Materials

This section looks at some of the key materials that are and can be used in the production of road going vehicles.

Key Materials in Brief

GRP – Glass Reinforced Polymer, also known as Glass Fibre

GRP is a low volume material. It is not suitable for mass production and is used most commonly in bespoke circumstances, such as hand built sports cars, kit cars and bus and train fascias. GRP is made using a manual layering technique; glass fibres are padded into layers of polymer in a mould. The polymer sets with an aerobic reaction, which means the mould is invariably tied up for a longer period of time than mass production would allow. The end result is a strong form that can be painted using cellulose paints in a traditonal way. Using a coloured gel-coat (the outermost layer), it is possible to pre-colour a component.

Pros:

Components can be formed relatively quickly with low-tooling time and cost. When using a disposable mould, complex shapes can be formed. GRP does not corrode.

Cons:

Only practical for low volume production. Production and preparation requires suitable safety precautions. Gel coat colouring is subject to colour fade. Poor performance impact, GRP is brittle and shatters rather than deforms like many common polymers.

Mild Steel

Mild steel is the most common material in use in the car industry. Despite the in-roads made by aluminium and polymers, steel still offers great value in mass production and more simpler production processes than its rivals. Improved galvanising and painting techniques mean that steel is holding more ground than ever before in terms of corrosion, one of its biggest problems. Steel is cheap to obtain by comparison to Aluminium and is readily available through recycling.

Pros:

Cheap, available and well understood. Mild steel can be prepared and processed using traditional tooling and responds well to standard joining processes such as spot and seam welding as well as bolting. Steel’s simplicity and ubiquity make it easy to repair.

Cons:

Steel requires good preparation to ensure it does not corrode in the presence of air and water, especially in cold climates where salt is also present on roads. Steel is heavier than its immediate rivals and adds to vehicle weight; this affects fuel consumption and to a lesser extent momentum (in impact) and handling.

Aluminium

Aluminium is hailed by Audi as a suitable material for all elements of a vehicle’s body. Whilst it can be used to save weight and resist corrosion, it does have its own concerns. Firstly, slightly more complex TIG welding processes need to be used in production (and repair) making it (currently) more costly than steel to work with. Where space-frame construction is employed, aluminium is often bonded with epoxy which creates strong bonds with good force distribution but is very difficult to repair.

Pros:

Aluminium is light and resistant to corrosion. Large weight savings can be made when using aluminium for the engine block.

Cons:

Aluminium is currently more expensive to manufacture with than steel. Corrosion can still take place if aluminium is placed on a less reactive metal or exposed to salt. Less well-established joining and bonding techniques can make aluminium awkward to repair.

Polypropylene

Polypropylene, or PP, is commonly used in bumper construction and is increasingly proposed as an alternative to metal bodywork. PP offers great elastic and plastic deformation characteristics compared to metals – making it ideal for use in impact. A combination of hard skin and softer foam PP has been flagged as a good alternative to traditional bonnets (hoods) where pedestrian impact is a consideration. Polypropylene is a thermoplastic which means it can be heated and melted for recycling, making it more attractive to manufacturers responding to European Vehicle End-of-Life Directives. Once considerable down-side of this material is its rate of expansion and contraction in response to heat. Current tolerances in car shut lines cannot be sustained with PP as during the summer panels will expand more than metals and will shrink more than metals in winter.