Internal cavities in the casting must be produced by placing properly shaped inserts, called cores, usually made from bonded sand, into the mold cavity. The core materials must be subsequently removed and, therefore, the internal cavity must communicate with the outside of the casting. The core also must be well-supported and anchored by the mold to avoid flotation or deformation during mold filling due to the difference between the density of the core and that of the liquid iron. One anchor point is the theoretical minimum. Normally at least two anchor points (core prints) are required.

Cores are also required when, due to configuration, the pattern cannot be withdrawn from the mold. Such cores can be readily made, but they add considerably to the cost of production. For this reason, the designer should be aware that undercuts prevent pattern withdrawal. Small cores exhibit relatively low strength especially at the temperature of liquid iron. For this reason, if holes of small cross section are required in the casting, it may be less expensive to machine the holes than to form them by the use of small (weak) cores. Channels of small cross section may be cast integrally by placing appropriately shaped steel tubing in the mold cavity in a core-like fashion. A portion of the steel tubing wall will fuse with the liquid Ductile Iron.

The volume changes that occur during the cooling and solidification of Ductile Iron are unlike those in any other alloy. The volume of the liquid decreases

with decreasing temperature until slightly above the solidification temperature. Upon further cooling, the contraction stops and a definite volumetric expansion starts. Unfortunately, the expansion phase prevails through only part, but not all of the solidification process. The expansion gives way to another contraction phase, "secondary shrinkage", which continues until all of the liquid is transformed to solid.

It is the craft of the foundryman to use the above-described pattern of volume changes to obtain sound castings. The temperature of the liquid iron should be high enough to provide for complete fusion of the separate streams and to avoid the entrapment of small gas bubbles. Each section thickness has its optimum pouring temperature range within which perfectly sound castings can be made. Ductile Iron castings with 3 mm thick walls may need to be poured as hot as 2,640°F (1,450°C), while 100 mm thick castings can be poured at between 2,300-2,400°F (1,260-1,320°C). Difficulties may arise when large differences in section thickness exist in one casting. Such castings can be made sound, with

considerable extra effort. The foundry will be pleased to explain.

It is in the best interest of both casting producer and user to design with as little difference in wall thicknesses as possible. It may be more economical to mechanically assemble two or more castings than to produce a one-piece casting with widely varying section thicknesses.