The temperature is increased to the "coking" temperature range of the seacoal present. During "coking," uncondensable reducing gases are produced, such as, hydrogen, methane, ethane, tars, light oils, and others which are distilled-off. The tar fraction plus the light oils burn and then contribute a carbon film or "sooting" action on the mold as a "bonus" factor, acting as an inhibiting agent. The volatile matter in seacoal consumes some of the oxygen immediately in the mold cavity as the metal ignites the seacoal upon entering the mold. This sooting prevents cutting action of the flowing metal against the molding sand and inhibits fusion of the silica sand grains. The gas content from the seacoal tends to form a cushion at the surface of the mold-metal interface, and the metal lies more evenly and quietly. Metal penetration is re­duced by the formation of the gas cushion which is aided by the coal's coking action. The gas pressure fills all the mold's voids between the sand grains giving the desired smooth casting surface but still permitting the venting of noxious gases. A certain amount of mold gas pressure is desired to help prevent metal penetration.

Sizing of The Seacoal
It is believed that seacoal should approximate the same size as the base sand grain in order to not alter the permeability of the mold­ing sand, but this is only an opinion. Seacoal on losing its volatile matter becomes coke, a fixed carbon. Some coals on coking swell to nearly three times their original volume, which may be detrimental to the surface of the castings. The coking theory has not been fully defined or proven, but the author believes it is a functioning part of the seacoal in helping to overcome apparent metal shrinkage. Some foundrymen claim that fines (those less than the U.S. No. 200 Standard Sieve) should be removed from commercial seacoal. It is the author's opinion that to narrow the range of particle sizes might exhibit a lag in the formation of the seacoal's gas cushion which it generates on heating. The reducing atmosphere which seacoal de­velops should form instantaneously, and removal of the fines may act as a disadvantage in certain cases. To cite an example, one cubic foot of coal in the solid state burns rather slowly when thrown into a flame; whereas, if it is ground to fine particle sizes, the coal would instantan­eously ignite, if subjected to the flame combustion. Time is a factor to be considered. Foundries find it detrimental to remove too many fines from foundry sands, as metal penetration and rough casting sur­faces can result. Since sand surfaces in the drag side of the casting is subjected to metal weight, the metal tends to force easier into the interstices of the sand grains, as the sand voids increase in size. Wash­ing and cutting of the metal may also result around the gates from too-open sands as they are more brittle. Open sands are also difficult to patch, or work. A carbon film is highly desirable in most cases to improve the finish of the castings.

Amounts Added
Too much seacoal increases the temper water demand due to increased surface area of the mixture. This causes rougher castings. Excess seacoal creates an evolution of gas which may cause blows, or porosity in the castings. Molding sand which is too rich in seacoal may promote defects called, "Map of Ireland," "Fins" or "Veining." Seacoal in moderate amounts is very beneficial, but as with the case of any other raw material used in the foundry, too much is detrimental. Seacoal depreciates hot compression strength at 1250°F. and 2000°F. (1010° C. and 1093° C.) rather effectively.

Seacoal is sold by grade designations namely: A, B, C, D, D-½. A and B grades are coarser and are used for molding heavier castings, C to D-½ grades are recommended for lighter to medium castings and for giving more detailed surface finish. Foundry facing suppliers have attempted to standardize on the best three commercial grades, namely: B, C, or D grades. "Dustless" seacoal (treated) and standard non-treated grades are available through most foundry supply houses. "Dustless" seacoal is a ground coal which has received a secondary treatment of oil or waxes to minimize dust when it is handled in the foundry. 

Analyses and Screen Tests of Seacoal

Seacoal Analysis

The analyses and screen tests (Table No. 25) are approximate. They may vary at least 5% on each sieve depending upon the sup­pliers choice of sieves, and arrangement of sieving with the customer.

The Properties of Seacoal

 Properties of Seacoal

Green Compression Strength-Increases (generally, as long as the temper water doesn't increase).
Dry Compression Strength-Increases, as temper water increases. (6% by weight of seacoal increases dry compression strength approximately 35%, as temper water is also increased to give workability-moldability.) 
Hot Compression Strength-Decreases, as seacoal furnishes a reducing mold cavity atmosphere.
Permeability-Generally decreases, due to the high "fines" content of the commercial grades of seacoal.
Flowability-Decreases, as the water demand increases in the molding mixture.
Temper Water Required-Increases by 10% of the weight of seacoal contained in the mixture. (Most of this water is held on the seacoal's surface, instead of being absorbed as in bonding agents. However, seacoal which is coked or formed into an ash sometimes acts as a sponge and absorbs an excessive amount of temper water. This build-up must be avoided by new clean sand additions, otherwise the molding sand becomes ashy and brittle.) 
Mold Hardness-Increases, if the same effort or work force is applied. (Even though the mold hardness increases, the metal tends to lie more quietly in the mold when seacoal is present, than when it is not. Avoid high mold hardness on thin section castings which freeze rapidly.) 
Deformation-Increases up to 6.5% by weight seacoal. The increase is dependent upon the seacoal's grain size. Temper water is gen­erally increased. If the water does not increase slightly, the mold­ing sands become brittle and break easily. Both wood flour and seacoal (coal dust) tend to make a smoother, softer and more moldable molding sand when used in regular amounts up to 4% to 5% seacoal, or 1% wood flour additions. Excessive amounts of seacoal and wood flour result in molding sands becoming low in resilience, they become brittle, they are difficult to handle and are poor in general practice. 
Refractoriness and Sintering-Increases, which is possibly due to the carbon or carbon film developed during pouring of the metal.
Volume Changes in Molding Sand-Less expansion and contraction characteristics of the mold is beneficiated by the use of seacoal. Many of the common defects which are directly associated to the expansion and contraction of sand mixtures seem to disappear when seacoal is present. Seacoal is related to the clay content, moisture content, sand grain distribution and particle grain size. The beneficial limits of seacoal when added to the molding sand tend to vary under different working conditions. 
Mold-Wall Movement-Effect of: ( 1) It is found that variations in mold materials influence metal solidification. Seacoal minimizes mold-wall movement. (2) As carbonaceous materials such as seacoal and wood flour are gradually increased in a molding mixture, within a definite limit, the piping of an iron riser of the casting decreases. (3) In comparison to western bentonite or fire clay, southern bentonite appears to lessen mold wall movement, but seacoal addi­tions improve all three bond mixtures. (4) In experiments, wood flour decreases the piping tendency of the metal, which is further aided by seacoal additions. (5) As the temper water of the molding sand mixtures increases, the piping tendency of the metal increases, but seacoal additions help to overcome excessive additions of temper water. (6) It appears that a dense, hard rammed mold results in less movement of the mold-metal interface and less seacoal is required. (7) Oil bonded core sands produce sound castings, but so does seacoal when added to green sand mixtures. Metal exudes slightly from the riser instead of piping when there is a lack of mold-wall movement. Sufficiently rammed molds in green sand act similarly. (8) Mixtures of molding sands and bonding agents are very complex. It is illogical to make definite comparisons between different mixtures. Each base mixture should be considered on an individual basis, but 5% by weight of seacoal in gray iron, ductile (nodular or S. G.) iron and/ or malleable iron castings helps to hold castings closer to pattern size and to have lesser mold-wall movement.
Refer to: Bohm, W.F., Metallurgist, Buick Motor Co., Div. GMC, "Mold Materials are Factors in Gray Iron Shrinkage," and Sanders, C.A. and Sigerfoos, C.C., "Gray Iron Shrinkage Related to Molding Sand Conditions," Reprinted from American Foundryman, January (1951) and February (1951).

A self curing binder is one which, when mixed with sand and an appropriate catalyst or accelerator, produces a castable sand mass, which then gradually hardens without external application of heat or chemicals. Sub-categories are: acid-curing resins, urethane-curing resins, oxygen-curing oils, alkyd resins and powdered metal-curing silicates. Some claim cement, sulphate resisting cement, dicalcium silicate and ferro-silicon powder, all or one, added to sodium silicate sand mixtures are self-hardening molds or cores.

Is a pale-yellow bland oil obtained from the pressing of seeds of the tropical plant-sesamum oriental. It is blended with other oils to form commercial core oils used in the core room. It has a specific gravity of 0.92 to 0.925.

This is a clay-like mineral formed under pressure and developed by nature's actions and reactions. It is largely composed of silica and alumina with various impurities present, such as iron oxide. Illite is the mineral constituent of most shales; some shales contain mineral oils and constitute a future supply of fuel oils. Shale is used with limestone for making Portland cement. Slate is another form of shale. As foundries sought higher metal pouring temperatures, less shale clays were used. Today, fire clays and bentonites are used principally for re-bonding foundry sands and for refractory purposes, as shale lacks many required properties.

Crude stick-lac is deposited by an insect. It is subsequently processed into a usable product. The majority of commercial foundry shellac comes from India. Shellac is about 91% resinous, but is often diluted with common rosin oil for foundry use. It is soluble in alcohol and in alkalies. It is used for various coating materials in the foundry and widely applied in certain pattern shops as a coating material. It is used as an ingredient in certain liquid coatings to give adherence to cores and molds. Shellac added to a light-off spray holds the refractory to the mold after it has been heated or lighted-off.

Are predominantly thermo-plastic phenol formaldehyde resins with hexamethylene tetramine (Hexa) in the mixture. Modifications to the base resins are made by the addition of wood resins and other ingredients such as aromatic carbolic acid.

Shell molding or Cronin C-Process is a method of forming molds which is based on the investment of coated sand on a hot 400°F. to 600°F. (204° C. to 316° C.) pattern plate to form a shell ¼" to ½" thick. The base sand is coated with a thermal-setting resin which, together with the processing and close control required, is quite expensive. For this reason the economic application of the Shell Process is somewhat limited and has been generally associated with forming cores. The resin melts, reacts with the hexa to increase the mol-ratio of formaldehyde to phenol and becomes a thermo-setting resin in the zone adjacent to the hot pattern surface.

A "Novalak" or "Novalac" phenol resin is used to coat heated sand grains while they are being mulled and agitated. When cooled these coated sands are ready for pattern application, heat forming, and molding. A typical resin formulation for shell core making is composed of 100 parts by weight phenol, 70-72 parts by weight formaldehyde (37%) and 0.5 parts by weight oxalic acid, as a catalyst.

Are produced in a number of physical forms. Most as powders, but some are liquid in an alcohol or acetone solution having 60% to 75% resin solids. They may also be water borne solutions (about 4% alcohol, 75% solids, the remainder water). Shell resins may be commercially offered in flake or granular shape.

A mineral having the composition (SiO2 ). Silica is found in most all minerals. It occupies over 60% of the earths crust. It has a Moh's hardness of 7 in the form of quartz and has a specific gravity of 2.65. Pure silica has a melting point of 3182° P. (1750° C.). Its purity determines its refractoriness. Silica occurs in various forms and shapes for use in the foundry as "foundry sand." It has wide industrial uses and one use is as a refractory in the foundry. Silica sand is the base aggregate used for making molds and cores. "Silica flour" is made by grinding silica sand into fines. It is used in a number of foundry operations.

What is It?
Silica flour is a finely ground silica that is available in several commercial grades. Silica flour is called "flint flour" in the ceramic industry. Most users prefer 95% to pass a U.S. No. 100 Standard Sieve. Other users prefer 95% passing a U.S. No. 140 Standard Sieve. Some prefer a still more finely ground product. The very fine silica flours are called "200 grade," "325 grade," or finer. These grades generally refer to the "mesh" or "sieve" size, but actually the grade indicates the bulk of the flour that passes that U.S. Standard Sieve number, or the equivalent Tyler "mesh" screen. Use Contrary to accepted thought, the addition of silica flour to a foundry sand mixture, or the presence of those fines that are finer than the U. S. No. 140 Standard Sieve in a system sand, does more than simply seal-up voids and increase hot compressive strength. Fundamentally, silica flour is an additive having smaller particle sizes which have a different order of magnitude, than those of the base sand. In order to properly understand the action of such small silica particles, it is first necessary to study their effect on unbonded dry compacts; then to determine what happens when the bond is added to the mixture. Finally, the effect of heat shock imposed by liquid metal on the bonded sand with silica flour in the mixture should be considered. The effect of the variations of particle sizes in an unbonded silica compact has been studied both theoretically and experimentally. The results of these studies can be accepted as authoritative. A mass of round uniformly sized, unbonded particles can be compacted to a mass density of 60.5 percent having 39.5 percent voids. Further compaction requires greater force, i.e., a force higher than usually available in the foundry. The mass density is unaffected by the size of particles, provided they are all of the same diameters, as the percentage of voids remains the same. Rounded grain foundry sands, such as those from the Ottawa, Illinois, or Portage, Wisconsin districts, closely follow this theoretical statement. These sand grains are not theoretically spherical, or of the same identical uniform size, but the test results are similar to the calculated results. For example, the calculated density of 60.5 percent is equivalent to a weighed sand mass of actually 99.8 lb./cu. ft. Dry, rounded grain sands have about this same mass density. Sub­angular sands are slightly less in density, as predicted by theoretical calculations. If higher mass density is required for any reason (resistance to metal penetration is one), higher density of the sand mass can only be obtained by increased applied pressures on the molding sand, or by spreading the sand particle sizes to fill the different sized void spaces. The addition of U. S. No. 200 sieve particles when added to No. 20 U.S. standard sieve particles, increases the density seventeen (17) percent.

Early Foundry History
Until the early 1940's steel foundrymen were taught to use very open sands having permeabilities as high as 300 to 500. It was the belief of many authorities of that period that permeability should be maintained this high or damaging gases could not escape and would impregnate the castings and cause losses. Most of these pioneer foundrymen were accustomed to working with naturally bonded sands, not synthetic (compounded) sands. As previously indicated, naturally bonded sands accompanied by a high clay content, tended to close-up the heap, unit, or system sand too rapidly and they absorbed an overabundance of temper water. 
Ramming affects naturally bonded sands to a much greater degree than when synthetic (compounded or blended) sand mixtures are used. The vent rod was formerly employed to a great degree where naturally bonded sands were used. This was done to insure open chan­nels so the gases could escape. Since this period, the vent rod or vent wire have virtually disappeared in the production foundries, but vents on the pattern are encouraged. Foundrymen formerly encountered excessive metal penetration, cuts, washes, and swells due to their fear of using a tighter molding sand. Foundrymen were afraid of the consequences if they rammed the molding sand too hard, or "too tight," as they stated. Gas developed in the mold has always been a fear to foundrymen.

New Slant
The foundryman's primary objective was, and is, to overcome cast­ing "burn-in." At all A.F.S. technical meetings much discussion arises on the subject of, "What is it - 'penetration,' 'burn-in,' or 'burn-on'?" Caine,1 has pointed out that most burn-in is nothing more than metal penetration in the sand voids. Burn-in is the wrong nomenclature, he maintained. He stated that the weight and pressure of the liquid metal forces its way into the interstices of the sand grains by the sheer force of static metal pressure. 

<1> J. B. Caine, Met. Sawbrook Steel Casting Co., Lockland, Ohio. Trans. of A.F.A., Vol. 50, "Report of the Sub-Committee on Sintering Test, Foundry Sand Research Comm. (1941-1942)." 

Other foundrymen do not concur; as some claim that metal temperature, mold-metal interface reaction, the metal's fluidity, and other chemical and gas factors further affect "metal penetration" conditions. Some investigators feel the chemical forces and multiple reactions involved at the mold-metal interface cause more of the "burn-on" than simply metal weight and pressure. Probably, all these reasons for metal penetration are sound. The author has proven in several past articles that: (1) the metal weight and pressure, (2) the chemical reactions and (3) the gas theories all contribute to metal penetration and metal burn-on. Silica flour has helped to prevent metal penetration in some cases, but failed in others.

Closing The Sand
Early foundrymen using open sand, (i.e., sand possessing very high permeability) desiring to correct metal penetration or burn-in had no alternative but to close the molding or core sand by filling the interstices or voids with large additions of silica flour or fines to the molding mixture. Many foundrymen used these fines in excess and thereby invited other types of trouble. Silica flour additions greatly affect the sand's mixture properties. It is important to control not only the fines but the temper water, bentonite, fireclay, binders and additives in the molding sand mixture. A fraction of a percent of silica flour added to molding sand affects the thermal properties of the sand mixtures tremendously. Silica flour must be rigidly controlled, whether it is used for casting one metal alloy or another. There are many opinions which can be expanded upon, but it is essential that one deals with facts.

Silica Flour Affects All Properties
A higher percentage of silica flour used by early foundrymen, particularly those casting steel, increased temper water requirements to the greatly increased surface area. Extra temper water and silica flour in sand mixtures exert considerable changes in the hot properties and expansion characteristics of the molding and core sands. Permeability of the sand mixture is decreased which prevents mechanical metal penetration, but the dry compression strength is greatly increased. The early foundrymen neglected to consider that the flowability of the sand mixture is also decreased by the preponderance of fines present. The hot compression strength increases greatly when as high as 20% silica flour is used in the mixture. Many castings having deep cylindrical pockets or deep cavities develop hot tears and cracks due to this greater resistant strength. Lumps and masses of molding sand are carried to the dump from excessive additions of silica flour, which gives increased dry and baked strengths.

Toughness And Flowability
Foundrymen gain sand toughness by incorporating silica flour in the molding mixture. Molds may be more carelessly handled by this increase in the toughness of the molding sand mixture. The increased toughness is primarily due to the extra temper water required from the silica flour addition. However, in making the sand mixture tougher, the change in its flowability is noticed by the necessary increased jolts or ramming at the molding machine. Five percent (5%) silica flour addition reduces flowability, if the temper water is also increased. Since labor costs are high, any increase in ramming must be considered before a final decision. The advantages and disadvantages must be carefully considered by management to determine if a change is warranted.

High Temperature Properties
The action of the molding sand must be studied at elevated temperatures, or the temperature to which the sand is subjected during the casting operation. Molding sand stability is increasingly recognized as an important element of control. The sintering point (regardless of importance) and refractoriness are both changed when silica flour is added to molding sands in normal working amounts. Mineralizers or ceramic fines sometimes act as fluxing ingredients and are responsible for lower sintering points. Further, fineness of all the ingredients influences the P.C.E. value, or refractoriness.

Volume Changes
Volume change and mold atmosphere seem to be the two most important factors to be considered in molding sand practice. When silica flour is added to certain molding sands, the expansion rate rapidly increases. More surface area is exposed and the increased temper water due to the fines present affects the expansion rate and its characteristics. High expansion causes the mold surface to be subjected to considerable compressive forces during casting. When conditions are irregular, high sand expansion develops which may cause the mold surface to spall, break or crack, causing defects such as scabs, buckles, rat-tails, veins, and sand inclusions, which are accompanied by excess casting scrap or rework. When sand grain distribution is not normal, the void spaces between the sand grains may either be too great or too small and the contraction of the molding sand may not be in balance with normal mold expansion. However, impervious dense sand may also have high damaging sand expansion. Both of these con­ditions are an extreme and tend to cause high sand expansion. Sub­sequently, contraction is decreased with additions of silica flour to molding sand.

Present Trend
To say that yesterday's foundrymen were incorrect in using large amounts of silica flour in early foundry practice, is contrary to the times. Although many foundrymen expected to minimize metal penetration into the mold, the real value of the silica flour may have been its contribution to improved higher pouring temperature properties of which the foundrymen may have been unaware. Castings that de­manded such mold properties benefited from very high silica flour additions. However, the modern day foundryman is finding it advantageous to use finer base sands (to date, the best overall working sand to supply good finish to the casting and furnish satisfactory properties is approximately an A.F.S. Grain Fineness Number 60), thus eliminating the expense of heavy additions of silica flour. Many prominent steel foundries have ceased using silica flour as an ingredient in molding sand. This elimination is considered more economical. Thus, when the base sand has proper sand grain distribution, silica flour may be unnecessary. Why add excess cereal, wood flour, organic bonds or southern bentonite to foundry sand mixtures to obtain lower hot strengths, then defeat the lower hot strengths by adding excessive silica flour or iron oxide additions which increase the hot strength? Some foundries use more than the normal amounts of silica flour and claim no ill effects, but they are paying a penalty by being forced to increase cereal, wood flour and clay bond to overcome the higher expansion of the added silica flour. In other words, the foundry is operating at a very high molding sand cost by adding excessive amounts of both cereal and silica flour, as each counteracts the other's prop­erties. Also the use of high percentages of very fine silica may be a health hazard in certain instances.

Casting Hot Tears
If a green sand is surrounded by liquid metal, the core is compressed as the liquid metal starts to contract upon cooling. If the sand core is rigid, or expands, and retains its hot strength too long, the casting may show a strain or a hot tear. Excessive cleaning (fettling) costs may also occur if the baked sand in the pocket of the casting is difficult to remove. However, if this same green sand core loses its hot strength and collapses too soon, or if the core has a low hot plastic deformation, the molding sand may become too brittle at the elevated temperatures. It may be assumed that the surface of the core sand mixture can break away and may be entrapped by the hot metal. Lower hot strength may cause the casting to deform, or the molding sand may cut, or sand inclusions may be evidenced in the casting. If the molding sand cuts and washes, 5% silica flour additions (over5 % the reaction is more valid) prevent these happenings from occurring.
Obviously, molding sand in light sections or pockets of the casting must collapse readily when the liquid metal is rapidly solidifying. The opposite is true of molding sand surrounded by heavy metal sections, and the solidification is slower. It is a time-temperature prob­lem which must be considered as such. It is needless to illustrate that molding sand costs increase with heavy additions of silica flour. Finer sands may solve the problem at less costs from a more economical viewpoint. Mr. Harry W. Dietert contributed much to the knowledge of silica flour used in molding sands, and his work is acknowledged.

Green Compression Strength-Increases (up to 5% silica flour tends to decrease green compression strength in southern bentonite bonded sand mixtures, but fire clay and western bentonite bonded sand mixtures show an increase). 
Dry Compression Strength-Increases.
Hot Compression Strength-Increases.
Flowability-Decreases (as temper water increases).
Expansion-Increases with certain base sands, reduces with others. The sand grain distribution and the clay content of the base mixture are important factors. Both expansion and contraction properties are influenced by the hot compression strength of the sand mixture containing 10% additions of silica flour, the free expansion increases over 100%. 
Contraction-Decreases significantly.
Toughness or Resilience-Increases.
Workability-Increases (as the temper water increases).

Usually a fire clay containing a high percentage of silica fines is called a "siliceous clay." It is not a good bonding clay if higher green compression strength is the primary consideration, but is a clay that can be used in many facets of the foundry, particularly where skin­drying of the mold is practiced. Siliceous clay is used in dry sand molds where fines, such as silica flour, can be eliminated. This type clay is used for various refractory purposes around the foundry.

Is a bluish, black crystalline material with a chemical composition (SiC). It has a Moh's hardness of 9.5. Its specific gravity is 3.12 to 3.20 and it is a very refractory aggregate withstanding temperatures of 4200°F. (2318°C.). It is used in some non-ferrous foundries as a base sand. 

A group of resin-like materials in which silicon takes the place of the carbon of the organic synthetic resins. Silicones have more heat resistance than organic resins and are water-resistant. They act as lubricants and parting compounds. (Also see: PARTINGS.)

Silicon esters are compounds of the general formula  SiOi] CO2• R, where R is a function organic group. A typical silicon ester is the silicone which has a formula of:


Though not complying with the above definition exactly, these are generally considered to be silicon esters due to the presence of a Si-O-C group.

A water-clay dispersion which may also contain foundry additives such as seacoal and cellulose. Slurries are used for rebonding sand systems or molding unit sands. Western or southern bentonite and/or fireclay bond may be used as the dispersed clay. A slurry is used to furnish both the temper water, bond and carbon to a reused molding sand system.

Is a talc including many varieties. In the foundry, it is used as a dry parting or a "dust-on parting" compound as well as being used in some core, mold, and ladle washes. It is known in mineralogy as "steatite." Some nonferrous foundries using only steel or metal ladles coat their ladles with a mixture of soaps

Is a common, commercial name for sodium carbonate having a chemical composition (Na2CO3 ). For household use it is called "wash­ing soda." Soda ash is also called, "sal soda.ff Soda ash is less expen­sive than caustic soda, a sodium hydroxide. It is also used for softening water. In the steel foundry it is used more widely to control pH value of sand mixtures. From 2 oz. to 5 oz. per ton of molding sand containing 5% western bentonite is the accepted amount of soda ash used. It has a melting point of 1560° F. (850° C.). "Purite" is a commercial trade name used by the foundries as a fluxing agent. Purite is a briquetted soda ash.


Is a gelling carbohydrate, extracted from giant brown seaweed, called "kelp." Sodium alginate is used as a thickener in mold washes, refractory coatings, core pastes and spray compounds. It is a sodium salt of alginic acid, (C6H8O6 )n, a polyuronic acid composed of B-D­mannuronic acid residues linked so that the carboxyl group of each unit is free while the aldehyde group is shielded by a glycosidic link­age. Sodium Alginate is soluble in water forming a colloidal dispersion. It is insoluble in alcohol when the pH is below 3.0. It is a stabilizing colloid, giving good texture and fine lubrication to refractory washes, coatings or sprays. (See: THICKENING AGENTS.)


Is a derivative of high purity cellulose from cotton or paper with the ability to thicken water strongly. It is used as a suspension or thickening agent in mold or core washes and as a film forming agent in various foundry coatings.

The chemical composition is (NaBF4). It is prepared from reactions of boric acid, hydrofluoric acid, and caustic soda. It is highly soluble in water, but sparingly soluble in alcohol. It is used as a fluxing agent in foundry use.

Is also called "chile saltpeter" or "soda niter." It is a mineral found in areas of South America and has the chemical composition of NaNO3. It has a Moh's hardness of 1.5 to 2 and a specific gravity of 2.29. It is readily soluble in water. It can be prepared synthetically and is used as a drier for core oils when oils are used as binders in the core room. It requires very little sodium nitrate (0.01 % to 0.05%) to hasten the baking action of the cores, but again it should be added to core mixtures with caution. It is best to purchase sodium nitrate (or ammonium nitrate) from a commercial source for use as a proper adulterant resinate since they have learned to make it less hazardous when heated in the oven.

Its chemical composition is (NaBO3 ) • 4H2O. Its molecular weight is 153.9 and is a white, odorless, crystalline powder. It is stable when kept cool and dry. It liberates oxygen in warm or moist air and is soluble in water. It is used in the foundry industry as an oxidizing agent for core oils or for certain molding and core processes.

A group of water soluble compounds commonly known as "water glass" or "soluble glass." Chemically, it is composed of silica and alkali in various proportions. The silica to alkali ratio (SiO2/Na2O) determines the physical and chemical properties of the compound. Sodium silicate has been used in the foundry industry as a binder for the CO2 process. From 2% to 4% high ratio colloidal silicate (normally between 2.00 : 1 and 3.00 : 1) is mixed with a clean silica sand, then subsequently hardened by carbon dioxide (CO2 ) gas passing through the formed mixture. To accelerate the hardening of sodium silicate mixtures, ground ferrosilicon may be added to hasten the curing. The reaction, however, is dependent on ambient temperatures and during certain winter months or high humidity days, the setting rate slows down considerably. Warm sand hastens the reaction. Sodium silicate is at times added to cement molding mixtures in 1% increments to aid setting time. CO2 gas will further accelerate it. To obtain better casting surfaces and collapsibility in sodium silicate-CO2 molds or cores, from 1% to 5% Klean Surf Iron Oxide is added to the mixtures. Also for better collapsibility, sugars, pelleted pitch, seacoal (coal dust), wood flour and other carbons may be added to such mixtures, but more gas is formed.

Cereals, resins and proteins are usually referred to as solid organic binders. These binders are used in mold and core mixtures singularly, or in combination.


Oils that are treated with sodium hydroxide, alkalis, or addition of soaps render them soluble in water. Oils treated in such a manner emulsify readily because of the formation of sodium oleate and sodium palamitate. Sulphonated lard oil is used to aid the solubility of oils because it requires about 1% of sulphur to ready the oils for water solubility. Soluble oils often contain pine oil, or oil of sassafras, to improve their odor. Creosote or cresol may also be added as the disinfectant. Rosin or rosin oil may be added in the form of sodium resinate to improve certain of their properties. Soluble oils are used in the core room for a number of purposes, even though their wider use in the machine shop as cutting oils is known. They exert a lubri­cating action and act as a parting between the core mixture and the core box.

Usually used in magnesium foundry practice. It is necessary to determine the total water soluble salts within set limits by additions of silica-fluoride and boric acid in the molding sand. In general foundry practice, any salts soluble in water in molding sand mixtures are called "soluble salts."

Any liquid having the power of dissolving various other materials are called solvents. Commercial solvents are organic substances used in the foundry and are generally alcohols, benzines, ethers, gasolines, and turpentine. One mold or core coating uses rubber as its base. The best solvents for rubber are dichloroethylene or carbon bisulphide. Benzine and turpentine are used for solvents where foundry supply houses wish to offer products which contain gums or resins. Quick drying materials sold to the foundry usually have solvents such as amyl alcohol or amyl acetate. Dioxan is an excellent solvent for cellu­lose compounds and resins which are used in the foundry. Water is even a solvent for organic and inorganic materials used in foundry compounds. Solvents are usually materials which evaporate or are re­moved by heating. They leave behind the products of combustion on baking. Foundry compounds which contain fats or greases may use dichloroethyl ether as a solvent. Many liquid parting compounds con­tain solvents. Certain core oils, mold and core washes contain solvents.

Southern (calcium) bentonite is used as a bond for sand mixtures used in the casting of all metals. Southern bentonite bonded mixtures help to prevent hot cracking and hot tears in castings, as the molding and core sands shake-out freely without lumping. Southern bentonite bonded mixtures have less dry, baked and hot compressive strengths than western bentonite bonded mixtures. Mixtures collapse more readily, hence molding sands bonded with southern bentonite accommodate metal contraction in interior casting sections. Panther Creek southern bentonite is mined in the northeastern section of Mississippi and central Alabama. It is chiefly prepared in powdered form for commercial use. The physical properties are distinctively different from Volclay western bentonite. About 85% of clay substance in Panther Creek is the mineral, "montmorillonite." The balance (15%) being fragments of other minerals similar to those found in Volclay except that the predominant mineral is glauconite, which does not appear in Volclay.

An oil obtained from the pressing of soybeans. It is used as a core oil ingredient and has certain inherent properties that improve compounded or formulated core oils.

Sperm oil is a fatty oil extracted from the head cavity of the sperm whale. It is purified for foundry use and is graded according to the temperature of pressing when obtaining and processing the oil. Foundry grade sperm oil has a specific gravity of 0.88 to 0.885. It has a flash point above 440° F. (227°C.). Foundry commercial sperm oil is likely to be ½ head oil and ½ body oil from the whale. Sperm oil differs from fish oil and whale oil as it consists chiefly of waxes, not fats. Sperm oil absorbs very little oxygen from the atmosphere and is not influenced by great variations in temperature of baking. It is, and was, widely used as a core oil substance some years ago, but has gradually lost its position to other competitive oils over the course of time.

A western (sodium) bentonite sized for dispersing in water. Sperser grade is a granular sized bentonite. It is practically dust free, the bulk of the sizes are between a minus U.S. Standard Sieve No. 40 and a plus U.S. Standard Sieve No. 140. Sperser bentonite is of the same high grade, standard properties as powdered Volclay western bentonite.

Is a large group of substances extracted from grains, fruits and tubers. Starch is used as a binder for adhesives, pastes, cores, or molding sand additives. Starch is a component of many foundry compounds. Foundry cereal (corn flour) may contain from 55% to 75% starch. Potatoes have about 18% of starch which is widely used in the European foundries. Most of the commercial foundry starch is extracted from corn. Starch has different chemical reactions and different chemical analyses. Specific gravity of starch is about 0.499 to 0.513. It is insoluble in cold water, but forms a paste with hot water. Dextrin, as a starch, has excellent adhesive properties. Starch is extensively distributed as granules in many plants. Those plants that serve as important reserves of starch are the seeds of wheat, corn, barley, rye, rice, the tubers of potatoes, cassava (tapioca starch) and the pith of the sago palm. The form and size of the starch granules are the characteristics of the plant on which it is found. When processed the starch grains are washed free from the cellulose and other materials in the grain by water. The suspension of starch is allowed to settle and the water is drawn-off. In the case of corn kernels, the oily germs float on the surface of the suspension and are separated for subsequent extraction of the oil. Starch consists of a number of glucose units attached to each other by oxygen linkage to the carbon atoms. Starch is an important food material consumed with the proteins of grain cereals or potato starches. Starch is used in the foundry as corn starch, and secondly, it is used for the further production of dextrin. In Europe, starch is more widely used in the foundry industry than in the U.S.A. where the Europeans have plentiful amounts of potatoes instead of the corn flour cereals which are more abundant in the U.S.A. In the U.S.A., the corn flour cereal starches are used in the core room principally as ingredients to core sand mixtures.

Stearic acid is a wax-like substance used in many waterproofing compounds in the foundry, e.g., dry parting shake-ons. Stearic acid is also used in liquid partings. It is a common constituent of hard fats and its composition is [CH3(CH2)16COOH]. It is soluble in alcohol. In liquid partings, it is first dissolved by a solvent and then added to the fuel oil or kerosene which is principally used as the main carrier. Its specific gravity is 0.984. It has a melting point of 156 °F. (69°C.) and a boiling point of 554° F. (291°C.). Stearic acid is usually diluted with paraffin or tallow for foundry purposes. It is melted into calcium carbonate, which is then ground and sized into fine powder and sold as a waterproof, dry parting compound for the foundry industry.


Steel-Flo Flour is another type wood flour produced and shipped from Aberdeen or White Springs, Mississippi. It is a specially prepared wood flour for use in the steel foundry industry. However, other foundries using wood flour or cellulose additives have found it satis­factory for their individual purposes. Steel-Flo Flour is a hard wood flour which is colored by iron oxide to help identify its use in mold or core sand mixtures. The iron oxide imparts slight physical and mechanical changes to this product which differs from Five Star Wood Flour and other cellulose. These properties, however, are preferred in foundry sand mixtures.
Recommended Use
It is used similarly as Five Star Wood Flour, or other wood flour additives. Approximately 0.5% to no more than 3% by weight is used in foundry mold or core sand mixtures. It is used as a direct replacement for wood flours, cob flours and other cellulose additives.
Its distinct advantage is due to its imparting color to sand mixtures. In certain sand mixtures it is difficult to determine which mixtures contain wood flour unless a color additive is present. Steel-Flo Flour's color additive helps identify its presence in any given core or molding sand mixture. The wood flour furnishes the buffing control to lessen sand expansion difficulties. Steel-Flo Flour contains less than 3% ash content; most of this is due to the coloring agent present. Some foundries have stated there is a certain amount of hot, plastic deformation connected with Steel­Flo Flour sand mixtures which is not found with other cellulose additives. This may be true, as the Klean Surf oxide added tends to create a certain amount of hot plastic deformation when the mold or core is heated. It is one of the best wood flours known to reduce rapid volume changes in sand mixtures when they are at elevated temperatures. It improves collapsibility and tends to eliminate surface defects that may be caused from sand expansion such as scabs, buckles or similar defects. Steel-Flo Flour has been particularly accepted by the steel, malleable and higher temperature foundries which are more sensitive to castings subject to hot tears or cracks. It is another tool for the foundrymen to use to lessen the danger of scrap.

A stalk or stem from grains, especially from wheat, rye, oats or barley. It is the stalk of grain after threshing. Straw is used less sparingly today in the foundry than in the past. Straw is used in compo mixtures, dry sand mixtures and certain large core sand mix­tures in Europe more so than in the U.S.A. and Canada. Straw aids in preventing large volume changes in molding sand mixtures and overcomes expansion difficulties. It was the substitute for manure in molding sand mixtures, formerly used for such purposes. 

Is another term for "Lignin" which is recovered from paper pulp sulfite liquor and used in furfural plastics as an extender for phenol in phenolic plastics. It is used in the foundry as a core or mold binder. It is used as a waste liquor from sulfite mills to resurface roads. It has strong advantages as a binder when used with clay in various core sand mixtures and mold compounds. Sulfite liquor is made by dissolving sulphur dioxide gas in an alkali solution such as lime to form calcium sulfite, which is then used as a lignin solvent in paper making.


It is also known as lignin sulfonate. A binder used with sand, clay, and water in core mixtures. It is a lignin dissolving liquor used in making pulp by the sulfite process. The sulfonates may be ammonia, soda, calcium, or others.

SULPHUR (See: INHIBITOR) A mineral having the chemical symbol (S). Its Moh's hardness
is 1.5 to 2.5. Its specific gravity is 2.05 to 2.09 and it melts at 232°F. (110° C.). At about 780°F. (416° C.), it is converted into a vapor which offers protection as an inhibitor in the casting of magnesium and its alloys in green sand molding.

Includes a large group of products. Synthetic resins are more properly referred to as "resinoids." The two best known foundry synthetic resins are the urea or phenol formaldehyde types. Both urea and furfural synthetic resins are widely used in the foundry industry. Hexamine may sometimes be substituted for formaldehyde in foundry resins, particularly when the resin is used to make shell molds. Urea-formaldehyde and phenolic resins are popular foundry synthetic resins. Synthetic resins require close control in core formulations. Mixing, and particularly the baking temperatures employed, must be closely governed. Foundries use them successfully as core oil replacements and use them in various forms of baking ovens. (Also see: UREA FORMALDEHYDE-PHENOLICS-FORMALDE­HYDE-PHENOL FORMALDEHYDE.)


Review of "Glossary of Foundry Additives" by Clyde A. Sanders, American Colloid Company

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