Chopped carbon fiber, polyamide and innovation redefine the modern pickup truck bed

When asked about the most challenging aspect of the CarbonPro carbon fiber-reinforced thermoplastic (CFRTP) composite pickup box, which debuted on 2019 short-bed (crew-cab) GMC Sierra AT4 (off-road) and Sierra Denali half-ton pickups, Mark Voss, engineering group manager of advanced structural composites and pickup boxes at General Motors Co. (GM, Detroit, Mich., U.S.), laughs. “The most challenging part?” he asks. “Every part of this project was challenging. Everything was new: we had new design criteria, new impact and rear-barrier performance specs, plus we had a new material and process. Every part of the design process was a challenge at one point or another. However, the results speak for themselves: the CarbonPro box is a game-changing execution.”

Team approach

Voss, who previously worked on composite applications for Corvette, knows a thing or two about innovation and talking management into trying new things. Starting in 2011, he was involved with initial negotiations and later joint-development work with Teijin Ltd. (Tokyo, Japan) to commercialize automotive applications for Teijin’s then newly developed Sereebo CFRTP sheet composite (see “Sereebo CFRTP sheets: ‘Saving the Earth’”). Three years of what Voss calls “learning cycles” — running trials and evaluations, finding and addressing issues, then running more trials and evaluations — followed before team members felt they understood how the material behaved and where to use it. That’s when they began looking for an application and platform for Sereebo’s automotive industry debut. By 2015, they’d identified the pickup box on the 2019 model year Sierra Denali as ideal.

There were a number of factors that led to the decision. First, there was the economy of scale, as light-duty trucks (pickups and sport utility vehicles) represent the fastest-growing, most-profitable passenger-vehicle segment in North America. Second, since trucks use body-on-frame instead of monocoque construction, the box isn’t integral to the body-in-white, so it does not require the thermal performance to survive E-coat — the electrophoretic corrosion coating applied to chassis components at the start of vehicle build. Third, since the box sits outside the passenger compartment, impact tests and load cases would be less severe than those for the cab structure itself, making it a less-risky location to try a new material and process. Last, since GMC’s customers generally embrace technology that’s both high-tech and luxurious, they are expected to welcome all the unique features the team was planning.

Voss describes a “true team approach” to the material and process development work that went into making the CarbonPro a reality. That team included GM, Teijin and molder Continental Structural Plastics (CSP, Auburn Hills, Mich., U.S.), which joined the effort in 2015 to help commercialize the process GM and Teijin codeveloped and to produce the CarbonPro box. CSP, which was purchased by Teijin in 2017, has a long history of producing other composite pickup boxes in sheet molding compound (SMC).

Evolving technology

Sereebo is a sheet-form composite featuring a polyamide 6 (PA6) matrix reinforced with discontinuous/chopped carbon fiber (25 millimeters, 24K tow). The fiber bed has been described as being very well distributed, giving the material isotropic properties depending on how it’s molded.

The thermoplastic matrix provides many benefits. First, because they’re supplied pre-polymerized, thermoplastics mold much faster than thermosets, which polymerize and cross-link in the tool. The downside of pre-polymerized polymers, however, is that molecular chains are longer, stiffer, and more tangled, so it’s harder to get good fiber wetout. Therefore, fiber-volume fraction tends to be lower than with thermosets. Second, thermoplastics also tend to have lower density than thermosets, contributing lightweighting opportunities. Most importantly, thermoplastics produce far better surfaces out of the tool, eliminating the significant post-mold finishing — such as sanding and painting — that are often necessary with thermoset composites. In addition, a “tough” polymer like PA6 extends thermal performance and increases damage resistance compared to polypropylene, the most common matrix for thermoplastic composites used in automotive. Another benefit is that thermoplastic offal/scrap is easily recycled (melt reprocessed) by grinding the material and putting it into another feedstream with the same resin system — although this does shorten fiber reinforcements.

Of course, carbon fiber contributes higher stiffness and strength than glass fiber, at lower weight and thinner wall sections — albeit with a small sacrifice in impact strength, which can be improved via resin selection. Heavier tows are more affordable than finer aerospace-grades and are commonly used in the automotive industry, where modulus is usually the limiting factor in designs rather than ultimate strength. By using chopped- rather than continuous-fiber reinforcement, ultimate strength is reduced, but remains more than adequate for automotive applications and can be improved via thicker sections or by adding geometry (for example, ribbing) or both. Reportedly, a single grade of Sereebo in two thicknesses is being used to mold most of the pickup box’s components.

Hybrid forming

Although Sereebo flows once preheated and placed in a tool, to maintain its natural isotropy, the team isn’t flow-forming it like conventional glass-mat thermoplastic (GMT), direct-long-fiber thermoplastic (D-LFT), or SMC. Rather, an interesting hybrid forming process is used. It combines an innovative preforming step accomplished using a robot-mounted preforming device (RMPD) followed by compression molding at “conventional pressures.” The RMPD is described as complicated end-of-arm tooling that is unique for each part being molded. Parts are molded larger than necessary — then trimmed to final size after molding.

“Sereebo’s isotropic properties are worth their weight in gold so we created a process to retain those material properties,” notes Voss. “Still, we’re achieving depths of draw of 14 to 16 inches [36 to 41 centimeters] on the side panels in structural materials,” he adds.

“The Sereebo material molds like GMT and D-LFT,” explains Steve Pelczarski, CSP engineering director for program and product development. “However, we deliberately keep flow low by limiting blank temperature during preheating — a choice that protects both resin and UV-stabilizer — and by preforming the sheet over the press just before forming. The depth of draw and features you can produce in Sereebo are endless as long as you preform the shape prior to presenting material to the tool.”

The four biggest CarbonPro parts — the headboard, right and left side panels, and the platform/floor — are formed on a new 3,600-metric ton Dieffenbacher press with a rapid (5-second) open/close cycle (see “CarbonPro box: New forming process”) at CSP’s Huntington, Ind., U.S. plant, 30 minutes from GM’s Fort Wayne Assembly plant (Roanoke, Ind., U.S.) where 2019 Chevrolet Silverado and GMC Sierra pickups are assembled. Several smaller CarbonPro parts — in virgin Sereebo as well as in some recycled-LFT (using ground Sereebo scrap plus some virgin PA6 to enhance flow) — are compression molded nearby on a smaller 1,200-metric ton press. Three sub-bonding steps join cross-car sills, wheel-wells and side-panel modules, then those sub-assemblies are brought together in a final main-box bonding step where final box assembly occurs. A two-part urethane structural adhesive (Pliogrip 8500 from Ashland LLC, Columbus, Ohio, U.S.) is used throughout.

“We’re getting 75 percent property retention in the recycled Sereebo for left and right stake pockets,” Voss adds. “This is a big win-win, because it helps with our business case while making the process more sustainable.” Depending on how the post-industrial recyclate (PIR) parts fare in the field, GM and CSP have plans to repurpose 100 percent of Sereebo scrap elsewhere in the vehicle, which would make the new process zero-waste.

CSP also is producing compression molded end-gate covers in glass fiber/PP D-LFT, injection molded wheel-house assemblies and front filler panels in glass fiber/PA6, mini-sills in glass fiber-reinforced epoxy pultrusions, and three of the box’s four cross-car sills in Sereebo.

Customer-focused features

The CarbonPro box also includes special features that enhance the vehicle and its cargo space (see “Discriminating features”). First, the box has proven to be incredibly impact resistant (see video below), which is a huge functional benefit that eliminates the need for a bedliner. Not only will it not rust or dent, but the molded-in-color (MIC) black composite needs no paint or coatings to protect it against scratching and weathering.

Second, much work went into designing the corrugated floor structure. A light texture is used in troughs so dirt and grime wash away easily, while a “grippy” aggressive texture is molded into crests to ensure good stability even when the bed is wet or dusty. Special motorcycle pockets in the headboard and bonded tie downs (each capable of 227-kilogram loads) allow customers to secure two dirt bikes on left and right sides, or a Harley-Davidson “Fat Boy” motorcycle in the center-front of the box. Additional tie downs are distributed strategically to help stabilize various loads. Integral lights illuminate the box interior around fender flares and the tailgate (either standard or six-position Multipro — see “Discriminating features”).

The composite box plays an important role in the Sierra’s mixed-materials construction (combining aluminum, high-strength and roll-formed steel, plus composite and plastic), a combination that shaves 163 kilograms off the outgoing model.