In general, it is more economical to produce several castings in a single production mould, because of material, energy and labor savings:

Less amount of mould material utilized per casting. Common feeders and gating system can be used, improving the yield. Reduced set up time during manufacture (for mould preparation, pouring, shakeout and fettling) per casting.

The savings must offset the higher cost of tooling manufacture. Multiple cavity moulds are therefore preferred only when the castings are very small compared to the smallest size of production moulds and the production quantities are large. The mould cavity is usually taken up after deciding the casting orientation and parting.

The minimum distance between cavities and from any cavity to the nearest edge of the mould must be set. The minimum distance must be sufficient to: (a) prevent damage to the mould, and (b) to allow adequate heat transfer so that local hot spots are not formed in the portion of a casting close to another cavity. The minimum distance ranges from 25 mm for small castings to 50 mm for medium size castings.

The number of cavities in a mould has to be optimized in terms of metal to sand ratio:


where, ?metal and ?sand are the density of casting and mould materials, Nc is the number of casting cavities, Vc is the volume of a single cavity, Vf and Vg are the total volume of feeders and gating, respectively, and Vmould is the overall volume of mould (based on its dimensions). A low value of the ratio indicates poor utilization of mould material. A high ratio must also be avoided, since this may lead to poor heat transfer, in turn leading to defects related to casting solidification. The minimum recommended value of metal to sand ratio for ferrous castings is 1:1. In practice, it ranges from 1:2 to 1:6.

The procedure is repeated for different sizes of mould, and the mould which gives the highest value of metal-to-sand ratio (but more than the minimum limit) can be selected.

In practice, the mould cavity layout will also depend on the position of feeders and gating, especially if they are common to more than one casting. It is also possible to design the layout with different types of castings in the same mould. For example, a large casting (but not large enough for a good metal to sand ratio) can be combined with one or more small castings in the same mould.