JEC COMPOSITES MAGAZINE - Issue #147 - September/October 2022 - 41

Serial CFRTP structural part for BMW Group body-in-white application
a
b
up additional lightweight construction
potential. Functional integration
considerably simplifies the subsequent
assembly processes and reduces the
number of components.
The comparatively low cost for an injection
mould as well as the simple
adaptation of the materials, e.g. by
varying the fibre content in the injection
moulding compound, open an
additional weight and cost reduction
potential.
c
Fig. 2: MAI Skelett concept component
(a), bending test (b) and results (c)
a skeleton construction concept was
created for carbon fibre-reinforced
thermoplastics (CFRTP). The core
idea is to convert a multifunctional
injection-moulded component into
a structural component. At the same
time, the use of continuous carbon fibres
has to be limited to the main load
paths. These are unidirectional carbon
fibres contained in the rectangular
profiles, which are impregnated with
a thermoplastic matrix (black rods in
the middle of the image).
The rods themselves are embedded by
injection moulding in a thermoplastic
compound that is also reinforced with
carbon fibres.
The ribbed structure possible in injection
moulding leads to a truss structure
with which the individual rods are
stiffened to form an overall profile.
For cost and heat deflection reasons, a
polyamide 6 (PA 6) matrix was chosen,
a well-known material in car body construction.
The innovation in the component
production lies less in injection
moulding than in the production
of 3D-shaped rods, their integration
and fixation in the injection moulding
tool and the process-reliable connection
between the materials. At the
same time, injection moulding opens
Concept studies
The idea was tested within the framework
of the MAI Skelett joint project [1],
which is part of the MAI Carbon leading-edge
cluster [2].
A variant for the skeleton construction
was derived from the BMW i3's front
roof bow (Figure 2), with the goal to
clarify many fundamental questions including
the development of the forming
technology for the rods and the boundary
conditions for the adhesion between
the rod and the injection-moulded
material. Another focus was the use
of fibres from the preform trimmings of
the BMW i3 production as reinforcements
in the injection-moulded compound.
The first component tests were
decisive, showing a good agreement between
the prognosis of the simulation
calculations and the bending impact
tests (Figure 2).
Thus, the MAI Skelett project also
technically confirmed the product geometry
developed for the frame and
the design calculation method.
Furthermore, the economic potential
of the skeleton construction method
was substantiated and the remaining
risk for a series application could be
estimated in a qualified manner.
Pre-development
The specification of the unidirectional
continuous carbon fibre-reinforced
PA6 base as the core of the skeleton
construction, which has to endure the
most diverse load profiles, was signifa
b
c
d
Fig.
3: Components of
the BMW iX's series front roof bow
icantly influenced by production considerations.
For reasons of lightweight
construction, a high fibre volume content
and a consistent, good impregnation
of the individual filaments were
desired. Among many possibilities, a
CFRTP rod with a PA6 matrix and a fibre
volume fraction of almost 50% was
chosen, whereby a constant rod quality
was achieved compared to the concept
phase by optimizing the impregnation
of the fibres and the profile formation.
After the primary requirements were
answered in the concept phase, they
had to be examined and confirmed in a
360° view of the product and its functionality
in the vehicle.
The challenge was on meeting the requirements
for its overall integration
into the vehicle, without the need for
new body construction and paintwork
processes. Lateral sheet metal tabs were
used for integration into the entire vehicle
(Figure 3b). These tabs reach clearly
into the skeleton component to ensure
force transfer. They can be used for the
standard spot-welding process during
the bodyshell construction. With these
sheet metal tabs on the front roof bow, it
was now possible to carry out complete
vehicle tests.
N°147 September - October 2022 / jec composites magazine
41
Feature Automotive
JEC COMPOSITES MAGAZINE - Issue #147 - September/October 2022

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