R_Bibb_Medical_Modelling_The_Application_of_Adv.pdf

72 Medical modelling
address trapped volumes. If the cavities are very small, totally closed and
of no interest to the clinician, the data can be edited to fi ll in the cavity. It
may be tempting to leave such cavities in place as they are fully closed;
however, this poses risks. If the model is broken, sawn or drilled into at a
later date, the unused material may leak out. This could be in the form of
loose powder, solvent or liquid resin which may damage the model or prove
a nuisance (or, depending on the material being used, even a minor health
risk) to the user or patient.
Where cavities are larger or deemed important, artifi cial openings can
be created to enable the waste material to be removed. When creating such
openings the location and shape should be chosen to make them obvious
so that they are not confused with the anatomy. For example, if holes are
made square they are less likely to be confused with naturally occurring
holes. Alternatively, as described above, the model can be sectioned into
parts that are built separately either to enable access to the cavity or to
eliminate it altogether.
5.2 Stereolithography (SL)
5.2.1 Principle
Liquid resin is selectively cured to solid by ultra violet (UV) light accurately
positioned by a laser. The laser scans the layers onto the surface of the
resin, the fi rst layers being attached to a platform. Successive layers are
cured by lowering this platform and applying an exact thickness of liquid
resin.
5.2.2 Detail
The Stereolithography Apparatus SLA ® was developed and commercialised
by 3D Systems Inc. in the 1980s. Models are made by curing a photopolymer
liquid resin to solid using a UV laser. Models are built onto a platform
that lowers by a layer thickness after each layer is produced. Wait states
allow the liquid to fl ood over the previous layer and level out. Then a
recoater blade will pass over the liquid, levelling the resin but also removing
any bubbles or debris from the surface. The length of the wait states and
speed of the recoater blade will depend on the viscosity of the resin. This
method also means that there are problems with building objects with
trapped volumes as the liquid in these areas is not in communication with
the resin in the vat and does not level out, leading to build failure. In an
attempt to remedy this, the recoater blade has a U-section that picks up a
small amount of resin with a vacuum and deposits it over the previous
layer.