R_Bibb_Medical_Modelling_The_Application_of_Adv.pdf

Case studies 249
However, the problem of how to support the sides and base was encountered
again. Although automatic support software is available to generate
supports for SLC fi les, it presented exactly the same problems as the previous
attempt. To create minimal supports, a combination of two approaches
was used. The fi rst involved the use of C-Sup to generate supports automatically
that would separate the bottom of the model from the build
platform. These supports were automatically generated to end just above
the bottom of the part as it was again assumed that the internal structure
of the model would be self-supporting. To support the sides, the crate
structure was used again (minus the bottom). Automatic supportgeneration
software was then used to create supports that would separate
the crate from the build platform. The STL fi les of these structures were
then converted into the SLC format (using Magics, Materialise NV,
Technologielaan 15, 3001 Leuven, Belgium). This resulted in four SLC fi les
that could be prepared for the stereolithography process. These are not
shown here, as the fi les are only contours and therefore cannot be rendered
and viewed from an angle.
However, this was not a simple task to achieve. The fi rst problem was
encountered when arranging the items in the correct positions for part
building due to the use of the SLC format. The SLC fi le is essentially the
contours of the model at the layer thickness intended to build the model.
Therefore, it has to be generated in the correct position and orientation
relative to the z-axis. Once generated, the fi les cannot be repositioned or
rotated relative to the z-axis. Initially this was overlooked resulting in
corrupt SLC fi les after repositioning. Therefore, the SLC generation code
was re-written to create the fi rst contour and 8 mm in z height to allow room
for support structures.
The second, more fundamental and diffi cult, problem was discovered
with the software that generated the bone structure SLC fi les. The SLC fi les
created were invalid and were not recognised by current stereolithography
software (3D Lightyear TM ). However, no error fi les are generated by 3D
Lightyear TM and so it was impossible to ascertain the nature of the problem.
When using the obsolete Maestro software (3D Systems Inc.), the SLC fi les
were again found invalid but error message fi les were generated. Reading
these error messages showed that the orientation of the contours was incorrect.
Crucially, when attempting to prepare a build using these SLC fi les
Maestro was able to re-orient them, resulting in valid slice fi les in the SLI
format. These error messages also highlighted the fact that the fi nal layer
had zero thickness. Although Maestro was unable to correct this error
automatically, once it had been discovered it was a relatively simple matter
to correct the code.
The reason that the SLC fi les were invalid in 3D Lightyear TM is that they
were generated according to an obsolete specifi cation from 3D Systems.