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Whereas short-fibre thermoplastics are routinely injection moulded in the plastics industry, the injection-moulding of long-fibre thermoplastics is reserved to a select few processors. The segment still concerns only a handful of automotive equipment manufacturers, who are only too happy to have invested in the technology.
(Published on January-February 2006 – JEC Magazine #22)
The impregnation of glass fibres with a thermoplastic resin gives a composite that is stiff, impact resistant, and temperature stable. Glass-fibre producers were quick to implement the concept by formulating long-fibre-reinforced composites for injection moulding. Polypropylene (PP) was a thermoplastic that was already in widespread use in the automotive industry; with the possibility of reinforcement, it has been able to move into new applications. Auto builders are also keen to reduce their production costs.
The introduction of a new PP-based material with 12-mm-long glass fibre reinforcement has considerably changed the scene. This injection-moulding grade has been extremely popular in the automotive sector, where it is replacing metals and short-fibre thermoplastics in many applications, such as frontend carriers, dashboard inserts, underbody shields and door modules. It is not the only contender in the coveted market for semi-structural automotive parts, however. The offering of long-fibre thermoplastics (LFT) has developed considerably over the past decade, and there are now a whole series of formulated grades based on polyolefins, polyesters, PC-ABS alloys, and polyphenylene sulphide (PPS), among others.
One thing that differentiates producers is the type of impregnation process they use. (To take one example, some propose longfibre pellets made by impregnating glass fibres with PP resin using hot melt techniques.)
The grades are differentiated by the type of additives used (stabilisers, pigments, coupling agents), fibre length (10-25mm) and glass content (20-50%). The chemistry is quite complex and supplierspecific. For example, one pellet variety where the glass fibres are commingled with thermoplastic filaments gives a high fibre fraction (75%) that is used with virgin resin. The solution has advantages for processors, as it is cheaper than buying ready-to-use pellets.
Processing concentrate LFT pellets differs from processing readyto- use pellets only in that it requires installing a gravimetric feeder in the injection press, for a relatively small investment.
A boom in applications
To squeeze material costs even more, two European equipment manufacturers have adopted in-line LFT compounding. At the Audincourt site in France’s Doubs department, Faurecia is producing front ends from glass roving and PP resin. In Germany, plastics processor Aksys is using the same process to produce front ends for the Golf V. Injected PP in the form of long-fibre pellets represents an estimated 16,000 tons in the European automotive market.
Adding in direct long-fibre thermoplastic (D-LFT) processed materials gives a total of 18,000 tons – which is still not very much, compared to the 1,700,000 tons of plastic materials used by the automotive industry. However, this market segment is growing faster by far than other composites, including thermosets, glass mat reinforced thermoplastics (GMT) and short-fibre thermoplastics. In 2004, European builders in the three processing segments (ready-to-use LFT pellets, concentrate LFT pellets and D-LFT) consumed an estimated 28,000 tons – or a 55% increase! At the Gorzow Wielkopolski site in Poland, Faurecia produces 1,200 dashboards per day for the Golf V. The dashboard inserts are injectionmoulded in PP-LFT pellets. Volkswagen appears determined to concentrate on LFT injection. “It is easier to recycle a PP-LFT insert than a metal one,” observes Marcel Frery, the manager of Ticona France. Grinding only reduces the length of the fibres, without degrading the thermoplastic’s properties, so materials can be recycled a number of times. VW decided to extend the use of LFTs to both front ends and dashboard inserts based on this criterion, among others. It remains to be seen whether other builders will follow suit.
The motives for individual engineering choices can be very different. The front end module for Renault’s Megane II and Scenic II models is contracted out to VPO (Valeo Plastic Omnium). The module is a hybrid injection-moulded design, with the front end and bumper beam in short-fibre-reinforced polyamide (PA) and the light bezel in long-fibre-reinforced PP. Renault opted for a single-material design on other models – for example, the Traffic and Master utility vehicles, whose front end modules are injection-moulded in PPLFT by Peguform in Burnhaupt in France’s Haut-Rhin department. Peguform also manufactures the Citroën C2 hatchback, which consists of a talc-filled PP skin and PP-LFT trim panel. “The entire module is manufactured and painted at Peguform and delivered to Citroën in Aulnay, where it is mounted on the body by screw assembly,” says Thierry Renault.
The potential of these materials is far from being exhausted. One of the emerging applications is for underbody shields, with a number of developments at Aksys, Rieter, and Trèves. In Spain, Faurecia is manufacturing door modules from concentrate LFT pellets for the Ford group, delivering 2,000 of the assembled modules every day. Another line of development is in injectionmoulded LFT hatchback trim panels. When PP is not adequate, it is possible to move up to a more upscale produce with polyester, polyamide, or PPS. In other words, the entire range of engineering thermoplastics can be used with long-fibre injection.
Parts and tooling design
LFTs are still a speciality among injection techniques, because working with fibres that have a residual length after moulding of 3-5mm involves strict constraints for part design. According to the head of development at Delachaux Systèmes d’Injection (DSI), having the fibre orientation in the direction of material flow gives the material anisotropic properties. To help their automotive customers develop applications in which polymers like PP-LFT replace metal, this company set up a rheological design unit that works upstream of the manufacturing process to optimise the design of parts and tooling. “Simulation analysis serves to optimise the number and position of injection gates so as to reduce the shrink variability of the moulded part,” explains one of the rheological engineers at Delachaux.
To increase stiffness locally in parts like front end carriers and dashboard inserts, ribbing must be added in the non-visible parts of the structure. Here again, this leads to constraints in the injection process, because, due to the ribbed areas, there are considerable differences in wall thickness. For this type of part, a sequential mould-filling process is used with a valve gate system that controls the opening and closing of the injection nozzles.
At Delachaux, the valve-gate control system was optimised to reduce friction with the fibres and keep the system from clogging up with amalgamations of resin and fibres. LFT processing requires a fluid material flow throughout the entire system. To allow this, the nozzles, gates and runners are oversized, and any components that could obstruct the flow – e.g. check valves, filters and right-angle elbows – are eliminated. Because the longer fibres are highly abrasive, a higher level of protection against tooling wear (nitriding, titanium coating, chemical vapour deposition, hard steel, etc.) is also required.
Builders, of course, want to reduce their costs, and contracting out subassemblies to plastics processors is a step in that direction. LFTs are relatively more expensive than metals, but this is compensated for by the lower weights achieved and the lower tooling costs. “An injectionmoulded thermoplastic subassembly costs from 30 to 60% less than a similar metal one,” says Olivier Godde, the injection simulation specialist who provides assistance to automotive-part suppliers in metalreplacement applications. The cost savings start with the tooling. To give an idea of the situation: a mould used for aluminium lasts only 400,000 cycles whereas, for the same level of investment, an injection mould will have a service life of 1.5 million cycles. Reinforced thermo- FEATURE “AUTOMOTIVE” 40 JEC Composites Magazine / no22 January-February 2006 plastics have other advantages, too. They require few finishing steps, if any; there is no need for corrosion-resistant paints; the lower part weight means lower handling costs; and, especially, functional integration leads to more cost savings. The Syntes cockpit system marketed by Faurecia provides an example of the progress achieved in injectionmoulded engineered subassemblies. To meet frontal impact requirements, the lower part of the two-piece-moulded module was reinforced with an overmoulded steel tube. “With this development, more than a million cockpits are being delivered each year the world over,” says a representative from the Faurecia Interior Systems division. While production of the complete module is reserved to a handful of equipment manufacturers with just-in-time capability for delivery to customers worldwide, processors that cannot claim tier one status in the automotive sector can still participate in this technology boom by contributing their expertise in smaller engineered components, where the TP/long-fibre combination can tip the balance away from metals in terms of cost and weight savings for under-the-hood parts, pedal assembly brackets, seat frames and the like.
As for builders, they are open to innovation – especially where weight reduction is concerned, because they know that the trend is for reducing vehicle fuel consumption.