Biomedical Engineering Reference
In-Depth Information
Figure 3.27 Polished longitudinal
section through an ENDOLIGN screw
manufactured by composite flow
molding, illustrating an optimal fiber
orientation. Photo courtesy of Icotec
AG.
Helix area (torsional stability & thread
resistance)
Contour area
(low notch sensitivity)
Transition area
anchor function of the
thread into the core)
(
Axial area
(tensile and flexural strength)
more clearly illustrated diagrammatically in Fig. 3.28 .
This fiber pattern means that the mechanical proper-
ties of this part are absolutely optimized to achieve
the best mechanical performance.
Another example of a composite flow molded
part is a bone plate (snake plate), as illustrated in
Fig. 3.29 . This product has to be strong and stiff to
support the applied loads without breaking or
failing by fatigue, or overly deflecting, and has to be
mechanically capable to levels much beyond those
that can be achieved by conventional injection
molding of carbon fiber compounds. Here, the holes
through which fixation screws are to be located are
molded “in situ” when the fibers are forced to flow
into the cavity and around inserts, reinforcing them
more effectively than if the holes were machined
afterward. This would result in fibers being cut
through and the plate weakened. Again, fiber
orientation is optimized as the fibers flow around
the holes. This is illustrated in an X-ray in Fig. 3.30 ,
for which metallic wires have been added to render
the material some visibility. The process is not
limited to axisymmetric devices such as pins,
screws, and plates, but can be applied to more bulky
three-dimensional objects such as fusion cages, as
illustrated in Fig. 3.31 , courtesy of Signus Medi-
zintechnik GmbH.
High-Pressure Forming
This is another process for making composite parts
using pre-preg materials made with continuous
carbon fibers at a high fiber volume fraction. Again
the pressure involved in forming the part is signifi-
cantly high, since the material has a high viscosity
even when the polymer is above its melting point.
In this process the carbon PEEK pre-preg is
chopped into square or rectangular pieces of 1 e 2 cm.
These are then placed randomly in a mold and the
mold is closed and heated such that the PEEK matrix
melts. There is then further compaction of the mold
to consolidate the material and drive out any air. The
material flows and fibers redistribute as all parts of
the cavity are filled. This randomizes the fiber
distribution to an extent and when cool forms a solid
component with considerable strength and stiffness.
This process can be used to create true three-
dimensional parts with varying wall section thickness
while retaining fiber length at high concentrations.
3.5.2.5 Machining
Extruded rods and sheets of carbon fiber-reinforced
PEEKmay be machined by substantially conventional
Figure 3.28 Schematic illustration of fiber flow in
a helical pattern around the screw flights for optimal
mechanical strength. Photo courtesy of Icotec AG.
Figure 3.29 Composite flow molded bone plate “snake
plate.” Photo courtesy of Icotec AG.
 
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