To cast a part you first need a set of molding boxes. This consists of two parts, namely a top half and a bottom half, and sand is pressed into each of these two parts against the pattern. The resulting mould corresponds to one-half of the casting that will eventually be made. The two halves fit perfectly together, which ensures that we can obtain a block of sand with a hollow that has exactly the same shape of the casting to be produced. If the casting itself has to be hollow then the molding box also includes a core. The core, also made in chemically-bound sand, is slightly smaller than the pressed mould, meaning that the cast iron can flow evenly into the space between the core and the mould box. The principle is simple: where there is no sand, we eventually get cast iron.