R_Bibb_Medical_Modelling_The_Application_of_Adv.pdf

Case studies 247
supports became available. Although they would have been an improvement
in this respect, they would still have resulted in an excessive number
of supports with an even larger support STL fi le.)
To avoid these problems, a novel strategy for producing the necessary
support was attempted. The approach was based on two fundamental
assumptions. Firstly, as bone is a naturally occurring, load bearing structure
it is made up of self-supporting arches. Therefore, the structure should, in
theory, support itself except for the sides of the model where the structure
has been sliced through. The second assumption was that the open spaces
would all be inter-communicating and, therefore, there would be no ‘trapped
volumes’ (a recognised problem in stereolithography).
As automatically generated supports were impractical, a very thin crate
was designed using CAD that would support the sides and base of the
model yet still allow good draining, see Fig. 6.94. This is necessary to avoid
the ‘trapped volume’ effect that adversely affects the stereolithography
process. The crate was then exported as an STL fi le. Curtain support structures
were then automatically generated (using 3D Lightyear TM , 3D Systems
6.94 The crate structure.