Designing castings and pouring systems

The development of a component by means of a foundry process naturally presupposes the choice of material and dimensioning according to the rules of mechanical design (casting design stage). Once this stage is complete, it is necessary to design the geometry of the mould in which the casting will be made, taking into account cavity filling phenomena (influenced by the temperature and viscosity of the alloy), minimising the effects of turbulence (if the liquid alloy is poured into the cavity too quickly, air pockets may develop or mould “erosion” and the incorporation of sand particles may occur), appropriately evaluating the behaviour of the material during its solidification.

Solidification is a key step in foundry processes: if volumetric shrinkage phenomena associated with the transition of the alloy from the liquid to solid state is not controlled, defects such as shrinkage cavities will form.

The extent of volumetric solidification shrinkage strongly depends on the composition of the cast irons and their type. It must also be considered that volumetric shrinkage of the “ferrous” part of a cast iron (that is, the austenitic structure) can be partially or fully compensated by “graphite expansion”, that is, the increase in volume associated with the formation of the graphite. Therefore:

  • White cast iron has a variable volumetric shrinkage in solidification of between 4 and 5.5%;
  • Grey cast iron can vary between a volumetric shrinkage of 2.5% and expansion of 1.6%;
  • Spheroidal graphite iron can vary between a volumetric shrinkage of 4.5% and expansion of 2.7%.
Cast iron component design Zanardi Foundries

For all of the aforementioned reasons, the geometry of the piece to be developed must necessarily be integrated with the insertion of sprues, risers, and cores (for more information, see the page Microstructural constituents of a cast iron). Risers are “reservoirs” capable of supplying liquid to compensate for any possible cavity shrinkage, and are positioned in the areas most at risk of generating said defects.

To support casting and pouring system design, once based solely on operator experience, modern numeric simulation codes are used, capable of viewing and predicting, in the utmost detail, cavity filling, the solidification of the alloy and cooling of the casting.
Cast iron component design Zanardi Foundries

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