JEC COMPOSITES MAGAZINE - Issue #117 - November/December 2017 - 33

Automated manufacturing of stiffened composite shells using dry fibre material

the layup of larger and more complex
preforms at a higher layup speed. The
part complexity varied from 2D unidirectional preforms with constant thickness to 100-ply preforms with multiple
orientations and several ramps with
ratios from 1:10 and 1:20. The trials
showed that the initial layup parameters
for smaller preforms and lower layup
speeds could not easily be transferred to
the layup at higher speeds. In that case,
larger preforms were used to modify
the heating law of the IR heating device.
Compared to prepreg slit tapes, the
chosen dry fibre material was less stable
and resulted in high bulking effects at
the course beginning during the layup.
Minimizing this effect was a challenge,
especially on the edge of parts and in
patch areas. A modification of the machine parameters (lower acceleration
and less tow tension) in combination
with an optimized ramp design helped
to handle this effect.
The full-scale demonstrator was manufactured based on the results and modifications of the pre-trials. The design
locally consisted of up to 100 plies with
different thickness areas from 7 mm to
20 mm and several ramps with ratios
1:10 and 1:20. Similar to the pre-trials,
a standard release film was applied
on the tool surface by vacuum-based
deep drawing. The compaction force
and heating energy for the deposition

were varied between the first ply and
the ongoing plies to enable a secured
infusion process. The finished preform
weighed approximately 400 kg and
consisted of nearly 1440 m2 of dry fibre
UD material.
Infusion process development
After laying up the preform, filling the
part with resin was the next major production step. CTC developed a concept
for the infusion of a wing demonstrator
consisting of a DFP skin laid up with
the DLR GroFi® device and coproduced
T-stringer reinforcements made of noncrimp fabrics (NCF). NCF enabled
much higher stringer production rates
with sufficient drapeability compared
to unidirectional deposition processes
on complex-shaped profiles or flat
stacking and subsequent forming.
The vacuum-assisted resin infusion
(VARI) technique was used to fill the
preform. Here, the dry fibre preform is
closed in a cavity, the air is evacuated
and the hot liquid resin flows into the
reinforcement due to pressure differences between the part and the resin
sprue. Then, the epoxy resin is heat
cured to produce a hard part with the
required properties. To achieve the
properties required for aerospace components, the infusion has to take place
at high temperatures, between 100°C
and 150°C.

Vacuum infusion is an open-mould
process where only one side of the part
is defined by a hard-shaped mould. A
stack of auxiliary materials with a vacuum foil on the top sealed to the mould
is placed over the fibres. This separates
the part from the environment so that a
vacuum can be applied. The difference
between the vacuum pressure in the
part and the atmospheric pressure in
the resin inlet is the driving force that
pushes the resin from the infusion
machine into the part.
In an investigation for filling large parts,
the vacuum infusion process was adapted to the special needs of thick and
large DFP preforms. To gain experience
with these new materials and show the
process capabilities, a 14-metre-long
wing cover demonstrator was produced
using the VARI process. One of the
main challenges was to infuse 400 kg of
fibre material with 200 kg of two-component resin. Small coupon trials were
conducted to investigate the interaction
between layup parameters and infusion
behaviour. Finally, the layup parameters associated with the highest resin
volume flow were chosen to minimize
the infusion time for the full-scale component. In close cooperation with the
material and machine suppliers on one
hand and research institutes on the other, this could be accomplished in a very
short time and with a low budget. DLR

Fig. 2: Successful manufacturing of a large-scale demonstrator by co-infusion of a DFP skin and NCF stiffener at DLR and CTC in Stade, Germany

N°117 November - December 2017

/ jec composites magazine

Table of Contents for the Digital Edition of JEC COMPOSITES MAGAZINE - Issue #117 - November/December 2017