Sewing carbon

Materials World magazine
,
2 Jan 2019

Advances in the tailored fibre placement process have reinvigorated interest in the technique, which could inspire widespread adoption of carbon composites in the mainstream automotive industry and beyond. Richard Harrington investigates the latest refinements.

The strength and lightweighting benefits of carbon fibre have been demonstrable for over 50 years, since it was first widely used in military and aerospace industries. Use of the material is particularly fashionable in the automotive industry that, in the face of increasingly stringent regulations, sees lightweighting as a crucial enabler to reduce emissions of new vehicles – less weight equals greater efficiency and improved fuel economy.

Cost has traditionally been seen as a significant barrier to entry for vehicle manufacturers, upon which unit costs weigh heavily. This has ensured that carbon composites have principally remained the domain of high-end, high-value manufacturers, or used in small quantities as aesthetic enhancements to help fill the lengthy options lists adorning glossy new vehicle brochures. In addition, carbon composites can be difficult to recycle at end-of-life, an ever more important requirement for vehicle manufacturers.

Tailored fibre placement (TFP) has been heralded as a technique that can reduce the cost of carbon composite manufacture to a more accessible level. Developed within the embroidery industry, TFP stitches carbon fibre onto a base layer in a net shape, rather than the more traditional laminate method of assembling multiple woven layers of carbon fibre cut from large sheets. The cost advantages include very low wastage of the raw material, relatively low capital start-up costs, and automated, scalable production. Design benefits are also numerous, such as the ability to accurately place fibres for maximum strength and clever fibre laying to allow the creation of complex 3D shapes that conform to the applied stresses on the part.

TFP refinement

Julius Sobizack, Managing Director of German TFP machine manufacturer ZSK, said, ‘TFP is an established technology that has existed for over 20 years, but we believe that recent advancements make it a viable manufacturing technique that brings carbon composites – and their benefits – within reach of a larger audience.’ The company has made technical revisions centred on increasing TFP productivity levels, focusing on enhancing speed, reliability and scalability. This could make the process suitable for semi-mainstream automotive series production and transference to aerospace and defence.

‘After research and development, we have patented a process called fast fibre laying,’ explained Sobizack. ‘This is a variable stitching technique that alters the way carbon fibre rovings are deposited. The premise is to allow the sewing of intermediate layers of thread with a low number of stitches – essentially acting as anchoring points at changes of direction – before the top layer is stitched more securely, holding the material in place. This enables the optimisation of thread orientation and provides a reduction in machine time as every layer is not required to be stitched through to the base at regular intervals.’

TFP is carried out using specific industrial embroidery machines, which are able to provide precise control over stitch frequency, placement and speed. For example, latest generation machine heads are able to vary speed, depending on requirements – speeding up when rovings are being laid in simple sequences, or slowed for intricate precision and repeatability. The automated process also reduces the risk of human error, providing exact control over footpad pressure and stitch spacing.

An evolution of the hardware and software that supports TFP manufacture offers further productivity improvement. For example, connectivity and control systems remove the reliance on manually switching threads or changing bobbins for different tasks, and jobs can be controlled, begun or ceased remotely.

Flexibility can also be improved by adopting a twin-head system that allows for two rovings to be laid in parallel on the same part. This is the ability to lay two different materials at the same time, or interchangeably. Practical applications for this capability include carbon fibre alongside a second reinforcing material, such as glass fibre, to help further reduce unnecessary costs by eliminating the use of expensive materials where they are not essential. TFP also reduces waste by producing only what is required, rather than cutting a shape from a much larger piece, the rest of which finds its way either to landfill or a complex, expensive and inefficient recycling process.

‘Thinking laterally, a further use for the twin-head system is the stitching in of an embedded component,’ said ZSK Technical Embroidery Manager, Melanie Hoerr. ‘Electrical wiring, heating elements, strain gauges or antennae can all be stitched. Complex wiring can be laid using conductive thread. For automotive applications in particular, the potential for use as part of the vehicle interior of the future is considerable. Hidden embroidered switchgear in the dashboard, seat controls lost in the leather of an adjustable bolster and heated elements stitched through the seat’s underside to prevent the irritating circuit failure that is so typical of current systems. It’s an exciting time. Using previous single-head machines, similar projects would require sequential laying of fibres, which could more than double process time,’ she added.

Early adoption and automotive application

In terms of automotive applications, TFP is still in its infancy. However, while it’s not the mass-produced C-segment vehicle that the technique could soon be benefitting, advancements in machines and technology have already enabled the use of carbon composites for a vehicle where traditional methods had previously rendered the material unsuitable.

Elemental Motor Company sought to extend the use of carbon composites in its track-focused RP1 road car. Elemental Composites Manager Peter Kent said the company had discounted carbon composites for this component, as it forms an integral part of the rear structure of the car, on a cost and complexity basis. And that using TFP, the company was able to implement carbon composites into the structure of the car.

‘There are heavy costs to adding lightness,’ he said. ‘We found the material could be quickly produced and was extremely durable. The component in the RP1 is directly in the line of fire from road debris and easily withstands everything that is thrown at it. With a complex 3D structure, including compound curves, it is completely at odds with the traditional image of a carbon composite component.’

Peter McCool, Managing Director of SHAPE Group, which manufactured the relevant parts for the RP1, explained, ‘Carbon fibre is a wonderful material that is hampered by traditionally very limiting properties. High unit cost, both in terms of design time and financial investment, brittle properties and a lack of durability – carbon composites have always been an expensive experiment to get wrong.

‘To combat the inherent brittleness of the material, traditional production methods are labour intensive with a high probability of over-engineering a solution. Hand laying works around the material’s weaknesses to a degree, but TFP enables the precise laying of directional fibres for optimal load-bearing strength. This reduces waste and accelerates development and manufacturing time, but results in a far better component. The RP1 rear bodywork is a vital structural part that easily withstands severe forces.’

As a demonstration of TFP’s potential in the real world, the RP1’s tale is compelling.  Powertrain development and alternative forms of motive power, such as electrification, is key to the emissions challenge faced by the automotive industry, but TFP as part of a lightweighting strategy has competitive advantage-providing potential, especially if the emissions goalposts continue to move and manufacturers seek every incremental advantage possible.

TFP’s wider industrial use

Individual preforms can be produced in TFP up to 2.0x1.8m, but within those size boundaries are almost limitless possibilities in terms of geometries and intended application. The compound structure of the material lends itself to wider use, due to a high level of impact resistance achieved by using thermoplastic resins, as opposed to thermoset resins found in traditional carbon fibre.

‘To prevent ageing, thermoset resins are normally refrigerated, whereas thermoplastics can be stored at room temperature,’ explained Hoerr. ‘Not only are thermoplastics easier to handle in terms of storage and management, they can be up to 10 times more shock absorbent and are easy to recycle. Heating a thermoplastic-based component to around 250–300° will generally lead to fibre separation, whereas thermoset resins are more difficult to dislodge. There’s also the added bonus that recycled carbon fibre can be used as the base material for the TFP process, further reducing the environmental impact and increasing the process’s efficiency.’

This inherent component strength underlines the material’s suitability for the production of lightweight, robust parts, including suspension components, body mountings, structures and panels, brackets or gears. Most significantly, this is not limited to the automotive industry.

‘An industry that benefits from reduced weight and increased durability can profit from the TFP manufacturing processes,’ concludes Hoerr. ‘Studies have shown that a 10% weight reduction can result in a 6–8% improvement in automotive fuel economy, but the savings are even more remarkable for aerospace. For example, a major airline operator has stated that every kilogram taken from its fleet saves the company US$20,000 per year. As air travel is a mode of transport with significant emissions, literally the whole world can benefit from improving efficiency through reduced weight.’