R_Bibb_Medical_Modelling_The_Application_of_Adv.pdf

78 Medical modelling
means that there are no problems when building objects with trapped
volumes.
As in SL (but upside-down on the Perfactory ® ), overhanging or unconnected
areas have to be supported. Supports are generated by the build
software and built along with the model. When a model is complete, excess
resin is washed off, sometimes using a solvent, and the supports are removed.
Unlike SL, the models do not require post-curing, although if required this
can be achieved without special equipment as the resin will cure in daylight.
Also unlike SL, there is no laser, which leads to considerable savings in
maintenance and running costs. The projector bulbs will require regular
replacement, but the cost of around £300 is negligible compared to the costs
of replacing a laser.
The accuracy of the process is a function of the projected image of the
cross section. The DMD has a pixel array of 1280 × 1024 (SXGA), but the
image can be scaled using optics. Therefore, the resolution of the model
will be defi ned by the overall size of the build area divided by the number
of pixels. Typically, this resolution is in the order of 0.06 mm but may range
from 0.15 to 0.032 mm depending on the parameters and optics used.
The materials used are similar to SL resins and produce models that are
solid, tough and transparent, although usually orange or red in colour. An
example is shown in Fig. 5.13. Materials have been developed and approved
for specifi c medical application in dental technology and hearing aid manufacture.
An example of a hearing aid shell manufactured by DLP TM is
shown in Fig. 5.14. The advantages and disadvantages of DLP TM are summarised
in Table 5.2.
5.4 Fused Deposition Modelling (FDM TM )
5.4.1 Principle
Thermoplastic material is fed in fi lament form to a heated extrusion head.
Layers are made by molten material deposited as a fi ne bead. The build
table lowers an exact amount and the next layer is deposited on to the
previous layer, bonding due to partial melting.
Table 5.2 Advantages and disadvantages of DLP TM
Advantages Disadvantages
High accuracy, excellent surface fi nish Resin handling requirements
Little material waste Limited material choice
Low maintenance and running costs Limited build size
Transparent models