Mention plastic cars, and most people think of the leaky, creaky and crazed
bodywork so common on glass fibre cars of the 1960s.
Plastic still has a poor image even today, being connected more with flimsy
toys than the leading edge of
industry technology. But plastics are
becoming increasingly used in making cars, largely as a result of the drive
towards weight reduction for lower
consumption and greater economy.
For many years the problem has been that plastics are not strong enough to
be used without reinforcement. Now the scene is changing, with more
complicated, composite materials taking over from simple plastics. These new
materials bring changes in design and manufacturing processes but, most of all,
offer greater driving economy.
Reaction injectio moulding
In reaction injection moulding, two types of
plastic are injected at the same time. These react chemically and set hard inside the mould. The process is very fast, taking only about 90 seconds to complete each panel.
The wings and bqmpers of the Reliant Scimitar SS1 are made in this way, but they have additional refinement—glass fibre matting is laid into the mould first and the liquid plastic squeezed in around it. Reinforced reaction injection mouldings (RRIM) can be made to be very flexible while retaining high strength.
Glass fibre cars
Glass fibre - more correctly known as GRP, which stands for glass-reinforced
polyester - combines lightness with strength. Weight for weight, glass fibre is
much stronger than steel, so the panels can be made lighter. For many years it
was the only plastics material used for structural car parts, and it is still
widely used today.
In a glass fibre bodyshell, glass fibre matting is used to reinforce a
polyester or epoxy resin panel. The resulting material is relatively stiff, but
still has sufficient 'give' to withstand low-speed
. In the 1950s,
manufacturers started to use the material for car bodies. The first car to
appear with a glass fibre body was the Chevrolet Corvette of 1957, but
underneath it still used a conventional steel
to give it strength.
The following year, Lotus introduced the original Elite - the world's first,
all-glass fibre monocoque. It had no steel chassis, and the
suspension were bolted directly into the glass fibre shell. Among the cars
currently in production with this construction is the Midas, a low-volume
specialist car that takes much of its running gear from the BL Mini and
GRP vs steel
As an alternative to steel, glass fibre construction has considerable
advantages. Though of much thicker section the structure is light, and rust
becomes a thing of the past (except where it is bolted to metal). For the
manufacturer, tooling costs are lower than for car construction in steel
because large presses are not usually needed.
Despite the advantages, glass fibre cars have only been produced by small
manufacturers, usually attracted by the low tooling costs. For mass production,
it has never really been a viable proposition. Manufacturing is relatively
slow, since each body section needs to cure for a couple of hours. Car body
manufacture in steel, on the other hand, has had the benefit of much more
investment and development.
Since the early days of glass fibre, the use of plastics and other
composites has come a long way, with techniques such as reaction injection
moulding allowing a much faster manufacturing
Most manufacturers now use some form of plastic for the bumpers of their
cars. In the early days of plastics, the bumper moulding would have been a
single piece of ugly heavy-gauge plastic (polypropylene or thermoplastic) in
black or grey, usually with metal reinforcement behind to give adequate
Nowadays the strength is more likely to come from moulded-in box sections: a
piece of lightweight foam is used as a base, around which the rest of the
bumper is moulded. The foam has no real strength of its own, but when it is
used as a
it moulds the plastic into a strong 'hollow' section. Rover
make the bumpers of the 800 series in this way but Lotus have developed the
technique to the extent that they can use it for the main structural sections
of their car bodies. Steel parts can also be moulded-in to give localized
reinforcement for door hinge or lock mounting, and for protective parts such as
door girders and roll bars.
Many of the plastic-bodied cars in production now use different types of
plastic for different parts. Bumpers need to be deformable to absorb impact,
and elastic to avoid permanent damage, so they are made from a variety of
specially modified plastics with some rubber-like properties (such as
polypropylene or modified thermoplastic polyester). For panels like the boot
lid it's better to use more rigid material such as fairly heavy gauge glass
fibre or cold-pressed reinforced polyester so that the driver feels solidness
when slamming it.
Using many different materials is easy to arrange if the body is made up
from a number of different parts bolted to a central structure, but Lotus, who
make their bodies in two main parts, manage to combine several different
materials into the same casting. With vehicles that have individual panels
bolted on to a separate steel
, such as the Reliant Kitten and Scimitar
and the Renault Espace, each panel may be made of only one material or a
composite of two.
Provided that all the different materials used can be made to accept the
same type of paint system, there will be no problems with finish.
Glass-reinforced polyester will take conventional paint, as will other
materials such as rigid polyester, used for non-flexible body parts. Most
plastics cannot withstand paint-bake temperatures, however, so a common way
around the problem is to use a two-component paint system that sets by chemical
reaction rather than with heat. Renault use this type of paint system for the
Espace, which also has minute balls of polyester incorporated into the paint
for high-wear areas such as the lower panels, which are prone to attack from
dirt, grit and stone chippings.
It is not possible to self-colour high-density polyurethane and conventional
paint won't stick on it, so many of the early bumpers were unattractive. Since
then, new materials and special primers have been introduced which allow the
bumpers to be coloured.
The bumpers of the Austin Maestro and Montego, which are made from PBT
(polybutadiene teraphthalate), are sprayed at the same time as the body to
ensure good colour matching. They are painted away from the body and then
Although plastics are being used more and more for body parts and small
have meant that plastics could soon be in
widespread use for more major components. Glass fibre
been used on the 1980s Chevrolet Corvette and in the Sherpa van.
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As well as replacing a door because it has become rusted or been damaged in
, you may also need to do this job before a respray. With the doors
off, the door pillars and inside surfaces can then be resprayed properly.