R_Bibb_Medical_Modelling_The_Application_of_Adv.pdf

5.7 Jetting head technology
5.7.1 Principle
Physical reproduction 89
Build material is deposited discretely by jetting heads, similar to those used
in inkjet printers. The material is ejected as a liquid, solidifying on contact
with solid material or the build platform. The head moves in X and Y-axes
to build the layers. The build platform lowers by a layer thickness and
material is deposited on to the previous layer.
5.7.2 Detail
There are several companies, such as Objet, Solidscape and 3D Systems,
producing small offi ce-based machines that utilise deposition processes
similar to those found in inkjet printers. Machines such as the 3D Systems
ThermoJet ® and those from Solidscape deposit wax-based materials in their
liquid state that instantly cool and solidify to produce the model, whilst the
3D Systems InVision TM and those from Objet use photo-polymerising resins
that are simultaneously solidifi ed by UV light as they printed. An InVision TM
model is shown in Fig. 5.22.
Generally, the machines and materials are clean and safe and do not
require extraction or special handling. Most machines build support structures,
but the Solidscape and InVision TM machines surround the model with
a secondary wax support material that is either melted or dissolved away
from the completed model. The Solidscape machine is also unique in
employing a milling head to level each layer.
As might be expected from technologies that rely on printing technologies,
the resolution of these machines is often quoted in terms of dots per
inch (dpi). Some of these machines, such as the ThermoJet ® and InVision TM
from 3D Systems, are not as accurate as the more expensive systems, such
as SL for example, but they are intended more for the concept design stage.
A ThermoJet ® medical model is shown in Fig. 5.23. Correspondingly, the
material strength and surface fi nish are not as good as SL for example.
However, the Objet models are comparable to SL models.
One of the advantages of printing technologies is the ability to produce
very thin layers. This reduces the stepping effect, and models are very
smooth compared to other layer manufacturing techniques. Layer thicknesses
of 0.040 mm are typical but can be as thin as 0.016 mm. This means
that models can be made that show excellent surface detail as described in
Section 6.17 Research applications case study 3. Another advantage is that
each layer is created during a pass of the print head regardless of the geometry
of any given cross section, which makes prediction of build times relatively
simple and accurate.