Sand-Casting

The two basic types of sand-casting are:

• Removable/re-usable pattern (usually wood or metal)
• Disposable pattern (e.g. polystyrene patterns, which vaporise when the metal is poured)

There are many vital aspects of the sand-casting technique, which are necessary to produce good castings. For example, the sand must have:

• Adequate binding qualities: To achieve this, a small percentage of clay is added.
• Suitable porosity characteristics: This permit the escape of gas/steam formed in the mould. There are different requirements for different metals (e.g. steel and aluminium).
• Correct grain size and sufficient strength: The sand is graded by means of a sieve and the strength is controlled by the amount of bonding agent present.
• Suitable temperature resistance: The sand must withstand the molten metal temperature without fusing/melting.
• Adequate hardness: The hardness may be checked by the resistance to indentation by a spring-loaded ball.
• Acceptable moisture content levels: This is usually in the range of 2-8% and is checked by weighing the sand before and after drying.

While the characteristics of the sand are important, the design of the mould must also meet certain standards, as stated below.

• The top and bottom halves of the mould (cope and drag, respectively) must incorporate positive alignment features.
• The pattern must be shaped, such that withdrawal from the sand leaves a perfect impression. Therefore, tapered faces are better than perpendicular faces.
• Suitable feed channels must be provided for the molten metal to enter the mould. These channels are called as the sprue and the runners.
• Strategically placed reservoirs (called risers) must be incorporated to ensure proper filling of the mould, as the metal shrinks and begins to solidify. Typical steel shrinkage is around 3-4% and aluminium shrinkage, 6-7%.
• Where necessary, the vents must be present to permit the escape of gas and steam when the molten metal contacts the sand.
• Local ‘chills’ are sometimes included in the mould, to encourage more rapid, local solidification of the metal.

The advantages of sand-casting are that it is a simple process, economical for small batches and also suitable for most metals. The major shortcomings are that the process is not very rapid, not particularly accurate (due to lack of sand rigidity) and is not suitable for thin-wall sections.