Good tactile sense
Multi-component control switches
Modern vehicles are being fitted with more and
more equipment which – even in the era of the ‘controller’
– is turned on and off as required using push-button switches. These
switches are made by injection-moulding plastic around thin wooden, metal
or aluminium inserts. Switches made entirely of plastic can also be produced
using a multi-component injection moulding process. It is possible to produce
both two-component and three-component switches using multi-component injection
moulding technology.
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| Fig. 3: Switches made from polybutylene terephthalate are dimensionally stable and have a high quality surface finish (photos: Degussa) |
The switches are used mainly to activate the rear windscreen
heating, fog lamps or heated seats. The backlit symbol consists of a transparent
or translucent plastic such as polyacetal (POM) or polycarbonate (PC), surrounded
by a base material, usually black, e.g. polyamide 6 (PA6) or preferably polybutylene
terephthalate (PBT).
For a long time, switches made entirely of plastic consisted of a polyamide
supporting base with a transparent insert. However, malfunctions were common
in the case of components with complex geometries featuring long control domes.
Because of the high water absorption of the polyamide 6 used (at room temperature
and relative humidity of 50%, PA6 absorbs water 3 % of its weight) the thin
domes warped and in extreme cases the switch jammed. In addition, there was
inadequate adhesion between the polyamide base and the function symbol injection-moulded
around it.
Fibreglass-reinforced PBT is the material of choice
Polybutylene terephthalate moulding materials reinforced - depending on the
rigidity, viscosity and flowability required during processing - with 10%
or 20% cut glass fibre are more suitable (e.g. Vestodur GF10 or GF20 from
Degussa, in Marl/Germany). A polybutylene terephthalate manufactured with
isotropic fillers and fibreglass has proved outstanding where low-distortion
switches with greater rigidity are required.
Switches made from Vestodur X7095 have a high degree of dimensional accuracy,
show low levels of distortion and adequate dimensional stability, even when
heated. The material adheres well to the transparent POM or PC inserts around
which it is injection moulded. Its flowability is such that even millimetre-thick
extrusions are not deformed. In terms of flammability, the moulding material
meets the automobile industry’s requirements for vehicle interior components
with a rate of combustion of less than 100 mm/s.
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| Fig. 1: The high degree
of dimensional stability of the material used for production guarantees
the long operating life even of switches with complex geometries. |
Fig. 2: The stem of the injection-moulding coated PC is bound firmly in the PBT matrix without deformation (view of a test specimen using light-optical microscope) |