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2.2 Properties for processing – casting

The casting (or pouring) group of processes is one of the most convenient for making three-dimensional shapes, especially if repeated copies are required. However, you do have to be able to get your material into liquid form, and it has then to be 'runny' enough to be poured.

What do these conditions require?

To get a liquid, you have to either melt the material; or dissolve it in a solvent which is subsequently evaporated off (the 'solution route'); or pour liquid precursors into a mould where they react chemically to form a solid (the 'reaction route').

Some materials (e.g. thermosetting plastics) decompose rather than melt on heating. Others react with oxygen when heated, so need to be melted in inert atmospheres (which may prove expensive). Yet others have such high melting points (see the database) that the energy costs of heating them is only justified in special cases.

The solution route needs a suitable solvent, which you then have to be able to evaporate safely (many coatings such as paints are applied this way), but you can have shrinkage problems as the solvent is removed. The reaction route is used for both thermosets and thermoplastics and for concrete, but chemical reactions can produce considerable quantities of heat, so you must allow for this in the design of the process.

Once you have the liquid, can you pour it?

The physical property that determines the 'runniness' of liquid is called viscosity. This varies with temperature and is not all that useful for describing how well a mould will be filled if the temperature of the liquid is falling as it runs into the cold mould. In the casting of metals a more useful property is fluidity, which takes into account not only the viscosity changes but also the effects of cooling rate, surface tension of oxide films and the temperature range over which the alloy filling the mould actually freezes. Eutectic alloys have a high fluidity as they melt at a single temperature. Many of the alloys used for casting products are based on eutectic alloys.

Water and most liquids at room temperature have low viscosities, so can be poured easily, as can thermoset precursors. Molten thermoplastics, freshly-mixed concrete and clays have much higher viscosities. Although concrete can be poured, the others generally need to be pushed into their moulds, which is why injection-moulding machines for plastics are much 'beefier' than their pressure die-casting machine counterparts for metals.