This process uses compressed air to form the softened sheet. In its simplest form the material will naturally form a smooth bubble shape but it is also used a great deal commercially in the manufacture of plastic (polyethylene (PETE)) bottles and containers including petrol tanks. One of the major advantages of this process is that there is less restriction on the shape that can be produced compared to metal pressings giving the designer the freedom to fit the tank around other components.
Glass Reinforced Plastics (GRP)
For prototypes and small batch manufacture GRP moulding is a cost effective way to produce complex, large-scale plastic mouldings. It involves combining 2 materials to form a composite material, the basic ingredients consisting of a glass mat (thin strands of glass woven into a mat, and the reason why mouldings produced this way are often called fibreglass mouldings) and a liquid polyester resin which is worked into the glass mat using a brush. The shape is built up in layers on top of a wooden mould which has been coated with a release agent (a non-stick coating, usually wax). The resin sets as the result of a chemical reaction with a third substance or catalyst.
Variations of this include using Graphite or Carbon Fibre which is more expensive than glass but with a greater strength to weight ratio and stiffness. The bodywork for formula 1 racing cars is made by this method as is high performance sports equipment.