R_Bibb_Medical_Modelling_The_Application_of_Adv.pdf

80 Medical modelling
5.4.2 Detail
This process was developed in the USA by Stratasys Inc. (14950 Martin
Drive, Eden Prairie, MN 55344-2020, USA) and the technology is used in
all their machines including the less expensive Dimension branded machines
(FDM TM is a Stratasys trademark). Models are made by extruding thermoplastic
materials through a heated nozzle. The nozzle moves in the X and
Y-axes to produce layers then the build table lowers by a layer thickness
and the next layer is produced. Supports for overhangs are built up and are
removed when the model is complete. The materials are thermoplastics
such as acrylonitrile-butadiene-styrene, nylon, polycarbonate and polyphenylsulphone
which are fed in the form of a fi lament from a spool. The
models produced, therefore, can be handled directly and require no special
cleaning or curing. The physical properties are close to injection moulded
plastic components and so are very strong and durable compared to other
RP models. The diffi culty of removing the supports has been addressed by
the use of a second support material, which does not adhere to the build
material, enabling the supports to be more easily removed. On some
machines, the support material can be dissolved away in an agitated water
bath. This not only reduces labour but also reduces the risk of physical
damage to delicate models.
The process also means that the machine is very quiet and clean and is
suitable for an offi ce environment. Because the process is relatively simple,
there are few moving parts and the machine is, therefore, reliable and
comparatively easy to maintain. Surface fi nish and accuracy are not quite
as good as for example SL, but the machines cost less than similar sized SL
machines.
As with SL, FDM TM models are hard and completely solid making them
suitable for sterilisation for use in theatre. FDM TM models are opaque
(usually white) which hides internal details that could be visible in an SL
model. A typical FDM TM medical model is shown in Fig. 5.15.
FDM TM is well suited to medical modelling. FDM TM models show reasonable
dimensional accuracy and surface fi nish. The models are particularly
tough and can withstand repeated handling, making them ideal for teaching
models. The fi nished models lend themselves well to cleaning and sterilisation
and the use of medically acceptable materials is an advantage.
Unlike SL, the unused material is solid and, although it does not adhere
to the model, it can be diffi cult and sometimes impossible to remove from
the internal cavities often found in human anatomy. The option of the
water-soluble support material has made removing supports from medical
models much easier. Stratasys has also recently introduced a material that
has been tested to internationally recognised USA standard (USP 23 Class