R_Bibb_Medical_Modelling_The_Application_of_Adv.pdf

114 Medical modelling
• Road widths (extruded thermoplastic width) between 0.250 and
0.965 mm.
• Layer thickness (extruded thermoplastic height) from 0.178–0.356 mm.
The above specifi cation indicates the precision of model building that is
achievable with FDM TM . It can be seen that the results are broadly similar
to those achieved by SL. However, SL can achieve thinner layers and more
precise control over the laser position compared to the deposition of plastic
material in FDM TM .
Computerised Numerically Controlled (CNC) milling
Although generally not considered one of the many rapid prototyping
technologies, CNC milling can successfully build some medical models (24).
This technology was applied in the construction of custom titanium implants
for cranioplasty. CNC milling uses a cutting tool, which traverses a block
of material removing it on a layer-by-layer basis. Fig. 6.5 shows a model of
a skull defect (only half the skull has been created). The complexity of
models that can be achieved using CNC milling is limited as it only cuts on
one side of the model data. If the model required has any internal features
or complex surfaces facing a number of directions, then CNC milling would
6.5 Half skull model created by CNC milling demonstrating a large
cranial defect.