Pioneering plastics

The blueprint for a 'unique' lightweight commuter bicycle presented a major challenge to a specialist injection-moulding firm.

A new slant on the lightweight commuter bicycle has been developed by a UK company using injection-moulded, glass fibre reinforced plastic parts.

Although the design of the bicycle is based on a frame manufactured from magnesium, a specially formulated nylon filled with long glass fibres is used for crucial mechanical parts like the rear suspension unit. The expertise to injection mould this lightweight yet strong material, which is 60 per cent glass, was provided by UK-based Protomold.

For the Shropshire company it was a challenging project. According to John Tumelty, managing director of Protomold, it wasn't that the moulding process was unusual, it was the materials they were using. The company's senior engineers had never previously worked with a material incorporating such high glass content. For this reason, the engineering team was uncertain how well the material would flow and pervade the cavity within the injection-moulding tool.

"With the GoCycle parts we were really pushing the boundaries of what is possible with plastic, and therefore the materials specified were, in the main, exotic thermoplastics," explains Tumelty.

"A very dominant material in the range is 60 per cent long-fibre glass-filled nylon, which is pretty extreme in terms of the glass content, coupled with the fact that it's long fibre. During the moulding process, those fibres have a tendency to align with flow direction. The way the fibres are aligned affects the mechanical properties of the end part," he adds.

Working closely with Karbon Kinetics, the bike's UK designer, Protomold scrutinised CAD geometry of all the components that required moulding. The information was entered into Protomold's inhouse flow analysis software, Protoflow, so that a visual prognosis of the injection moulding process could be established.

The company's engineering team then tested different location points for where the plastic enters the cavity (known as gates), and analysed the various pressure and temperature conditions required to achieve successful component moulding. At the end of the project Protomold had developed a total of 47 individual injection-moulding tools, which were used to manufacture the required parts.

"It was quite entertaining," says Tumelty of the work. "Our mould technicians here have hundreds and hundreds of years experience between them yet hadn't worked on anything like this, so it was interesting work.

"We knew we had incredibly short lead times, which also added to the challenge. On a simple, run-of-the-mill plastic part we can turn that around in 24 hours, but with the GoCycle components we had identified that we were going to have engineering challenges ahead. With that in mind, we pulled out all the stops so that we had more time in the mould shop to experiment and play with the moulding parameters.

"We took the viewpoint that there was a solution there, and we had to find it. We couldn't read about it in a book because no one had done it before. So we worked faster in the run-up stages, to create time to find the solution," Tumelty says.

The engineers feel they have learnt a lot from working with this plastic's features and also on a product that's pushed boundaries of its own in regards to its propulsion system, hub-based electric motor and lightweight magnesium frame.

"Various parts, from the suspension system to the sprockets, are made out of plastic in a variety of different odd grades," says Tumelty. "The employees have not only been able to learn about the different plastics, but been able to see their effort come to something new in the marketplace."

Aesthetically pleasing

On top of achieving mechanical properties, the engineers also had cosmetic requirements to achieve. Alongside GoCycle's durability ethos, it was also intended to be fashionable and lustrous. With this in mind, the majority of parts feature a shiny black specification, meaning Protomold engineers had to determine the precise velocity at which to inject the material into the cavity to ensure consistency of colour.

"As with most things, the more mechanical property-orientated a material becomes, often the less aesthetically pleasing it becomes," says Tumelty. "Glass fibres can have a tendency to show up on the surface of a moulded part, which on a black plastic will give a silvering effect. In layman's terms, you're looking at the black plastic through a fibrous glass layer. Obviously that's not very attractive.

"There's also quite a lot of effort and experimentation that went into the parameters of moulding the part in order to not only attain the required mechanical properties, but also to get the cosmetics to an acceptable level."

Protomold says it enjoys a challenge and has found, as a company, that it is working in the more extreme end of the marketplace, with components going into vastly different end products from Euro fighters through to desktop calculators.

"We're all engineers, and engineers like the challenge of achieving something that's not been done before and coming up with new concepts and ways of gating and thinking about things differently," says Tumelty.

"We've been able to apply our work with GoCycle to other customers who've come up with other bizarre materials. One thing that's surprised me slightly since starting Protomould is the sheer number of different plastics that we're asked to mould. We've got 800 different resins on stock now, and it's growing every day. People generally aren't prototyping a simple component out of [the thermoplastic] ABS [acrylonitrile butadiene styrene]".

Protomold has learnt a lot from working on the GoCycle project, and has benefited greatly from the challenge it has overcome. The company now has moulding parts in place for further manufacturing runs for the bike, and also has the knowledge to provide 'exotic' services to other consumers with specific, unusual demands.

"The reason they want to make a prototype moulding is because they don't quite understand how the material is going to behave or how their product is going to behave. Customers often have some extreme requirement that forces us to push the boundaries.

"Plastic technology and the material science behind plastic is evolving constantly. Very high temperature plastics are available now and our challenge is adapting them and making them mouldable through aluminium tools on a high-speed basis - exactly what we've geared our company up to do."

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