R_Bibb_Medical_Modelling_The_Application_of_Adv.pdf

Physical reproduction 67
However, there are some obvious examples where orientation makes a
considerable difference to surface quality. The long bones of the arms and
legs, for example, are essentially cylindrical in form. Orienting a model of
a long bone such that it lies fl at will minimise build time and cost, but the
layers will be readily apparent in the model. Orienting the build in an
upright sense will increase build time and cost considerably, but the layered
effect would be drastically reduced in comparison, leading to much better
model quality. Fig. 5.5 illustrates the effect of stair stepping on a model of
a proximal tibia (the relative thickness of the layers has been increased to
exaggerate the effect). The upper model was built lying horizontally; whilst
this minimised build time, the thickness of the layers has had a negative
effect on the reproduction of the contours of the model. The lower model
was built upright, and consequently the layer thickness has not had such a
detrimental effect, at the cost of increased build time.
Support
All RP processes provide support to the model as it is built. Some processes
build supports concurrently with the model whilst others utilise the unused
material to support the part as it is built. Usually, parts that are supported
by the unused material may be oriented to provide the minimum cost, or
5.5 The stair step effect of build orientation.