Casting furnaces

Cast iron is melted in special casting furnaces, the function of which is to supply the charge with the amount of heat required to bring it to a temperature above its melting point, making it sufficiently fluid and smooth that it can easily fill the cavity of the mould.
Cast iron casting furnaces can be classified according to the methods used to supply the required heat:

  • Use of solid fuel (cupola furnace);
  • Use of electricity (induction or resistance furnaces);
  • Use of natural gas.

Moulding techniques

Moulding” refers to the process by means of which the mould is made. The refractory material (which is usually silica sand), after being mixed with the binders (a widely used mixture is water and bentonite), is placed in a container (half-mould), in which the half-model has been previously placed. The refractory material must be pressurised around the model to create the casting cavity after the model has been removed. Moulding can take place using manual, mechanical or automated means.

Designing castings and pouring systems/h2>

The versatility of the foundry processes gives designers and end users a significant amount of operating flexibility, to meet the requirements set by the production of the component. The possibility of designing castings and pouring systems offers distinct advantages and benefits, especially when taking into account the alloy being poured and the type of component being produced.

Generally speaking, the following aspects must be considered for the correct selection of a foundry process:

  • surface quality of the casting
  • required size accuracy
  • number of castings to produce
  • type of moulding equipment necessary (models, core boxes, etc.),
  • costs of making the mould or die
  • interactions between the process identified and the design specifications of the casting and/or component
Cast iron component design Zanardi Foundries

Pouring mould process

There are different criteria for classifying pouring mould processes; the simplest identifies three categories on the basis of the material used for the mould in which the metal will be cast:

  • pouring processes in a “disposable” sand mould or, more generically, in refractory material, which has been given a suitable consistency by means of binders; this mould can be produced by using a permanent (i.e. reusable) or a “disposable” model, i.e. one that cannot be reused;
  • processes using permanent moulds (proper moulds), usually made by machining steel.
  • special processes with ceramic, plaster or other moulds.

Preparation of the moulds and cores

The mould is produced by shaping a suitable refractory material (sand, foundry sand), using a model to obtain the desired cavity shape, into which the liquid alloy is then poured. Cores can be inserted in the moulds (i.e. inserts made of refractory material and binders), which are used to create a gap (between mould and core) that will then be filled by the alloy. It is essential that the mould cavity retains its shape during the pouring and solidification of the alloy.

Post-process operations

The first post-foundry process operation is decoring: through the use of suitable vibration systems, the casting, which has now solidified and has suitable mechanical properties, is separated from the moulds and cores (the binders have meanwhile lost their ability to aggregate the refractory material due to the high temperature).

The various pouring appendages are removed (sprues, pouring channels) by mechanical or thermal cutting.

Cast iron part thermal treatments

Depending on the type of cast iron, heat treatments may be appropriate. This can be simple stress-relieving annealing (to remove any tension induced by the foundry process) or hardening followed by tempering treatments. One of the special features of ADI austempered ductile iron is the austempering treatment, aimed at optimising the microstructural characteristics of the matrix. In components which require strength and surface hardness, thermochemical treatments such as nitriding can be performed.

Mechanical and surface finishing

Additional mechanical and/or surface finishing can be carried out on the cast iron, especially sandblasting (for simple aesthetic and “cleaning” purposes) and shot peening (to harden the surface).

Other processes: lost wax, shell moulding, centrifugal casting and lost foam

In the cast iron foundry, in addition to silica sand (or green sand), other types of processes are used, such as:

  • lost wax casting
  • shell moulding process
  • centrifugal casting
  • lost foam

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