3.3 Forming v casting
As the stresses needed to make solids flow are considerably higher than those required for liquids, forming processes normally require a lot of energy and strong, resilient tooling. This means high expenditure on capital equipment as well as tooling and energy. As a result, forming is often economically viable only for production volumes large enough to justify the high tooling costs.
So when do we use forming rather than casting? There are three reasons why, for many products, forming is preferable to casting.
Geometry. Products with one dimension significantly different in size from the others are most suitable for forming processes – 'long' products such as rails or 'thin' products such as car-body panels are usually made by a forming process. Imagine trying to cast a 50 metre length of pipe; large forces would be needed to squeeze the metal down into the mould, and it might be difficult to keep the metal liquid for long enough. There would also be a lot of scrap to discard.
Microstructure. As noted earlier, the microstructure of the material has a direct bearing on the properties of the final product. Controlling the microstructure is easier during forming than during casting. Also, the type of microstructures produced by forming are inherently stronger than those produced by casting, as the products of forming do not contain the dendritic structure and porosity inherent to castings. We will return to this point later.
Some materials are difficult to process as liquids, e.g. they may have high melting points, or react with the atmosphere.
Why are car-body panels produced by forming and not casting? (Hint: think in terms of the shape of the final product and the form of the starting material.)
Producing thin products such as a car-body panel is very difficult by casting. To fill the mould would require the casting liquid to be very fluid and to stay that way while it filled all of the mould cavities. This is difficult to achieve. Hence it is easier to form the body panels, probably starting with a sheet of material that is pressed into shape.