Lightweight Electric/Hybrid Vehicle Design

Lightweight construction materials and techniques 181
reinforcement for flat mouldings such as a bonnet may not be absolutely necessary. The handling
of the fibre mats, however, can be difficult as they are rarely self-supporting. A bonnet can be
designed as a slim sandwich structure instead of inner and outer mouldings. The rigid foam core
thus provides a convenient method of supporting the fibre reinforcement during transportation of
the fibre to the RTM mould, (b).
Epoxy resin systems, with their low volume shrinkage of between 1–3% together with their
good mechanical properties, are ideally suited to class ‘A’ surface body panel applications. Painting
at temperatures up to 150°C can also be met with specifically formulated epoxy resin systems.
The table at (c) gives the most important mechanical properties of a typical resin system developed
for RTM. Faster curing systems based on vinyl ester resins can be used for RTM structural moulding
where surface finish is less critical. Vinyl ester resin-based systems are capable of giving mould
closed times of under 3.5 min. High speed mixing and dispensing technologies developed for the
polyurethane industry are also being adapted for the RTM process.
Metal inserts to spread loads at high stress areas, such as hinge attachment points, can be
incorporated into the RTM moulding. The shape of the polyurethane core moulding can provide a
method of locating and transporting these inserts during the preform process. A particular design
feature of the bonnet is the designed failure line. During high speed impacts the bonnet fails along
this line.
RTM was successfully exploited by PSA in their Tulip concept battery-electric car, Fig. 7.5.
The car maker worked with Sotira Composites Group to develop the body structure which comprises
Polyurethane foam core Glass-fibre preforms
Assembly of preforms to core Resin injection
Fig. 7.5 Tulip concept car: (a) structure; (b) sandwich configuration.
(b)
(a)