Benefits and drawbacks of Pultrusion

Pultrusion is much less common as a plastic-processing method than its more familiar relative, extrusion.

The processes are similar in that they allow continuous lengths of a set and an unchanging profile to be formed, but one of the main differences between them is that extrusion can be used for aluminium, wood-based composites and thermoplastics, while pultrusion is used in the forming of composites that use long strands of fibre as reinforcement.

As the name suggests, the process is based on pulling the blended materials of the composite through a heated die. This differs from extrusion, which is based on pushing the material. The continuous lengths of reinforcing fibres, which can be made from glass or carbon, are saturated with a liquid resin mixture as they are pulled through the die, which, besides shaping the component, also acts to cure the resin as it is heated. Sometimes, pre-impregnated ('pre-preg') fibres are used, removing the need for a resin bath.

Manufacturers of plastics have, in recent years, experimented with many applications that traditionally used metals, and pultrusion is a typical example of the benefits such experimentation can bring. Pultruded plastics display an increased range of physical properties that can benefit both engineering and design applications, because they offer the toughness of metals with the advantages of low weight and corrosion-resistance. Pultrusions are incredibly dense, hard and rigid sections – they even 'clank' like pieces of metal when you knock them!

Volumes of production
Depends on the size and complexity of the shape. 500 metres is a typical minimum run.

Unit price vs capital investment
The cost is lower than that for some moulding processes such as injection and compression, but higher than for, hand lay-up moulding.

Speed
Depends on size, but as a rule of thumb it is possible to achieve 0.5 metre per minute for a profile measuring 50 by 50 millimetres, 0.1 metre per minute for chunky shapes and 1 metre per minute for narrower sections.

Surface
The surface finish can be controlled to a degree, depending on the reinforcement and polymer.

Types/complexity of shape
There are no problems with undercuts in pultrusion. Virtually any type of shape that can be squeezed through the die can be made, bearing in mind that the shape must have a constant thickness.

Scale
The maximum size for profiles is typically 1.2 metres wide, although there are specialist machines that make larger components. Minimum wall thickness is approximately 2.3 millimetres. The size of the manufacturing plant dictates the limit to the length of the pultrusion.

Tolerances
Vary depending on the profile, but on a standard box-section, with a wall thickness of 4.99 millimetres, the tolerance is ±0.35 millimetres.

Relevant materials
Any thermoset polymer matrix that can be used with glass and carbon fibre.

Typical products
Applications for pultrusions are varied and include permanent and temporary structural components for industrial plants, vandal-resistant indoor and outdoor public furniture, and funfair and exhibition stands. Smaller-scale applications include electrically insulated ladders, ski poles, racquet handles, fishing rods and bicycle frames. Perhaps surprisingly, pultruded plastics have a resonance similar to certain woods, which has led to them being used as replacements for hardwood frames for xylophones.

Similar methods
Extrusion and Pulshaping.

Sustainability issues
Parts can be produced with thin wall thicknesses as a result of the fibre reinforcement, which minimises material use without compromising strength. However, as the process is entirely automated and heat intensive, energy use can be quite high in relation to the fairly slow cycle speed. The combination of materials makes the composites non-recyclable.

Courtesy of Making it manufacturing techniques for product design (2nd Edition) By Chris Lefteri, Laurence King Publishing

Author
Chris Lefteri, Laurence King Publishing

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