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generation plastics have bodywork potential
by Gordon Crabtree ONE of the main problems in the use of reinforced plastics in the commercial and passenger vehicle industries is the achievement of production rates and quality of products which will meet the high standards required in both fields in sufficient quantity and at a comparable price with steel.
A recent paper by Mr B. Gibbons, technical sales manager of the composite materials division of Fothergill and Harvey Ltd, on the future role of pre-impregnatei claims new developments are leading to a second generation which are glassreinforced—these could play a more exciting and important part in the commercial and passenger as well as motor body industries in the next few years Mr Gibbons says that a tremendous amount of information has been published on wet lay and cold press moulding techniques and designers in the passenger and commercial vehicle sections are well aware of the properties these materials can offer. Similarly, manufacturers of glass-reinforced raw materials are aware of the industry requirements in terms of material performance.
Mr Gibbons adds that a considerable amount of effort has been put in by the resin and pre-impregnated manufacturers to increase their knowledge of these materials and in turn the constructive use of them as an engineering material.
It is only five year ago that pre-impregnates were first being spoken of as the glass-reinforced plastics material of the future and one which would revolutionize their use particularly in the commercial vehicle industry.
The most significant development in the sphere of pre-impregnates is one recently developed using low-profile polyester resins. These are two-phased plastics systems containing unsaturated polyesters, and modified with thermoplastics such as polystyrene, polyethylene, polymethyl methacrylate with normal conventional fillers and extenders. These materials, noted for their low shrinkage and smooth surface after moulding, have been developed in America, the resin systems being currently offered to the automobile trade among others by no fewer than four major resin producers.
There is no doubt that the use of sheet moulding compounds of this type will gain momentum and it can be envisaged that in the near future, claims Mr Gibbons, materials will be available which can be formed into complex shapes when using standard metal pressing techniques. Probably, different types of equipment may be needed and the use of infra-red pre-heating of materials may well be standard practice in a few years' time.
Mr Gibbons feels it is also interesting to note the comments from Ford of America's materials specialists Mr H. Spence and Mr G. Wolfe that the auto components achieved with thee moulding compounds are far superior to zinc die-castings. Furthermore, the one-to-one shrink factor of polyester permits direct translation of the lay-out draft of the part to the die models, tooling, checking fixtures and moulds. Essentially, many of the design principles employed in zinc die-castings are applicable to these new resins systems. At the same time Spence and Wolfe offer a word of caution that these moulding materials should be confined to parts where structures are not subject to undue stress, as the materials have a zero field compared to zinc die-castings and the parts cannot be loaded with forces which will induce cracking.
Mr Gibbons points out that as with all glass-reinforced plastics laminates, preimpregnates have a much lower tensile modulus than steel. This disparity can be overcome to a large degree by increasing the thickness of the section in mind. For instance, a sheet can be four times as thick as sheet metal and yet weigh no more.
In terms of stiffness, however, he admits metals are much better and that there is a very considerable disparity between the two groups of materials which does not look like. being narrowed down for a long time to come. However, the strength of the newer engineering plastics is generally accepted as being quite adequate for the purpose of advancement in commercial and passenger vehicle body design, particularly as the materials lend themselves readily to the development of shapes which because of their construction are inherently rigid.
Today, estimated production figures for 1969 in the UK show that some 2-I-3 million pounds weight of polyester pre-impregnated glass mat was used by moulders embracing a variety of products and end uses.
However, so far only 10 per cent of this production is going to the commercial vehicle passenger and motor industry. Currently, it is being used to produce fan cowls, gearbox covers, stone guards for disc brakes, facia panels on low-production passenger vehicles, steering column binnacles, headlamp surrounds, front and rear wings on commercial vehicles, and doors and other components on cabs. Mr Gibbons feels that this is a reasonable array of components, but also stresses that not one of these items is in anything like volume production.
In order to get the new material in its proper perspective, it is useful to consider the physical properties of currently available pre-impregnates and compare them against other types of glass-reinforced plastics. Increased stiffness can only be attained using initial design features such as double curvatures and ribs and this does not substantially increase the .cost of tooling or indeed of the finished component.
Stress loading has been, and still is, a problem in glass-reinforced plastics systems. Designers accustomed to working with metals know that the behaviour under stress and strain is readily predictable. However, when most designers new to this material are faced with more acute problems while endeavouring to forecast the behaviour of glass-reinforced components they are shocked to find that debonding of a glassfibre-reinforced polyester material can occur at 30 per cent of ultimate tensile strength.
Tooling, says Mr Gibbons, is a controversial subject. Design, tool material and ultimate cost are a source of constant debate and tool cost has been/the downfall on more projects than has the component cost. It has been accepted that a tool for a medium-sized plastics body moulding to be produced at a rate of 60,000 units a year would be approximately one-third that of tooling costs for a similar steel body; engineering costs would be about the same. The reason for lower tool costs is the smaller number required due to a reduction in the number of parts and also the simpler moulding techniques required.
Most customs moulders supplying to the commercial and passenger vehicles industries could produce them at about 5s 6d to 6s per lb finished weight. These costs would be based on raw materials running at 2s 5d to 2s 8d per lb. While it is difficult to forecast reductions of any significance in raw material costs, there is certainly room for the development of materials in plastics which will have quicker process times and which will require less finishing after moulding. Current pre-impregnated systems process at about I30°C with cure times of between three and five minutes for mouldings of between 5 and bib. For passenger and commerical vehicles pre-impregnates offer excellent design potential: the material is made to controlled limits, in particular in the sphere of glass-resin ratio, therefore vehicle designers can work with extremely low safety factors.
In spite of the positive advantages and predictions of five years ago the question may be asked: Why is there at present only a low consumption of pre-impregnates and a slow reaction to their use as motor body components? From a scientific point of view the materials are undoubtedly a major advance in technology. At the moment, the major criticism from the commercial motor, passenger and car industry about them centres around their finish, cost of production, size of equipment required to press out large and subsequent heavy mouldings, and inadequate technology on tooling.
Some of these criticisms are accepted by the manufacturers. Nevertheless, it is certain that several large projects which would have involved many tons of material have been abandoned at stages of development due to internal commercial reasons.
In his paper, Mr Gibbons asks whether designers of commercial vehicles should educate theinselves to the use of this new material. He does not mean to suggest that the designers are inadequate, merely so used to conventional materials and to the limits of their performance.
New developments have led into the second generation of glass-reinforced pre-impregnates mentioned earlier. These will certainly be playing an important part in the motor industry over the next few years. Improvements in flame retardment have helped to produce a new pre-impregnate which more than satisfies the new BS.476, part one, class one, legislation in this sector, and will be of supreme importance to commercial vehicles in the next decade. To sum up: at present United Kingdom, American and Japanese manufacturers are involved in major research and development programmes on these new resins; and the level of surface finish of plastics motor bodies being obtained is extremely high. These materials, in fact, have a lower shrink factor than conventional unsaturated polyesters. Hence the finished mouldings have accurate surface and dimensions.
Mr Gibbons concludes that the prospect of good penetration in the commercial and passenger vehicle sections Of the trade for these new glass-reinforced plastics systems are better now than ever. However, in addition many barriers still exist, particularly in the case of designers of vehicles who will not accept the vast changes that these materials can bring. And they will take place.