Plastics Assembly: Composite Joining Pros and Cons

May 27, 2010
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Demand for composite materials is growing rapidly, as manufacturers in many industries scramble to find new ways to reduce weight, eliminate corrosion and enhance durability. Composites have high strength-to-weight ratios that often exceed steel and aluminum, which promotes weight savings. In addition, composite materials are dimensionally stable, provide design flexibility and create part consolidation opportunities for engineers.



Demand for composite materials is growing rapidly, as manufacturers in many industries scramble to find new ways to reduce weight, eliminate corrosion and enhance durability. Composites have high strength-to-weight ratios that often exceed steel and aluminum, which promotes weight savings.

In addition, composite materials are dimensionally stable, provide design flexibility and create part consolidation opportunities for engineers. And, the tooling investment needed for composites is often less than what’s required for metal stamping.

“Composites can be molded into much lighter-weight components than alternative substrates,” says Jamie Hubbard, market development engineer at Henkel Corp. Other key advantages are the ability to achieve more complex shapes than a stamped steel part and the ability to create larger components with fewer assembled parts. “While composites cost more than metals, trade-offs such as reduced weight and improved strength outweigh the initial material cost,” Hubbard points out.

Despite all those benefits, some engineers are reluctant to use composites. They view it as a relatively brittle material that is prone to fractures, cracks and delamination. In addition, manufacturing with composites tends to be more labor intensive than with alternative materials, such as traditional plastics, which can be injection molded.

“Composites are considered risky, design codes may not exist for a particular application, manufacturing operations may not be set up for composites and, for certain applications, their up-front cost or material costs may be higher,” says Ajay Kapadia, section manager in the adhesives, composites and sealants section of TWI Ltd. “Composites are sometimes referred to as ‘plastics,’ and this makes people think of consumer plastics, which are usually unreinforced [materials] not designed for load carrying.

“Certainly, composite materials are not suitable for every application,” adds Kapadia. “However, with society moving toward a low-carbon economy, some of the inherent properties of composite materials offer solutions to new problems. More applications in composites are inevitable.”

Adhesive is used to attach composite panels to the side of a bus.

Common Misperceptions

“Some of the common misconceptions of composites include the idea that the material exhibits uniform strength, all composites act similarly, and that working with the material is easy,” says Ray Toosky, president of Polaris Fastening, a consulting firm that specializes in the aerospace industry. “Composite materials, specifically carbon fiber, have directional properties due to the pattern and orientation of the fibers. Thus, while strong in one direction, the material may not be able to take the same loads in a secondary direction.

“This orientation of the fibers, which is very specific to different composites and composite manufacturers, creates very unique materials, with unique characteristics,” warns Toosky. “Manufacturing and handling composite materials is rather difficult, since the material does not have malleability. Incorrect machining and handling can crush the fibers and make the structure defective. Repairing of damaged fibers is difficult and unreliable.”

Toosky and other experts believe that many engineers still need to be educated about the pros and cons of composites. “It’s still outside the comfort zone of a lot of people who are used to metal,” explains Michael Favaloro, technical marketing manager for Fortron PPS composites at Ticona Engineering Polymers. “Parts also need to be designed differently.”

Joining composite parts presents a number of challenges to engineers, not the least of which is fastening composites to other materials. “Fastened joint design and bolt stack analysis are areas where engineers untrained in composites can run into real problems,” warns David Hornick, director of advanced composites technology at Gulfstream Aerospace Corp. “Another area of concern with composite joining is bonding and the requirement for a prepped surface. If not done correctly, the joint can fail at well below predicted values. This is an area where process control is critical.”

“There is a lack of understanding [when it comes to] material characteristics, properties and the advantages of composites, which makes designing with composites difficult,” adds Gabrielle Hampson, director of marketing at the American Composites Manufacturers Association (ACMA).” One of the biggest misperceptions that engineers have about composites is that they are aerospace materials and not suited for high-volume production.”

Dave Archer, president of Archetype Joint LLC, says he has been seeing more applications for composites every year, especially in the automotive industry, where engineers must tackle a wide variety of weight- and fuel-saving challenges.

“However, there is some confusion about what composites are,” Archer points out. “People tend to use the term differently. For some, it’s anything having to do with thermoplastics. Other engineers consider composites to be materials comprised of carbon fibers and impregnated resins.”

According to ACMA, not all plastics are composites. A true composite is a combination of a polymer matrix resin, such as polyester, vinyl ester or epoxy, and a fiber reinforcement, such as glass, carbon or aramid.

Composite parts can be joined with mechanical fasteners, adhesives and welding. Thermoset composites are usually joined by adhesive bonding, while thermoplastic composites are typically joined by welding. Fasteners are primarily used to join aerospace composites.

“Composites are easily joined to dissimilar materials, as long as the process is carried out correctly,” says Kapadia. “For instance, carbon fiber composites work best with titanium and are used extensively in the aerospace industry.”

Composite demand is expected to skyrocket in the auto industry, as engineers focus on smaller, fuel-efficient vehicles.

Growing Demand

Composite materials are used to produce everything from golf clubs and skis to sailboats and wind turbines. Everyday items that are made from composites include office chairs, car door handles and laptop computer cases.

Because they’re nonconductive and nonmagnetic, composites are also popular with electrical component and telecommunications equipment manufacturers. Recently, there’s been growing interest from aircraft, automotive and railway manufacturers eager to reduce the weight of their vehicles.

All-composite commercial airliners such as the Boeing 787 and the Airbus A350 have dominated the headlines, but many smaller aircraft are also being developed. For instance, composites are used in unmanned air vehicles such as the RQ-7B Shadow and single-engine piston aircraft such as the Cirrus SR22.

Composites allow aerospace engineers to create fuselages, wings, tails, landing gear, nacelles and other components that are smooth and curvy, without the rivet lines found on traditional aluminum airframes.

“The future of composites is promising in all aircraft,” says Jacques Cinquin, senior composite materials engineer at EADS Innovation Work, which is the R&D arm of Airbus and Eurocopter. “The weight and fatigue advantage are so strong that aircraft will never come back to metallic structures, but will [evolve toward] more extensive use of composite materials.”

Demand for composite materials is expected to skyrocket in the aerospace industry during the next decade, due to the growing need for fuel-efficient, durable, low-maintenance aircraft. In addition, composites allow engineers to slash assembly cost and complexity by reducing the number of components and subassemblies needed to produce an airplane.

“While composite materials currently represent a relatively small segment of the aerospace industry, enormous potential exists for composites to become a more integral component within the industry in the future,” says Norman Timmins, director of consulting at Lucintel, a market research firm. By 2018, total demand for composite materials in the aerospace industry will top $35 billion.

Innovative manufacturers such as Gulfstream Aerospace are leading the charge. The company has been using modest levels of composites in its business jets for more than 20 years. They are used in floor panels, furnishings, fairings, cover pieces and some primary structures, such as the aft pressure bulkhead section of the fuselage.

Gulfstream is currently using large amounts of composite material to assemble key parts of its new G650, such as the tail section and structural wing assembly. “When used correctly, composites can reduce an aircraft’s weight and cost, the fuel it burns and the carbon dioxide it emits,” says Hornick. “Composites can also improve the aircraft’s efficiency, durability and skin appearance.

“And, the single-piece design of composites reduces the number of parts and increases the speed of manufacturing,” Hornick points out. “The large reduction in part count due to the use of consolidated composite pieces has had a positive effect [on our] assembly process.”

The use of composites has also affected the layout and flow of Gulfstream’s assembly line in Savannah, GA. “It’s important to control the process and hold tight tolerances when working with composites,” notes Hornick. “So, we’ve replaced hand riveting with an integrated panel assembly cell.

“Additionally, components move through the manufacturing process on precision build carts, which maintain the fixture-fuselage reference points throughout the build process,” adds Hornick. “To ensure strict adherence to process specifications and to deliver manufacturing repeatability, we [also] developed specialized robotic tooling using a computer-controlled proc-ess.”

Engineers at NASA are also eager to leverage the benefits of composites. They’re experimenting with large composite structures that could make it easier and cheaper to transport heavy payloads to and from space.

“In space travel, where every additional pound of weight drives costs higher, any weight savings provides increased payload capacity and potentially reduces mission expense,” says Mike Kirsch, manager of the composite crew module project at NASA’s Langley Research Center. “We pressurized the module to twice the Earth’s atmosphere to demonstrate the ultimate design capability of the structure, and followed that by pushing and pulling it to simulate the forceful tug of different mission phases. There were no anomalies.”

The crew capsule, which is currently undergoing additional testing to gauge its resistance to extraterrestrial damage, is comprised of two halves. During buildup, Kirsch says many of the critical orthogonal joints were assembled by the use of preformed three-dimensional weaving technology, which NASA calls Pi joints. “We tracked the strain across the joint and verified that the composite was fully capable of handling the pressure and vehicle loads in the crew cabin,” he explains.

This fiber composite damper weighs half as much as an aluminum damper.

Down-to-Earth Applications

As aerospace applications increase, new manufacturing processes evolve and the cost of composites continues to drop, the auto industry will be the next big frontier for the material. Automotive engineers have been dabbling in composites for more than a decade. But, most applications have focused on low-volume vehicles, such as high-end sports cars like the Chevrolet Corvette and the Dodge Viper.

“Significant opportunities exist for composites in the North American automotive market,” claims Lucintel’s Timmins. “The market is growing due to a number of factors, including the continuing demand for lighter weight materials to replace metals; the use of more environmentally friendly, recyclable materials; and continued technology improvements in fabrication techniques, material formulations and product design.” But, high cost and lack of high-speed manufacturing processes will limit short-term growth.

“[Cost-effective] carbon fiber prices will open up new potential for car manufacturers,” says Orhan Imam, market development manager at Dow Automotive Systems. “There are [also] new resins in development to ensure fast curing cycles for automotive parts.”

European automakers are already jumping on the composite bandwagon. For instance, when BMW launches its much-anticipated Megacity vehicle in 2013, the electric car will be ultra lightweight, due to carbon-fiber body panels.

“This vehicle will radically alter the motor industry as we know it,” claims Norbert Reithofer, BMW’s chairman of the board. “BMW is currently the only company that will be launching a volume-production vehicle on the market that features carbon fiber-reinforced material.”

The German automaker and SGL Group are building a $100 million plant in Moses Lake, WA, to supply composite parts for the Megacity car, which will be assembled in Europe. “This new plant is a milestone in the use of carbon fiber for large scale production in the automotive industry,” claims Robert Koehler, CEO of SGL. “It will be the world’s most cost-efficient carbon fiber plant using state-of-the-art technologies. We will ensure that carbon fibers play a revolutionary role in lightweight automotive construction.”

Several German automotive suppliers recently unveiled composite components that will probably find their way into future mini and micro vehicles such as the Megacity. For instance, engineers at ZF Sachs have developed a fiber composite damper than weighs half as much as traditional aluminum dampers.

“Weight savings are generated by using lighter materials instead of steel parts in the piston rod, internal parts and module assemblies on the one hand, and with the integration of functions on the other,” says Peter Ottenbruch, head of ZF’s powertrain and suspension components division. “The newly designed components make previous heavy steel components like the spring cap unnecessary.” A

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