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Kalpana Mistry: Kalpana Mistry is Project Leader at The Welding Institute, Abington Hall, Abington, Cambridge CB1 6AL, UK. Tel: 01223 891162; Fax: 01223 892588; E-mail: [email protected]
Introduction
The rapid development of plastics as engineering materials over the last 50 years has influenced the development of plastics welding and joining technology. Plastics joining is a strategic issue. Designers and engineers have found that, although great advances have been made in plastics processing methods such as injection moulding and blow moulding, new components and devices would not have materialized from the design stage without novel welding methods. This is especially true for manufacturers on the leading edge of innovative business sectors such as the automotive, medical and packaging industries.
The 1930s utilized spin welding to manufacture plastic compasses. The 1940s introduced hot gas welding of PVC for military applications. Later, as applications became more diverse and demanding, high frequency welding, ultrasonic welding and hot bar welding were developed. Since then, the total number of welding techniques has risen to 20; five of which are currently being developed at TWI: laser welding, microwave welding, focused infrared welding, forced mixed extrusion welding and friction stir welding.
Plastics materials
Plastics materials can be divided into two major groups; thermosets and thermoplastics.
Thermosets: the non-weldable plastics
Thermosetting plastics comprise large chains of repeated chemical molecules, which undergo an irreversible reaction to form close-networked structure with strong covalent bonds ("cross links"). This cross-linked structure provides the material with rigidity when cool, but cannot be softened again by the application of heat, thus preventing flow. If excess heat is applied, thermoset materials will degrade. Consequently, they are not weldable. Typical examples of thermosets are urea formaldehyde, melamine formaldehyde and phenol formaldehyde (also known as "bakelite").
Thermoplastics: the weldable plastics
Thermoplastic materials also comprise long chains of repeated chemical units called "mers" which when linked together form "polymers". This reaction is known as "polymerization"; for example polyethylene (known commercially as "polythene") is produced when the ethylene monomer is polymerized. The links between long chain molecules are essentially linear although branched molecules can also be formed by varying the polymerization process variables. Thermoplastics do not possess cross links, thus allowing the molecular chains to diffuse and flow. Thermoplastics can usually be dissolved in solvents and...