Fibre reinforced materials are used in products where weight savings, lower production costs, low maintenance costs and freedom of design are an issue. In these cases, traditionally thermosets are used reinforced with glass or carbon fibres. In many cases the properties of these materials allow application as a contruction material where they replace e.g. metals.

A second group of fibre reinforced plastics are fibre reinforced thermoplastics. Here thermoplastic material is used as a matrix with fibre reinforcement. Compared to thermosets, these materials often show better impact properties, increased toughness and can be processed in a fast and clean way.

Large-scale application of these materials has so far been limited to (short) fibre reinforced injection moulding materials. The material properties in these cases are about 2-3 times higher than the unreinforced material. In the automotive industry glass mat reinforced thermoplastics (GMT: glass mat reinforced PP) are widely applied as semi-structural, compression moulded parts. ‘High-end’ applications can be found in for instance the use of high quality thermoplastic matrices like PEEK with carbon fibre reinforcement. In spite of the excellent properties of this last group of materials, their high costs and limitations in processing methods allow only limited application of these materials in for instance aerospace products.

Considering price and properties there is a huge gap between short fibre reinforced thermoplastics and GMT’s on the one hand and continuous fibre reinforced thermoplastics (CFRP’s) on the other hand. This keeps fibre reinforced thermoplastics from wider application as (semi-)structural materials in relatively cheap and fast processes for large production series.

A relatively new group of materials, Long Fibre reinforced Thermoplastics (LFT’s), can fill exactly this gap. LFT’s are discontinuous long fibre (a few cm’s of fibre length) reinforced thermoplastics that (unlike GMT’s) are available in different fibre and matrix combinations. As a result, for every application the right material combination can be used. Precisely this characteristic enables LFT’s to fill the above-mentioned gap between GMT’s and CFRP’s (see figure 1: new material combinations). Furthermore some specific advantages make LFT’s a serious price competitor for products for which traditionally GMT’s are used (see figure 1: Savings). Finally compression moulding of these materials is a process that has not yet been fully optimised. A better control of the effect of the process on fibre distribution in the final product and therefore on the material properties can result in considerably improved product properties while using the same materials (figure 1: Control).

The potential cost reduction compared to the use of GMT’s while improving the product quality, has meantime proven itself in the fact that for instance the front-ends of the Volkswagen Passat series are currently made of LFT’s, at the expense of the thus far used GMT’s.

Here some determining advantages of LFT’s as compared to GMT’s were [1]:

Already in the Western-European automotive industry only, it is expected that in 2008 out of the total amount of fibre reinforced thermoplastics used, 20% will be LFT’s [2]. The fact that this only covers one of the three areas (Savings, see figure 1) at which the total development of LFT’s aims makes it clear that these materials awaits a promising future. Products in non-automotive areas, like construction, civil, industry, etc are especially suited for the application of LFT’s. This includes among other things, street furniture, self-supporting housings, machine parts, etc.

The processing of LFT’s shows much resemblance with that of GMT’s and to a smaller extent SMC’s, and can be described in a simplified way by four steps: melting the material, ejection and insertion in the mould, compression moulding and cooling under pressure and release of the product (see figure 2). Vital in the processing of LFT’s is the conservation of fibre length and hence the mechanical properties of the final product. The first step, the melting of LFT’s, therefore is not done using a traditional extruder. Traditionally extruders work with high shear forces in the material, which causes considerable fibre breakage. Especially for LFT’s several companies [3-5] have developed extruders that melt the material whilst conserving the fibre length. However, such a machine requires a significant investment (about 0.5 - 1 million Euros), reason why application so far has been mainly limited to the automotive industry.

In processing LFT’s for the same process two different expressions are used that differ mainly in differences in initial materials used. Indirect LFT-processing uses semifinisheds supplied by several manufacturers in pellet form that are molten in the extruder. Manufacturers that supply LFT’s with different fibre-matrix combinations are among others: Flex Composites, DSM, LNP, Asahi, RTP and Ticona.

To take advantage of the economy of the LFT-process to the greatest extent fibres and matrix material can also be mixed in the extruder, so-called direct LFT-processing. This eliminates the need for semi-finisheds. Due to this cost-reduction this is the preferred method in the automotive industry (still only in combination with glass-polypropylene), where traditionally costs play a dominant role.

After melting, the desired quantity of material is ejected from the extruder and cut-off by a cutting device. The molten LFT-material is then inserted in a ‘cold’ mould, which afterwards closes at relatively high speed. At closing the LFT-material will start to flow during which it fills the mould cavity. During cooling down the mould is kept closed at its pressure of at least 50-100 bars, after which the mould is opened and the product can be taken out.

Because of the large investments that go with the purchase of the required extruder and press for the processing of LFT’s, their application has mainly be limited to the automotive sector. It’s evident that in this way the many opportunities that lie outside the automotive sector are not being utilised.

Within Lightweight Structures B.V. a major part of the research effort is aimed at LFT’s. As part of this research a patented piston-blender has been developed for melting the LFT’s while conserving fibre length (see figure 3). Due to the machine’s simple construction, its costs are only a fraction of those of the usual LFT-extruders. The machine therefore offers the possibility to process LFT’s at only minor investments.

Using this machine LFT’s with different fibre matrix combinations ranging from glass fibre reinforced PP to carbon fibre reinforced nylon have been processed (see figure 3) and tested. The possibilities that LFT’s offer, as described above, are clearly shown in figures 4 and 5, where the tensile stiffness and bending strength, respectively, of the tested LFT’s (in this case the two extremes: 12,5 mm glass fibre reinforced PP and 25 mm carbon fibre reinforced PA-12) versus fibre volume fraction are plotted. It is clearly shown that by modifying material combinations and fibre fractions a wide range of stiffnesses can be attained (the shaded area). In this way properties can be tailored to the final desired product properties. Extension of this area is possible by further change of the parameters mentioned.

Note that in figure 5 at higher fibre fractions the strength of the glass-PP material decreases. Too high a fibre fraction does not necessarily result in better properties.

For comparison in the figures also the properties of GMT are given, which clearly shows the position of LFT’s relative to this material.

Apart from possible applications of LFT’s and attainable properties (figure 1: New material combinations) furthermore the research at Lightweight Structures B.V. aims at process control (figure 1: Control): the relation between processing parameters, fibre orientation, fibre length and distribution and product properties.

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References