Sand casting is one of the oldest and most versatile metal casting processes. It involves creating a mold from a sand aggregate and pouring molten metal into the mold cavity. The different types are primarily classified based on the sand used and the method of mold production.
1. Classification by Sand Bonding Method & Mold Type
This is the most common way to categorize sand casting processes.
A. Green Sand Casting
This is the most widely used sand casting process.
Description: The mold is made from a mixture of sand, clay (typically bentonite), water, and other additives (like coal dust). The term “green” refers to the fact that the mold is not baked or cured; it contains moisture during the pouring of the molten metal.
Process: The sand mixture is packed around a pattern in a flask. The pattern is removed, leaving the cavity. The mold is used while still “wet.”
Advantages:
Low cost and recyclable mold material.
Fast production cycles, suitable for high-volume production.
Good collapsibility (the mold gives way as the casting shrinks).
Disadvantages:
Lower dimensional accuracy and surface finish compared to other methods.
Higher moisture content can lead to gas defects (blow holes, porosity).
Applications: Engine blocks, cylinder heads, manifolds, large gears, and many other industrial and automotive components.
B. Cold Box Casting
In this process, the sand mold is cured at room temperature using chemical binders and gaseous catalysts.
Description: Sand is mixed with a liquid organic resin binder (e.g., phenolic urethane, furan) and a catalyst gas (e.g., amine, SO₂) is pumped through the packed sand to trigger instantaneous hardening.
Process: The sand mixture is blown into a core box or mold box. The gas is introduced, curing the mold in seconds.
Advantages:
Very high production speed for molds and cores.
Excellent dimensional accuracy and good surface finish.
High core strength.
Cures at room temperature, saving energy.
Disadvantages:
The binder systems and gases can be hazardous, requiring ventilation.
Generally more expensive than green sand.
Applications: Complex engine cores, hydraulic parts, and high-precision castings.
C. No-Bake (Air-Set) Casting
This process uses chemical binders that cure at room temperature through a chemical reaction, without the need for gas catalysts.
Description: Sand is mixed with a two-part binder system (e.g., a resin and a catalyst). Once mixed, the chemical reaction begins, and the mold hardens over time.
Process: The sand mixture is packed into a flask. It is left to air-harden, which can take from minutes to hours depending on the chemistry.
Advantages:
Produces very large and heavy molds and cores that are difficult to make with other processes.
Excellent dimensional accuracy and surface finish.
Low gas evolution during pouring, reducing defects.
Disadvantages:
Slower production cycle compared to Green Sand and Cold Box.
Binders have a limited “work time” (bench life) after mixing.
Applications: Large, one-off castings like pump housings, valve bodies, and heavy machinery components.
D. Shell Molding (Croning Process)
This process uses a thin, hardened shell of sand as the mold, rather than a solid sand block.
Description: A metal pattern is heated and covered with a sand mixture coated with a thermosetting resin (e.g., phenolic). The heat melts the resin, bonding the sand grains into a hard shell around the pattern. The shell is then ejected from the pattern.
Process: The two shell halves are assembled and often backed up with shot or sand in a flask for support during pouring.
Advantages:
Very smooth surface finish.
High dimensional accuracy.
Good permeability and reduced cleaning and machining.
Disadvantages:
High pattern cost.
Limited size of castings due to shell thickness.
Resin can be expensive.
Applications: Small to medium-sized parts requiring high precision, such as camshafts, connecting rods, and gearbox parts.
2. Other Specialized Sand Casting Types
A. Lost Foam Casting (Expanded Polystyrene Process)
A unique process where the pattern is made from foam and evaporates upon metal pouring.
Description: A pattern is made from expandable polystyrene (EPS) foam and is coated with a refractory ceramic. It is then placed in a flask and surrounded by unbonded sand. Molten metal is poured directly onto the foam, which vaporizes and is replaced by the metal.
Advantages:
No parting lines or cores are needed, allowing for incredibly complex geometries.
Excellent dimensional accuracy.
Simplified process and reduced cleaning.
Disadvantages:
Pattern costs can be high.
New patterns can only be used once.
The process can produce defects if the foam decomposition is not controlled.
Applications: Complex engine blocks, cylinder heads, and artistic castings.
B. Vacuum Molding (V-Process)
This Japanese-invented process uses a vacuum, not a binder, to hold the sand together.
Description: A thin plastic sheet is heated and stretched over a pattern. A flask is placed over the pattern and filled with dry, unbonded sand. A second plastic sheet covers the top, and a vacuum is drawn through the flask, compacting the sand and forming a rigid mold. The vacuum is maintained until after the metal has solidified.
Advantages:
Extremely smooth surface finish.
No moisture-related defects.
Sand is completely dry and easily recycled.
Disadvantages:
Process is more complex and slower.
Limited for very high-production runs.
Applications: Panels, art castings, and plates requiring a fine finish.
C. Loam Molding (Sweep Molding)
An ancient method used for creating large, symmetrical castings like bells and pulleys without a solid pattern.
Description: A rough brickwork or frame structure is built and covered with a wet, loam sand mixture (clay-rich). A sweeping board or template (a stencil shaped to the desired profile) is rotated around a central spindle to scrape the loam into the final mold shape.
Applications: Primarily for large, axisymmetric parts where a full pattern would be impractical or too expensive.
In summary, the choice of sand casting type depends on factors like the required production volume, part size, complexity, desired dimensional accuracy, surface finish, and overall cost considerations. Green sand remains the workhorse for high-volume applications, while Cold Box, No-Bake, and Shell Molding are chosen for higher precision. Lost Foam and V-Process are specialized techniques for unique applications.
