3.1 Computer-aided design model
All AM products must start with a digital model of the external physical shape of the final component. The term external not only refers to the outward-facing surfaces but also to the internal surfaces. You can think of it as any boundary between material and air.
A digital model can take a number of forms. It could be a professional engineering drawing of a component done with proprietary CAD software; it could be a model created with consumer software such as Google SketchUp; or it could even be a 3D laser scan of a physical object (Figure 7).
The model data is used to create a file with stereolithography data (an STL file). STL files were originally implemented in software created by a company called 3D Systems. The company needed to provide its RP machines with a digital representation of each layer to be added. For the purpose of this course, a knowledge of the intricacies of the STL file format is not necessary.
OpenLearn - Additive manufacturing 
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