Why are so many engineers still reluctant to use engineering plastics?

There are three main drivers for changing materials in the current engineering and industrial climate. The first, and most obvious, is cost. The second is to lower weight. And the third is to increase performance.

These three trends are being driven by rising oil prices, increasing legislation, and also competitive pressures in the market place. Gone are the days of any parts being over engineered, today parts are vigorously analysed and re-engineered to optimise price, weight, and performance.

The practical effect of all of this is that many traditional steel parts are being put under review to see if there is a better material. This has prompted plastic producers to come forward with innovations to show the marketplace just what is possible when it comes to metal replacement with engineering plastics.

"When engineers changed from making die cast aluminium air intake manifolds to plastic ones, the design driver was that it offered a cheaper solution," says Craig Norrey, head of design (EMEA), at DuPont. "It was, however, also lighter. But, what they didn't expect was a 2-3% increase in power output from the engine. The smooth walls of the manifold actually gave better airflow resulting in the higher performance. If that happens, it is a great reason to change, but it is finding those applications that are critical."

Plastic manufacturers and suppliers have been busy in identifying these kinds of applications in a number of industries. It wants to offer comprehensively superior materials over metal to convince engineers to make the move. It has also been busy innovating by modifying plastic materials that can offer better mechanical performance under hotter, harsher, and dirtier conditions to help open even more market opportunities.

"The metal guys don't worry about temperature or humidity," says Norrey. "So, we have produced a high temperature nylon called Zytel that gives constant stiffness and strength between -40 to +90°C."

Automat Irrigation has recently taken advantage of DuPont's Zytel Nylon when it redesigned a hydrocyclone, an irrigation filtration technology. It successfully redeveloped the device, replacing primary metal components including the main body. The new design offers multiple benefits including greater service life, enhanced robustness and lighter weight.

Vijay Warke, vice president of Operations at Automat Irrigation, says: "For the last five years we have been converting many of our products and components to plastic... thanks to its stiffness, creep and impact resistance and its ability to cater for the design requirements necessary."

The case of not just replacing, but actually improving, is also a stance shared by Quadrant Engineering Plastics. Paul Simmons, business development manager at Quadrant Engineering Plastics Products, says: "An additional benefit having less weight is less stress on the surrounding equipment, and an overall energy reduction in the system. Take a crane, for example, the large flywheel and cable sheath it is a lot of weight on the top of the structure that needs a lot of power to operate. So you can reduce that by considerably by using lighter materials. And that also reduces stress on the surrounding structure."

Quadrant has been working with the aerospace industry in developing suitable plastic materials that can work at very high loads and at high temperatures. Perhaps, most impressive is on landing gear which experience tremendous temperature fluctuations and vibrations. While normal operating temperatures range from -40 to +70°C, the temperature near the brakes can exceed 650°C. Quadrant has developed a range of materials that are self lubricating, vibration, chemical and wear resistant to be used for brackets, bushings, seal rings and wear surfaces that offer many advantages.

Change of mindset
Getting plastics in the mix when it comes to material specification has no doubt come along way, with many industries increasingly looking further than traditional steels. However, many engineers design and work with metals and changing their mindset remains a challenge.

For many parts, components and products, specifying a change in material is risky and potential costly. Testing and approval of a new material type can be a long and costly process, and while many are well aware that an existing way of doing things is probably far from optimum, investment in change to something unfamiliar is sometimes too difficult to justify to senior decision makers. However, many feel that a corner has now been turned with the next generation of engineers now coming through.

"The use of engineering plastics in many environments is becoming more acceptable," says Simmons. "We have been trying to convert people to use a plastic rather than a metal by promoting the benefits, and now engineers coming through university understand more from that plastic point of view. So we are finding more original equipment is being specified in plastic than we use to."

The selection of suitable plastic is often tricky given the number of different types supplied, that offer many different properties and can be quite varied in cost. Plastic materials also call for a slightly different approach to design. However, suppliers are keen to help here and demystify many aspects of the material type by offering advice.

The automotive industry is perhaps one of the strongest markets that metal replacement is happening. The sector faces significant CO2 targets that must be reached by 2020 otherwise OEMs will face major fines. This has created a market pull for different materials and that is helping other industries to embrace polymers by solving some of the fundamental concerns.

One big question that is always raised around metal replacement with plastic materials is how to join them. There are a host of plastic welding technologies for plastic to plastic parts but plastic to metal is not quite as clear cut, as many production engineers from the high volume world of automotive want to use existing machinery such as welding robotics on an assembly line.

"What DuPont has done here is attach metal welding tabs to the plastic component to enable a spot weld," concludes Norrey. "However, we've seen moves by the automotive industry away from spot welding, anyway, to self piercing rivets as it is a cleaner technology, with no emissions, and can be done cold. At the same time adhesives are starting to be used more often. Both of these are playing in to the hands of engineering plastics as an increasingly viable, and advantageous, material to replace metal."

Justin Cunningham

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