Chevrolet’s seventh-generation Corvette uses an updated SMC that cuts 40 lb from the vehicle’s weight, compared to traditional materials.
Photo courtesy of General Motors

While Ford’s F-150 is the latest example of swapping steel for aluminum to cut weight, the trend has been taking place quietly for several years with many truck hoods, SUV liftgates, and sedan trunks moving to lightweight metals.

Following that trend, automakers are also swapping out metal enclosures and structural components with high-strength composites, and that trend should accelerate rapidly throughout the next five years.

“Program plans in the pipeline with OEMs for vehicles that will come out in 2020, 2021, and 2022, now is the time that they are deciding on the best material selections,” says Frank Silvagi, vice president of engineering at Continental Structural Plastics Inc. in Auburn Hills, Michigan. “We’re engaged in so many of those conversations. We’re in the middle of decision-time right now.”

Tough, easily formable plastic components have been showing up in vehicles for several years in bolt-on parts – fascias, pickup beds, and fenders. Next-generation parts are more integrated into cars and trucks – lift gates on SUVs, roof panels for Jeeps, and even structural components that must face rigorous crash testing.

Critical structures

Ask most engineers at General Motors (GM) what vehicle has done the most to advance composites, and they’ll point to the Chevrolet Corvette. Back to its 1953 roots, designers promoted the vehicle’s glass-fiber reinforced plastic body panels. Continuing development of materials has kept the performance car’s weight low for decades.

Silvagi, however, points to a different Chevy when talking about signs of things to come – the Spark EV. The gasoline version of the Spark car, about halfway in size between a Smart Fourtwo and a Mini Cooper, is an inexpensive vehicle that GM targets toward young, first-time buyers. Though production of the electric version ended this year, the $18,500 vehicle (before federal tax rebates) had an 82-mile electric-only range and was sold primarily in California.

It also has a sturdy plastic enclosure surrounding its lithium-ion battery pack.

“This is the most structural part we’ve ever made,” Silvagi says. The enclosure, made from a pre-preg directional fiber, is significantly stronger than traditional sheet-molded compounds (SMCs).

Mark Verbrugge, director of research and development at GM’s Chemical and Material Systems Labs, says designers are considering a wide range of materials to find the most weight-efficient, manufacturing-friendly fit for each component.

“We’re going to see more of this. For battery enclosures, composites are really an ideal material,” Verbrugge says. “The combination of ductility and strength gives you the required toughness.”

He adds that high-strength steel and aluminum still tend to be the preferred materials for structural, underbody load paths, but he could see that changing in the near future as companies continue to explore ways to shed weight.

“Right now, there’s no OEM that’s selling, in high volumes, a vehicle with underbody load paths made from fiber-reinforced composites,” Verbrugge explains. “But there’s so much activity and progress being made with carbon-fiber composites that I wouldn’t be surprised to see some underbody paths in high-end luxury vehicles at some point.”

Composite advantages

Silvagi says with new vehicles, materials companies are fighting to show that their products are ideal for each part. Steelmakers have developed new, higher-strength, lower weight alloys; aluminum producers are developing processes to make that material easier to form into complex parts; and composites suppliers are working to lower the costs of carbon-fiber materials and increase the strength of glass-fiber.

“Weight is certainly an advantage for us, especially compared with steel, and we can be competitive with aluminum,” Silvagi adds. “The tooling and investment required for composites is significantly less than for steel or aluminum – as much as two-thirds less. Our tooling processes do not require the same capital investment as what aluminum parts need to make 3D shapes.”

GM Verbugge says automakers are carefully considering all of those issues when they spec parts for future vehicles. He adds that designers are much more open than they were a few years ago to mix-and-match materials, as long as parts can be cost-effectively manufactured to stringent quality standards.

Tina Oaks attaches wiring harnesses on a Spark EV battery pack at General Motors’ Brownstown Battery Assembly Plant near Detroit. The electric vehicles’ batteries are housed in a pre-preg, directional-fiber composite enclosure supplied by Continental Structural Plastics.
Photo courtesy of Continental Structural Plastics

“It’s certainly part of our strategy – the right material, the right place, the right application,” Verbugge states.

Today, designers tend to favor composites in cosmetic applications that require complex shapes.

“A lot of liftgates these days, because of the aggressive styling from the OEMs, if you’re going to go with a metal piece, the suppliers are going to have to go to a three-piece design. If you go with composites, we can do the same kinds of aggressive design in two pieces – a full inner and a full outer,” Silvagi says. “There are liftgates out there that you couldn’t even dream of stamping out of aluminum because the shapes are so complex you could never draw and form it.”

Fitting into the current assembly process is key. Composite body panels, for example, must be tough enough to withstand minor collisions on the finished vehicle and able to handle high-temperature processes in the paint shop such as electro-coating (e-coating). While plastic parts don’t need the corrosion protection, the traditional plant build process involves assembling the entire vehicle body in the body shop and running the entire structure through e-coating.

“There are composites that are capable of handling our e-coating processes for the metals embedded in them,” Verbugge says. “In the past, that would have been a bigger issue. I wouldn’t say it’s not an issue any longer, but it’s much less of an issue.”

William R. Rodgers, light polymer systems technical fellow with GM research and development, adds that as designers and manufacturers increase their use of composites, they’ll likely move some attachments off of parts, creating body panels that don’t need to go through high-temperature processes. Such design changes will lower manufacturing costs by eliminating processing steps and give designers a wider range of options when choosing which composites to use.

Increasing familiarity

One of the biggest challenges to getting more structural composites into vehicles is a lack of familiarity with the materials, Rodgers adds. Most automotive structural engineers have spent their careers working with steel, occasionally dabbling in aluminum. They’re in the early stages of studying composites for those functions.

“A lot of it comes back to learning how to model composite structures better,” Rodgers explains. “Modeling influences our designs, so we need to have a better understanding how composites perform and why they perform the way that they do. Our vehicles are completely designed in math before we build them. With composites, we’re still in the learning curve to understand the constituent relationships to understand them so we can feed them into our computer-aided engineering tools.”

He adds that automotive designers are working with modeling software companies to improve design and simulation tools for a better understanding of how and where to use composites instead of metals.

Silvagi says composites producers are working with automakers to show how they can use materials today without redesigning vehicles or reworking manufacturing processes.

Chopped glass drops into a resin mixture on a Continental Structural Plastics sheet-molded compound (SMC) line. Improvements to SMCs are allowing automakers to lower weights and use reinforced plastics in more applications.
Photo courtesy of Continental Structural Plastics

By 2025, many auto plants will have to retool to work with multi-material structures, swapping out weld-only robots for ones that can weld, rivet, and apply adhesives. Until then, Silvagi says composites must be body shop-friendly. Because most of CSP’s parts are bolt-on components, attachment hasn’t been a major issue, but he says he expects different manufacturing processes in the future.

“Long term, the further you consider what you can do with different processes, the more design changes you’re going to see. If you decide to use a composite B-pillar instead of welding in a metal one, you might be able to replace a welding process with a simpler mechanical one. It makes us rethink the entire process for manufacturing a vehicle,” Silvagi says. “We have strategies, shorter-term, that offer benefits while staying within the constraints of current plant systems, but to get the full cost and weight benefits from material replacement strategies, you’re going to need to rethink how you manufacture vehicles.”

Carbon fiber developments

The vast majority of composites being used for automotive structural and exterior applications are glass-fiber reinforced plastics, and there’s a limit to how strong those materials can get. The next generation of composites will use carbon fiber reinforcement. Already popular on low-volume, expensive sports cars, the material is too costly to consider for mass-market cars and trucks today.

In March, Continental Structural Plastics and Mitsubishi Rayon announced plans for a joint venture to develop lower-cost SMC and pre-preg compression-molded composites (PCM). Silvagi says the venture is in its early stages so he can’t share much detail on plans, but composites makers see huge potential in the market if they can lower material costs.

Chevrolet’s Spark EV electric car ended production earlier this year.
Photo courtesy of General Motors

“There are multiple proposals we’re working on with OEMs around the world who are looking at carbon fiber for mass-market vehicles. It’s pretty easy to use and rationalize for supercars, but using that in every day, higher-production vehicles is a challenge,” Silvagi says. “Keeping the cost-per-pound down is key.” GM’s Verbugge agrees, adding that carbon fiber will eventually land on mass-market vehicles. Predicting the timing of that is difficult, though.

“We look at everything from cost-per-gram of CO2 emissions. We’re going to see more composites in the future, especially carbon fiber composites,” Verbugge says. “It’s our full intention to move these materials into the mass market as it becomes possible to do that. The timing is going to be when it makes sense. We have to keep customer value in front of us.”

Silvagi concludes that 2025 fuel-economy mandates and competition have automakers willing to consider a wider range of material and powertrain options than ever before, so composites will have the opportunity to compete for more parts.

“Ford really shook things up with the aluminum F-150, but not in the way they expected. Everybody’s jumping on and saying ‘there’s something to this aluminum thing.’ But instead of the OEMs all trying to figure out how to use aluminum, they’re trying to figure out what’s better than aluminum,” Silvagi says. “We’re getting calls from people looking to do better than aluminum on weight and see how much they can save on tooling.”

Continental Structural Plastics Inc.

www.cspplastics.com

General Motors Co.

www.gm.com

About the author: Robert Schoenberger is the editor of TMV and can be reached at 216.393.0271 or rschoenberger@gie.net.