Biomedical Engineering Reference
In-Depth Information
the injection molding machine temperatures along
the barrel and also, in some cases, the mold
temperatures.
Table 3.3
shows typical molding
conditions for natural PEEK polymer, barium
sulfate-filled PEEK, and 30 wt% short carbon fiber-
filled PEEK compound. These are typical values and
will change depending on the specific grade of PEEK
used, the mold geometry, and molding machine. As
with unfilled PEEK polymer, compounds have to be
pre-dried in a drying oven.
Injection pressures of 7
e
14 MPa (1000
e
2000 psi)
are initially used with holding pressures of
4
e
10 MPa (570
e
1430 psi). To create a homoge-
neous melt and to aid consistency of shot size,
a nominal back pressure of 0.2
e
5 MPa (30
e
715 psi)
should be used.
A screw speed between 50 and 100 rpm is
optimum for transporting and melting polymer.
Screw speeds lower than 50 rpm are to be avoided,
as this will result in longer cycle times. Screw
speeds higher than 100 rpm are not recommended,
as they may result
specific molding parameters, mold geometry, and
material.
A consequence of fiber flow orientation is that the
material properties are not fixed, but instead are
uniquely determined at the point of manufacture,
under the specific conditions of production. The
tensile, shear and compression strength, and modulus
do not correspond with those measured on an
injection-molded test bar, because the fiber orienta-
tion distribution in such a test bar has usually been
substantially optimized to be somewhat aligned
along the length of the bar, parallel to the direction of
applied load. The real fiber arrangement in a molded
part could be somewhat less optimized in the weakest
areas and here the strength is usually only a fraction
of the maximum possible. This has important design
implications and has been the subject of much
research as attempts have been made to predict
polymer flow, fiber orientation, and mechanical
properties for given part geometries, materials' flow
characteristics and molding conditions. Nowadays,
sophisticated finite element software packages make
such predictions more reliable.
in excessive localized shear
heating.
A second major difference between the injection
molding of filled (particularly short fiber-filled) and
unfilled polymers is that the filler is subject to flow
orientation as a result of the flow fields that develop
during mold filling. Flow patterns may be complex,
particularly where there are changes in cross-section,
or changes in shape, around bends, along ribs, and
around holes, and the fibers rotate and become
aligned to a greater or lesser degree depending on the
specific flow path. The fibers may change direction
through the thickness of the part, being predomi-
nantly aligned in one direction close to the surface
and in another direction within the core, although the
specific distribution in real parts depends on the
3.5.1.2 Extrusion
Often extrusion is an intermediate step in the
production sequence whereby granules of pure
polymer or compound are consolidated and shaped
by melt extrusion into some form of “stock shape”
(rod or plate) from which individual components can
later be machined, as will be described in the next
section. Extrusion melt processing again uses
a rotating screw melt pump to convey, compress,
melt, and pressurize polymer (or compound) gran-
ules in a heated metal barrel to a sufficient pressure
such that the material can be forced through the die
Table 3.3
Typical Injection Molding Temperature Profiles for PEEK-OPTIMA
Rear Temperature
C(
F)
Middle Temperature
C(
F)
Front Temperature
C(
F)
Nozzle Temperature
C(
F)
Material
Unfilled
PEEK
355 (670)
365 (690)
370 (700)
375 (707)
Radiopaque
PEEK
355 (670)
365 (690)
370 (700)
375 (707)
CF PEEK
365 (670)
380 (716)
390 (730)
395 (743)
