Plastic + Metal = Exceptional stability
Plastic-Metal Hybrid Material
Rhodia Engineering Plastics, for example, has
developed a range of application concepts focusing on hybrid solutions that
combine metals and engineering plastics for high-load bearing (semi-) structural
components. The technology is targeted particularly at automotive applications,
where it brings further benefits such as greater integration depth, weight
and cost savings, enhanced energy absorption, and added-value appearance.
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| Fig. 1: The metal part
to be over-moulded (frontend) |
Fig. 2: The plastic moulded
part and … |
Fig. 3: … the assembled
frontend (photos: Rhodia) |
Existing plastic/metal solutions can be divided into open
and closed rib structures. While open rib designs, where thin metallic sections
are moulded over with thermoplastic ribs, provide a good strength-to-cost
ratio, their effective mechanical strength and stiffness is highly dependent
on the direction of the applied load. This results in complex mould requirements,
which must be met by means of sophisticated hot runner technology and/or the
use of very low-viscosity resins. Furthermore, the open rib approach cannot
be used for applications that need to be safely protected from dirt and water
penetration.In contrast, closed rib and related hollow designs have a more
balanced high stiffness and strength regardless of the load direction, that
is both in tensile stress and in compression areas. In addition, the hollow
sections can be used to integrate various additional functions. Rhodia has
been investigating three different closed hybrid technologies – Metal
Over Moulding (MOM), Plastic & Metal Assembling (PMA), and Metal &
Gas or Water Assisted Injection Moulding (MGAIM or MWAIM) – each providing
a number of distinct advantages depending on specific application requirements.
Heavier but better loadbearing
In MOM, one or more metal inserts are directly over-moulded over with an engineering
thermoplastic resin. The final reinforcing design is then created in a second
step by one or several plastic covers welded on. The resulting hollow section(s)
within the structure can serve to integrate cables, flow channels or fluid
containers, for instance replacing the need for a separate cooling water expansion
tank in automotive front-ends. While MOM components will be somewhat heavier
than open rib designs, this is more than compensated for by their superior
load-bearing, energy absorption and integration capabilities. The time and
cost for the additional injection mould and the jointing process is justified
by lower mould complexity, lower injection pressures, and lower mould wear.
PMA also involves two process stages, by combining a rigid, ribbed injection
moulded plastic part with a metal section by means of clips, rivets and/or
ultrasonic welding. Again, this requires additional capital expenditure, but
the space within the final product offers significant potential for cost and
weight saving as well as functional part consolidation. The mechanical strength
compares well with that of open rib structures at lower overall costs.
Additional costs but simpler technology
In MGAIM or MWAIN, the complete hybrid component is manufactured in a single
integrated process, by over-moulding a metal part and then coring the plastic
material by use of the gas or water assisted technique. The result is a very
stiff and lightweight, energy absorbing structure with outstanding surface
appearance (no sink marks). The cored-out sections lend themselves to functional
integration.
Of particular note, the freedom of design offered by MGAIM and MWAIN significantly
exceeds that of any deep-drawing metal, leading the way to a number of high-end
applications replacing expensive aluminium or pure steel components while
also minimising the number of jointing and assembly steps. Rhodia sees a wide
range of automotive applications for these three technologies, from structural
front ends and instrument panel carriers to highly integrated door modules
as well as seat shells and an array of smaller applications, such as brake
pedals.
www.rhodia.com