Advances in tailored fibre placement technology are making carbon fibre a more viable mainstream option. Cheaper, stronger and far more adaptable, the manufacturing possibilities are vast, as Richard Harrington explains
Adding ‘lightness’ is an effective method for increasing efficiency and enhancing performance. Composites have been a fashionable go-to for achieving lightweighting results, but recent refinements to the tailored fibre placement (TFP) process further broaden the traditional benefits brought through using carbon fibre, including increased strength. Modern TFP also reduces the costs and makes the composite suitable for wider application.
“Beyond a handful of low-volume applications, the automotive industry has found that the relatively low productivity, high costs, material wastage and labour-intensive manufacturing processes have been prohibitive for wider-scale adoption of carbon fibre as a material for large or complex components,” explains Julius Sobizack, managing director at ZSK, the German embroidery machine manufacturer responsible for the innovative refinements made to the TFP process. “TFP has been around since the 1990s, but its benefits are only now being extracted through advancements in the way materials
are laid and understanding of their complex properties.”
In essence, TFP increases the level of automation associated with carbon fibre reinforced polymers (CRFP) manufacture whilst also drastically reducing material wastage. Although TFP was an attractive option at inception, it initially failed to deliver the productivity levels required to become a mainstream technique. Updates to the process promise to solve these concerns and adoption is already becoming more widespread across aerospace, defence, medical, clean energy, smart clothing and sports equipment manufacturers.
TFP offers virtually limitless freedom in terms of applications and brings composites’ benefits within tangible reach. For example, CRFP can be 10 times stronger than steel while weighing a fifth as much. This translates into major economy savings for automotive and aerospace, in particular. “Independent studies show that a 10% weight reduction can result in a 6-8% improvement in fuel economy,” explains Sobizack. “The savings are more marked in aerospace, where according to a major airline operator, every kilo taken from its fleet of aircraft saves the company US$20,000 per year. Of course, these benefits are perfectly aligned with the requirement for ever-reducing emissions.”
One of the most obvious downsides of carbon fibre is cost. Using traditional manufacturing techniques, components can cost 20 times that of an equivalent steel part. It is also unsuitable for complex or load-bearing shapes: the physical properties of carbon fibres are immensely strong only when forces are applied along their length. Due to this, carbon fibre layers are applied at different angles to build up a component’s strength in multiple directions, which is challenging for complex shapes and is extremely labour-intensive. Each layer is cut from sheets of carbon fibre, which is often pre-impregnated with the matrix resin (so-called ‘prepreg’), leading to high wastage of a costly material – rates of anything up to 60% in some cases. TFP directly addresses these concerns.
TFP utilises embroidery-based techniques to manufacture composites. Unlike traditional laminate construction methods, TFP begins with the reinforcement material in its strongest and generally most affordable form: dry fibres. With no plies to prepare before creating the preform, the cutting stage is entirely eliminated. By laying fibres in place and stitching periodically to the base layer, waste materials are reduced to the extent that the material scrap rate on a TFP part is in the region of 1 or 2%.
“One of the main advantages of TFP is that individual fibres can be placed exactly as required, without the need for multiple layers, giving designers almost limitless freedom to optimise a structure based on the forces acting upon it,” notes Melanie Hoerr, ZSK’s technical embroidery manager. “Its level of automation makes TFP entirely repeatable which minimises variation in dimension, density or fibre position, and eliminates human error, ensuring consistent structural performance.
“Using this process, layers of thread can be deposited without stitching them to the base material at regular intervals,” she continues. “To maintain position, layers can be anchored at a few key points, which enables the preform to deform into complex 3D shapes when pressed into a mould, resulting in geometrics that would be almost impossible to replicate using alternative automated methods. This enables carbon fibre to become a cost-effective option for manufacturers designing lightweight, load-bearing components such as suspension components, body elements, mounting brackets, and other structural and semi-structural items that would have traditionally been made from steel or aluminium. In such instances, the weight-saving is considerable.”
Before TFP, it hasn’t always been possible to satisfy a full range of loading conditions with fixed fibre orientations, even through the build-up of multiple layers. Each layer must be individually cut, often from prepreg, which results in considerable waste and is very labour intensive. Prepreg materials also have their own nuances and barriers to entry: they must be kept at a low temperature to prevent the thermoset resin from ageing, and the cutting technology required can be complex and expensive. This limits the scope for mass production and raises the cost of finished products. By comparison, the thermoplastics that are generally used as part of a TFP matrix can be kept at room temperature and have much improved impact resistance when compared to thermoset resins.
One of ZSK’s innovations that has resulted in the evolution of TFP’s accessibility is the introduction of its Fast Fibre Laying technology. This technique allows intermediate layers of thread to be laid down extremely quickly and with minimal stitches, focusing on anchorage and changes of thread orientation. The top layer is then stitched down thoroughly, fixing the layers below. This is much faster than stitching through every layer to the base layer at regular intervals and complements the flexible manufacturing footprint that is inherent to TFP.
The automated process extends to switching between thread or bobbin changes and is highly scalable. Each ZSK machine head can lay between 1 and 3kg of carbon fibre preform per hour and can handle two rovings of up to 60,000 fibres each. Machines with up to eight heads can create components simultaneously to considerably reduce cycle time.
TFP is a simple concept, but a complex one to refine. Designers need to understand the draping simulation (where to add or remove material to ensure that 2D preforms will successfully press into a 3D structure) and where and how stitches should be applied for optimal distortion and intended strength. Due to this complexity, ZSK has established research and training centres both in Europe and Seattle, USA. These centres enable engineers to familiarise themselves with the process, but also to better understand its significant potential.
“TFP principles can be applied to more than just the manufacture of carbon composites,” concludes Sobizack. “For a range of industries, a twin-head embroidery machine, for example, can be used to stitch an embedded component such as electrical wiring, heating elements, strain gauges or antennae. This enables a more widespread utilisation of smart textiles embedded with elements such as RFID components. Even complex wiring systems can be embroidered for applications such as next generation automotive or aerospace interiors; the potential is vast.”