JEC COMPOSITES MAGAZINE - Issue #96 - April 2015 - 42

thermoplastics

tion are injection moulded, extrusioncompression moulded or processed in
other ways. In LFTs, the fibre aspect ratio
(fibre length divided by fibre diameter)
approaches 2000, hence over 90% of the
strength of a continuous fibre is achieved
at affordable cost [7]. The fibre content of
the hot-melt impregnated material ranges
from 10 to 70% (by weight). The higher
the fibre content, the higher the strength
and stiffness of the resulting product [1-2,
7-8].

Fig. 1: Representative range of materials and
product forms for advanced tailor-engineered
thermoplastics, including pellets, tapes, fabrics
and hybrids

shape- and performance-optimized
aerospace products (see Figure 1).
Engineered thermoplastic polymers
such as polyetherether ketone (PEEK),
polyether ketone (PEK) and polyphenylene sulphide (PPS) have been used in
aerospace applications. However, their
high-temperature processing requires
expensive autoclaves which have limited
their use to high-end applications. Carbon fibre reinforcements offer a wider
range of possibilities for manufacturing
new products for aerospace applications.
Thermoplastic polymer composites are
melt-processed to produce intermediate
composite forms, such as pellets and
tapes, that can be processed downstream
into high-performance structures. In the
hot-melt process, fibre tows (carbon,
glass, Kevlar/aramid, etc.) are impregnated with a wide range of thermoplastic
resins such as PEEK, PPS, PEK, among
others. In this work, the processing
options used for the development of
aerospace parts include extrusion-compression moulding, long fibre injection
moulding, sheet thermoforming, tape
placement, thermoplastic pultrusion, and
tailored sheet blanking, placement and
stamping [4-6].
The long fibre thermoplastic (LFT)
pellets produced via hot-melt impregna-

In extrusion-compression moulding
(ECM), the hot-melt impregnated
pellets are fed into a plasticator, i.e. a
low-shear extruder that creates the intermediate form referred to as a "charge".
The charge is a viscous melt of polymer
and fibres oriented preferentially in the
circumferential direction of the extrusion
screw. The charge is stamped in a closedcavity tool (mould) to produce complexshaped parts.
There are several advantages of using
carbon fibre thermoplastic materials for
aerospace applications:
- The process is amenable to providing
tailored fibre orientation in specific
directions;
- Due to the ability of the fibre/resin
charge to flow through the cavity, parts
with geometrical complexity can be
produced, while maintaining superior
structural properties;
- The process allows the integration of

electromagnetic shielding materials
and inserts, and the encapsulation and
integration of components as part of the
materials solution.

Thin-walled carbon fibre LFT parts
Several studies have been carried out
on processing of thin-walled parts using
short glass fibre-reinforced thermoplastics; however, limited studies have
been reported on thin-walled parts/
shells using carbon fibre-reinforced LFT.
Carbon fibre-reinforced thermoplastics
cool much faster when they touch mould
surfaces due to the high thermal conductivity of carbon fibres. The manufacture
of thin-walled LFT carbon fibre-reinforced thermoplastic composites (< 2 mm
thick) can pose a challenge due to the
high ratio of the frozen skin layer (that
touches cooler mould walls) to the molten core layer during material flow.
Case 1: The studies carried out by the
UAB team has helped them better
understand how to process carbon LFT
into thin walls. The ability to produce
parts with complex geometry such as
curvature, flanges, sharp corners and
a wall thickness of ~2-3 mm has been
demonstrated. Figure 2 represents a
carbon LFT composite aerodynamic
shell (aeroshell) and baseplate for an
aviation electronics application. The
composite insertion replaces an investment-cast aluminium aeroshell and
aluminium baseplate at reduced weight,

Fig. 2: Baseplate assembly (left); aeroshell assembly (right). The baseplate is in two halves (1, 2)
assembled by a hinge joint; the aeroshells are in four quadrants (1, 2, 3, and 4). UAB holds the exclusive
license for the manufacture of these parts (USPTO 8,277,933)

42 jec composites magazine / No96 April 2015

JEC COMPOSITES MAGAZINE - Issue #96 - April 2015

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