4.9.1 Creating shapes
With modern AM techniques, the starting point is a digital description of the product in the form of a 3D CAD model (Figure 68). This then has to be broken down into units – the building blocks of the AM process – but rather than the rectangular blocks I have been discussing, the building block of all AM processes is a 2D layer or slice through the object. These layers are themselves generated from a file created by the CAD software that defines all the surfaces of the object in question. This can be deconstructed into the 2D slices needed to drive the AM machine.
Dividing a complicated 3D object into layers presents an interesting challenge. There are two main issues to do with shape, rather than to do with data processing, which is not our concern here. The first is similar to the question of how big to make a house brick. How thick should each layer be? The second is something more to do with geometry. Figure 69 shows an abstract object made by an AM process. Imagine what each layer taken through the object looks like in two dimensions. Many of them will consist of a more or less random set of disconnected bits, depending on where the slice cuts through the object.
So if such an object really is to be built out of layers, there has to be a way of placing each bit on top of the previous layer and holding it in the correct place as the following layer is formed.
We shall revisit these two issues as we look at a selection of different AM processes next.