If you think about it, the number of different things you can do to a raw material to get it into a desired shape is pretty limited (Figure 57).
You could start with the raw material in liquid form, pour it into a mould that replicates the shape you want and wait for it to solidify – think of making ice-cubes or casting concrete.
You could squeeze , squash, hammer, bend or stretch the material into its required shape – similar to rolling out a piece of dough or modelling with clay, as the car body designers do in a car factory when they are working with a new body design.
You could start with a lump of solid material and carve or cut it to shape, in the same way Michelangelo transformed a block of marble into the statue of David.
Finally, you could build your shape by taking different pieces and joining them together using any number of methods: screwing, nailing, gluing, welding or stitching for example. Innumerable products involve at least some joining, ranging from a skirt to a car body, and from a desk to an aircraft wing.
So, starting with a given mass of raw material, whether it is a pile of granules of plastic, an ingot of steel, a lump of clay, a block of stone or whatever, the basic process routes for manipulating it into a specified shape nearly all fit into one of four categories:
- a.pouring, which we will refer to more precisely as casting
- b.squeezing and bending, which we will call forming
- c.cutting (sometimes referred to as 'machining')
However, life is rarely simple. To start with, the wide range of engineering materials means that there are many, many variations on each of these process routes. And you shouldn't worry about trying to fit every process you encounter neatly into one of the four categories above. You will see processes that combine elements of more than one approach. The categories simply provide us with a convenient way of grouping similar processes together and examining the underlying scientific principles that unite them.