R_Bibb_Medical_Modelling_The_Application_of_Adv.pdf

Physical reproduction 73
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 using a solvent and the supports
removed. The model is then post-cured in a special apparatus by UV
fl uorescent tubes. All lasers used in stereolithography emit in the UV spectrum
and are, therefore, not visible to the naked eye. The laser represents
a considerable cost and has a limited life, leading to high running costs.
Lasers are replaced on an exchange basis with the manufacturer.
The speed of the machine depends on how much energy the resin requires
to initiate polymerisation, as the power of the laser is more or less constant;
if more energy is required the laser must travel slower. Material properties
and accuracy also depend on the resin characteristics. As the material
polymerises, there will be some degree of shrinkage; this can be compensated
for in the build parameters but may also lead to other problems, most
notably curl. This was especially true early in the development of SL when
most systems used acrylate-based resins. These problems were partially
eliminated by altering the build style, i.e. the way the laser scans the layers.
The development of epoxy-based resins eliminated these problems as it
shows very low shrinkage; this gives very accurate models, although it
requires more energy to polymerise and therefore builds slower. New materials
are becoming available with physical properties more similar to
thermoplastics.
Solid resin models proved unusable as sacrifi cial investment casting
patterns, because they swell with heat and crack the ceramic shell. To
enable investment casting, quasi-hollow build styles were developed. These
produce models as a thin skin with a delicate supporting structure inside.
The vent and drain holes are incorporated into the skin allowing the uncured
resin to be drained and centrifuged out. These holes are then plugged with
wax. The surface area of this type of model is very large, and models will
absorb moisture readily so they must be used quickly and stored in dry
conditions. When these models are heated, the supporting structure softens
so, upon burn out, the models collapse in on themselves and therefore do
not crack the investment shell.
Typical SL medical models are shown in Figs 5.9 and 5.10. In medical
modelling terms, SL is in many ways ideal. SL models show good accuracy
and surface fi nish. The transparency of most SL materials enables internal
details such as sinuses and nerve canals to be clearly seen. The fact that
unused material remains liquid also means that it can be easily removed
from internal spaces and voids. This is crucial when considering that
the majority of medical modelling is of the human skull, which possesses
many such internal features as well as the cranium itself. These advantages
can be clearly seen in the examples illustrated here. The solid, fully
dense, fi nished models lend themselves well to cleaning and sterilisation