R_Bibb_Medical_Modelling_The_Application_of_Adv.pdf

66 Medical modelling
particular RP system. Compared to engineering products that have been
designed for manufacture, models of human anatomy may be much more
challenging to prepare for a successful build, even for those experienced in
the operation of their RP machines.
The following sections describe some of the most common RP processes
and highlight their key technical considerations. However, there are some
basic principles that apply to nearly all RP technologies when building
medical models. The most important consideration is the orientation of the
build. This will have an infl uence on the surface fi nish of the model, the
time it takes to build, the cost of the model, the amount of support required
and the risk of build failure. All of these factors are interdependent, the
key to producing a high quality medical model is a thorough understanding
of them, correctly identifying the priorities, and reaching a compromise
solution that best meets the needs of the clinician. The effect of orientation
on these factors is explored below.
Build time and cost
As all RP processes work on a layer-by-layer basis, the builds consisting of
two repeated stages; drawing or creating the layer and recoating or depositing
material for the next layer. Generally, the material deposition stage
takes longer and often poses the greatest risk of build failure. Therefore,
orienting a model such that it minimises the number of layers will reduce
the build time.
Generally, RP model costs are directly related to the build time.
Therefore, the longer the build time the more the model will cost. In many
cases, the automatic option is to orient a model for minimum height and
therefore minimum cost. However, as described below, this may have an
undesirable affect on the quality of the model.
Surface fi nish, model quality
As described in Section 5.1.3, the layer-by-layer building process results in
a stair-step effect on sloping or curved surfaces. Depending on the shape
of the object, the orientation may have a great effect on the degree of stair
stepping on the model surface. However, the mechanisms that create the
layer geometry usually offer better resolution. An example of a model that
illustrates the effect of layer thickness in an exaggerated manner can be
found in Section 6.2 Implementation case study 2. When considering engineering
parts, the most important feature is identifi ed and the build is oriented
to provide the optimum surface quality for that feature. However,
human anatomy usually possesses curved surfaces in all directions and
the optimum orientation may depend on other, more important, factors.