Sand casting is a process that utilizes non-reusable sand molds to form metal castings. It is a common production method for metal components of all sizes, from a few ounces to several tons. Sand casting isn’t only versatile in the size of its products – it can also create exceptionally complex or detailed castings, and can be used to cast nearly any metal alloy.
What is Molding Sand?
Molding sand has three important advantages over other mold materials:
- it is inexpensive,
- it is easily recycled, and
- it can withstand extremely high temperatures.
Sand casting is one of the few available processes for metals with high melting temperatures such as steels, nickel, and titanium. Because of its flexibility, heat resistance, and relatively low cost, sand casting is the most widely used casting process.
Castings are produced by pouring liquid metal into a mold cavity. For a casting to be successful, the mold cavity must retain its shape until the metal has cooled and fully solidified. Pure sand breaks apart easily, but molding sand contains bonding material that increases its ability to resist heat and hold shape.
Green sand (an aggregate of sand, pulverized coal, bentonite clay, and water) has traditionally been used in sand casting, however modern chemically bonded molding systems are becoming more popular. The most widely used casting sand is silica (SiO2).
Molding Sand Properties
The ability of the sand mold to hold its geometric shape under the conditions of mechanical stress.
The ability of a sand mold to permit the escape of gases and steam during the casting process.
Moisture content affects a mold’s strength and permeability: a mold with too little moisture may break apart, while a mold with too much moisture can cause steam bubbles to be entrapped in the casting.
The capacity of the sand to fill small cavities in the pattern. High flowability creates a more precise mold, and is therefore useful for detailed castings.
The size of the individual particles of sand.
This property evaluates the shape of the individual grains of sand based on how round they are. Generally, three grain categories are used in foundry sand:
- Rounded Grain sands provide relatively poor bonding strength, but good flowability and surface finish.
- Angular Grains have greater bonding strength because of interlocking, but poorer flowability and permeability than rounded grain sands.
- Sub-angular Grains are the middle road. They possess better strength and lower permeability relative to rounded grains, but lower strength and better permeability than angular grains.
The ability of the sand mixture to collapse under force. Greater mold collapsibility allows the metal casting to shrink freely as it solidifies, without the risk of hot tearing or cracking.
The mold must not melt, burn, or crack as molten metal is poured into it. Refractory strength measures the ability of molding sand to withstand extreme heat.
The ability of molding sand to be reused (after sand conditioning) to produce other sand castings in subsequent manufacturing operations