Options and opportunities

The proliferation of engineering plastics in to all industrial sectors continues at speed. These days, nothing seems to be off limits and some incredible products have come to market replacing components traditionally made from metals. The automotive market seems to be on the frontline of this materials sea change, with everything from interior panels to structural parts being made from polymers.

For design engineers and material specifiers, the word 'plastic' has taken on new meaning over the last 10 years or so. It's no longer associated with cheap, low grade parts but precision components and impressive physical properties.

Leicester based Nylacast has been beating the drum for this change in attitude and has capitalised on its acceptance as a high-end engineering material. It's seen numerous applications open up that were once dominated by metals.

"It's been a gradual process of getting engineers comfortable with what the materials can do," says Professor Malcolm Fox, research and development manager at Nylacast. "While engineers have always known its potential, we have found some markets are now really opening up.

"Some of the metal replacement projects even surprise us, and we have made a lot of progress in terms of what's possible. We are able to hone the properties of a polymer so it does exactly what we want, and that means doing it better than metals."

An example is the company's wormwheel gear used on an automotive power steering assembly. The part is created using a co-polymer attached to an inner steel hub. This is an important reminder that polymers can do a lot, but they can't do everything.

The steel provides the dimensional stability and strength needed to withstand the torque between the hub and driveshaft. However, the toothed ring has been replaced by a cast grade of co-polymer 6.12, giving the gear self lubricating properties when working alongside the mated steel worm, ensuring the whole component runs smoothly throughout its lifetime. Other specific properties include high shock resistance and temperature operation between -40 and +120°C.

It is a classic case of multi-material engineering offering a superior solution. And the safety critical component has now been used on over 11 million vehicles across 45 different platforms worldwide.

"We utilise and exploit the inherentproperties of each material, which are normally considered as being opposites," says Professor Fox. "Metals excel where polymers fail and polymers excel where metals fail. The combination and integration of both materials results in a high degree of engineering tolerance and therefore the best possible solution for the application."

Embracing polymer

One of the main drivers for metal replacement with plastics is ultimately driven by the need to lower tailpipe CO2 emissions, but the real engineering challenge lies in reducing the weight of cars. And by replacing steel components with specialist polymers, the overall impact is significant.

It is this ability to lighten that has allowed Arpro foam to venture in to new metal replacement territory. The expanded polypropylene foam supplied by Windsor based JSP has been a popular choice for bumpers for years, as it boasts two fundamental physical capabilities. First, it is isotropic so has consistent performance irrespective of direction, but also it can be deformed on multiple occasions and still return to its original shape.

However, recent material developments and improved tooling have pushed the boundaries of feasible 3D shapes, as well as allowing smaller and thinner parts to be produced.

The recently launched BMW i8 used Arpro for the car's standout door panels. The upward swinging doors are a visual highlight of the car and emphasise its sporty characteristics. However, such a design had to be incredibly light to even get off the ground.

The door is complex and needs to provide stability and strength, as well as component integration such as trims, loudspeakers, switches, and sound insulation in a very thin wall. All this means that heavier materials were discounted straight away to ensure the door would function as planned.

Arpro was eventually selected due to its ability to reduce weight and testing at JSP's facility was rigorous, overcoming a number of challenges. The engineering process was particularly complex because of multifunctional requirements in one component. The radical design required multiple iterations to achieve perfection but the resultant part is smooth, sleek and contributes to the success of the interior design.

Manipulation of material properties is vital when designing plastic components. And while strength and stiffness has been the domain of metals, Staffs based EMS Grivory is even able to challenge this view with its longfibre thermoplastics (LFT). These materials combine very high stiffness and strength with low weight, but critically, without the expensive of carbon fibre reinforced polymers.

With a length of up to 10mm, the fibres form a fibre 'skeleton' that improve thermomechanical properties without affecting polyamide-specific features such as resistance to chemicals or surface quality.

LFT materials exhibit high mechanical values even beyond the glass transition point and the fibre skeleton ensures improved energy absorption, higher heat deformation temperatures, improved behaviour at very high and low temperatures as well as low warpage.

The company's long glass-fibre reinforced Grilamid LCL-3, for example, combines a density of only 1.15g/cm3, but a strength of 250MPa. EMS has also been able to further increase those properties relevant for metal replacement using glass-fibre rovings and has developed an injection-moulded material that exceeds 300MPa tensile stress at break.

Design with intent

These improvements by material developers and suppliers are impressive, but it's only half the story. Engineers need to be mindful when applying plastics to metal replacement projects, particularly if experience in this area is perhaps somewhat lacking.

By combining functions, reducing the number of processing steps, incorporating colours and providing corrosion protection, a more cost effective and functional part can normally be produced.

"We have to try and educate the market," says Ian Nott, managing director of KD Feddersen UK. "For some customers we do run a basic course where we will talk about the differences in all the various engineering plastics, what you can do, and what sort of applications you should be looking for."

These days, metal replacement is becoming a much more defined art. With the aid of CAD and in particular finite element analysis, the process of metal replacement starts to become less of a black art, and a more routine exercise.

"People are becoming much more aware of the steps they need to take to bring it through and have success," says Nott. "It's not a case of saying, 'I'm going to replace this with plastic', and then go off and choose the wrong material, the wrong design, have a tool made and then it doesn't work."

It is clear that metal replacement and design is more than just the geometry of a component and a flick through a data sheet for materials info. The entire process of design and materials specification has become blurred and integrated, and both need to be dealt with early on. Even questions of manufacture need to be dealt with right at the frontend, by the designer, material specifier, CAD operator, and all the other job titles now dominated, normally, by one in the same individual - the engineer.

Cargo chains get lightweight treatment

DSM Dyneema and Load Solutions have launched a lightweight chain (pictured) made of synthetic fibre. The TYCAN chain is a non-metallic link chain made with 'Dyneema', claimed by the company to be the world's strongest fibre. It has been developed to replace steel chains that are used to lash and secure cargo during transportation. Dyneema is a patent-protected ultrahigh molecular weight polyethylene (UHMWPE) fibre that is 15 times stronger than steel on a weight-toweight basis, and up to 40% stronger than aramid materials.

The chain has been hailed for its outstanding performance by users across a wide range of industries, from shipping, mining and oil and gas to commercial fishing. TYCAN chains weigh one eighth that of the conventional metal counterpart, which means trucks can safely carry more cargo while remaining within the safe operating limits.

Alex Rock, marketing manager at Load Solutions, says: "There is a major cost benefit for painted surface and sensitive cargo. Using TYCAN chains to lash cargo eliminates the dents and scratches often caused by lashing with steel chains, and can significantly lower the cost burden of damage claims."

Commercial field trials are ongoing globally in various industrial branches to further expand the commercial market opportunities for TYCAN.

Justin Cunningham

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