How Different Types of Molding Sands Are Used in Sand Casting
Sand casting dates back thousands of years, with the earliest recorded use of molding sand appearing in 1540. While foundries have modernized with newer technologies and equipment, molding sand remains popular for producing high-quality and detailed components that go into finished goods.
Properties of Molding Sand
Molding sand generally contains four primary ingredients — silica, moisture, a binding agent and additives.
Silica forms the majority of molding sand and combines with organic or inorganic binders before introducing moisture to help the sand bond properly and become stronger. When needed, additives promote certain sand properties required for a specific project. Popular additives include coal, pitch, silica flour, sea coal, corn flour, dextrin and wood flour.
As a result of the combinations, molding sand types differ slightly in factors like shape, color, density, specific gravity, moisture level and permeability. Foundry sand quality considers multiple factors, including variability, durability and chemical makeup.
Some properties are specific to the foundry industry when evaluating sand's suitability for a particular casting. These include:
- Collapsibility: Once the liquefied metal hardens, the sand mold must permit the casting to contract freely to prevent the metal from tearing or cracking.
- Cohesiveness: This property refers to how the sand particles attract and bind with each other, and the level of their bonding contributes to the different strength characteristics.
- Adhesiveness: Sand must have enough adhesion to stick where desired, such as to the molding box's inner walls.
- Plasticity: This characteristic gauges how well the sand flows or compacts to fill inner structural details.
- Permeability: Permeable sand allows for gases, air and moisture to escape to promote accurate solidification and prevent defects.
- Dry strength: Once a foundry professional introduces molten metal, moisture in the mold begins to evaporate. The level must remain high enough to allow the mold wall to hold its shape.
- Green strength: This property refers to the molding sand's combined adhesion and cohesion after the introduction of water.
- Refractoriness: Among the most important molding sand properties, refractoriness is the sand's ability to withstand high heat without degrading.
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Modern Molding Sand Types and Uses
There are several commonly used sand types, each with different applications in the foundry, including:
- Facing sand: Foundries use facing sand to form the mold's face — the part of the pattern contacting the molten metal during casting. It contains no previously used sand but may have carbons to help guard against burning as the hot metal makes contact.
- Backing sand: Backing sand reinforces the facing sand to fill the remaining molding vessel. This sand is usually dark since it contains previously used sand or coal dust.
- Core sand: Core sand is often mixed with an oil-based binding agent, pitch or flour for cost-effective core-making.
- System sand: More automated foundries use less facing sand and instead rely on cleaned and reconstituted used sand — called system sand — to completely fill the mold.
- Green sand: Green sand is a mixture of silica, clay and water that forms a high-strength, lightweight bond. It's highly malleable, similar to dough and can retain shapes and impressions to create the casting pattern.
- Parting sand: Foundries use parting sand to prevent the green sand from sticking to the pattern. It's simply silica with no additional moisture, binders or clay, giving it a dust-like property.
- Dry sand: Dry sand is green sand exposed to heat to remove moisture. This drying produces high-strength molds with sufficient rigidity and stability for large castings.
- Loam sand: Loam sand contains higher amounts of clay than green sand and has a thinner consistency. Its primary use is for larger castings.
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