Aluminum Casting Techniques

Background

Aluminium casting processes are classified as Ingot casting or Mould casting. During the first process, primary or secondary aluminium is cast into rolling ingot (slab), extrusion ingot (billet) and wire bar ingot which are subsequently transformed in semi- and finished products.

The second process is used in the foundries for producing cast products. This is the oldest and simplest (in theory but not in practice) means of manufacturing shaped components.

This section describes exclusively Mould casting which can be divided into two main groups :

·         Sand casting

·         Die casting

Other techniques such as "lost foam" or "wax pattern" processes are also used but their economical importance is considerably lower than both listed techniques.
Sand Casting

In sand casting, re-usable, permanent patterns are used to make the sand moulds. The preparation and the bonding of this sand mould are the critical step and very often are the rate-controlling step of this process. Two main routes are used for bonding the sand moulds:

·         The "green sand" consists of mixtures of sand, clay and moisture.

·         The "dry sand" consists of sand and synthetic binders cured thermally or chemically.

The sand cores used for forming the inside shape of hollow parts of the casting are made using dry sand components.

This versatile technique is generally used for high-volume production. An example of half sand mould is given in Figure 1.


Normally, such moulds are filled by pouring the melted metal in the filling system. Mould designing is a particularly complex art and is based on the same principle as gravity die casting illustrated in Figure 4.

In the "low pressure" sand casting technique, the melted metal is forced to enter the mould by low pressure difference. This more complicated process allows the production of cast products with thinner wall thickness.