More that a weight issue

Paul Boughton

The main reason for specifying plastic in cars is to save weight – but they can deliver other benefits too. Lou Reade reports.

The overwhelming reason for using plastics in cars is the same as ever: to save weight. If a fenders, bumpers, accelerator pedals and other parts can be made from plastic – which is around half the density of an equivalent steel part – this frees up ‘space’ to add other elements such as more robust safety features.

But plastics must do more than simply save weight. They must also meet the ever-more stringent requirements of the automotive industry – whether it is higher engine temperatures, easier recyclability, design freedom or, as ever, reduced cost.

Under-bonnet advantage

Dutch chemicals specialist DSM has developed a hydrolysis resistant grade of polyethylene terephthalate (PET). Arnite A HR could be used to make dimension-critical components under the bonnet, in place of metal or more expensive polymers.

Continuous engine downsizing, especially for compact cars, has led to higher under-bonnet temperatures and puts higher demands on plastics components. DSM says there is a demand for materials that retain properties such as strength, stiffness and impact strength under these conditions.

“Injection moulded parts from conventional PET lose as much as half of their tensile strength after 1000 hours at 85°C and 85 per cent relative humidity,” said Wouter Gabrielse, automotive segment manager for DSM. “Parts made from Arnite A HR keep around 90 per cent of their initial strength under the same conditions. The performance we have achieved is spectacular.”

Arnite A HR can replace metals or higher cost engineering plastics such as polyphthalamide (PPA) and polyphenylene sulphide (PPS) in parts that require high dimensional accuracy, and good chemical resistance and mechanical properties. The material is available with either 35 per cent or 50 per cent glass fibre reinforcement.

The new grade is aimed at a number of under-bonnet applications, including throttle valve bodies, sensors, air control valve housings, electronic throttle control (ETC) and exhaust gas recirculation (EGR) covers, and ignition systems. As well as hydrolysis resistance, it has high stiffness, strength and dimensional stability.

“Arnite A has been the material of choice in automotive safety applications like brake booster valve bodies for many years” said Gabrielse. “The new grade can satisfy applications that are closer to the engine, which until now have never been achievable with PET.” 

Indian debut

Meanwhile, Sabic Innovative Plastics says that its Noryl GTX resin – a blend of polyamide and polyphenylene ether – has helped automotive manufacturer Mahindra & Mahindra to develop India’s first injection moulded plastic fender.

The fender, developed with Tier 1 supplier Plastic Omnium, is featured on the new XUV500 sports utility vehicle (SUV). Switching from steel to Noryl GTX helped Mahindra to improve fuel economy, reduce emissions, expand design freedom and enhance resistance against minor impact.

The resin allows significant freedom to create flared fenders with accentuated curves in line with the XUV500’s ‘cheetah-like’ design. It can be up to half the weight of steel, and reduced the fender weight by 0.9kg (1.98lbs) – a saving of 27 per cent.

The resin’s enhanced resilience compared to steel allows the XUV500’s fender to flex and absorb energy to regain its original shape, reducing repair costs. At the same time, the design flexibility provided by the resin eliminated the multiple steps, tools and operations that are needed to make steel fenders.

Noryl GTX combines the dimensional stability, low water absorption, and high heat resistance of MPPE with the chemical resistance and flow of PA to create a material with high stiffness, impact strength and heat performance required for online painting.

Racing rubber replacement

For many years, Evonik of Germany has supplied materials for components on the Lotus Exige – which are then tested under the demanding conditions of motor racing. This year, Lotus will use a multi-layer tube system that incorporates a bio-based Evonik resin for the first time.

Evonik has supplied grades of its Vestamid polyamide to Lotus before, for use in its multi-layer line for charge-air cooling. Now, the line incorporates the company’s new Vestamid Terra, which is derived from sustainable sources.

The multi-layer tubes are lightweight replacements for rubber hoses and reinforced lines. The MLT 8000 multi-layer tubing system has since proven its performance in vehicles worldwide. The current car uses MLT 8000.3 with an orange outer layer. This system is around 870g lighter than cooling line systems with steelflex tubes, a weight reduction of more than 70 per cent.

The coolant lines have three layers: the inner layer is a specially adapted grade of polypropylene; the second is an adhesion promoter layer; and the outer layer consists of the Vestamid (or bio-based Vestamid Terra).

Evonik’s bio-based polyamides have been used commercially since 2010 as mono-layer tubes in air brake lines of utility vehicles, and other applications, but this is the first time they have been used in multi-layer tubing systems for coolant lines.

Composite gain

BASF has moved into a new area, with a new method for making fibre-reinforced thermoplastic automotive parts. The Ultracom concept has already been used to create a seat pan for the Opel Astra OPC.

Ultracom combines three components: a continuous fibre woven nylon fabric; a range of glass-filled nylon and PBT moulding compounds; and engineering support, including elements such as computer simulation.

The fabric is placed into a special injection mould and heated until it softens. The moulding compound can then be used to ‘overmould’ the fabric, which creates structures such as ribs to add strength.

BASF is initially aiming to create two types of component: those with high stiffness, or those with high energy absorption. This will be done by changing the overmoulding compound in each case.

The company has built a pilot line to create the parts, and says the cycle time of around 60 seconds could be reduced to 50 seconds with the use of improved automation.

“The Opel OPC part was made in series production but not in extremely high volume,” said Reinhard Jakobi, head of processing technology at BASF Engineering Plastics.

He adds that the mould to make the parts is around 20-30% more expensive than a conventional mould.

More ‘overmoulding’ material grades will also be developed in the near future, says BASF. Before that, examples of moulded parts will be seen at the forthcoming K2013 show in Germany, which takes place in October.

On the bounce

Car maker Fiat has redesigned the jounce bumper system on its latest Punto. Made from a modified version of DuPont’s Hytrel thermoplastic elastomer (TPE), it integrates several components into one to boost performance and reduce system cost.

The jounce bumper is designed to absorb impact and dampen noise, vibration and harshness (NVH) by preventing the metal shock absorber spring from fully compacting under load.

The redesigned part uses a TPE in place of foam polyurethane (PUR) or rubber, for high elastic recovery, durability and low stiffness variation over a wide temperature range. It eliminates the need for metal or reinforced polyamide saturation cups and acetal or rubber rings that are normally used to increase energy absorption.

The modified Hytrel improves several characteristics, including mechanical performance (particularly under high strain conditions), shape recovery, a balance between stiffness and low-temperature properties, and thermal stability.

Several OEMs have tested the new design on a variety of vehicles. The tests indicate a similar performance to PUR, but with lower part damage. After traversing 3,000km of cobblestoned roads, the Hytrel jounce bumper showed less than 3 per cent permanent deformation – some way below the industry target of 10 per cent, said DuPont.

Steel replacement is likely to continue in automotive. But this example, of substituting one plastic for another, is destined to become more common – as car manufacturers see that can be used to do more than just saving weight

Plastic limo

Sometimes, there is nothing other than plastic that can be used to create a particular car design.

‘Mood Indigo’, the latest film from French director Michel Gondry, features a transparent limousine that incldues extruded and thermoformed acrylic parts.

A design team from Peugeot spent more than six months assembling the ‘LimoVian’ car – named after Boris Vian who wrote the novel on which the film is based – by grafting pieces and accessories onto a Peugeot 404.

The transparent parts were made in ShieldUp from Altuglas International, part of the Arkema Group. The acrylic polymer combines two components – PMMA and an elastomer, structured at the nanoscale – which provides complete transparency while optimising other properties such as mechanical properties and chemical resistance.

ShieldUp took 10 years to develop, and won last year’s Potier prize for innovation.

The material is heated to 180°C for 15 minutes to soften it, then is either vacuum formed or blown with compressed air (8 bar). It took many attempts for the plastic converter ABP to obtain the ideal shape for each part.

ShieldUp has also been used to create a panoramic roof on the Renault Twizy, a two-seater electric car.

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