Composite leaf springs: Saving weight in production

Composite leaf springs are not new to the automotive industry. In fact, the leaf spring itself dates back to the horse-drawn carriage. By design, leaf springs absorb vertical vibrations caused by irregularities in the road. Variations in the spring deflection allow potential energy to be stored as strain energy and then released more gradually over time. Composites are well suited for leaf-spring applications due to their high strength-to-weight ratio, fatigue resistance and natural frequency. Internal damping in the composite material leads to better vibration energy absorption within the material, resulting in reduced transmission of vibration noise to neighboring structures.

The biggest benefit, however, is mass reduction: Composite leaf springs are up to five times more durable than a steel spring, so when General Motors (GM, Detroit, Mich.) switched to a glass-reinforced epoxy composite transverse leaf spring (supplied by Liteflex LLC, Englewood, OH, US) on the 1981 Chevrolet Corvette C4, a mono-leaf composite spring, weighing 8 lb/3.7 kg, replaced a ten-leaf steel system that weighed 41 lb/18.6 kg. This reportedly enabled GM to shave 15 kg/33 lb of unsprung weight from the Corvette, yet maintain the same spring rates. The leaf spring was transverse-mounted; that is, it ran across the car’s width at each axle. This eliminated the coil springs that sit up high in a spring pocket on the frame. Thus, the car can sit lower to the ground, which improves car handling.

Today, GM continues to employ transverse GFRP composite leaf springs on the front and back of its Corvette models. The 2014 Chevrolet Corvette Coupe includes a double-wishbone suspension, which, at GM, goes by the name short/long arm (SLA). SLA refers to the fact that the upper control arm is shorter than the lower one. A transverse composite leaf spring presses against the lower arm and spans the width of the car. In fact, the spring is always loaded against the subframe. This design directs shock loads into the frame side, eliminating the standalone rear antiroll bar that must be incorporated into models with standard suspension packages. The spring’s camber curve also is said to improve tire contact with the road during cornering.

Composites also have the potential to replace steel and save weight in longitudinal leaf springs (see “Building a stronger longitudinal leaf spring,” under "Editor's Picks," at top right). These run parallel to the length of the vehicle, providing suspension as an integrated part of the wheel guidance system. “Longitudinal leaf springs have a higher safety factor,” claims Frank Fetscher, head of business development, Benteler-SGL (Ried, Austria), a joint venture of Benteler Automotive and the SGL Group – The Carbon Company (Wiesbaden, Germany, see “SGL Automotive Carbon Fibers opens new plant in Washington,” under "Editor's Picks"). “They can have a linear spring rate or a progressive spring rate — multistage springs — and must perform better with respect to torsion and side stiffness than transversal springs.”

Higher speed, greater volume

To date, commercial glass- and carbon-reinforced composite leaf springs have been limited to low-volume production models. “When resins were first being used in the automotive industry, epoxy systems already proven in the aerospace industry were the first to be selected,” explains Scott Simmons, business development specialist for chassis, Henkel AG & Co. K (Madison Heights, Mich.). “While these epoxy systems provide a very high-performing part, the prepreg manufacturing process primarily employed with these resin systems is better suited for the low-volume production associated with aerospace.”

Epoxy prepreg systems weren’t fast reacting because they didn’t need to be for autoclave processing, which, for purposes of quality assurance to high aerospace standards, necessarily involved slow and carefully controlled applications of temperature and pressure. However, much research has gone into expediting the production process through the use of faster molding processes and the development and use of suitably fast-reacting resin systems. These emerging systems show promise for economical mass production of composite leaf springs.

Polyurethane & HP-RTM

“In automotive, RTM [resin transfer molding] is the go-to process,” asserts Simmons, “and maximizing the speed of processing is critical for high-volume manufacturing to become a reality.” To that end, Henkel has developed a polyurethane matrix resin system designed for fast automotive high-pressure RTM (HP-RTM) processes. “Our goal was to mimic the performance characteristics of epoxy, while increasing processing speed and flexibility,” explains Simmons, noting that, ultimately, “Automotive OEMs want a composite system that will allow 100,000 to 250,000 parts per year at a relatively low capital investment cost.”
Henkel’s Loctite Max 2 matrix resin reportedly provides an answer: A high modulus (2,800 MPa) in combination with an elongation-to-break of 5 to 10 percent, with tensile strength of 80 MPa. Due to its specific polymer backbone structure, which combines “soft” polymer segments with strong H-bridging of the urethane moieties, the neat polyurethane resin is said to exhibit intrinsic toughness. According to Henkel, this eliminates the need for additional toughening agents that increase cost and viscosity. The toughness properties of the resin translate, practically, to fatigue resistance. This is critical because automotive leaf springs are subjected to dynamic loading under driving conditions, and are required to pass tests that require 700,000 recurring load cycles. The use of flexible materials with high fatigue tolerance prolongs the life of the leaf spring considerably.

Henkel has collaborated with Benteler-SGL to commercialize mass production of a lightweight, fiber-reinforced leaf spring using a polyurethane-based HP-RTM process. The process combines unidirectional (UD) glass fiber preform technology with Henkel’s Max 2 resin system. The result is a leaf spring that, Henkel says, weighs 65 percent less than the conventional steel option — 6 kg vs. 15 kg (13 lb vs. 33 lb).
When Henkel approached Benteler-SGL with its polyurethane process, the latter was developing a front-axle composite leaf spring for the Mercedes-Benz Sprinter, a lightweight cargo van manufactured by Daimler AG (Stuttgart, Germany). The Sprinter has sported a composite leaf spring for a number of years. As with previous iterations, the part was designed with glass-reinforced epoxy. “Benteler-SGL had already designed the orientation and density of the fabric,” says Simmons, “and we presented an alternative resin that could work with the design already in place.”

“Replacing the existing epoxy system with Max 2 polyurethane was appealing to Daimler because polyurethane is tougher and can withstand bending and flexing better than epoxy,” he maintains. “It also offers improved resistance to crack propagation, meaning that if a rock pops up and strikes the leaf spring, any chip or crack that might occur is less likely to propagate.”

“Benteler’s interest was in regard to speed,” says Simmons. “The existing epoxy resin required a mold time of approximately 30 to 35 minutes. With a program requiring 100,000 to 150,000 parts annually, a 30-minute cycle time would require a large number of molds to meet demand, which then impacts capital investment costs significantly,” he says, noting that “the Max 2 resin system offers a faster injection time — from minutes with the epoxy to seconds with the urethane — and a faster mold time — from 30 to 35 minutes with epoxy down to eight minutes with urethane.” 

“With HP-RTM, we have an economic[al] process that offers geometric design possibilities,” explains Fetscher. “In the end, the final product has the same properties as it would with an epoxy system.” According to Fetscher, the rheological behavior of the polyurethane matrix resin as a function of temperature and isothermic cure kinetics were evaluated to determine a process window for injection at minimum resin viscosity. The optimum processing window proved to be 70°C to 110°C (158°F to 230°F). “Under optimized processing parameters, it is possible to inject the mixed polyurethane matrix resin at viscosities as low as 30 mPas, [30 cps]” claims Fetscher. “Using high-pressure RTM equipment, low matrix resin viscosities enable an ultrafast injection rate of 100g to 300g of resin per second. At the same time, the unique flow behavior of polyurethane matrix resins doesn’t lead to undesirable fiber displacement effects that can be seen with matrix resins of higher viscosities.”

Henkel recently introduced its Max 3 polyurethane-based system, which it developed with input from Benteler-SGL. Notably, the new system also includes an internal mold release to enable easier processing. “Typically, some type of mold release is required in RTM or compression molding, so we have integrated the internal mold release into the product to eliminate the need for that step,” explains Simmons. Further, optional accelerators can be added to the base isocyanate and polyol to enhance processing speed.
Max 3 also offers an increased glass transition temperature, which improves finished-part temperature resistance. “Increasing the temperature resistance continues to be a target for our future polyurethane systems,” Simmons emphasizes, noting that the continued research amounts to an insurance policy of sorts. “In automotive, a temperature resistance between 150°C to 180°C [302°F to 356°F] would allow the parts to go through the e-coat process,” he explains. “Not that the composite parts necessarily need the e-coat process,” he observes, “but our goal is to allow for composite parts that can withstand the same processing temperatures as the other components on the car, to streamline production.”
    
Epoxy formulators respond

Over the past several years, substantial progress also has been made in epoxy resin technology and the processes used to mold epoxy composites. Momentive Specialty Chemicals (Columbus, Ohio) has developed so-called “snap-cure” epoxy resin systems, designed to allow medium- to high-volume production of structural composites, including leaf springs. The new systems retain the properties of traditional epoxy-based composites, according to Momentive, yet process in a matter of minutes when used, like the polyurethanes, in HP-RTM.

“The advanced formulations are unique in that they provide a long-enough injection window for a robust impregnation of the reinforcing fibers while still enabling an extremely short cure cycle,” claims Dr. Roman Hillermeier, who, with Momentive research partners Dr. Tareq Hasson, Lars Friedrich and Cedric Ball, presented findings at the Society of Plastics Engineers’ 2012 Automotive Composite Conference and Exhibition (see end note). “The entire process requires a short cycle time to be viable for automotive mass production volumes,” said Hillermeier. In practical terms, he said, that means less than five minutes.
One key to enabling these faster production speeds is the preform binder. “In the case of rapid RTM processing, it is particularly important that there is good compatibility with the resin matrix, the reinforcement’s permeability is not negatively affected, and the binder provides enough strength to prevent distortion of the fibers during injection,” explained Hillermeier. “The higher level of performance was achieved by means of a ‘reactive’ or ‘crosslink-able’ binder.”

At production speeds of five minutes or less, the time required to fill the mold and complete fiber wetout is a challenge with epoxies. Structural composite parts require relatively high fiber volumes of 50 percent or more, Hillermeier noted. “Very low viscosity and having sufficient time for impregnation are the two key characteristics that are needed to achieve quality finished parts. The ideal injection viscosity of an RTM resin should be below 100 mPas [100 cps] for at least 60 seconds at processing temperature.”

In answer, Momentive has developed two fast-reacting epoxy systems with enough designed-in thermal latency to allow time for thorough fiber wetout of large or geometrically complex parts. Both systems are designed for HP-RTM processing. The first, EPIKOTE 05475 resin with EPIKURE 05443 curing agent, reportedly cures within five minutes at 120°C/248°F. The second, EPIKOTE 05475 resin with EPIKURE 05500 curing agent and Heloxy 112 internal mold release agent, reportedly cures within two minutes at 115°C/239°F. Most recently, Momentive introduced its EPIKURE 05500 fast-cure epoxy and EPIKOTE 04695-1 binder/EPIKURE 05490A curing agent for production of Class A composite auto parts using a gap-impregnation RTM process (see “Composites Europe: A lot of car hoods," under "Editor's Picks”).

Momentive also has worked with molders during the development of its new epoxy systems. A noteworthy example is IFC Composites (Haldensleben, Germany), which has been mass-producing glass-reinforced epoxy-based leaf springs since 2005. The company uses a semi-automated prepreg manufacturing system, during which continuous fiber is impregnated with resin. IFC has reportedly supplied more than 1.3 million composite leaf springs for light-duty trucks, including Daimler’s Sprinter cargo van. The Sprinter front axle leaf spring manufactured by IFC measures 1400 mm/55 inches long, 75 mm/3 inches wide, 30 mm/1.18 inches thick and weighs 5.5 kg/12.1 lb compared to the 25-kg/55-lb steel front leaf spring it replaces.

Next step: Multilink systems

New developments also include changes in the manufacturing approaches to auto suspension systems. “The next step for transverse leaf springs,” predicts Benteler-SGL’s Fetscher, “will be a move away from single component suppliers in the direction of systems suppliers.”

“A multilink axle system with a composite leaf spring covering jounce and roll function is the most effective weight optimization for a complete rear-axle module and the next step in weight reduction,” he adds. Development goals for Benteler’s leaf-spring rear module include weight reduction through replacement of the coil spring and antiroll bar by a transverse composite leaf spring, with no reduction in the vehicle’s handling behavior and an improvement in the suspension system’s acoustic damping. Bump and roll stiffness would be supported by the leaf spring. Benteler believes vehicle dynamics would be improved. Reportedly, weight savings would be 4 kg to 8 kg (8.8 lb to 17.6 lb) per system and costs are within an acceptable range in relation to the weight reduction. Currently, Benteler has developed system integration within the axle suspension concept to include the function of the antiroll bar into the leaf spring. The system is fully developed and ready for program integration.

ZF Friedrichshafen AG (Schweinfurt, Germany), a global supplier of driveline and chassis technology, is taking it one step further with the development of a wheel-guiding transverse leaf spring. The system is designed to perform spring, antiroll and wheel-control functions. This leaf spring, however, is manufactured via heated compression molding, with an epoxy-based resin system and continuous glass fiber reinforcement.

According to ZF, the loading on the spring is complex, making process control, in terms of fiber content and orientation, a key to success. The spring’s design eliminates a number of conventional steel components — an antiroll bar with mounts, two antiroll bar links, two control arms and two conventional coil springs. ZF reports that the composite leaf spring suspension system is approximately 12 percent lighter than a conventional MacPherson strut suspension, approximately 10 percent lighter than a conventional twist-beam suspension, and can be as much as 60 percent lighter than a steel multileaf spring. Key to precise wheel control and desired spring rates is the design of the leaf spring cross-section and the placement of the mounts. ZF’s design is targeted to the compact car class, and the company is expecting first production applications in 2014.
    
Moving forward with composites

Because transverse composite leaf springs are already in use in lightweight trucks and cargo vans, as well as high-end sports cars, “the main focus for the future of transverse leaf springs,” says Fetscher, “will be the system integration of body-suspension (coil springs) and antiroll bar functions into a multilink leaf spring suspension concept.” These will be a key factor in widespread adoption. “This system will target mainly passenger cars in the C- and D-class segments,” he says, referring to mass-production compact cars and large cars, respectively.

On the longitudinal leaf spring side, composites are used primarily on higher clearance pickup trucks, large cargo vans and heavy-duty trucks. Here, prospects are a bit less promising. “For longitudinal leaf springs, we expect to see more of a component substitution of steel by FRP rather than system integration,” explains Fetscher. That said, the likelihood that composite leaf springs that debuted in the rarified reaches of high-dollar 1950s-era sports cars will reach commercial production in everyday automobiles has, after 50 years of composites research, never been higher.

Dr. Hillermeier and his co-authors published their finding in “Advanced Thermosetting Resin Matrix Technology for Next Generation High Volume Manufacture of Automotive Composite Structures,” Momentive Specialty Chemicals Inc., 2012.