The Elemental RP1 roadster uses carbon-fiber wheel arches fashioned from TFP carbon fiber. (Elemental)
Linking embroidery and carbon fiber to save weight and cost
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The story of carbon fiber’s use by the automotive industry in medium- to high-volume manufacturing has for decades moved through chapters of high expectation and low likelihood. Carbon-fiber still remains–mainly for cost reasons—essentially a super-premium material.
But now, a carbon-fiber application called Tailored Fiber Placement (TFP) has emerged—via the unlikely technology route of the embroidery-machine industry.
The TFP material is not for ornamental embroidery of seats, dashboards or door cards, but for series-production components such as a gearbox casings and pedal box or engine ancillary brackets, saving weight and adding strength without cost penalties.
Julius Sobizack, CEO of ZSK, a TFP forming and embroidery machine manufacturer near Dusseldorf, Germany, explained, “TFP allows complex 3D shapes to be created from a 2D preform in a quick and consistent way, with a lower cost structure. It unlocks significant design and engineering potential, with dramatically reduced carbon fiber wastage and facilitates the combined use of the material with thermoplastics—in a single part.”
He is confident that TFP using technical embroidery has ever wider potential.
Carbon fiber, stitch by stitch
Instead of weaving all the fibers into a perpendicular arrangement then cutting to the required shape, TFP allows the functional fibers to be arranged in “bundles” exactly where they are most needed for structural performance and stitched into position on a compatible textile or polymer base layer. Production is entirely scalable through the addition of more multi-head laying and embroidery machines, ZSK said.
Melanie Hoerr, ZSK Technical Department Manager, said: “The benefits of being able to lay the fibers in the direction of load—instead of cutting across the fibers of a pre-woven sheet—have been clearly demonstrated in tests on components with holes: TFP allows the fiber to be laid around the hole.”
Under tensile testing, instead of failing at the opening, the part fails as though no hole was present and withstands up to 50% higher load before failing.
Hoerr added that carbon fiber roving can be tailored exactly to a requirement: “For a composite, we know the direction of load. This is important, because carbon fiber can take the most load in the fiber direction. With standard technology using non-crimp fabric, you are limited to specific degrees: 90o to 45o or minus 45o”
Using tailored fiber placement, there is an “endless” roll and the fiber can be laid precisely as needed, shaping a part exactly to how it is required to look as a finished item.
“With the embroidery machine, we lay the carbon-fiber roving on a base fabric and use zig-zag stitching every 5 mm to 8 mm to place it. So it is flexible and can be folded or pressed out in a heated mold later to create a 3D shape, the design capability being integrated into the fiber placement.”
Improved performance, reduced waste
Most composites begin as a sheet of woven fabric which is cut into pre-forms, laid up in a mold and impregnated with resin. This condemns all the fibers to be arranged in two directions, at 0o and 90o, and to take the form of a rectangle. For many applications there is huge wastage, and the action of cutting to shape means that the remaining fibers are often not arranged in the best way to carry the loads present. This leads to the addition of further layers, with their fibers in the preferred direction, making the finished part heavier, less efficient and more wasteful—all increasing cost.
A key advantage of TFP is that, through selective stitching, it provides absolute freedom of positioning, ensuring that the fibers do not move during processing yet still permitting the pre-form to be folded where required. This means that a complex carbon composite 3D component can be produced economically and consistently, with short cycle times. The ability to vary the stitching properties locally means the preform can be stretched, bent or folded without wrinkling. Furthermore, fiber wastage is only 1-2% of the total instead of an average 20% - 30% depending on the part.
Although a special embroidery machine is used for carbon fiber laying, it is multi-role, able immediately to work with another material such as glass fiber. In this fashion, carbon fiber can be used where strength is needed, glass fiber—which is cheaper and has good recycling potential—for lower or non-stressed areas.
The CNC-controlled TFP machines are available with multiple heads; in the time it takes to make one preform, an 8-head machine can make eight preforms. Large multi-head TFP machines often can be installed for less than the cost of a typical automotive sheetmetal forming tool, said Hoerr: “One ZSK head can lay between 1 and 3 kilograms of preform per hour and can handle two rovings of up to 60,000 fibers each.”
TFP itself is not a new process. It has been used in industry—including automotive—to help facilitate such processes as placing wiring in heated vehicle seats and antennae for RFID components. It also has been used in the aerospace industry for CFRP parts.
In the UK, the Shape Group, manufacturer of tooling and high-carbon composite parts to the transport industry, has used ZSK’s embroidery machines to develop an automatable method of manufacturing fiber preforms combined with thermoplastic fibers. CEO Peter McCool, who formerly held senior positions with Super Aguri F1, Lola Cars and Amlin Aguri Formula E, explained that a commingled preform comprised of thermoplastic filaments and carbon rovings is easily consolidated into a net shape part using a heated press. “The ability to align and orientate fibers for required strength and stiffness with zero waste will enable TFP to become the dominant carbon fiber manufacturing process for optimized preforms. Combining multiple materials in a single preform and thermoplastic matrix offers so many advantages and opens new opportunities for component design,” McCool said.
He regards TFP as a new material in its own right: “The thermoplastic matrix means the finished component has new levels of toughness against shock and impact compared to thermoset resin composites.”
Shape has produced an example of this—a TFP wheelarch liner fitted to the ultra-low-volume Elemental RP1 sportscar that combines track capability with road legality. Said McCool: “The TFP component is very light, yet tough enough to resist stone and grit impact and acts as a structural component. The preform is very close to the final component shape and the complex curves and corners are readily formed under low pressure.”
At ZSK, Sobizack stated that with TFP, the improvement in production rates and consistency are equally marked, helping composite manufacturing to compete more effectively with sheetmetal. “Startup costs should not be seen as a barrier. With scalability using multi-head machines and no requirement for complex cutting table technology, TFP is a cost-effective way into the composites market.”
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