Long fiber reinforced thermoplastics

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In the specialist literature, the terms short, long and continuous fibers are very often differentiated from one another. Depending on the definition, one speaks of long-fiber- reinforced thermoplastics (LFT) if the reinforcing fibers (e.g. glass fiber , carbon fiber , aramid fiber ) in the finished component are at least several millimeters long .

There is no upper limit for the length of the fibers, for example in the case of fiber layers or fiber braids one speaks of a quasi-infinite fiber length.

The advantages of components made of LFT compared to fiber-reinforced thermosets are higher toughness (tougher breakage behavior), lower emissions during processing, virtually unlimited shelf life (without cooling), lower cycle times and better recyclability.

Disadvantages are the reduced compressive strength and rigidity , the surface quality and the problem that thermoplastics tend to creep, especially under long-term loads and elevated temperatures.

Compared to short-fiber reinforced thermoplastics, LFTs have a higher impact strength and the mentioned tendency to creep is much lower, which enables this type of fiber-reinforced plastics to be used with increased quality requirements (such as in the interior area in the automotive industry, e.g. instrument panels or door panels).

Manufacturing

The processes that are used to manufacture components from LFT can be divided into two groups with regard to their processing, compression processes and injection molding processes. Within these groups, a further distinction can be made between processes based on semi-finished products and direct processes.

LFT processors who use semi-finished products from third-party manufacturers must heat them up before processing in order to achieve adequate adhesion to the matrix material. While glass mat reinforced thermoplastics (GMT) are only processed by pressing, stick granules are used both in pressing and (and above all) in injection molding. Direct processes, in which the manufacturer compounds the molding compound himself, are becoming more and more popular. Especially because, in addition to the possibility of determining the additives and the fiber content of the compound itself, this results in considerable cost savings compared to the use of semi-finished products. In addition, through direct processes, longer fiber lengths on average can be achieved in the finished component, which is accompanied by an improvement in the mechanical properties.

Semi-finished product based LFT

The semi-finished products can either be in the form of sheets or as long-fiber-reinforced granules (rod granules , abbreviation LFT-G or LFG). In both cases, fibers are incorporated into the polymer matrix either by extrusion or pultrusion . The plates are manufactured using special variants of the extrusion process. To produce the granulate, the extruded strand is chopped into pellets approx. 8-25 mm long .

LFT in the direct process

In the direct process for the production of long-fiber-reinforced, thermoplastic components, the use of energy and cost-intensive intermediate stages (semi-finished products) is dispensed with. The molding compound, which is to be processed into the finished component by pressing or injection molding, is produced immediately before it is processed. The fiber content, the additives and the matrix material can be set variably, so that the material can be optimally adapted to its later application.

In the direct process, a distinction can be made between single-machine and two-machine technology. With two-machine technology, two extruders connected in series are used to produce the molding compound. The first is responsible for compounding the polymer, while the second extruder is responsible for incorporating the fibers (drawing, impregnating, consolidating) into the melt. Here the two processes can be optimally adapted (better than with single-machine technology) to their respective task. This enables more flexible process management. With the direct process, it is also possible to reuse the waste that arises (in-house recycling). With an additional recycled extruder, this can even be achieved while maintaining the fiber length.

With one-machine technology, the steps mentioned must all take place on one machine. So there is a need to use polymer compounds that have to be stabilized for both work steps and the use phase. In order to ensure process stability, with only one extruder a compromise has to be found between optimal incorporation of the fibers and optimal compounding.

processing

The semi-finished products or the extrudate produced in a direct process are processed either by pressing or by injection molding. In principle, both processing methods can be used for all products, with different advantages and disadvantages. The advantages of the pressing process lie in a shorter cycle time, longer fiber lengths and thus improved mechanical properties. A major disadvantage is that pressing always creates a press burr, which means that post-processing is always necessary. The advantages of injection molding are that insert parts can be incorporated directly during injection molding, better surface quality can be achieved and that there is greater design freedom for the product (more complex cavity in contrast to pressing, which mainly uses flat components). The disadvantage is that a longer cycle time and poorer mechanical properties than when pressing have to be accepted.

Manufacturer

  • FACT: FACTOR®
  • RTP Company
  • SABIC : SABIC® STAMAX
  • TechnoCompound
  • Ticona : Celstran®, Compel®
  • PolyOne : OnForce®

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