Biomedical Engineering Reference
In-Depth Information
Figure 3.19
Tensile elastic
modulus of unidirectional and
multidirectional
250
225
compared with other implantable
materials
ENDOLIGN
200
175
and
cortical
bone.
Source: Invibio.
150
125
100
75
50
25
0
3.4 Preparation of Materials
holes in the heated die fixed to the end of the barrel to
form continuous extruded strands.
The
compound
is formed by introducing the filler
(powder or chopped fibers) part way along the length
of the barrel through a port, whereupon the filler is
incorporated into the polymer melt by the action of
the rotating screw. Fibers are introduced in discrete
bundles that have been pre-cut to 6 mm lengths, as
illustrated in
Fig. 3.20
.
The strands, now comprising the mixture of
polymer and filler, are conveyed from the die,
allowed to cool and solidify, and then chopped into
discrete lengths of a few millimeters to create pellets
as shown in
Fig. 3.21
for subsequent processing by
injection molding or extrusion into the final device,
or blanks for machining.
This is a simplified review of the compounding
process. Refinements include the use of a twin-screw
compound extruder comprising two co-rotating
screws to achieve better mixing of the constituents
and the use of “loss-in-weight feeders” for accurate
metering of the filler and polymer to ensure that their
proportions are correctly blended. Chopped strand
pellets may be subjected to metal detection and the
whole procedure may be conducted in a clean envi-
ronment to minimize the risk of contamination.
Figure 3.22
shows the compounding setup used for
short carbon fiber PEEK-OPTIMA. Note the very
clean conditions employed in this facility.
The setup and design of the compounding system
are critical in determining the quality of the com-
pounded product. The chopped fibers need to be of
3.4.1 Powder and Short Fiber
CompoundsdExtrusion
Compounding
The advantage of powder-filled and short fiber-
reinforced thermoplastic polymer materials, as
a consequence of the filler geometry and relatively
low volume fraction, is their ability to be melt pro-
cessed using conventional plastics processing
equipment, developed, and refined by the polymer
industry over many years. The principles of com-
pounding are quite simple as explained here;
however, a more detailed review of this process is
provided in the literature
[31]
.
Basic equipment comprises a horizontal heated
metal cylinder with an internal rotating screw driven
by an electric motor. There is a hopper containing
polymer at one end of the barrel and a metal die block
with holes for the molten polymer to flow out of at
the other.
In operation, the polymer, in the form of powder or
granules, falls from the hopper into the flights of the
rotating screw, which then transports it in “Archi-
medes style” along the internal length of the heated
barrel. Frictional heating, generated as the polymer
presses and moves against the internal wall of the
barrel, and some of the applied heating cause the
polymer to soften and eventually melt to form
a viscous fluid. The pressure developed by the
rotating screw pumps the viscous polymer through
