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    The Long and the Short of Fibre Reinforcement of Thermoplastics

    Fibre Reinforcement. The advantages of adding glass fibre to thermoplastics to increase stiffness (modulus), strength, heat distortion resistance and dimensional stability are well known.

    LFRT automotive 2

    Because of the method of compounding, conventional glass fibre filled grades have fibre lengths of the order of 1 – 3 mm. That’s a bit too short to spread applied loads effectively between fibres to maximise strength potentials.

    Increasingly moulders are now being offered Long Fibre Reinforced Thermoplastics (LFRT). These are based on a range of thermoplastics (polypropylene, polyamides, polyesters, polyphenylene sulphide and even PEEK).  The glass fibres are the same length as the pellets, typically 12 – 25 mm.

    Despite some attrition in fibre length during the moulding process, the mouldings show a significant increase in tensile and flexural strength. They also show a dramatic improvement (x 5) in impact resistance, compared to short fibre grades at the same glass content.

    Stiffness, purely a function of volume loading, will not be much different. However, the long fibre grades exhibit reduced shrinkage, minimal warping and better dimensional stability.

    Adding all these benefits together can compensate for the price premium associated with long fibre grades.   The price-to-properties advantages permit reduced wall thickness, weight reduction and low ‘per piece costs’. This is a combination ideal for metal replacement applications.

    Typical LFRT applications include propellers, robotic gearboxes, wind turbine blades and automotive consoles. However, LFRT is also suitable for more humble and less technical items such as a broom handle.

    Designers should note that, although stiffness increases over the range of glass content (20 – 60 % by weight), strength and impact benefits can be marginal above 40% loadings. This is because there is more attrition of fibre length during the moulding process at higher glass loadings.

    Moulders need to be aware of processing problems. These include the longer pellets bridging in the feed hopper and the importance of minimising shear by using low compression screws, well designed check rings, generous runners and gates and no sharp corners.  Interestingly long fibre grades cause less wear on the screws and cylinders because there are fewer fibre ends.

    You may wish to view a glossary of electrical and some plastic terms here: .

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    Coping with weld lines

    In injection moulding, weld lines (knit lines) form when two melt fronts meet. If the melt fronts do not coalesce completely, at best there will be a cosmetic flaw. At worst there will be a mechanical weak-spot, with strengths of the order of 10 - 90 % of the material potential.

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    What determines friction between thermoplastic components?

    Friction is an important property for thermoplastics in bearings and gears but also has a part to play in assembly of plastic parts (snap-fit and interference-fit) and ejection during moulding.

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    Polypropylene: the Workhorse of the Plastics Industry

    Polypropylene. Little did Karl Zeigler or Giulio Natta realise, 60 years ago, when they were developing a catalyst system to produce a useful thermoplastic from the inexpensive monomer, propylene, that their work would have such far reaching consequences.

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    Creepy things can happen with long term loading of thermoplastics.

    Deformation is defined as the change in the shape of a body caused by the application of a force (stress). It is proportional to the stress applied within the elastic limits of the material.

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    What makes medical grade plastics so special?

    Toughness and transparency are important properties for the constituents of intravenous lines.

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    What causes mouldings (and moulders) to be off-colour?

    When mouldings are not the intended colour, the first thing to check is the raw material, particularly the dosing rate, if you are using masterbatch, and the quality of regrind.

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    Do your mouldings suffer from jet lag?

    When explaining the moulding fault commonly known as ‘jetting’ to part-time students from the plastics moulding sector, I usually prefaced my remarks by saying that ‘jetting’ was not a fault that you should see these days. Then one of the students appeared the next week with a classic example of ‘jetting’.

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    Transparent ABS can be a clear winner

    Transparent ABS. Mentioning transparency in the context of ABS moulding materials can raise a few eyebrows. This is because ABS is normally taken to be opaque and indeed the vast majority of grades of ABS are opaque.

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    Understanding the difference between SBS & SEBS thermoplastic elastomers?

    The difference between SBS and SEBS thermoplastic elastomers explained.

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    Is MFR really much help to moulders these days ?

    Often a moulder has to change material grades. One of the first properties to be consulted on the new data sheet is usually the MFR. This is to establish if the new grade has the same melt viscosity performance.

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