Excerpt from the Complete Casting Handbook (Second Edition) 2015 by John Campbell.
Placing a block of metal on the pattern in a sand mold and subsequently packing the sand around it to make the rest of the mold is a widely used method of applying localized cooling to that part of the casting. A similar procedure can be adopted in gravity and low-pressure die-casting by removing the die coat locally to enhance the local rate of cooling. In addition, in dies of all types, this effect can be enhanced by the insertion of metallic inserts into the die to provide local cooling, especially if the die insert is highly conductive (such as made from copper alloy) and/or artificially cooled, for instance by air, oil or water.
Such chills placed as part of the mold, and that act against the outside surface of the casting, are strictly known as external chills to distinguish them from internal chills cast in and become integral with the casting.
In general terms, the ability to chill is a combination of the ability to absorb heat and to conduct it away. It is quantitatively assessed by
heat diffusivity = (KρC)1/2
where K is the thermal conductivity, ρ is the density, and C is the specific heat of the mold. It has complex units Jm−2K−1s1/2. Take care not to confuse with thermal diffusivity = K/ρC normally quoted in units of m2s−1.
From the room temperature data in Table 5.1 (unfortunately, high-temperature values are less easily obtained), we can obtain some comparative data on the chilling power of various mold and chill materials, shown in Table 5.2. It is clear that the various refractory mold materials—sand, investment and plaster—do not act effectively as chilling materials. The various chill materials are all in a league of their own, having chilling powers orders of magnitude higher than the refractory mold materials. They improve marginally, within a mere factor of 5, in the order steel, graphite and copper.
The heat diffusivity value indicates the material's action to absorb heat when it is infinitely thick, being unconstrained in the amount of heat it can conduct away and store in itself, i.e. as would be reasonably well approximated by constructing a thick-walled mold from such material.
This behavior contrasts with a relatively small lump of cast iron or graphite used as an external chill in a sand mold. A small chill does not develop its full potential for chilling as promised by the heat diffusivity because it has limited heat capacity. Thus, although the initial rate of freezing of a metal may be in the previous list's order, for a chill of limited thickness its cooling effect quickly becomes limited because it becomes saturated with heat; after a time it can absorb no more. The amount of heat that it can absorb is defined as its heat capacity. We can formulate the useful concept of heat capacity per unit volume ρC in terms of its density ρ and its specific heat C so that the heat capacity of a chill of volume V is simply volumetric heat capacity = VρC
In the SI system, its units are JK−1. Figure 5.11 illustrates the fact that if chills are limited by their relatively low heat capacity, there is little difference between copper, graphite and iron. However, for larger chills that are able to conduct heat away without saturating, copper is by far the best material. The next best, graphite, is only half as good, and iron is only a quarter as effective.
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