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Even as we speak of “green” cars and energy issues are giving cause for thought, conventional ways of thinking are hard to get rid of. For all of this, composite materials can tout some genuine advantages.
(Published on November-December 2008 – JEC Magazine #45)
M. REYNE, ENGINEER, ENSIAS
In automotive construction, 15% of the average weight of current vehicles consists of synthetic materials. The breakdown for the three major functions is as table 1:
Interior trim panels already consist mostly of plastics, and temperatures restrict the use of plastics in the engine assembly. Neither of these cases applies to body parts, however, so the share of plastics here should rise significantly in future. Who knows, perhaps one day we will see an all-plastic/composite body shell.
In the automotive body industry, you have to paint steel to keep it from rusting, but there is something illogical about painting colour-fast, age-resistant plastic components just so that they will resemble.... painted steel. The industrial approach is to allow a material to become established on its own merits, but marketing channels tend to conceal the use of polymers instead of promoting their characteristics to customers.
Cost price seems to be the only barometer by which the automotive industry takes the plunge or not. It is difficult to imagine, therefore, that a synthetic material that costs more than steel per weight unit and takes longer to process than metal could replace metal just because it reduces weight. And yet, value analysis shows that what eats up the most cost are:
As an illustration, here is a concrete example: before 1970, all car bumpers were made of chromium steel, and the chromium coating accounted for 70% of the bumper’s cost. At the time, Renault put out its R5 model, which featured the first front bumper made of SMC composite, and many Renault executives were saying, “We’ll never sell a car without chrome bumpers”. The SMC bumper was a key element in the car’s image, however, and the car was a great success. It sold very well, and Renault’s competitors all adopted SMC bumpers later on. Due to cost and recycling issues, these bumpers are now made of polypropylene (unfortunately, they are often painted, making them more expensive and more difficult to recycle).
The thing to know for an automotive plant is that it is not the drawing presses which constitute the largest investment, but rather the E-coating lines and accessories, including pollution control. These have a major impact on cost price, due to the high energy consumption for a 20-minute E-coating step at 170-200°C, a 20-minute primer step at 160°C and a 20-minute painting step at 140°C, added to the environmentally costly pollution caused by volatile varnishes and residues, difficult to control and eliminate.
To help synthetic materials hold their own against steel, we need to get away from the Class A concept, which practically doubles the cost of the drawn or moulded part. There are several solutions for dramatically reducing these costs. One is opting for matte rather than brilliant Class A-type finishes, which really have no functional raison d’être, but create distracting reflections and, barring continuous maintenance, lose their shine after a few months of use. We could easily save three to five hundred euros per vehicle by using matte paint finishes, like the ones on military vehicles or electrical appliances. Cost assessments in the automotive industry calculate down to eurocents, but the whim of a designer can raise that same cost by as much as a hundred euros − the justification being “the customer won’t like it if....” (see R-5 bumper above). By definition, styles change.
Another, even better solution is to adopt an all-plastic/composite body with in-colour surface that is not subject to deterioration.
20-30 kg of paint
Besides saving cost on investment (no more painting line), labour (no more varnishing step) and materials (no more primer, paint and varnish) and eliminating the need to recycle effluents, the use of polymers reduces the weight of the vehicle, both through the use of the more lightweight polymer and through the elimination of 20-30 kg of paint per vehicle! All the shell parts for Mercedes’ little Smart car are made of injected thermoplastic (a blend of PC and PBT), illustrating that it is possible to obtain attractive colours with moulded-in colour (a protective coat of varnish is applied, as well).
For very large structural parts, the best solution is the use of compression-moulded, long-fibre-reinforced thermoset composites (SMC). SMC material is very suitable for auto bodies, but its natural colour is matte. Above all, a major recycling handicap is curbing its use: like all thermoset plastics, SMC is not reprocessable. It must be recycled in the form of fillers, for which there is limited use. Recycling SMC also turns out to be a losing proposition, cost wise. Sooner or later (and the sooner, the better) it will become absolutely necessary to switch to thermoplastics, which have a much shorter production cycle (only half as long) and can be reprocessed. For now, steel has the advantage because it is easily recycled.
There are two options open to achieve the required mechanical properties, especially stiffness, using synthetic materials:
Injection moulding would be the most cost-effective solution, but we don’t yet have the capability to make a TP composite incorporating long fibres using that closed-mould process.
Today there is another potential solution for creating a shell using a thermoformed or pressed long-fibre-reinforced thermoplastic composite, such as Twintex (long-glass-fibrereinforced PP), as shown in the estimated cost comparison below. The example is based on 2-m² body panels of roughly equivalent stiffness.
The costs are roughly equivalent, with a slight advantage for the TP composite that should enhance the economies of scale on materials consumption, and increasing productivity with acquired experience.
Neophytes will object that a metal shell makes us feel safer, forgetting that Formula One race drivers are protected by composites. Whether or not the body is in steel or plastic has no effect on safety. At speeds upwards of 100 km/h – with or without seat belt or airbag – a frontal impact with a fixed obstacle like a wall or a tree will leave the car’s occupants either dead or in very bad shape for the rest of their lives, because internal organs can burst. The energy to be absorbed increases proportionately with the weight of the vehicle and the square of its velocity: the kinetic energy = 1/2 mV² (m = mass, V = velocity). What is never said is that the heaviest and fastest cars are also the most deadly in case of impact with a fixed obstacle. (Of course, every driver has the illusion of being better than the others and of driving in a Formula One race!)
Once the problems associated with processing and recycling long-fibre-reinforced composites are resolved, it is likely that car builders will eventually escape from the “tyranny” of E-coating and switch to the use of synthetic materials for car bodies to save on weight and lower prices. For now, (assuredly) skilled deep drawing specialists with mechanical backgrounds still prevail in that area.
Besides changing ingrained driver habits (including with respect to observance of speed limits!), we need to acquaint designers with functional value analysis, which is the only realistic concept from the consulting world that can be applied to all the many techniques used in this industry.
There should be a special note on the Citroen Mehari, which was the first medium-production-run car with a thermoplastic (thermoformed ABS) body, mounted on a metal chassis. This concept will likely make a comeback with improvements in the future. French drivers in coastal areas still regret this simple car, which was highly resistant to the aggressive saline environment. Another pioneer was the Traban, a car with a two-stroke motor and body made of phenolic resin reinforced with cotton fibres. Three million of these cars were built between 1958 and 1990 in the ex-GDR (German Democratic Republic). Although it was the first mass-produced car with a composite body, the Traban was nevertheless scorned in Western Europe.
In the United States, the Fiero and Saturn models mass-produced by General Motors demonstrated the potential for fibreglass reinforced composite bodies on sedans, while Chrysler’s Composite Concept Vehicle (CCV) prototype gives an idea of what the future might hold: four short-glass-fibre-reinforced PET body-shell panels that are injected on an 8,000-ton press.
Composite Concept Vehicle (CCV) prototype gives an idea of what the future might hold: four short-glass-fibre-reinforced PET body-shell panels that are injected on an 8,000-ton press. Three things must be achieved in order to make more progress:
There will certainly be errors committed in plastic body making, and some of the suggestions made here will perhaps be invalidated. These are proffered mainly as not-unrealistic ideas that could be developed by plastics processors with the capability, and who could most benefit from them. Surely some of these ideas will come to fruition in the near future.
We can only hope that the practice of painting non-rusting materials will be abandoned, as it raises cost and makes recycling more difficult. We can also hope that the principle of sustainable development advocated by auto makers is not just a myth! In the long run, the only uncertainty is on who will produce these plastic shells: equipment manufacturers or auto makers? Given that equipment manufacturers already account for 70% of the cost of a vehicle, the relationship between the two will certainly have to be readjusted.