R_Bibb_Medical_Modelling_The_Application_of_Adv.pdf

Physical reproduction 63
will display a stepped effect (shown in Fig. 5.2). Consequently, when building
objects, careful consideration should be given to the orientation so that
the optimum features are formed in the XY plane and stepping is avoided.
For example, a cylindrical shape should be built upright if the circular
section is to be faithfully reproduced.
The extent of the stepping can be diminished by creating thinner layers
but, as the overall build time tends to be more dependent on the layer
addition process, a larger number of thinner layers leads to longer build
times. Therefore, a compromise between surface fi nish and speed is established
for each RP process.
5.1.4 Beam compensation
Y
X Z
5.2 The stepped effect of layer manufacturing.
As RP devices are additive processes, the method used to create the layers
has a fi nite minimum size. In the case of the printing-based technologies,
this is usually expressed as the pixel size that the printer is capable of
reproducing, which in turn is usually expressed in the same terms as paper
printers in dots-per-inch or DPI. However, with laser-based technologies,
such as stereolithography and selective laser sintering, the diameter of the
laser beam provides the minimum capability. For maximum accuracy, the
software that controls this process offsets the path of the laser by half of its
diameter so that it draws a path within the boundary of the object. This is
illustrated schematically in Fig. 5.3. However, this means that small, thin
features below that minimum size cannot be reproduced. A similar effect
is shown by fused deposition modelling, where the minimum thickness of
the deposited bead of molten plastic is compensated for in the same
manner.
In practice the size of offset is very small, typically small fractions of a
millimeter; however, the effect should be considered separately to dimensional
accuracy. Whilst dimensional accuracy may remain very good, large
offset values will have a detrimental effect on the ability of the RP system
to reproduce intricate detail, crisp edges and sharp corners, as illustrated
in Fig. 5.4.