Post-process operations

Upon completion of the foundry process, the sand mould incorporates the newly produced casting. The relatively long permanence at process temperatures causes the binders, which have exhausted their sand-aggregating function, to decompose and thus fail to ensure cohesion between the sand grains used for the moulds and cores. These are therefore the ideal conditions in which to extract the casting from the sand. This separation, normally defined as “decoring”, is more easily achieved using conveyor belts with vibration systems or by inserting the flaking castings and moulds inside rotating cylinders.

Attention must naturally also be paid to the cooling speed of the casting, and therefore the moment when decoring occurs. The permanence of austenitic structures up to temperatures below 600°C (for example, due to rapid decoring) may in fact cause troostite to form. This is a microconstituent characterised by a matrix of ferrite with very fine cementite needles, which can be likened to a very fine type of pearlite. It is harder than conventional pearlite and can therefore generate inconsistencies in the mechanical characteristics of the casting.

After decoring, the so-called “casting appendages” must be removed either mechanically or by oxy-fuel cutting to achieve the desired final geometry.

Desanding detail

The casting obtained after decoring is in fact a single cast iron “object” inclusive of risers, sprues and even the “burrs” (that is, the protrusions generated when the liquid cast iron slips between two half-moulds, then solidifies). All of these “casting appendages” must naturally be removed before carrying out the necessary finishing on the casting (this process is also known as “removal of risers”). Risers and casting appendages can be added to the charge of a melting furnace and then recycled.

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