Metal replacement simplified

The long carbon fibres form a fibre skeleton inside the component, thus improving the thermo-mechanical property values of the material.

Ems-Grivory first entered the metal replacement market in 1991, with the introduction of Grivory GV. The company particularly highlights its long fibre reinforced thermoplastic materials (LFT) which are said to stand out from the range, asserting that they offer strength values similar to those of metal but at much lower weight. The reinforcing fibres, which are up to 10mm long, form a skeleton inside the component, providing sustainably improved thermo-mechanical properties without affecting the PA’s resistance to chemicals or surface quality. Ems-Grivory claims that LFT materials exhibit mechanical property values even beyond the glass transition point and, further, that the fibre skeleton ensures improved energy absorption, higher heat deformation temperatures, improved behaviour at high and low temperatures, as well as less warpage. The company says that up to 10% fewer fibres can be used to achieve property values comparable to those of compound materials – a density advantage of around 9%. Long glass-fibre reinforced Grilamid LCL-3 offers a claimed density of 1.15g/cm³ with a strength value of 250MPa.

Major weight savings

These advantages direct LFT products towards use in challenging metal replacement applications in automotive construction, where weight and cost savings are paramount. For example, the Audi A8’s rear seatback is made of Grilon TSGL-50/4 reinforced with 50% long glass fibres, which offers enormously high stiffness values, even at high temperatures, and ensures high toughness in collisions, and thus provides elevated levels of protection for passengers in the event of side impact. It also has a low tendency to warp round off the material profile.

Audi A8 rear seatback made of Grilon TSGL-50/4

Tensile stress at break

Through new developments in the field of long fibre technology, Ems-Grivory maintains that it has been able to extend and increase the properties relevant for metal replacement and to extend its applications into new fields. With newly developed glass-fibre rovings it developed an injection-moulding material (Grivory GVL-6H HP) with no carbon fibre content but which exceeded the “sound barrier” value of 300MPa for tensile stress at break (tested to ISO 527). The new rovings have a different kind of geometry and, therefore, a different length-to-thickness ratio – both in 10mm granules as in the injection moulded finished component. This enables improved surface quality, along with extraordinary mechanical properties, says Ems.

Elimination of weak points

Ems Tape Technology (ETT) offers another new option for lightweight design. It involves oriented endless fibres that are completely impregnated and delivered in the shape of tapes (bands). With their help a component’s weld line strength, for example, can be massively increased. In cooperation with the HSR Advanced Technical College at Rapperswil, Switzerland, a technology has been developed for introduction of these tapes into components. Tests carried out on demonstration parts developed specifically for this purpose have generated key data. Examination of these findings show that areas around weld lines have up to twice the strength of an unreinforced weld, when a single tape is applied. The bending strength values achieved were practically independent of the structure of the basic component, with more or less orientation of the fibres or even when weakened by a weld line. Other properties such as creep, long-term alternating or pulsating fatigue strength were also improved.

Ems-Grivory says that these tapes can be inserted into those areas of a component where the highest mechanical stressing occurs. With this local reinforcement, a less high-performance basic material can be used, thus allowing costs to be further reduced.

Improved handling of thermosets

One well-known problem in the manufacture of thermosetting composite components is the shifting of fibres inside the component mould or during preform manufacture. If the fibres do not lie at exactly the intended position, loss of stability results. Binding agents are used to prevent this. Ems-Griltech’s Griltex CE is a range of polymers specifically developed for easier handling. It can be used for an epoxy matrix and with materials such as two-component polyurethane, phenol or cyanic ester resins. Griltex CE stabilises fibre structures or tapes and prevents shifting of the fibres in resin transfer moulding (RTM) processes. The product does not need a curing time and it is possible that several fibre layers can be bonded onto each other and processed in only one step. The company claims that it also improves mechanical properties of the composite component. It exhibits maximum compatibility with epoxy resins, thus creating a strong bond. Residual fracture strength values in compression after impact tests (CAI) increase by up to 65% and the fracture surface is smaller. Shear strength measured in interlaminate shear strength tests also increases by 33%. Composite fibre-reinforced sheets can be pre-manufactured using Griltex CT and fibre material.