Biomedical Engineering Reference
In-Depth Information
used. As mentioned previously in this chapter, PEEK-
OPTIMA LT1 is recommended for the majority of
machining and injection molding of medical device
components. PEEK-OPTIMA LT2 demonstrates
good melt strength with reduced viscosity and is
therefore recommended for the extrusion of thin-
walled parts such as tubing. PEEK-OPTIMA LT3 is
especially preferred for the injection molding of
thin-walled parts. The problem of machine wear is
common to all engineering plastics, and it is therefore
recommended that screws, dies, and barrels should
be hardened to minimize wear, especially when pro-
cessing fiber-reinforced materials.
Table 2.3
Traditional Plastic Processing Methods
and Applicability to PEEK
Processing Method
Applicable
Injection molding
Yes
Extrusion (profiles, sheet, and
monofilament)
Yes
Compression molding
Yes
Powder coating
Yes
following. A comparative plot of melt viscosity
versus temperature for a range of conventional
polymers in comparison with PEEK-OPTIMA LT1 is
shown in
Fig. 2.4
.
Although commercial PEEK materials are
supplied nominally dry, the pellet form of the mate-
rial typically absorbs 0.5% w/w atmospheric mois-
ture. It is therefore recommended that for processing
operations such as injection molding, the polymers
should be dried to less than 0.02% w/w moisture.
Typically, suitable drying of PEEK pellets can be
achieved by 3 h of exposure in an air-circulating oven
at 150
C. If the oven is only capable of lower
temperatures, a longer drying time will be necessary
(e.g., 12 h of exposure at 120
C).
PEEK-OPTIMA materials are provided in a range
of viscosities in relation to the processing technique
2.5.1 Injection Molding
Injection molding is an attractive manufacturing
technique suitable for mass projection of PEEK
implant components (
Fig. 2.5
). Injection molding is
typically performed using pellets or granules, which
are poured in a hopper in the machine. The pellets are
then automatically introduced into a heated screw
assembly that melts and pressurizes the molten
polymer, so that it flows into a heated mold. Once the
PEEK component has consolidated, it is automati-
cally ejected from the mold, so that a new cycle can
take place.
Injection molds are optimized and specially
designed for each part, taking into account the details
of the part geometry and flow and pressure capabil-
ities of
the molten polymer
injection system.
Figure 2.4
Shear viscosity versus
temperature
for
a
range
of
thermoplastics. Source: Invibio.
