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The growth of advanced composites in the manufacture of automotive bodywork components is limited by a number of factors such as labour-intensive lay-up, trimming and finishing processes, long processing times, and high raw material costs. Delta-Tech has developed a range of prepreg materials that reduce the cutting and trimming work while improving impact behaviour and other safety aspects.
(Published on November-December 2008 – JEC Magazine #45)
PAOLO NIERI, PRESIDENT, DELTA-TECH SPA
The use of composites in the fabrication of automotive body panels is limited by a number of factors associated with open mould processes, while recent developments in RTM and vacuum infusion have opened new opportunities allowing lower temperature processing, less labour-intensive layup and faster tool turnover. Yet these alternative technologies do not completely resolve all problems, or may require a massive production effort in order to make up for the higher development and tooling costs.
Another issue related to the use of composite materials in automotive bodywork components is their poor ability to dissipate impact energy. Solid panels that have the required flexural stiffness to remain stable in the presence of aerodynamic forces often exhibit stiff and elastic behaviour in case of an impact. If the impact is against a human body, it may cause serious personal injuries. To improve this behaviour, several types of micro-sandwich configurations have been developed, where a tough woven or non-woven core, often made of polyester or aramid fibre, is placed between skins made of woven carbon or glass fibre. In other cases, the core is made of a tough syntactic resin layer.
Over the past few years, Delta-Tech has been committed to providing cost-effective high-performance solutions to the automotive industry using prepreg technology. In particular, Delta-Tech has focused on reducing the cutting and trimming work for parts made in open moulds, reducing the cost of bodywork lay-ups and improving the impact behaviour and other safety aspects related to the use of composite materials in automotive applications.
Kordo™ − Net-shape components directly off-the-mould*
A new technology developed by Delta-Tech now makes it possible manufacturing components in open moulds with no need for subsequent cutting and trimming, by forming a round edge directly in the mould.
Kordo™ is an impregnated pre-formed edging ribbon which can be laid around the part contour during the lay-up process. This is very easily done, due to the very soft and pliable nature of the ribbon, enabling bending even around a complex part contour. Kordo™ is made of a prepreg strip folded around a soft polyester braid in such a way that the two strip edges are then matched and stuck together. The resulting preform (see Figure 1) is supplied in 20-50 metre long coils.
During the lay-up process, the thick side of Kordo™ is pointed outwards, while the thin side overlaps with the panel plies. It must be noted that a little dam with the required edge profile needs to be built in the mould in order to precisely place the ribbon during lay-up and get the right shape in the final part. This way, the edge shape is controlled only up to the vertical sidewall. If a completely round shape is required, a semi-rigid caul pad can be used. In this case, a positioning groove must be created on the tool in order to precisely match tool and caul pad.
Kordo™ can be made of different fibres such as carbon or glass, and is available in different thicknesses. The typical edge diameter is 5 mm or 3 mm. Kordo™ is impregnated with matrix systems that are designed to match with the wide range of Delta-Preg prepreg systems, specifically addressed to the automotive market. Figure 2 shows a typical application for the bodywork components of the X-bow, a high-performance car from the Austrian car manufacturer KTM-Sportcar Produktions GmbH. The edges of the carbon fibre bodywork panels for this modern and aggressive car design comply with the safety regulations thanks to a continuous 5 mm-thick edge made out of Kordo™. Edge cutting and trimming has been eliminated as well, for considerably reduced labour. As a further advantage, the thick edge surrounding all the body panels significantly contributes to part stiffness and dimensional stability.
Semi-impregnated micro-sandwich (SIMS™)*
PET fleece has been used as a core layer in micro-sandwich construction for many years. Whether the fleece was impregnated or used dry, all the past experiences aimed at achieving full impregnation by the matrix in the final laminate in order to obtain the desired stiffness. Delta-Tech studied micro-sandwich constructions with PET needle-punched fleece core, showing that exceptionally high toughness could be achieved in the presence of residual dry (therefore not bonded by the matrix) domains within the fleece thickness.
An optimum sandwich configuration, with the appropriate resin content, was selected and tested to achieve the right balance of stiff bridging between the skins while maintaining the desired amount of dry fleece within the laminate thickness. Once the right resin content in the prepreg was set, it was demonstrated that the minor resin content fluctuation in commercial prepreg production does not affect the final properties of microsandwich structures.
The typical section of SIMS™ laminates is reported in Figure 3, together with a microscope image of an actual panel section. The skin fabric can be made of carbon or glass fibre, with the latter representing an extremely cost-effective solution that can provide the required stiffness due to the thick sandwich configuration, low raw material cost and low weight afforded by the light, partially impregnated PET core.
The dry fleece is pre-matched with a prepreg skin directly at the Delta-Preg factory. The micro-sandwich structure is then completed by adding a ply of woven prepreg. This means that only two plies are laid-up to produce the laminate.
The prepreg product range includes woven fabrics, multiaxial fabrics and unidirectional tapes fabricated by solvent coating or hot-melt technology. Most of the production is based on carbon fibre, but glass, aramid and special fibres are also used.
Delta-Preg’s production mainly targets the automotive market, including competition and high-end GT cars. Other important market segments are motorcycles, sporting goods, boat building and industrial machinery.
As the SIMS™ laminate cross-section shows dry PET fibres, the component edges need to be sealed. This can be ideally accomplished using Kordo™ all around the perimeter of the part. Alternatively, the fleece-containing ply can be left short on the edge and overlapped by a woven prepreg strip.
The resulting structures are characterized by extremely high toughness. In case of a strong impact, the panel retains good stiffness until the first failure occurs; then it keeps yielding in the same way as a thermoplastic material. This results in high energy dissipation while the maximum yield force remains low. This will reduce personal injuries in the event of an accident. The advantage of SIMS™ over conventional micro-sandwich structures with impregnated fleece core is well illustrated by comparing the results of drop ball impact tests carried out on SIMS™ laminates and an equivalent laminate where the fleece core is fully impregnated by the matrix. Figure 4 shows a typical force profile versus time during an impact test. The comparative results are reported in Table 1. Here “SIMS™A” and “SIMS™ B” stand for different degrees of fleece impregnation, “A” being lower than “B”. The impact energy dissipated by the fully-impregnated panel could not be measured as it was lower than the minimum energy detectable by the testing machine. The most striking difference was noticed for the maximum impact force, which is less than half for the SIMS™ panels than for the other one.
The development of construction techniques capable of reducing labour and enhancing the mechanical and functional properties of composite bodywork components can broaden the application potential of these materials in the automotive market.
While matrix injection technologies remain the foreseeable best option for large-scale manufacture, Kordo™ and SIMS™ can significantly extend the advantages of prepreg technology, by providing: