THE SCOPE FOR LARGE COMPONENTS

By ARTHUR C. BENNETT. The Financial Times (London)

Issued by British Information Services – The Certificated Engineer November 1965.

One aspect of the increased productivity of the British plastics industry is the development and successful production of very large components. It is being said that developments could bring injection-moulded car bodies much nearer reality and would certainly lead to easier methods of producing industrialised building panels, kitchen units, baths and even larger components. 

This is certainly the view of one company which has begun production of what it claims to be the largest acrylic mouldings in the world. These are roof domes which have areas up to 2 100 square inches and weigh up to 17 pounds. 

The company says its experiments point the way also for the manufacturers of injection moulding machines, who could now think in terms of machines for producing larger mouldings than ever before. One basic need, it says, is for a machine having a means of heating greater quantities of material and the ability to move it rapidly from the heating chamber to the mould. 

Reinforced Plastics 

The production of large components is easier when using polyester resins reinforced with glass-fibre (although it is a comparatively slow process); the popularity of these materials could be seen at the recent International Boat Show in London where nearly 40 per cent. of the craft on display were moulded from them. Among the bigger boats was a 35-foot six-berth cruiser, the hull for which was moulded in a glass-fibre-reinforced resin. 

It is in the field of boat production, especially that polyester resins reinforced with glass-fibre come to the fore. The material has a better strength-to-weight ratio than either wood or steel, and all shaping is achieved while it is still in the mould. Complicated hull shapes, including double curvatures, can be obtained economically and mouldings can be made in one piece. No painting or varnishing is needed. 

Another example of a large plastics moulding is a glass-fibre-reinforced polyester cover for a water storage tank. It is 28 feet in diameter and strong enough to support the weight of a man as well as that of cathodic protection equipment. 

For the convenience of manufacture, the cover was made in twelve radial sections with one central circular section even-feet six-inches in diameter. All sections, which were made from Crystic 189 polyester resin were moulded a quarter-inch thick with integral flanges each bonded and bolted to the other to form stiffening ribs. The total weight of the cover is 2 500 pounds. 

High-Temperature Application 

Reinforced plastics are also being used for the housing of television studio lights. One range of high-power floodlights and spotlights has a rating of five kilowatts with a specification calling for housings able to withstand a continuous running temperature of 390 degrees Fahrenheit. The housings are moulded using Crystic 199. This is a heat-resistant, isophthalic polyester resin which is also widely used in the aircraft industry. 

There are several reasons why reinforced plastics are better for this application than the more usual sheet metal. Weight is probably the most important consideration, but since the plastics material is an electrical insulator, it also has safety advantages when used with high-power equipment. Furthermore, the thermal conductivity of reinforced plastics is much lower than that of metal. 

The use of reinforced plastics has also resulted in a more functional design. A similar housing in sheet metal would have required the use of press tools or panel beating, which are both costly production methods. When using plastics, only one comparatively inexpensive mould need be made for the production of hundreds of housings, it is stated. 

Large Products from Sheet 

Polyvinyl chloride (PVC) sheet also lends itself well to large fabrications. Good examples of this are a hexagonal tank - for etching printed circuits - and two large louvres, made from Vybak. 

Plastics were chosen for the tank because many metals do not withstand the solution used for the etching process and because earthenware does not readily lend itself to such a complex shape. The tank is virtually a tank within a tank, the central tank is designed to hold the etching solution and the outer one is for washing the etched articles. 

The louvres, each measuring nine feet by six feet, have been made for the air extraction system of a public swimming bath. Polyvinyl chloride was chosen for these structures because of its non-corrosive qualities and ability to withstand moist, chlorine-laden air. The outside of the louvre is protected by polyester resins reinforced with glass-fibre chopped-strand mat. 

A final example of a large plastics product, or rather of one that is small, to begin with, but can be expanded many times larger, is a flexible fuel tank. 

The tank is made from Hycathane polyurethane and designed for use in aircraft, hovercraft and boats. The material has a high resistance to abrasion, great tensile strength and good resistance to ageing and bacteriological attack. 

In a strength test, an eight-gallon tank made from the material had 188 gallons of water pumped into It before the seams fail