Compositions: coating or plastic – Coating or plastic compositions – Molds and mold coating compositions
Reexamination Certificate
2002-11-08
2004-04-13
Marcheschi, Michael (Department: 1755)
Compositions: coating or plastic
Coating or plastic compositions
Molds and mold coating compositions
C523S139000, C524S492000, C164S520000, C164S523000, C427S215000, C427S221000
Reexamination Certificate
active
06719835
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to foundry techniques used to create sand cast metal parts. More particularly, the present invention relates to a new and improved sand casting foundry composition and method using shale as an anti-veining agent to prevent veining defects in the cast metal parts.
BACKGROUND OF THE INVENTION
Sand casting is a process used in the foundry industry to produce cast parts. In sand casting, disposable foundry shapes are made by forming a sand-based foundry composition into predetermined configurations and curing the composition to preserve those foundry shapes. A binder in the foundry composition maintains the predetermined configuration of the foundry shape. The foundry shape which defines the exterior of the resulting cast part, known as a mold, is positioned relative to the foundry shape which defines the interior of the cast part, known as a core. With the mold and the core foundry shapes oriented as desired, molten metal is poured between them. The foundry shapes confine the molten metal while it cools and solidifies into the resulting cast part.
The binder must have the capability to preserve the predetermined configurations of the mold and core foundry shapes while those foundry shapes are oriented in the appropriate relationship to create the cast parts and during the time while the molten metal solidifies into the cast part. The typical type of foundry sand used for this purpose is silica sand, although other useful foundry sands include chromite, zircon and olivine sands. Two basic types of binders are commonly employed: inorganic binders, such as clay; and chemical binders, such as phenolic resin binders.
The most widely used inorganic binder for a sand-based foundry composition is bentonite clay. The foundry composition of the sand and bentonite clay binder is referred to as green sand. Green sand is a water tempered sand mixture having plasticity. A green sand foundry composition is typically formed by mulling silica sand, bentonite and a small amount of tempering water. The tempering water allows the bentonite to become sufficiently plastic so that it may be smeared relatively uniformly and thinly over the sand grains during the mulling process. The thin coating of the bentonite on each sand grain interacts with the thin coating on the adjacent sand grains causing the sand grains to be held in place in the mold and core foundry shapes. Green sand molding is economical and is widely used to cast ferrous as well as non-ferrous metal parts. Green sand molding permits high quantity, high quality foundry production, particularly for smaller cast parts.
Chemically-bonded, sand-based foundry compositions use a variety of polymerizable or curable organic and inorganic resin binders to hold the sand grains together in the desired mold or core shape. Chemical bonding involves mixing the sand and a polymerizable or curable binder. Once the mixture of the sand grains and the uncured binder have been shaped into the desired configuration, the chemical binder is polymerized or cured by the addition of a catalyst and/or heat, resulting in converting the shaped configuration into hard, solid, cured mold or core foundry shapes. Examples of curable resin binders include phenolic and furan resins. In a typical no-bake process, i.e. one which does not involve the addition of heat for curing, the sand, binder, and a liquid curing catalyst are mixed and compacted to produce the desired configurations of the mold or core foundry shapes. A commonly used no-bake binder is a polyurethane binder, derived by curing a polyurethane forming binder material with a liquid tertiary amine catalyst.
When subjected to the heat of the molten metal, the sand grains in mold and core foundry shapes expand. If the sand grains in the molds and cores are too close together, the sand grains expand in size and push on the adjacent sand grains. The thermal expansion opens up small cracks and fissures in the molds and cores, and the molten metal penetrates into those cracks and fissures. When the molten metal solidifies, raised, narrow ridges on the surfaces of the cast part result at those locations where the molten metal penetrated into the small cracks and fissures. The resulting narrow ridges are referred to as “veins” or “veining.” The veining may make it necessary to surface grind or machine away the projecting veins. Of course, such surface grinding or machining increases the cost of producing the cast part.
Another type of foundry shape defect is caused by gas formation, particularly within core foundry shapes. Water in green sand casting foundry compositions will volatilize into steam in the presence of the hot molten metal. Trapped steam may cause pin holes or cracks in the foundry shape, resulting in the metal penetration into the foundry shape. The gas may also create an uneven or discontinuous surface in the cast part. Gas pressure also results from the volatilization of certain chemical constituents in foundry compositions. It is desirable to use chemical binders which are not susceptible to excessive volatilization, particularly in core foundry shapes.
Expansion and cracking from gas pressure is more of a problem in core foundry shapes, because core foundry shapes are typically surrounded by the molten metal due to their internal position. Those binders which produce significant amounts of gas when exposed to metallurgical temperatures may only be used in foundry shapes where the confined gas has an avenue to escape, otherwise the gas itself may induce cracks, fissures and pin holes. Mold foundry shapes are exposed to the ambient atmosphere and therefore provide avenues for the gas pressure to escape, although the gas pressure may nevertheless create defects in mold foundry shapes. To avoid excessive gas creation where a clay binders is used, the amount of tempering water used to activate the clay binders and allow it to be smeared over the sand grains is limited.
A wide variety of different agents have been added to sand casting foundry compositions in an attempt to improve the properties of core and mold foundry shapes to avoid veining and other casting defects. These additives, known generically as anti-veining agents, include starch based products, dextrin, fine ground glass particles, red talc and wood flour, i.e. particles of wood coated with a resin, granular slag, pulverized sea-coal, alkaline earth or alkaline metal fluoride, and lithia-containing materials, among many other things. Iron oxide, including red iron oxide, also known as hematite (Fe
2
O
3
), black iron oxide, also known as magnetite (Fe
3
O
4
), yellow ochre, and Sierra Leone concentrate, is also another widely used antiveining agent.
Each of these anti-veining agents are theorized to function in a different way to avoid or reduce the incidence of cracks, fissures and the other defects in the foundry shapes which cause veining. It is generally believed that the iron oxides increase the hot plasticity of the sand mixture by the formation of a glassy layer between the sand grains. The glassy layer deforms without fracturing at metallurgical temperatures, to prevent fissures in the foundry shapes. Grains of slag are thought to become soft at metallurgical temperatures permitting the sand grains to expand. Sea-coal and other combustible anti-veining agents are believed to form volatile gas at metallurgical temperatures leaving void space into which the sand grains expand.
SUMMARY OF THE INVENTION
The present invention relates to the use of particles of shale as an anti-veining agent in a sand casting foundry composition used to create foundry shapes for casting metal parts. Phyllosilicate mineral components of the shale particles will undergo crystal structural collapse when the foundry shape is heated by the molten metal during casting. With a sufficient concentration or volumetric quantity of shale particles distributed within the foundry shape, and with sufficient sizes of the shale particles, the collapse of the crystal structure of the phyllosilicate mineral components of each shale
Marcheschi Michael
Wyo-Ben, Inc.
LandOfFree
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