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Honeycomb/polyurethane sandwich enables one-step roof for smart fortwo car
BASF (Ludwigshafen, Germany) says its polyurethane foam
system, Elastoflex E, is being used for the first time to massproduce
an in-mold-decorated exterior car part, featuring a
honeycomb sandwich structure with a Class-A surface film.
The roof module on the standard model of the new smart
fortwo commuter car consists of a paper honeycomb and
two surrounding glass fiber mats.
The mats are sprayed in an impregnation
process with the low-density,
thermally activable Elastoflex E 3532
and pressed together with a solidcolored
Class-A film. In this way, a
single operation produces a roof
bonded or back-foamed in the mold because Elastoflex E
reportedly shows good adhesion to films.
“In contrast to conventional composite parts ... the individual
layers are not glued together in a multi-stage process,
but are instead produced in a single manufacturing step,”
says Gao Kwintmeyer, global purchasing at Fehrer.
module that is 30% lighter than the
standard roof on the previous model,
but retains the same strength and flexural
rigidity. The roof was developed
by Fehrer Composite Components
(Großlangheim, Germany).
The honeycomb technology has
previously been used in the car
interior, for example for load floors,
roof linings and rear shelves. For
use in exterior components, BASF
adjusted the viscosity and reactivity of
Elastoflex E; it is said to offer uniform,
thin wetting of the glass fiber mats
and does not drip. Once the semifinished
product has been impregnated,
it is pressed into shape in a
heated mold together with the Class-A
film. This causes the PU system to
foam up slightly at the edge of the
sandwich and creates a solid material
composite between film, reinforcing
mats and paper honeycomb core.
BASF says the reactivity of
Elastoflex E has been adjusted so
that long spray times of up to 120
seconds are possible for large-scale
parts, along with demolding times of
up to 60 seconds. Moreover, decorative
materials and films can be directly
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