R_Bibb_Medical_Modelling_The_Application_of_Adv.pdf

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As there is a huge variety of CAD programmes available and a number
of RP systems, each with different software requirements, industry required
a standard translation format that could enable the RP process to build
models designed in CAD. Stereolithography was the fi rst RP process to
market and employed a mathematically simple approximation of threedimensional
CAD models, called the STL fi le, which is fully described in
Section 4.6.2. The format describes models by closely approximating their
shape with a surface made up of a large number of triangular facets. As it
was the fi rst convenient transfer format to be offered by CAD software
developers, the STL was adopted by other RP manufacturers and has since
become a de facto standard in the industry. Consequently, despite some
shortcomings, all RP machines can use the STL fi le. However, many other
formats are also available, and a description of some of the most common
is provided in Chapter 4.
In practice, RP software slices the STL (or equivalent) fi le into a number
of cross sections. The software then creates control fi les that will instruct
the RP machine how to construct each layer and how to deposit subsequent
layer material. Depending on the sophistication of the process, there may
be a degree of user interaction at this point that can help to optimise the
build. The result is one or more fi les that are transferred to the RP machine
itself. Typically, the build fi le is then checked and the machine prepared
for a new build.
Although the models may take many hours to build, the machines typically
operate unattended and non-stop. Therefore, a build will frequently
be started last thing in the working day and the machine will run overnight,
often the model will be completed by the next morning.
All of the systems have some method of supporting the build in progress,
and this will require some degree of post-process cleaning and fi nishing
after the model is built. Finishing is usually done by hand to remove residual
material, remnants of support structures and the step effect. The level
of fi nishing employed depends on the end use of the model.
RP technologies are constantly being developed and incrementally
improved, so the descriptions here are intended to provide an overview of
how the technologies work and compare their relative strengths and weaknesses.
However, when assessing RP technologies from manufacturers or
service providers it is always advisable to obtain the latest specifi cations.
RP manufacturer websites are the best source of such information and the
most popular are listed in the bibliography.
5.1.6 Basic principles of medical modelling – orientation
By defi nition, every medical model is unique and the characteristics of each
model need to be considered carefully when selecting and utilising a