Following 10 years developing gas shielded arc welding techniques on aluminium alloys at the research laboratories of the British Aluminium Company, Mike joined the then British Welding Research Association in 1967. He spent two years as an experimental officer in the Arc Welding Department before transferring to the School of Welding Technology as Senior Welding Instructor.
In 1978 Mike transferred to the Process Operation and Control Department to carry out process development on AC square wave and MMA welding and AC MIG welding. This was followed by investigations of pulsed MIG welding on very thin section aluminium and very thick section titanium alloys. On completion of a study of orbital TIG welding on stainless steel pipe Mike transferred to the Plastics Joining Department.
Over the past four years he has worked on friction, hot plate and ultrasonic welding of thermoplastics with the emphasis on process development.
The last half century has seen a plastics revolution in vehicle manufacturing. Mike Murch looks at today's use of plastics in cars, trucks, buses and motorbikes, and puts the case for their future use.
Plastics and polymer based composite materials have found many applications in modern vehicle construction through their recognised merits of light weight, corrosion resistance, opportunities for component integration and production economics. These advantages have increased the amount of plastics used per vehicle from less than 2% by volume before 1950 to an average above 20% today, representing over 100kg weight in the present day car.
The automotive industry, encouraged by legislation, has adopted plastics mainly for non-structural applications. Following the oil crisis in 1973, when fuel prices quadrupled, targets were set in the USA to reduce fuel consumption under CAFE (Corporate Average Fuel Economy) regulations. Up to that time plastics had been used almost exclusively for vehicle interior trim, but to reduce weight and thus improve fuel efficiency, car manufacturers began to replace metal trim by plastics on external applications. Further regulations were introduced to increase crash safety that required bumper systems to withstand an 8 km/hr impact without crumpling or permanent deformation and this led to the plastic bumper predominating on today's cars.
Interior trim
Typical applications for plastics that currently account for 65% of use in vehicles are in the passenger compartment. Plastics parts include the dashboard, steering wheel trim, door trim panels, seat padding and trim, pillar trim panels, central console, rear parcel shelf, roof liner and sun visors. Many different plastics, but generally commodity thermoplastics, are used in these applications.
The prospects for further growth are good. Plastics are being considered for seat shells and vehicle flooring to replace steel parts.
Exterior body parts
Growth in applications of thermoplastics and thermosets on car exteriors has accelerated throughout the 1980s with improved processing techniques and material performance. Initially plastics were confined to small parts such as wing mirror housings, hub caps and front and rear light clusters. More recently large plastics body parts have become common with front and rear bumpers, radiator grilles and front spoilers now common on most cars.
Growth in plastics body applications continues with the bonnet and boot lid of several car models being produced from unsaturated polyesters (UP) as sheet moulding compound (SMC). A major example of SMC use is seen on the Citroen BX series where both the bonnet and boot lid are thermosets.
An important reason for the growth of plastics in external body parts is associated with potential production economies - where styling changes for model update can occur annually, the cost of tooling for steel body panels becomes uneconomic. Complex plastics parts can readily be designed and processed on lower cost tooling, and although material costs are generally higher, reduced production costs favour plastics for low to medium volume run (<200 000 units) models.
Under bonnet components
The earliest application of plastics in motor cars occurred in the engine compartment, where the high temperature properties and excellent electrical resistance of phenolic thermoset resins encouraged their use for magneto bodies, distributor caps and ignition coils in the early 1900s.
However, use of plastics remained relatively static until the development of nylon (polyamide-PA) in the mid-1930s, which was followed by numerous other engineering plastics. The advent of new plastics has caused a major revolution in material substitution under the bonnet over the past 20 years.
Thermoplastic materials have predominated with wash bottles in low density polyethylene (LDPE), and battery cases, air filter housings, heater boxes and fuel control linkages in polypropylene (PP). For more rigorous higher temperature applications glass reinforced PA is used extensively for radiator header tanks, water pump bodies, cooling fans and more recently for air inlet manifolds. The trend to increasing use of plastics continues with rocker box, oil sump and gearbox covers being made from reinforced PA substituted for previously used thermoset materials.
A recently developed materials form, glass mat thermoplastic (GMT), based initially on PP matrix, has been applied in semi-structural applications as engine shields, bonnet locking platforms and battery trays. The significant advantage of this material is that it is readily formed by stamping using existing tooling made for steel parts.
A major development under the car has been plastic fuel tanks formed by blow moulding in high density polyethylene (HDPE) incorporating a thin PA barrier layer. The fuel filler pipe, also in PE, is a good example of a hot plate welded component, ( Fig.1).
Welded parts
Many plastics applications on cars are fabricated to completion by welding. However, because plastics can be processed into large complex shapes, the amount of welding required is greatly reduced compared with fabrication of metal parts. Examples of welded plastics parts in the passenger compartment include door pockets and sun visors in PVC. These are fabricated using high frequency (HF) welding, also known as dielectric welding.
On car exteriors, front and rear light units are constructed from polymethylmethacrylate (PMMA) (acrylic) lenses joined to acrylonitrile-butadiene-styrene (ABS) backing using either hot plate or ultrasonic welding.
On a number of car models bumper fascias are welded to support beams by linear vibration welding ( Fig.2). In the engine compartment items welded using the hot plate process include water header tanks in glass reinforced PA ( Fig.3), and battery cases in PP. A recent example of welded construction is an air inlet manifold in glass reinforced PA fabricated by injection moulding in two parts and joined by linear vibration welding (Fig.4).
Future growth
For the foreseeable future the percentage weight of plastics used in cars will continue to increase. Two areas in particular, the engine and the body shell, are receiving detailed attention for substitution of steel by plastics.
A mainly plastic engine is currently under test for endurance and reliability by a major car producer. The engine, of two litres capacity, developed by Polimotor Research and Amoco Chemicals, weighs 76kg and contains 60% by weight (90% by volume) of plastics. An improved power to weight ratio of 1:1.7 compared with a conventional engine of about 1:4 is claimed.
Although plastic engines are a reality, the costs of materials and production are currently prohibitive and they are unlikely to feature on production vehicles until the economics of manufacture are improved.
Perhaps the most immediate area for further plastics usage is in car body panels, where particular emphasis has been placed by material producers on providing thermoplastics and thermosetting polymers and blends with mechanical and thermal properties demanded by the automotive industry. Several projects are being progressed and shown on concept cars, to demonstrate the improvements made in plastics and processing techniques.
TWI is currently involved in developing and assessing welding techniques suitable for joining thermoplastic support structures to selected body panel material. This initiative, through a Group Sponsored Project, started in February 1991, with backing from automotive manufacturers and material producers in the UK and Europe.
