Biomedical Engineering Reference
In-Depth Information
Component design should also take into account the
shrinkage that occurs as crystalline regions formwithin
the cooling polymer melt. Shrinkage is therefore
dependent on the level of crystallinity and therefore the
mold and polymer melt temperature. If operating
under recommended injection molding conditions,
commercial grades such as PEEK-OPTIMA can
produce consistent parts with dimensional tolerances
as low as 0.05%. Commercial manufacturers of
implantable PEEK materials provide full guidelines
and technical support for molding PEEK medical
components.
2.5.2 Extrusion
Extrusion is a manufacturing process for
producing long stock shapes, such as rods, sheets,
and monofilament fibers ( Fig. 2.9 ). PEEK pellets or
granules are typically the starting, raw material for
extrusion. Similar to injection molding, the pellets
are poured into a hopper that feeds into a heated
screw assembly that melts and pressurizes the molten
polymer. The molten, pressurized polymer is then
forced through a heated die and slowly cools to room
temperature along an extrusion line.
The extrusion of PEEK can be accomplished
using conventional extrusion equipment and dies at
similar temperatures as those described for injection
molding. The drive motor typically requires a power
output of 0.25 HP/kg h. This power requirement
is similar to that required for polycarbonate,
poly(ethersulfone) (PES), or high-molecular-weight
polyolefins.
PEEK is a very stable polymer and as such is not
sensitive to prolonged exposure (up to 2 h) to
temperatures above its melting point. However, for
optimal results, residence times should be of the
order of 5 e 10 min. Dead spots, areas of material
hold within the barrel, should be avoided. Therefore,
careful design of the screw and barrel is necessary.
The extrusion of PEEK-OPTIMA stock shapes
followed by machining remains the predominant
method of manufacturing medical device compo-
nents. PEEK-OPTIMA can be purchased in rod or
plate form ranging from 6 to 150 mm in diameter for
rod and 40 mm in plate thickness ( Fig. 2.10 ). In
addition to this, thin-walled implantable PEEK
tubing is also available from commercial suppliers.
The thermal processes involved in extrusion and
annealing of stock shapes also result in slight
mechanical property variances when compared with
Figure 2.7 Amorphous regions within an injection-
molded part as a result of low mold tool temperatures.
Photo courtesy of Invibio.
mold temperature results in rapid cooling of the part,
with insufficient time for crystallization, as demon-
strated in Fig. 2.7 . Additional detailed guidelines
for injection molding and mold design for PEEK
implants can be obtained from raw material
suppliers [10] .
It is possible to increase crystallinity by annealing
amorphous or low-crystallinity PEEK moldings
( Fig. 2.8 ). However, the process of crystallization
may lead to distortion and dimensional changes.
Figure 2.8 Effect of annealing after processing on
injection-molded test specimens. The amorphous
sample was created by heating to 400 C and then
immersed in liquid nitrogen. Image courtesy of David
Jaekel, Drexel University.
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