R_Bibb_Medical_Modelling_The_Application_of_Adv.pdf

68 Medical modelling
to fi ll the available build volume with the greatest number of parts. However,
for those processes that construct supports, their effect on model quality
and their subsequent removal should be considered.
Typically, the more supports that are present the more time will be
spent manually removing them, which can add considerably to the overall
time to delivery and labour costs. Surface fi nish and quality will also be
affected as most supports leave a witness mark on the surface after their
removal. For example, the effect of support removal from stereo -
lithography models is shown in Section 6.2 Implementation case study 2.
Another consideration when building medical models is the presence of
internal cavities. The skull, for example, contains many such cavities, and
supports within them may prove diffi cult or impossible to remove by hand.
Some RP processes utilise soluble supports, which can be removed by
immersing the model in a solvent. Whilst this still adds to the overall time
to delivery, labour costs are reduced and supports inside cavities are easily
removed.
Therefore, it is advantageous to orient a model to minimise the
amount of support but not to the degree where the build process is
threatened.
Risk of build failure
Whilst RP machines are generally reliable and able to operate for long
periods unattended, the build process can be threatened by pushing parameters
to their operational limits. As has been stated previously, building
models of human anatomy poses challenges to RP due to the highly complex
nature of the forms being built. This makes the risk of build failure higher
when attempting medical modelling compared to engineering parts. Often
the risks to build failure depend very much of the specifi c RP process being
used, but some general principles apply. The overall stability of the model
is important in most RP processes. It therefore makes sense to orient the
model such that it is most stable, i.e. wider at the bottom than the top. This
will almost certainly also directly affect the supports required. Generally,
the less support needed the lower the risk of build failure. The principle of
stability and the subsequent effect on the supports is shown clearly in the
example in Fig. 5.6, where the orientation on the right offers a more stable
build and a much-reduced amount of support.
Economic factors are also important. Most RP parameters are set to
provide the fastest, and therefore cheapest, build possible. However, setting
parameters for speed usually increases risk of build failure. The complex
nature of medical modelling means that parameters may have to be altered
to lower risk and, consequently, build time and cost may increase compared
to an engineering model of similar size.