1.14 Getting into shape: some basics
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.
You could melt or liquefy the raw material and pour it into a mould that replicates the shape you want – as if making ice-cubes.
You could squeeze, squash, hammer or stretch the material into its required shape – similar to modelling with clay or Plasticene, or rolling-out a piece of dough.
You could start with a lump of raw material and cut it to shape, in the same way Michelangelo transformed a block of marble into the statue of David.
Finally, you could assemble your shape by taking different pieces and joining them together using any number of joining methods: screwing, nailing, gluing, welding or stitching for example – innumerable products are made in this way, ranging from clothing to cars and from computers to aircraft.
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 are essentially limited to:
pouring, which we will refer to more precisely as casting
squeezing, which we will call forming
However, it's not quite as simple as that. To start with, the wide range of engineering materials means that there are many, many variations on each of these process routes. So far we have principally considered materials just to be 'stuff' that has a series of properties. We have seen that these properties vary from material to material but we have not really started to think about why they vary. We are not going to go into the material science behind this in any real depth in this course but what is important to realise is that materials, and hence products, exist on a whole series of size scales. We are all familiar with the sizes of tangible products ranging from a teacup all the way up to a suspension bridge or the Millennium Dome! We can call this scale macro structure. You should also be familiar with the concept that the properties of materials are controlled by the type and arrangement of their individual atoms and molecules, usually called atomic structure. Much of materials science and engineering is concerned with a size scale in between, too small to be seen with naked eye, but much larger than individual atoms and molecules. This middle ground is termed microstructure.
The properties of solid materials can be profoundly influenced by their microstructure and because the microstructure is often changed by processing, the properties of materials, and hence products, are dependent on how they are processed. Examples of these different size scales are given in Scales of material structured.
Even where a particular type of material and process combination is feasible, it could just be hopelessly uneconomic to contemplate it as a manufacturing option. Finally, the shape of the product is also important. Some manufacturing methods are better suited to particular shapes than others. Indeed, the shape of a product is a good attribute to begin with when deciding which processes are feasible. So one of the first things we must do is think about how we describe shape. One approach to this problem is given in Classifying shapes.