Biomedical Engineering Reference
In-Depth Information
2.2.1.1 Generation of a CAD File for Selective Laser Melting
In SLM, parts are built directly from 3D model data. For biomedical appli-
cations, the CAD file of the implant to be fabricated is normally generated
either by designing the part using CAD software packages or by re-
constructing the structure of the damaged part from CT or MRI data. A set of
CT/MRI data can be combined using commercially available software such
as Mimics (Materialise NV) to convert the two-dimensional (2D) DICOM files
to a 3D surface tessellated (STL) file. In some cases it can also be reverse
engineered by laser scanning the damaged part.
SLM is capable of building net-shape parts with complex geometries;
however, there might be some areas of the part that overhang and may need
a support material during building. In such cases, supports should be
generated in a way that they are easy to remove without damaging the part's
quality during post-processing. For example, SLM 250 (Renishaw PLC) ma-
chines use the same material for supports as they do for fabricating the part.
However, they use a lower laser power for the supports than they use to
fabricate the actual part so that the supports can be easily removed. Also it is
important to position supports in a way that they can be removed easily
and they cover all major overhangs. Once the part is designed and
supports are generated (where required), the CAD file is then exported in STL
format. The machine interface software is used to slice the STL file
into several layers according to the predefined layer thickness along the Z
direction.
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2.2.1.2 Equipment and Principle of Selective Laser Melting
A schematic of the SLM process is shown in Figure 2.1. A typical SLM ma-
chine consists of a scanning laser (usually a fibre optic laser with wave-
length, l
ΒΌ
1070 nm) a hopper attached to a wiper (re-coater), an elevator
that lowers a build platform to adjust the layer thickness, and a lens that
focuses the laser to the build area. Before starting the build, the chamber is
made to be inert by pumping in an inert gas such as argon. The powder from
which the part is to be fabricated is spread over the build platform from the
hopper to a pre-defined layer thickness. Depending on the surface quality
and fabrication speed requirements, this layer thickness can be fixed to be
between 20 and 100 mm.
Shortly after a layer of powder has been spread, the laser beam scans the
powder in the areas specified by the layer of the model file and fuses them.
Once the scan is complete, the build platform moves downwards by a pre-
defined layer thickness for a new layer of powder to be spread over the
previously scanned layer and this process continues until the part is com-
pleted. Once fabrication is complete, the build platform is raised and the
part is removed from the substrate. The un-fused material on the build area
can be sieved and recycled.
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The part is normally sonicated with deionised
water for 30 min to remove non-sintered particles.
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