5.1 Introduction
Currently polymeric materials account for approximately 25% of the mass of a modern car. Key reasons for
this include the lower density of plastics compared to metals, which assists in weight saving and the
possibility to simplify manufacture by reducing parts count or by simplifying assembly - eliminating
separate fasteners. Applications include interior trim, bumpers, petrol tanks and lights. The requirement
for easy recycling is also influencing the choice of plastic and assembly methods (as easy dis-assembly
and identification of plastic type is essential).
One large component (50 kg) made from fibre reinforced polymer is the roof panel for the Ford Transit van
with a coloured polyester or polyurethane skin.
One complication when introducing plastics into auto body designs is that traditionally most paint lines operate at about
200oC, which is fine for steel and aluminium, but too high for plastics. Pigments may be added
to plastics to aid the production of a high quality finish from a coloured gel coat.
For engineering applications plastics require resistance to one or more of the following:
- fuel
- hot air
- hot coolant
- hot lubricant
A popular material which meets these requirements is nylon 6 reinforced with 30 - 35% glass fibre which can
operate at temperatures up to 130oC. One application which is becoming more widespread is for
inlet manifolds, to replace cast aluminium, manufactured by injection using the lost core process.
A weight saving of up to 5kg can be made and as the surfaces are smoother, engine aspiration is improved.
For highly loaded structures, some work has been done with filament wound reinforced plastics for
drive and propeller shafts (Renault) however the high costs of manufacture is limiting their wider
application.
5.2 Metal Matrix Composites (MMC)
MMC materials have been used to replace cast iron for brake discs, helping to reduce critical
unsprung weight in some specialist vehicles, Lotus Elise, racing cars and motorcycles. Another
application is for studs in snow tyres, replacing tungsten carbide.
Although there are two types of MMC, one based on powder, the other based on fibre, only the powder based
materials, PMMC, are presently financially viable - at a material cost of about £8/kg, ref A1. The manufacturing
route used is mixing the (usually) silicon carbide powder (up to 30%) with molten metal (aluminium -
7% silicon or magnesium) under strict temperature control. The cast PMMC can be rolled, forged and extruded.
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5.3 Panel Cutting and Forming
The introduction of FEA software able to analyse sheet metal operations has helped to reduce the time
needed to introduce a new model. Other benefits are more accurate designs as the effects of locally varying
blankholder pressure and interface friction can be investigated. Other important trends are:
- the reduction in panel numbers, partly facilitated by the use of tailor welded blanks.
- programmable CNC techniques - applied to methods of cutting panels - plasma, laser, water jet and flame.
- CNC punch presses that include simple forming operations into the blanking process
It is now possible to simulate multi stage deep drawing processes, predicting material thickness variations and
forecasting potential problems such as tears or wrinkles.
Hydroforming - is a process particularly suited to manufacturing complex closed sections from simple
tubes. The tube is initially bent to a shape similar to that of the final product. It is then put into a die
which is closed and the tube filled with liquid which is pressurised. This forces the tube to fill the die space.
This means that the flanges and welding operations used to make box section form 'U' channel and a covering
plate are no longer needed. Hydroforming can give a cost saving while producing a component which is closer to the ideal.
Parts made in this way include door reinforcing members and suspension sub frames.
Superplastic Forming
Certain metal alloys are able to display superplasticity which means that at certain strain rates
their strength is very low and elongations of some 1000% are possible. Alloys require a very fine grain size
(1 to 15 micro m) which is stable up to a temperature of at least half of the melting point. One popular alloy
contains 78% zinc and 22% aluminium which at 250oC will become superplastic at a strain rates in
a limited range of 0.0001s-1 to 0.01s-1. Because of these low strain rates, forming
times can be seconds to hours which means cycle times are much longer than those of conventional forming processes.
Sheet material can be formed by vacuum forming and blow moulding techniques using low cost tooling.
Link to Superform Aluminium - manufacturers of components in
superplastic aluminium alloys.
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