Preparation of the moulds and cores

Cast iron foundries essentially use silica sands to make the mould in which the casting will be poured, and cores to allow castings with complex geometries, with internal cavities. The mould is therefore essentially a die. In typical foundry terminology, the term “mould” is used when the latter is made with refractory materials (silica sand or other ceramic materials) and is “disposable”, that is, it is destroyed each time a casting is produced. The term “die”, on the other hand, is used to speak of “permanentmoulds (that is, they can be reused many times), made of steel. In cast iron foundries, given the high casting temperatures, the only possible solution is to use moulds in silica sand or other ceramic material.

The silica sands are mixed with binding substances (inorganic such as bentonite, or organic such as amines) and inserted using different methods into metal containers, typically in steel or cast iron, called “flasks”. The flask is used to give mechanical stability to the mould, which, once subjected to the weight of the liquid cast iron during casting, may be deformed or break. Using models (that is, reproductions of the entire casting system including the casting) the sand-binder system is appropriately shaped, creating the cavity in which the liquid cast iron will be poured.

The models can be made of wood (medium-small series, and even large-sized castings), of polymer (medium-sized series), or metal, typically aluminium alloys (large-sized series), taking into account their possible deterioration due to contact with the sand.

It is important to bear in mind that it must be possible to extract the model, once the sand has been shaped, from the sand. Therefore, no undercuts must be generated (in which case, extracting the model would lead to damage and breakage of the mould). However, cores, similarly made of sand and binders, can be placed inside the cavity. The cores, inserted in the mould, allow the creation of castings with more complex geometries and with internal cavities. Using cores, in fact, the liquid cast iron no longer fills the entire mould, but only the cavity between the core and the mould. Like the mould, the core too must guarantee suitable consistency during casting and the initial solidification stage, but must gradually flake off and be removed during decoring.

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