Lightweighting is not the only way to achieve fuel-efficiency goals

Compromises must be made to achieve volume production requirements of carmakers

Road to fuel efficiency graphic

Automotive OEMs know the mandated goals for the Corporate Average Fuel Efficiency (CAFE) standards: 38.6 mpg by next year and 54.5 by 2025. According to David Trudel-Boucher of the Advanced Polymer Composites group of the National Research Council Canada, OEMs want low-cost solutions in short cycle times. Yet, the “automotive industry needs to adopt processes that meet the realities of today’s requirements,” said Trudel-Boucher, who presented a plenary lecture, “Advanced Polymer Composite Manufacturing Processes for Automotive Applications,” at the Plastics-In-Motion conference in Charleston, SC, which ran from May 8 to 11. 

Trudel-Boucher noted that while “regulations are putting pressure on the industry,” there are more and different ways to achieve fuel efficiency. “Lightweighting is just one,” he said, underscoring some of the barriers to polymer composite materials in automotive applications. First, the industry lacks true knowledge of the materials available, which makes it difficult to design new, lightweight vehicles. “As the industry gets more familiar with the materials, designers can introduce these into the designs of their vehicles,” he commented, noting that several car manufacturers are relying on the introduction of continuous fiber composites to achieve their weight reduction objectives.

But that leads to a second barrier. To incorporate these materials into automotive components, said Trudel-Boucher, a compromise must be made between the processes to produce polymer composite parts with the volume production that automotive OEMs require. Some of the processes Trudel-Boucher discussed include resin transfer molding (RTM), an old process used for decades in aerospace and automotive (sports and luxury vehicles). RTM offers a high fiber content (up to 60%), high dimensional stability and a good surface finish (up to Class A). That sounds like a winner until you get to the “long cycle time” the process requires—from 30 minutes to two hours. That’s something the automotive OEMs can’t live with because of the high volumes required.

One alternative might be high-pressure resin transfer molding (HP-RTM), a variation of the RTM process, to meet the series production requirements of the automotive industry. HP-RTM offers reduced cycle time, the ability to use automation and compatibility with carbon-fiber reinforcement. A preforming process is involved, which can be done in several ways: A slurry deposition, in which chopped fibers are deposited into the mold (3-DEP, or dimensional engineered preform). Recycled carbon fibers can be used in the process, which makes it attractive to OEMs, Trudel-Boucher explained, noting that BMW and several other OEMs are already using it. “BMW was one of the first to adopt this process,” he added.

High pressure ensures low permeability of the carbon-fiber reinforcements, a high fiber content for structural applications and snap-cure resins (epoxy and polyurethane) that offer cure times of less than five minutes. “The high pressure is required to fill the mold cavity and impregnate the preform before the resin reaches its gel time,” said Trudel-Boucher. There are two methods of HP-RTM: Injection and compression. “With the injection method, you can achieve very complex, 3D parts; however, you need to predict the flow of the resin to get high-quality parts,” he added.

Compression HP-RTM is a process similar to the injection process, in that the resin and hardener are mixed and then injected into a partially closed cavity (as opposed to a completely closed cavity in injection HP-RTM), and the material is injected at medium pressure as the resin flows within the cavity gap. The mold is then closed completely and the resin impregnates the preform in the z-direction (through the thickness). This process is best suited for 2D parts. “The geometry to achieve quality with this process will be less complex, such as you see in roof and hood structures,” Trudel-Boucher explained.

Trudel-Boucher also discussed stamping/overmolding using “organosheets,” a process that is similar to metal stamping but adapted to advanced composites. A pre-impregnated, continuous-fiber thermoplastic composite sheet is heated above its softening point in an external oven, then transferred to the press, where the “blank” is deformed under pressure (in a short 30-second time frame). The stamping can then be over molded, if required, using an over-molding operation after the stamping. The same tooling is used to obtain “welded” bonding between the hot composite and over-molding material, which provides the addition of functional features, he explained. This also provides new possibilities for lightweighting and part integration to achieve “cost-neutral” parts.

Another process is in-line compounding direct long-fiber thermoplastic that uses fiber rovings and thermoplastic granules. In this process, the base polymer (PP mainly) and additives such as heat stabilizers, lubricants, and coupling agents are added in the plasticizing zone of a twin-screw extruder. Fiber reinforcements are added in the form of rovings fed directly to the twin-screw extruder (glass fiber mainly, carbon fiber, natural fiber, aramid). The fibers are broken by the rotation of the extruder, and move to the compounding section for optimal glass wet-out and dispersion.

The direct, long-fiber thermoplastic (D-LFT) molding process has several advantages: Low cost of raw materials, possible complex geometry, fiber length retention compared with injection molding and high throughput. This process is largely dependent on geometry, and the high fiber length needs high-pressure (5-10 MPa/725-1450 psi) and, therefore, a large press, noted Trudel-Boucher, adding that the process can also be hybridized with advanced thermoplastic composites. The D-LFT process is good for under-body shielding components, end carriers, door panels and tail gates.

The continuous lamination process is used less in automotive but Trudel-Boucher believes it has “strong potential” in that market. “It’s under-used in automotive but it’s a cheap option, as the process yields several meters per minute,” he said. There is the “double-belt lamination process and the roll forming (calendering) process, which are good for fabrication of sandwich panels.” Applications include inside walls in the trucking industry.

OEMs need to weigh the complexity of the component vs. the performance requirements, noted Trudel-Boucher. Additionally, recycling is always important. “That’s the big issue for thermosets—scrap at the end of life, and the material’s cost, are problematic,” he said.

“You have to identify the right process for the application,” Trudel-Boucher concluded, “and then you have to accept that there will be compromises you have to make.”

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