Materials

9-1999

Technical Feature

Material advantage

Plastics are gaining in popularity, but their properties are still a mystery to many engineers. Lou Reade reports

Plastic materials have a lot of catching up to do. While metal has been used continually since the Iron Age, the first synthetic plastic only appeared in the last century.

This ‘added history’ means there is still a gulf in knowledge between the two types of material. Research into metals outweighs that into plastics many times over. For many engineers and designers, plastic is still an unknown quantity.

This is why Eureka helped to develop the Designing in Plastics forum, which ran as part of the recent Interplas plastics exhibition. Many engineering designers are demanding better, more detailed information on plastics. In some cases they need basic help; in others, they are looking to replace existing metal parts with new materials. Eureka helped Reed Exhibitions – the organiser of Interplas – to devise a format that will bring design engineers and plastics experts together.

A key feature of the forum was the ability to book one-to-one sessions with the experts. Instead of being handed a free spec sheet, designers sat down to talk through plastics-related topics with the expert.

Designers who want advice on moulding in a new material, for example, could sit down with one of the relevant companies and receive, in effect, a free consultation session.

Advice on nylon could be had from DuPont, which has helped to design a shopping trolley that uses nylon instead of metal. Victrex is the sole European supplier of Peek, a plastic with incredibly high temperature resistance that ideal for applications such as medical equipment. GE Plastics supplies a range of materials and is active in many end markets, including automotive and packaging. Owens-Corning, while not a plastics company, is a leading manufacturer of glass fibre – which is used to strengthen and enhance the properties of many plastics. Rhodia, formerly Nyltech, provides a range of nylons, mainly to the automotive industry.

The supporters of the forum are major materials companies. They all have examples of how their materials have been used in new and innovative ways. Each will present a case study on the subject.

Supermarket sweep

DESIGN.JPEG (14367 bytes)DuPont’s shopping trolley is a typical example of how plastics can replace metals – and lead to improved performance.

Aside from the physical performance– the trolley should be as durable but around half the weight of a metal one – plastics could form the basis for a true futuristic product, allowing groceries to be scanned automatically without being taken out of the trolley.

According to design consultant Mike Woodhall of the Industrial Design Consultancy: "Designing a supermarket trolley for the 21st century capable of embracing all developing retail technologies could only be achieved by producing it in an engineering polymer."

Previous designs of plastic trolley, according to IDC, tended to just replace metal with commodity plastics such as HDPE and PP. Specifying a higher grade plastic – DuPont’s Zytel nylon – allowed IDC to design a better trolley and use innovative techniques such as gas injection moulding.

"Gas injection provided us with the ability to create internal cavities, allowing us to get away from consistent sections and to design and handle three-dimensional masses of material," says Woodhall.

The trolley is moulded in three parts, which are then ultrasonically welded together. And despite being described as an ‘all plastic’ trolley, the team admits that – on the first model, at least – a standard metal castor will be used.

"A fully moulded plastic castor will be designed for the future production requirements, to complete the total plastic trolley concept," says Woodhall.

This will then make it fully recyclable. Marks and Spencer has first option on the trolley – which will soon be seen at a ‘flagship’ M&S store.

Phone home

Mobile communication is one of the fastest growing consumer businesses in the world today. Mobile telephones are just the tip of the iceberg, with notebook computers, pagers and in-car systems also taking off. The boom in the wireless market has been accompanied by a much stricter regime regarding electromagnetic interference. Sensitive electronic components – such as those found in mobile telephones – must be protected from stray radio frequency interference. At the same time, they must not affect other equipment.

Because all electronic circuits will produce some kind of interference, equipment such as mobile phones must be shielded. Shielding takes the form of a conductive enclosure – and can be anything from a metal box to a wire mesh.

Now glass fibre manufacturer Owens Corning has developed an integral solution that relies on a conductive composite plastic, which comprises a thin, continuous sheath of electroplated metal bonded to a graphite core. It is compounded with polymers such as polycarbonate, PET or nylon into long fibre pellets and injection moulded to create plastic parts with an inherent shielding ability.

These conductive composites are physically as strong as other graphite-filled plastics; the metal skin on the fibre enhances electrical conductivity. The result, called OC Shielding Solutions, is a moulded-in conductive shield. The composite can be moulded like any other plastic, so will allow the designer to specify shielding within a design, but without having to specify a separate component.

According to Owens Corning, this will mean fewer process steps and reduced time to market – as there is no need for operations such as painting or plating. The fact that shielding is inherent in the plastic also means that the final part can be made smaller.

Medical materials

Plastics can be used to replace more than just metal. In medical applications, Victrex Peek has replaced glass and a number of metals across a range of applications – taking advantage of its resistance to wear, chemicals and, most notably, heat.

Peek is also highly biocompatible and is fully FDA compliant. Victrex has recently extended Peek’s range with the launch of Peek-Optima LT – which, says the company, is set to be the first high performance thermoplastic that can be used in long-term in vivo applications, such as hip, spinal and dental implants.

Jorg Schlegel, medical market developer for Victrex, says: "In terms of medical engineering, Peek-Opticma can be tailored to match the stiffness and impact performance of human bone. Its inherent lubricity could make this material interesting as a sliding surface partner in artificial joints."

Peek’s combination of tribological performance and high mechanical properties over a wide temperature range has helped it to replace metals in many under-bonnet automotive applications. Examples include oil screens that are supplied to the truck manufacturing unit of Mercedes-Benz, a piston unit for air flow regulation in engine control systems and thrust washer seals for power transmission.

Designed to last

Thermochromic plastics – which change colour depending on their temperature – may be ready to break out of the novelty market.

GE Plastics has recently set up a separate division at its Netherlands headquarters that will look to transfer its plastics expertise to industry. The company says that it has developed new dyes that can be compounded with engineering thermoplastics such as polycarbonate. To date, thermochromic plastics have only been used in less demanding applications, such as children’s toothbrushes, which do not involve elevated temperatures. The new dyes mean that they might now be used in demanding applications such as oven door handles.

The company has also helped a customer to develop a motor end cap in thermochromic plastic, which will allow an instant visual signal as to whether the motor has overheated.

Keeping a foot on the brake

The plastic footbrake pedal may be with us within four years, according to a leading supplier of plastics to the automotive industry.

Rhodia Engineering Plastics, which supplies nylon to a number of leading car manufacturers, has recently helped to develop the plastic handbrake used in the Ford Focus.

Fico Cables, which manufactures the hand brake, used 50 per cent glass filled nylon in place of metal for the lever mechanism. This has allowed a reduction in weight, number of parts and assembly time.

Luigi Cermesoni, who was involved in the project, says: "All the other foot pedals and even the pedal box have now been made from plastic. Manufacturers are reticent to do the same with the brake pedal just in case it breaks."

He says that all the manufacturers are testing the idea and have found plastic to be up to the job.

"It is up to one of them to take the plunge," he says.

Back to Materials