JEC COMPOSITES MAGAZINE - EXPERT’S DIGEST - ADDITIVE MANUFACTURING - 2020 - 86

EXPERT'S DIGEST
ATL/AFP
conditions in which the lay-up temperature is
set to about 420°C, it has been found that it
is possible to achieve full crystallinity in-situ.
This arises from residual heat in the deposited
layer and from subsequent passes of the laser
as the layers are built one upon the other. This
opens the possibility of achieving relatively
fast lay-up speeds (~20 m/minute) with low
porosity (below 2%) and high crystallinity
(>25%) for an in-situ process on heated
tooling. The company believes that there are
further process optimisation opportunities
available that will maximise productivity and
reduce manufacturing costs.
To conclude, increasing the layup velocity
by 6.5 times could generate significant cost
savings that would be difficult to achieve
with conventional UDT processed at limited
speed. On a generic fairing part, cost savings
are anticipated mostly due to a productivity
increase leading to reduced capital investment.

VICTREX AE 250 tape were all substantially
fully consolidated with zero porosity (established using density measurements) per the
ISO1183 standard, whereas the reference
material contained significant porosity (3%),
which is above the acceptable limits for
aerospace applications (2%). In addition, the
reference material required more laser power
(2.1kW vs. 1.6kW) and higher lay-up temperatures (450°C vs. 380°C) compared with the
subject material. Higher lay-up temperatures
can lead to higher residual stresses upon cooling and consequential warping of the part. The
results obtained at higher lay-down speeds
are illustrated in Table 2. Here, the tool was
heated in all cases at a temperature of 140°C.
The highest laydown speed possible using the
Coriolis Composites C1 AFP machine was
20 m/min and at this speed, the VICTREX
AE 250-based panel was of good quality with
porosity levels around 1.9%, which is below
the accepted standard of 2% porosity. 20m/
min is 6.5x faster than that achieved with the
reference material, which additionally contained more porosity than the VICTREX AE
250 panels, even at the slow laydown speeds.

Out-of-autoclave post-processing
While in-situ lay-up is an attractive proposition, the use of heated tooling and the layup of
very complex 3D shapes may be impractical in
certain circumstances. In these cases, it might
be beneficial from a manufacturing throughput standpoint to lay-up faster and later optimise crystallinity, porosity and mechanical
properties using a post-processing step.
The Victrex and Coriolis trial shows that
VICTREX AE 250 polymer-based prepreg
tape can be laid-up at high speed (at least
60 m/minute) using a Coriolis CSolo AFP
machine on room-temperature tooling, giving
laminates with zero porosity when consolidated in an oven under a vacuum bag to apply 1
bar (100 kPa) consolidation pressure. Table 3
illustrates the lay-up conditions used. In this
type of oven consolidation, the part is held on
suitable tooling and typically heated to 350380°C so that the material passes through the
melt phase, at which it is held for a period of
time to achieve melting throughout the thickness of the laminate (typically 15-30 minutes

Crystallinity (in-situ processing)
An important material characteristic that
influences overall mechanical performance
is the degree to which the material crystallises through the manufacturing process.
VICTREX AE 250 polymer reaches a suitably
crystalline state upon cooling from the melt
at rates approaching up to 50°C/minute,
with the maximum level of crystallinity being
typically in the range 25-30%. Faster cooling
reduces the ability to fully crystallise at the
point of laminate manufacture and secondary
heating above the glass transition temperature
is needed if maximum levels of crystallinity are
to be achieved in the final part. This can come
from a post-AFP annealing step of the whole
manufactured part or happen during AFP
as the laser can heat up above Tg the layers
underneath the deposited one. Under certain

for thin laminates of below 5mm thickness).
Then, the part is cooled at a rate usually
governed by the oven's cooling ability. Such a
process " re-sets " the polymer condition such
that any processing history resulting from the
AFP process is erased, stresses are relieved
and maximum consolidation is achieved. The
mechanical properties of panels made from
VICTREX AE 250 polymer-based composite
tapes, consolidated in an oven under just 100
kPa, can be shown to match those from the
same material processed in a consolidation
press at some 6x higher pressure (600 kPa), as
shown in Table 4.
This work shows that a consolidation cycle
with VICTREX AE 250 is 20% shorter than
with another PAEK-based UDT (Figure 3).
This reduction in cycle time may be converted
to cost savings when manufacturing aerospace
composite parts. The resulting laminates
following AFP lay-up and OoA processing
display excellent consolidation and freedom
from voids/porosity, and even show sufficient
polymer flow to fill the spaces often associated
with the edges of ply drops (circled in Figure
4).
Conclusion
In-situ consolidation can run 6.5x faster than
another similar polymer intended for a similar
application and, with high tool temperatures
(in excess of 140°C) and high lay-up temperatures, it is possible to achieve acceptable
levels of crystallinity although the mechanical
properties may be compromised with interlaminar shear strength being some 50-60%
of the pressed panel standard. Generally,
VICTREX AE 250 polymer requires less
laser power and can be laid-up at lower temperatures than other similar materials because
of its lower melting temperature. These
factors can lead to energy savings and or the
specification of lower power lasers. For OoA
consolidation, the low melting temperature
and easy flow characteristics of VICTREX AE
250 polymer enable fast lay-up by automated

Tab.3: Post-consolidation of VICTREX AE 250 unidirectional tape laid-up at increasing speeds (48-60 m/min)
Panel #

Description

13
12

Head Velocity

Tool temp.
(°C)

Layup temp.
(°C)

Average
Laser power
(Watt)

Porosity
(%)

(mm/s)

(m/min)

A250 High Speed

800

345

3400

A250 High Speed

1000

345

3600

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JEC COMPOSITES MAGAZINE - EXPERT’S DIGEST - ADDITIVE MANUFACTURING - 2020

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