JEC COMPOSITES MAGAZINE - Issue #93 - December 2014 - 59

OoA option proved to be the lightest
design (-19%) and lowest overall cost
option to produce (-25%), taking 18
hours less time (-43%) to fabricate,
thus providing a significant productivity benefit (see Figures 1 and 2).

Comparative production
times and costs

Fig. 1: Cost comparison NLG door

higher elongation (9-10%) and minimized resin absorption, along with
the best impact damage resistance and
visual detectability, hence its suitability for this type of exterior aircraft
application.
To obtain meaningful comparative
processing data, CTC evaluated three
design options for aircraft carbon
fibre reinforced sandwich construction: (1) autoclaved honeycomb core
(64 kg/m 3, 4.8mm)/epoxy prepreg
resin system; (2) autoclaved HERO
core/epoxy prepreg; and (3) infused
HERO core/liquid epoxy resin. The
results of the three design evaluations
proved that both foam core options
took significantly less time and cost
to produce. The autoclaved HERO/
epoxy prepreg option was calculated
to be 21% cheaper overall than the
honeycomb design. However, the
HERO core/epoxy infusion resin

The production time of 42 hours
for the honeycomb/prepreg design
was reduced to only 24 hours for the
infused 'single-shot' HERO foam
core design option. This was primarily due to the elimination of two
key steps in the 'two-shot' curing
process needed for the autoclaved
honeycomb/prepreg design. Both
the lengthy prepreg core preparation
time and the need to separately first
cure, demould and then integrate the
carbon fibre reinforced outer skin
with the honeycomb prior to the
second-stage cure are avoided with an
infused closed-cell foam core laminate design. The net result was a near
doubling of moulding capacity (see
Figure 3). CTC pointed out that these
significant time and cost savings have
been achieved for a part with a comparatively simple geometry. As the
complexity in the shape and curvature
of a part increases, the savings in processing time and costs exponentially
increase in favour of an infused foam
core over an autoclaved honeycomb
laminate design.
With the new HERO core, production
costs could be reduced as it enabled

CTC to use a simplified manufacturing process compared to honeycomb.
The characteristics of this foam made
it possible to adapt the design for better manufacturing without a weight
penalty. A key redesign requirement
was equal performance, which CTC
expects to be confirmed in further
planned tests. Additional potentials to
further reduce weight may be possible
by milling the foam core to a load path
optimised shape.

Production of the infused
foam core design

The nose landing gear doors using the HERO core/epoxy infusion
resin design option were fabricated
by INVENT GmbH (Braunschweig,
Germany). The resin infusion process
is known from previous commercial
projects to be significantly faster
and more cost effective than using a
prepreg and autoclaving. The optimised infusion laminate design used
Rohacell 71HERO core foam (the
lowest density HERO grade at 75 kg/
m3), woven carbon fibre fabrics, and
RTM 6, an Airbus-qualified Hexcel
epoxy resin. The foam core material
was supplied pre-shaped as an insert
piece, NC machined to the precise
dimensions needed from a sheet of
71HERO core foam. The 'ready-touse' 71HERO pre-shaped core parts
were produced by Evonik's Rohacell®
SHAPES department and shipped to
INVENT to build the evaluation part.
Unlike a honeycomb core, all Ro-

Fig. 2: Comparison of time by process step

No93 December 2014

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JEC COMPOSITES MAGAZINE - Issue #93 - December 2014

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