R_Bibb_Medical_Modelling_The_Application_of_Adv.pdf

62 Medical modelling
of the layer thickness and height of the model. The layers are created by
some form of precision controlled plotting, scanning or deposition mechanism.
The accuracy and resolution in the XY plane is therefore dependent
on this mechanism. Most are accurate to fractions of a millimetre, so that
any geometry in the XY plane should be faithfully reproduced.
The layer thickness is dictated by the mechanical process that adds the
build material and may vary according to the material being used. Layer
thickness is usually in the order of 0.05–0.30 mm, although some printingbased
technologies offer much thinner layers. This will lead to a stepped
effect in geometry perpendicular to the XY plane.
As an example, consider a cube with two perpendicular holes through it,
as shown in Fig. 5.1. The hole in the top of the cube, as viewed from above,
formed by the scanning mechanism in the XY plane will be formed perfectly
(within the capability of the mechanism). However, the hole in the
side of the cube, as viewed from the side, formed by the addition of layers
5.1 Cube with circular holes.