The basics of sand casting


This very old and common casting process begins with a pattern made in the shape of the desired part. The pattern can be made from wood, plastic, metal or other material.  Simple parts can be made from a single piece or solid pattern.  More complex items are made in two parts called "split patterns."  A split pattern has a top or upper section called a cope, and a bottom section called a drag. Both solid and split patterns can have cores inserted to complete the final part shape. Where the cope and drag separate is called the "parting line."  Typically a well-designed pattern has tapered edges so that the pattern can be removed without breaking the edges of the mold.

The pattern(s) are packed into sand that is mixed with binding agents.  This helps harden the sand into a semi-permanent shape.  Once the sand mold cures, the pattern is removed, leaving a hollow space in the sand in the shape of the desired part. The pattern is purposely made larger than the cast part to allow for shrinkage during cooling. Sand "cores" can be inserted in the mold to create holes and improve the casting's final shape.  Simple patterns are usually open on top, and the melted metal is poured into them. Two-piece molds are clamped together, and the melted metal is then poured into an opening called a gate. Sometimes, vent holes are created to allow hot gases to escape during the pour. 

The pouring temperature of the metal is a few hundred degrees higher than its melting point to assure good fluidity.  This ensures that the part won't cool prematurely, resulting in voids and porosity.  Bruin Metal Works casts from a variety of aluminum and brass alloys.

When the metal cools, the sand is removed and the metal part is ready for use or for secondary operations such as tumbling/deburring, machining, polishing, plating, etc.  One of the biggest advantages of sand casting is that it's the least expensive of the range of casting processes.  Sand casting is ideally suited for low-volume runs, prototypes and similar applications.

 

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