The long and the short of fibre reinforcement of thermoplastics

This month, polymer expert and technical blog author, Dr Charlie Geddes, looks at fibre reinforcement in terms of properties, products and price

The advantages of adding glass fibre to thermoplastics to increase stiffness (modulus), strength, heat distortion resistance and dimensional stability are well known. Because of the method of compounding, conventional glass fibre filled grades have fibre lengths of the order of 1 – 3 mm, 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) 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 and 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 but 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’, a combination ideal for metal replacement applications. Typical LFRT applications include propellers, robotic gearboxes, wind turbine blades and automotive consoles but 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 percent by weight), strength and impact benefits can be marginal above 40 percent loadings because there is more attrition of fibre length during the moulding process at higher glass loadings.

Moulders need to be aware of processing problems such as 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.