R_Bibb_Medical_Modelling_The_Application_of_Adv.pdf

Table 5.3 Advantages and disadvantages of FDM TM
Advantages Disadvantages
Physical reproduction 83
Relatively cheap to buy and run Poorer accuracy and surface fi nish compared
to SL
Reliable Small features diffi cult
Clean and safe process Opaque material
Very strong, tough models
Medical standard material
Can be sterilised
scanned on top of the previous layer. Local melting also forms an interlayer
bond.
5.5.2 Detail
Models are made by selectively sintering thermoplastic powder material
using a laser. The materials are heated to near melting point and the laser
scans the cross sections, locally heating the powder enough to fuse the
particles together. The build platform lowers each layer and fresh powder
is spread across the build area by a roller. The inherent dangers of handling
fi ne powders are controlled by purging the build volume with nitrogen gas.
Models are supported by the unused powder. Overall, build times are comparatively
slow to allow for heat up, around 1.5 hours, and cool down of
the powder, around two hours. When completed the model is dug out of
the powder and bead blasted to remove any powder adhering to the model’s
surface. The machines are large and heavy and require water cooling,
extraction and nitrogen supply. Consequently, the operating costs are considerable
and, therefore, a high throughput is required to justify the purchase
of the technology.
Although these machines are produced by two manufacturers, the SLS ®
abbreviation is a 3D Systems registered trademark. EOS GmbH refers to
their machine as a laser sintering machine. LS machines typically use thermoplastic
materials. The most commonly used glass-fi lled nylon material
gives relatively low porosity, resulting in strong, robust models compared
to SL for example. Surface fi nish is relatively poor compared to SL as may
be expected from a powder. Accuracy is almost as good as SL and comparable
with FDM TM . Specifi c materials allow LS models to be used as sacrifi
cial patterns in investment casting. An elastomeric material is also available
for prototyping fl exible components, and aluminium-fi lled materials are
available for enhanced physical properties. When installed it takes quite a
while to set up, the machine parameters, involving a certain amount of trial