Biomedical Engineering Reference
In-Depth Information
block. The die block takes the form of a regular mass
of heated metal with one or more shaped holes, or
slots, cut through it, oriented axisymmetrically with
the extrusion direction, such that the polymer product
moves as a continuous length away from the extruder,
carrying with it the shape of the desired profile. Some
typical equipment and processing considerations are
described below.
the flow history will be different among these
regions. This has important implications when
machining parts from extruded products, because
there will be variation in fiber orientation, and
therefore strength and stiffness, among parts
machined from different areas of the profile.
3.5.2 Processing of Continuous
Fiber PEEK Composite Materials
3.5.2.1 Hot Pressing and Autoclave
Molding
Pre-preg materials do not lend themselves to being
injection molded on conventional equipment because
of the long fiber length and high fiber volume fraction
content. Conventionally, continuous fiber-reinforced
thermoplastic composite materials are more usually
processed by hot pressing. Stacks of pre-preg are
assembled one on top of the other with their
constituent fibers aligned in preferred directions,
according to the design, to achieve optimum strength
and stiffness. These layers are then heated to melt
the polymer (typically up to 30
or so above the
melting point,
T
m
, approximately 370
C) and
pressed together to compact and consolidate the
material such that full contact is made between the
layers, air is excluded, and voids are eliminated. In
some cases the press is then force cooled using steam
to a point at which the polymer resolidifies and cools
below its glass transition temperature,
T
g
(143
C for
PEEK). In other cases, the hot stack of material is
transferred to a second press set to a cooler temper-
ature, again around
T
g
. Pressing again and holding at
this temperature enable the part to solidify without
having to cycle one press through energy inefficient
heating and cooling phases. As the laminate cools in
the second press, a further one can at the same time
be heating in the first press, and so on.
In consideration of thermal heat transfer there are
limitations to the thickness of the laminated stack
that can be efficiently processed. Thicknesses may be
of the order of 1
e
40 mm or so.
An alternative to pressing in a platen press, which
can be ideal for planar laminates, is to use an auto-
clave. Here there is more scope for making shaped
parts with out-of-plane curvature. It is not impossible
to do this on a press, but on a press there is a need for
matched tooling with male and female halves and
usually there is a need for supplementary heating to
achieve polymer melt temperatures.
Barrel Temperatures
Cylinder heaters must be capable of reaching
400
C (750
F) and maintaining set temperatures to
within
2
C. Therefore, cast aluminum heaters are
not suitable and should be replaced with either high-
temperature alloy or ceramic heaters. Cylinder
heaters should cover all exposed metal surfaces to
ensure an even temperature distribution. Areas that
cannot be heated directly should be covered with
high-temperature thermal insulation to prevent the
formation of “cold spots.”
Drive Motor
The power output of the motor must be at least
0.25 HP/kg/h.
Barrel Capacity and Residence Time
The size and the output of the extruder should be
matched to obtain a short residence time, typically
5
e
10 min. There should be no “dead spots,” that is,
gaps around flanges or badly fitting blanking plugs.
All internal surfaces should be cleaned and polished
before extrusion commences.
Screw Design
The materials in the PEEK-OPTIMA polymer
grade range are compatible with most conventional
screw designs. The only screw specifically not rec-
ommended is a continuous compression “PVC” type.
This screw has virtually no feed section, which
results in the compaction of granular polymer leading
to excessive torque.
Depending on the shape, cross-sectional area of
the extruded profile, and extrusion conditions, the
material will undergo flow orientation, which in the
case of short fiber-reinforced materials will lead to
preferential orientation of the fibers and loss of
mechanical isotropy within the product. The fibers in
the material at the outer edges of the profile will be
oriented differently from those in the core, because
