Expendable-Mold Processes

In expendable-mold casting processes, the mold is used only once and is destroyed to release the part; hence great complexity of shape is feasible. Expendable-mold processes are further divided into those using permanent patterns and those using expendable patterns.

Sand casting is used as a generic term for all processes in which an expendable mold is formed of a granular ceramic (ranging from natural sand to synthetic ceramics) by consolidation around a permanent pattern (wood, metal, or plastic), which is removed prior to filling the mold cavity with metal, necessitating the use of draft for the release of the pattern. The "sand" is bonded temporarily by clay (green clay or dry sand mold), sodium silicate (carbon dioxide, or CO₂, process), cement, or vacuum. In principle, all metals can be cast by these techniques provided the sand is refractory enough. Because cores are also destroyed upon completion of solidification, great shape complexity is possible. As a broad rule, a mold made with firmer bond allows the use of finer sand, the production of smaller parts with thinner walls, smaller cores, and greater surface detail. The low thermal conductivity of sands favours complete filling.

Cost of production in these processes can vary greatly. A great advantage of sand casting is low investment in sand preparation, distribution and the possibility of mechanized molding, and sand reclamation. The ratings in Table 1 reflect these variables; the lowest-cost equipment and pattern will generally go with highest labour cost, skill requirement, and lowest productivity. The range of products is very broad, from small engine blocks to large machine-tool beds.