R_Bibb_Medical_Modelling_The_Application_of_Adv.pdf

92 Medical modelling
Table 5.7 Advantages and disadvantages of jetting head techology
Advantages Disadvantages
Cheaper, smaller machines Low accuracy (ThermoJet ® )
Easy to use Fragile models (ThermoJet ® ,
Solidscape)
Suitable for offi ce environment Slow build times (Solidscape)
Thin layers
Fast build times (ThermoJet ® , InVision TM SR)
High accuracy (Solidscape, InVision TM HR)
Transparent models (Objet)
Medically appropriate material (Objet)
an X-Y plotter mechanism. Build time is relatively quick as only the perimeter
of the layers is drawn. Builds can be speeded up by cutting more than
one layer of paper at a time; the paper is nominally 0.1 mm thick. The paper
is adhesive backed and the layers are adhered by a heated roller. The paper
is fed from a roll and passes over the build table; unused paper is taken up
by another roll. The laser cuts a support wall enclosing the model; material
that is outside the model and inside the support wall is cut into squares.
The build table lowers, fresh paper is fed over the top of the build and heat
bonded, and the next layer is cut and so on. When the model is complete,
the block is removed from the table and the support wall is removed. The
waste material is broken away to reveal the model. A typical LOM TM
medical model is shown in Fig. 5.24.
To obtain a cut that penetrates only one layer of paper requires the speed
of travel and laser power to be adjusted manually. Good bonding between
layers depends on the compression, temperature and speed of travel of the
heated roller, which is again adjusted manually until it is satisfactory. These
require regular checking and, generally, the machine needs a lot of maintenance.
Extraction of the smoke produced creates a tolerable amount of
noise. Although there is a certain amount of waste material, the paper is
relatively cheap. Helisys machines are increasingly uncommon and not
available in some markets, although many are still in operation around the
world.
The Kira system (Kira Europe GmbH, Heinrich-Hertz-Str. 8-10, D-40699
Erkrath, Germany) is similar except that the paper is fed in sheet form by
a photocopier mechanism; the adhesive is applied (as toner would be in a
photocopier). A hot plate bonds the layers and cutting is by carbide knife
using an X-Y plotter mechanism. There may be advantages to this approach
compared to the Helisys method. Most notably, less lateral stress is induced
on bonding because the hot plate presses evenly downwards as opposed to
the roller, which imparts a lateral force as it travels across the model. There