JEC COMPOSITES MAGAZINE - Issue #135 - July/August 2020 - 39

Overmoulded interfaces for composite aircraft parts

Fig. 8: Flow simulation at 30% filling time; the white dotted line shows the temperature measurement point

Fig. 7: Micrographic section at the interface

campaign are summarized in the table shown
in Table 1.
The micrographic section in Figure 7 shows
the interface. The upper material is an injected PEKK polymer reinforced with 30% carbon. From left to right, the interface meets a
continuous fibre, then sheer PEKK polymer
contained into the laminated composite and,
finally, an upright continuous carbon fibre.
Influence of process
parameters on interface failure
The increasing mould and injection temperatures raise the average cohesion strength at
the interface whereas the injection pressure
is lower. However, the pressure has a positive
effect on the signal/noise ratio, that is to say
the standard deviation.

Results analysis
The injected polymer transfers thermal energy to the overmoulded laminated composite.
If this energy makes the PEKK matrix melt,
molecular creeping between parts ensures
the interface cohesion.
At the measurement point, the [235°C;
380°C; 700 bar] and the [260°C; 400°C;

1200 bar] coupons remain beyond 330°C
for 5 and 13 seconds, respectively, and are
heated up to 332°C and 341°C, respectively.
The [235°C; 380°C; 700 bar] coupon shows
an average failure strength of 222 N while
the [260°C; 400°C; 1200 bar] coupon has
a significantly higher failure strength in the
Mode I test.
The design of experiments makes it possible
to determine the process parameters that
act upon the interface failure strength.
Furthermore, flow simulations confirm
the design of experiments, validate process
parameters and provide extra information
to better understand the phenomena at the
interface between materials.
The next investigation step could be a
structural analysis through numerical simulation, with cohesive elements for example. A
correlation between the test and simulation
results can ensure the reliability of numerical
interface parameters for future mechanical
sizing at the industrial scale.

Mode I and Mode II toughness. The design
of experiments helps to fine-tune process
parameters so as to provide improved
allowable values to the design team. The flow
simulation brings a physical understanding
of the relationship between failure strength
and process parameters.
Indeed, the time spent beyond the melting
point and the energy flux received by the
composite surface determine the interface
quality. These measurements and simulations
enlighten the field of design opportunities
and consequently cost reduction through
this fast, automated process.
The expertise illustrated in this paper paves
the way for finding out more about the potential of overmoulded advanced composite
parts.

Conclusions
Overmoulding in aeronautic applications
can be investigated by means of the proposed
specific coupon, which is helpful to measure

References
[1] COGIT Composites demonstrator
- Carbon/PEKK one shot stamped and
overmolded part

N°135 July - August 2020

More information:
www.cogit-composites.com

/ jec composites magazine

Feature Aeronautics

Flow simulation
Flow simulations (see Figure 8) for each
combination shed light on the thermal
history during the injection at the interface
point. They also propose a physical understanding at the local scale. The temperature is
measured at the interface, at the initial failure
initiation.
The melting point of the laminated composite matrix is around 330°C. Beyond this
point, the mould filling time is different for
each combination. Considering the polymer
temperature is above 330°C, an energy
flux can be extrapolated and compared to
strength at failure.

Fig. 10: Temperature [°C] versus time [s] at the measurement point for each set of parameters, [235°C;
380°C; 700 bar] in yellow and [260°C; 400°C; 1200 bar] in red, melting point in blue

http://www.cogit-composites.com

JEC COMPOSITES MAGAZINE - Issue #135 - July/August 2020

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