From Sand to Solid: A Step-by-Step Journey through the Sand Casting Process
Sand casting is a cost-effective process for producing metal parts weighing from ounces to hundreds of pounds. Best results are achieved with lower melting point alloys like cast iron and bronze.
The process is suitable for a wide range of production volumes. Large runs can be produced at low unit cost on automated equipment, while small quantities can be produced economically with more manual involvement.
This blog provides an overview of the complete sequence of steps involved.
Step 1: Produce a Pattern
The pattern is used to create a cavity in the sand the size and shape of the part being cast.
Patterns are reused, unlike the sand mold itself. Inexpensive patterns can be made in wood or plastic, or for greater durability, aluminum. Patterns are often made in two pieces, each slightly larger than the part being cast to allow for shrinkage as the metal cools. Vertical walls of patterns have a slight angle or “draft” so the pattern can lift out without pulling sand away.
Step 2: Form Two Halves of the Cavity
One half pattern is placed a box where sand is packed around it. The pattern is positioned so it can be lifted out afterward, which means the cavity only reproduces half of the pattern, and there can be no re-entrant features.
This process is repeated for the other half pattern. Additional inserts are added that will provide channels for the metal to flow into the cavity. The dimensions of this “gating system” determine how fast the cavity fills. Higher flow reduces the risk of nonfills, but at the cost of increased turbulence, which can cause casting defects.
The traditional sand casting process used “green” sand. This refers to it being damp, rather than the color. Green sand contains bentonite clay, which binds the grains together. Today, most sand casting is done with resin sand. This is sand that has been coated with a resin binder. It forms denser, stronger molds and more accurate cast parts.
Step 3: Insert the Cores
Cores take up space within the mold cavity that would otherwise be filled with metal. They are usually made from sand and are placed into the cavity before the top and bottom halves are joined.
Step 4: Assemble the Cope and Drag
The drag is the lower sand cavity, the cope is the upper piece containing the metal feeding channels. Together, the assembly is called a flask. Pins ensure good alignment between the two halves. The region in the cavity where the two sand surfaces meet will form a parting line on the cast part.
Step 5: Pour the Molten Metal
Metal flows into the cavity from a pouring basin on the top surface of the mold. As the cavity fills, metal rises up in a channel called the riser. This forms a reservoir that gets drawn back into the cavity to compensate for shrinkage.
Copper, bronze, brass and iron tend to pour and flow readily, but some other alloys are less fluid and some, like aluminum, can react with the sand. This is minimized by reducing exposure to air and adding fluxes to the molten metal.
Step 6: Demold and Knock-Out
After pouring, the metal is left to cool and solidify. On completion, the sand molds are removed from the flask and the cast parts knocked out. This also breaks up any cores incorporated into the mold.
The gating system and riser are then cut away, leaving metal castings that are ready for machining.
Robust and Economical
The sand casting process is adaptable to a wide range of alloys, part dimensions, and production quantities. Surface finish is dictated by sand particle size, and accuracy is determined by pattern precision and the alignment between cope and drag.
Limitations of the process are the need for draft angles on vertical surfaces and the parting line at the meeting point between the two halves of the mold. However, for many metal parts, the cost advantages outweigh the expense of machining these features away.