Observation and Analysis of Sand Casting

Sand casting
Mold type: Sand
The oldest casting method. Melted metal is poured into a sand mold. Molds can only be used once but the cost to make them is low, so this method is suitable for small lot production. The surfaces of parts made with sand molds are rough because of the sand particles of the mold. Also, the accuracy is not particularly high.

Die casting
Mold type: Metal
In this casting method, melted metal is injected into a metal mold under high pressure. Metal molds are expensive but can be used repeatedly, so this method is suitable for mass production. Surfaces of die cast parts are smooth, and highly accurate products can be manufactured quickly.

Permanent mold casting
Mold type: Metal
Melted metal is poured under only the force of gravity, without applying pressure. This method is called gravity casting or gravity die casting. Cast products using this method have high dimensional accuracy and excellent mechanical characteristics, so this method is suitable for parts that require high strength.

Lost-wax casting
Mold type: Plaster, ceramic
Melted wax is poured into a mold to make a wax model. The model is covered with plaster or ceramic. It is baked in a furnace to melt the wax. The wax melts and runs out, leaving a space in the baked plaster or ceramic which becomes the mold. Molds can only be used once but the cost to make them is low, so this method is suitable for small lot production. The surfaces of lost-wax cast parts are smooth, so highly accurate products can be manufactured.

Advantages

• Wooden patterns require low initial investment.
• Wooden patterns can be produced quickly.
• Various shapes can be molded, which allows for casting of complex and large products.
• Suitable for small-volume manufacturing of many product types
• Any metal material can be cast. (Die casting can use only non-ferrous metals such as aluminum alloys, zinc alloys, and magnesium alloys.)

Disadvantages

• Low dimensional accuracy
• Casting surfaces are coarse due to the sand molds.
• Running costs are high because the sand molds are destroyed each time, so this method is not suitable for mass-production.
• Lower mechanical characteristics when compared to permanent mold casting due to low cooling speed

The various types of sand mold are differentiated by the bonding method employed. The two most common sand mold types are:

Green sand mold
Clay (bentonite) is added to silica sand and the binding strength of water is used to bond the sand mold. Silica sand is cheap because it is produced naturally, but it is not suitable for mass-production.

Shell mold
A sand-resin mixture produced by mixing thermosetting phenolic resin with silica sand is used. This type of mold is called a shell mold because the mold is thin like a shell.
Thin shell molds form smooth surfaces and are often used for precise casting of automotive engine parts and similar parts. Shell molds can be destroyed easily, which makes them suitable for mass-production. However, they are more expensive when compared to green sand molds because metal patterns must be used.

The sand particle size greatly affects casting surfaces.
Smaller sand particles form smoother casting surfaces, but they hinder air flow and can confine gases which can result in defects. Larger sand particles form coarse casting surfaces, but they facilitate air flow and thus can produce castings with fewer defects caused by gases.
The sand particle size should be selected according to the needs of the application.