By David Reed, UT EditorMunich-Krauss-Maffei Kunststofftechnik GmbH, Munich, has adapted its long-fibre injection (LFI) process so that fibres based on flax, hemp or sisal can now also be used for reinforcement.Parts made using the new process-dubbed NFI for Natural Fibre Injection-can be cheaper and lighter than those reinforced using glass fibres, while retaining otherwise comparable performance, Krauss-Maffei indicated in a 26 Feb press announcement.Natural-fibres do not snap in the same way as those based on glass, so a special cutter is required, the firm explains. A specially developed arrangement of two intermeshing gear wheels both moves the roving along and cuts the natural fibres to length (at lengths of either 5, 10, 15 or 20 millimetres), as required by the parts involved.The cut fibres are then delivered into the centre of the mixed polyurethane stream from the high-pressure NFI mixhead and the air thus introduced expands and ensures that the mix is finely atomised and so provides effective wetting of the natural fibres. The resultant mixture is poured into the open mould.For more effective covering of surfaces, the processing unit’s jet stream can also be made to oscillate.Krauss-Maffei presently offers two LFI/NFI-mixing heads, one with an output from 80 to 300 g/s (LFI/NFI-22/28-MK8), the other running from 250 to 500 g/s (LFI/NFI-30/36-MK12); the natural fibre content can be as high as about 40 percent. For a component weighing about 4000g, the NFI pouring time is about 20s, giving a cycle time per mould of two to four minutes. Parts with a density of 700-1100 kg/mm2 and 3-4 mm thick, have tensile strengths of 32 to 38 N/mm2, elastic modulus 3000-4500 N/mm2, and impact strength of 10-30 kJ/m2. The modular design employed by Krauss-Maffei allows processors to transition from LFI to NFI at any time, the firm says.”
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