Cheaper carbon fibre: injection moulding thermoplastic and recycled carbon fibre to make BMW clutch pedal

BMW clutch pedal uses low cost carbon fibre pellet The water injection moulded clutch pedal uses low cost carbon fibre pellets
Carbon fibre remains a premium and expensive material, out of reach for most mainstream automotive manufacturers. However, recycling virgin fibres and mixing them with a thermoplastic offers an affordable alternative.

Over the last decade, the cost of carbon fibre has come down, and the exotic material is finding itself beyond originally foreseen applications within aerospace and defence. Despite this, however, the material is still considered premium, and cars in what might be classed as the ‘middle-income’ bracket, are unlikely to consist of prepregs cured in autoclaves for some time yet.

There are exceptions, but for the vast majority that haven’t or can’t, other options are becoming available. Here, there is substantial innovation and development to tick both the lightweighting and processing boxes.

One solution that combines the use of carbon fibre, but goes some way in trying to democratise its use, comes from engineering thermoplastic specialist, KD Feddersen. It wants to offer the market carbon fibre... but not quite as we know it.

The Staffs based material firm is supplied preprocessed carbon fibre off cuts and unused fibres, which it then repurposes. These virgin dry fibres are conditioned and chopped in a proprietary process, where they are then combined and compounded with a variety of possible thermoplastics. The result is carbon fibre loaded pellets that can be dropped in to standard injection moulding machines.

“It’s a lower cost alternative for those that want to lightweight, don’t want to go to glass or want improvement from glass, but don’t want the cost of virgin carbon fibres,” says Ian Nott, managing director of KD Feddersen UK. “It answers many of the lightweighting questions and challenges engineers have.”

Unlike traditional carbon fibre manufacture, the material is not woven, laid up, and does not use a thermoset resin that needs to be cured. Instead, the short fibres are combined with any number of thermoplastics such as Nylon, Polyketone, and Polypropylene.

It has led to a project to make a clutch pedal out of the material to see how effective it can be at replacing parts made from metal, glass fibre, or even engineering plastics. The clutch pedal was made using a AKROMID B3 ICF 15, the company’s Nylon 6 product, which has been loaded with 15% by weight of chopped carbon fibre.

Pellets of the material are then fed in to a moulding machine that uses a water-injection process to hollow out the centre, and save further weight. The moulded component is nearly 10% lighter and 25% stiffer than the same component made using a 30% glass fibre reinforced PA6.

Martyn Jocelyn, a technical sales manager at KD Feddersen, says: “This is offering a lot of the benefits of carbon fibre in terms of lower density and stiffness, but it is also giving improved cycle times and throughput.”

The superior stiffness of carbon fibre means, as a rule of thumb, half the fibres by volume need to be used compared to glass. For automotive applications, like the clutch pedal, the stand out product is indeed the B3 ICF 15. Though the cost of the material is more than a glass fibre equivalent, it is not as simple as a straight like-for-like comparison.

“It’s not just about the cost per gram of carbon vs glass,” says Jocelyn. “It’s a stiffer material so you can reduce the wall section and therefore you use less material. With a thinner wall section you can run it faster in the machine, so you reduce cycle time and reduce production costs. So you claw back some of that extra cost.”

An ENGEL moulding machine was used to produce a carbon fibre reinforced thermoplastic pedal

The cost of carbon fibre has been a prohibitive factor for years. At more than £15 a kg and some of the most labour intensive processes needed to manufacture it, the material is still out of reach for many.

However, pellets of repurposed chopped fibres combined with thermoplastic sit at around a third of that price. But, it is not just a straight cost benefit, but the elimination of multiple processing stages including weaving, laying up, machining moulds, wetting out, and curing. Feddersen’s ICF pellets by comparison can be used in a normal moulding machine as a straight drop-in replacement. The material is starting to build a cumulative case of advantages for automotive manufacturers.

“We would be able to come along with our material and put it in the same machine that’s making glass parts, and give you a weight saving,” says Jocelyn.

The materials currently being distributed by KD Feddersen UK have carbon fibre contents between 10% and 40%. And the results are impressive with the numbers speaking for themselves. For example, the AKROMID B ICF 40, a PA 6-based compound with a 40% loading of carbon fibre, achieves a tensile modulus of 30,000MPa, a flexural strength of 320MPa and has a breaking elongation of 1.8%. The AKROMID A3 ICF 40, a PA 66 based derivative compound achieves an even higher 33,000MPa and flexural strength of 370MPa.

The automotive industry is having to go beyond its comfortable cost envelope and is increasingly willing to pay more for lightweight material, especially if there are cost benefits to be gained elsewhere during manufacture. Other components being targeted include centre consoles, activated charcoal filters and control unit brackets.

The automotive sector at large is being pushed on all fronts and it is beginning to yield some impressive results. While this material alone is unlikely to lead to the holy grail of lightweighting – a low cost carbon fibre body in white – it certainly goes someway to enabling injection moulded parts to be made lighter.

“Our material isn’t going to compete against laid up carbon fibre structures because of the crash testing and the impact performance that’s needed,” says Jocelyn. “That is not really the target area. Where there is already an injection mouldable part and glass filled nylons, we can demonstrate a weight saving and an increase in stiffness.”

Author
Justin Cunningham

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