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Mold and coremaking materials.

As it has been for thousands of years, sand molding is the easiest, most inexpensive and reusable media to produce metal castings. This section discusses the consumables used in the mold and coremaking process--sand, binders and sand additives.

Sand is defined as granular particles resulting from the disintegration or crushing of rocks. Sand is an aggregate material essentially consisting of mineral matter no longer than 1/12 in. or smaller than 1/400 in. in diameter.

Sand denotes a class of several minerals--including silica, olivine, chromite and zircon--that are classified as sand when in the above size range.

Molding Sands

Molds for making a single ton of castings may require up to four or five tons of molding sand. Sand must be readily moldable and produce defect-free castings if it is to qualify as a good one. Classified by AFS grain fineness number (GFN) and screen distribution, the most commonly used sands in the TABULAR DATA OMITTED foundry industry are silica, olivine, chromite and zircon.

The GFN is the number of mesh per inch of the sieve that would just pass the sample if its grains were of a uniform size. It is proportional to the surface area per unit weight of sand, exclusive of clay.

Silica sand is the most commonly used aggregate in the industry, and is available in AFS GFN varying from about 30-180. These sands may be used with all binder systems.

Silica sands were accumulated through the deposition of sand along ancient seas. Where these deposits were buried, they were consolidated into sandstone. Silica sand is found throughout the U.S.

Olivine sand is an angular sand. AFS GFN is available from 60-180. This sand can't be used with an acid-catalyzed binder system due to its inherent alkalinity.

Olivine occurs in an ultra-basic igneous rock called dunite. Commercial deposits occur in Washington state, North Carolina and in Norway. Magnesium and iron content of the olivine mineral will vary with the composition of the magma. Olivine has lower and more uniform thermal expansion characteristics than silica and its uniform chill characteristics tend to minimize shrinkage defects.

Chromite sand is an angular sand used for its chilling tendency and resistance to metal penetration. The available AFS GFN range is between 50-80. Chromite sand is compatible with all binder systems.

Chromite occurs as magmatic segregations in ultrabasic igneous rocks. Present as masses, lenses or disseminated grains, chromite deposits are found throughout the world, yet most are mined in Rhodesia and South Africa.

Zircon sand is rounded grain sand generally used in the same manner as chromite sand. Available AFS GFN may vary from 65-130, depending on the sand source. Produced from beach sand deposits, these form as a result of weathering processes followed by stream transport to seacoasts where heavy minerals are further concentrated by wave and wind action. Zircon is used for its superior chill properties and minimal expansion characteristics.

Zircon and olivine offer special properties that often improve mold conditions. These sands are often more expensive than silica sand, and so are used only for special applications.

Special Properties

Chief among the concerns of foundry sands are permeability, cohesiveness and refractoriness.

Because steam and other gases are evolved in green-sand molding when molten metal is poured into the mold, the sand must be permeable--porous--to permit the gases to pass or else the casting may contain gas holes and other defects. Obviously, the finer the sand, the smaller the openings between the grains and ultimately the lower the permeability. However, finer grain sizes generally lead to better surface finishes, so a balance between grain size and permeability is needed.

Sufficient permeability is critical in permitting the metal to fill the molding cavity quickly and quietly. Too much permeability could lead to rough surfaces and metal penetration caused by the molten metal penetrating the open area between sand grains.

Cohesiveness is determined by the shape of the grains. The grains may be angular, rounded, subangular or compound.

Angular grains have a much higher surface area, requiring more bonding agents, especially in chemically bonded sands. The interlocking nature of these grains resists compaction, which helps avoid expansion caused by too much density (sand grains too close) while increasing permeability.

Rounded grains give maximum flowability, good permeability and compact easily--an advantage in mechanized core production. However, close packing of uniformly shaped grains encourages surface cracks and scabs on the face of the mold because silica grains expand when heated by molten metal (avoided with angular grains). Rounded grains require the least amount of binder and compact to a higher density. Rounded grain sands provide densities of up to 8-10% higher than angular sands.

Comparing rounded grains with angular grains of the same size, angular grains will always have a higher permeability because they resist compaction.

Subangular grains require less binder and compact to a higher density than angular grains. As the angularity of a sand increases, the amount of binder required to maintain a desired tensile strength value increases. This increase is due to the increase in surface area of the sand grains. (Rounded sand grains have the lowest surface area, and angular sand grains the highest.)

Compound grains are agglomerates of small grains bonded to form a large grain. This bonding is not generally strong enough to keep the grains together during normal foundry processes. Compound grains aren't suitable for foundry sands and should be avoided.

The refractoriness of sand is related to its ability to withstand high temperatures without breaking down or fusing. Impurities, such as metallic oxides, cause a lowering of the fusion point of some molding sands and result in sand fused with metal at the casting surface.

Other physical concerns for sand include:

* Green strength--After water and clay have been mixed into green sand, it must have adequate strength and plasticity for making and handling of the mold.

* Dry strength--As a casting is poured, sand adjacent to the hot metal quickly loses its water as steam. The dry sand must have strength to resist erosion and the metallostatic pressure of the molten metal, or else the mold may expand.

* Hot strength--After the moisture has evaporated, the sand may be required to possess strength at elevated temperatures. Metallostatic pressure of the liquid-metal bearing against the mold walls may cause enlargement, or, if the metal is still flowing, erosion, cracks or breakage may occur unless the sand possesses adequate hot strength. Too much hot strength, however, will cause cracked castings.

* Thermal stability--Heat from the casting causes rapid expansion of the silica sand surface at the mold-metal interface. The mold surface may then crack, buckle or flake off unless the molding sand is dimensionally stable under rapid heating. Some sands, such as zircon and chromite, are much more stable when compared to silica. Using these sands reduces expansion-type defects caused by poor thermal stability.

* Collapsibility--Heated sand that becomes hard and rocklike is difficult to remove from the casting and may cause the contracting metal to tear or crack.

* Sand should also respond to molding compaction processes, be reusable, offer ease of sand preparation and control and remove heat from the cooling casting.

Clay Binders

Any added material that imparts cohesiveness to the sand can be classified as a binder, such as clay. Molding sands may contain 2-20% clay. With a suitable water content, it is the principal source of the strength and plasticity of molding sands. Clays are defined as "essentially aggregates of extremely minute crystalline, usually flake-shaped particles that can be classified on the basis of their structure and composition into a few groups that are known as clay minerals."

These include fireclay (kaolinites) and sodium or calcium bentonites (montmorillonite). Sodium bentonite is commonly known as western bentonite because it is mined in Wyoming, and calcium bentonite is referred to as southern bentonite because it is mined primarily in Alabama. Although a few special clays exist, bentonites and fireclays are the most commonly used.

The bentonite bonding of sand grain results when a bentonite-water glue mixture is forced to coat individual sand grains. These coated sand grains are compacted to form bentonite bridges between sand grains, giving the mold its necessary strength.

The advantages to calcium bentonite is that it mulls or mixes easier, builds rapid green strengths and causes fewer problems with balling in the sand muller.

Sodium bentonites are used in sands requiring a higher level of dry and hot compressive strengths. For example, steel foundries typically rely on a sand mixture composed of sodium bentonite because of higher pouring temperatures. Maximum hot compressive strengths of 500-600 psi can be obtained with mixtures of fireclay and western bentonite.

Water, present in amounts of about 1.5-5%, activates the clay in the sand, causing the molding sand mixture to develop plasticity and strength. Water in molding sands is often referred to as tempering water. The water is adsorbed by the clay up to a limiting amount.

Chemical Binders

There are two broad categories of binders--organic and inorganic. Inorganic binder systems are those that do not contain carbon in the binder molecules. Inorganic systems are environmentally friendly, but unfortunately, are the least reactive, achieve the least strength and require the most attention during coremaking, moldmaking and casting. These systems are based on silicate and phosphate/metal oxide technology. All other binder systems are organic.

Chemical binders are classified into three groups by their processes: nobakes, heat-cured and vapor-cured binders.

* Nobake systems--Nobake systems (or air-sets) are those in which resins are mixed with a liquid catalyst that reacts with the resin at ambient temperatures. In certain instances, minimal heat is applied with infrared lamps or by other means to prevent lack of cure against the cold pattern surface or to accelerate the cure. Cures will generally develop from within several seconds up to several hours, depending on the binder system.

Types of binders used for nobake systems include: furan/acid, phenolic/acid, phenolic/ester, oil urethane, silicate/ester, phenolic urethane, phosphate/metal oxide, and polyol urethane.

* Heat-cured systems--Heat cured systems (hotbox, warmbox and shell) are those in which the binder is mixed with the sand and catalyst, and blown into a heated corebox or against a pattern to produce the core or mold.

With true hotbox binders, corebox temperatures of 400F or higher are typical, with the cure effected in seconds. Internal cure develops as a result of exothermic reaction that takes place after the cores are removed from the corebox.

Heat-cured systems include shell, core oil, phenolic hotbox, furan hotbox, urea formaldehyde hotbox and warmbox.

* Vapor-cured--Vapor-cured systems (gas cured and coldbox) are those that mix resin on sand grains and blow the mixture into a corebox. Then, catalyst is blown in the form of vapor, which sets the binder instantaneously. One of the differences between coldbox and nobake is that in nobake, a liquid catalyst is used, while coldbox systems use a vapor to cure the sand.

Vapor-cured systems include: phenolic/urethan/amine, silicate/C|O.sub.2~, furan/S|O.sub.2~, acrylic/epoxy/S|O.sub.2~, phenolic/ester TABULAR DATA OMITTED (methyl formate), and phenolic/C|O.sub.2~.

Sand Additives

In addition to the three basic ingredients of molding sand (sand, clay and water), other materials may be present. Additives are usually used to improve certain properties. These include cereals, cellulose, carbons and chemicals.

Cereal--Cereal and starch additives (normally referred to as flours) are natural organic carbohydrates produced from various grains, typically corn. They are generally added to increase the green deformation (ductility) of sand mixtures, making it easier to draw poorly maintained or difficult patters.

Cereals can also be used to eliminate severe sand expansion problems. However, they can cause flowability, shakeout and control problems, which may outweigh their benefits.

Cellulose--Cellulose is any cellular material derived from a plant or plant by-product. Many forms of cellulose are available to the foundry industry, including crushed corn cobs, ground oat hulls, wood flour, ground nut shells and rice hulls.

Cellulose acts as a cushioning agent between sand grains. When molten metal is poured into a mold, it rapidly burns out the cellulose between sand grains at temperatures well below the critical expansion of silica.

Carbons--When added to a sand mixture, carbons provide a release or peel of the sand from the casting at shakeout, and may improve surface finish. Typical carbons include seacoal, asphalt and gilsonite, causticized lignite and light petroleum distillates. They are generally used on iron and copper-based alloys.

Chemicals--Used for special problems and applications, chemical additives include soda ash, wetting agents, and polymers.

References

Cast Metals Technology Metalcaster's Reference & Guide Green Sand Additives Modern Casting, Various Issues Principles of Metal Casting Mold & Core Test Handbook Metalcasting Dictionary Aluminum Casting Technology Technical assistance from Scott Strobl, National Engineering Co.
COPYRIGHT 1993 American Foundry Society, Inc.
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1993, Gale Group. All rights reserved. Gale Group is a Thomson Corporation Company.

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Publication:Modern Casting
Article Type:Cover Story
Date:Nov 1, 1993
Words:2117
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