Plastics come of age at TWI

TWI Bulletin, March/April 1991

Maria Girardi joined the Plastics joining Department in October 1989, after completing a postgraduate research project at Bath University investigating the matrix modification of glass-fibre reinforced composites. Her first degree, in Materials Technology was obtained at Coventry (Lanchester) Polytechnic. Since she came to TWI, she has been involved in welding gas and water pipes, ultrasonic welding and adhesive bonding. She has also been actively involved in a programme with Johnson Matthey, to develop a transducer for ultrasonic welding of plastics using a new magnetostrictive material which was launched at the Interplas '90 exhibition. Maria has continued to be actively involved with committees of the British Composite Society, and the Institute of Metals. With both societies she successfully promoted activities for younger scientists.

Maria Girardi summarises industrial applications of plastics and composites as a lead in to future articles focusing on joining technology for these materials.

Since its inception five years ago, the Plastics Joining Department's work has concentrated on increasing understanding of the fundamental processes involved in a wide range of joining techniques now available. The department is also developing innovative joining techniques.

Development of plastics and composite materials has been very rapid over the last 30 years. They are now widely replacing conventional materials like metals, glass and wood, for non-structural and structural applications. Positive developments in the plastics industries have been sustained by a period of economic expansion, and by further penetration of plastics into established and new applications. A revival in the construction industry and growing use of lightweight, cheap materials in the automotive industry have both played particularly important roles in increasing the use of plastics over the last few years.

Plastics present and future

In traditional applications (packaging, electrical, and consumer goods) there has been a clear increase in demand over recent years. For example, the electrical industry has used plastics for many years for plugs, sockets, wire and cable insulation, and for encapsulation of electronic components.

Plastics are also extremely important materials in the packaging industry. Applications range from bottles and containers for liquids and solids, to plastics films, bags and sacks.

In the building industry, uses of plastics for non-structural applications currently include piping, guttering and conduit, damp courses, flooring insulation, wall cladding and, more recently, window frames. Allied to the building industry are the service distribution industries such as gas and water. Both of these now use plastic piping.

The automotive industry is a major consumer of plastics, the weight per car increases year by year. Initially the main uses were in car electrical equipment such as batteries, plugs, switches and distributor caps. Plastics then became established in upholstery and interior body trim. In recent times, there has been an increase in under-the-bonnet applications, such as radiator fans, inlet manifolds, petrol tubing and coolant water reservoirs. Requirements for fuel economy, which demands lighter cars, and for increased occupant safety have led to a substantial increase in use of plastic for bumpers, radiator grilles, and other body panels.

Use of composites has also increased in the marine, aerospace and automotive industries.

Glass-fibre reinforced plastics boats are widely adopted for their cheapness of manufacture, case of maintenance, lightness and, for military purposes, their non-magnetic characteristics. A particular advantage in boat building is that seamless structures can be fabricated and this reduces the cost of assembly, and the likelihood of leakage in the hull or deck. The good corrosion resistance of plastics has also led to their widespread use in both fixtures and fittings.

In aircraft, composites are particularly important because complex shapes can be manufactured easily. Other structural applications where composites are used include wings and helicopter rotors. Although the potential for use of advanced composites in car manufacture is very high, these materials are struggling to become established in the mass production environment.

Demonstrations of the serviceability of advanced polymer composites have been carried out on a number of significant motor car and truck components, such as leaf springs, torsion bars, drive shafts and bumper bars. However economics have dictated that these can be used only in low volume production vehicles. The high cost of materials and the lack of low cost fabrication processes currently prevents their use in mass produced vehicles.

Composites are also used in sports goods, such as rackets, surfboards, golf shafts, bicycle frames, oars, hockey stick shafts and fishing rods.

Joining - the enabling technology

Since plastics and composites have become an important and rapidly growing field, a need to produce larger and more complex parts has been created. Hence, there has been a greater need for joining these materials in industry. There is a variety of methods available for fastening and joining plastics and composite parts. These include use of mechanical fasteners, mechanical joints, adhesives and welding. Future articles will give an overview of these joining techniques for plastics and composites with a review of current R&D at TWI.