Another RIM rolls along

"PDCPD" may not sound familiar, but it's one plastics acronym that you may start to hear more often. Polydicyclopentadiene, as this liquid resin is more formally known, has started to gain ground in a variety of heavy truck applications as an alternative to fiberglass-reinforced plastics (FRP) and sheet molding compound (SMC). "PDCPD is a good fit for many parts that have to be large, tough, and durable," says Garland Lee, a vice president at Metton America Inc., which supplies the material.

Molded in a reaction injection process, this decades-old material saw its first use as a way to make large chemical containers, giant valves, and septic tanks in Japan. Other big, tough parts have followed over the years. The material has, for instance, gone into shipping racks for engines and transmissions. And a variety of heavy equipment exterior parts have also been molded from PDCPD, particularly by Japanese manufacturers like Komatsu. According to Lee, most of these PDCPD applications spring from the material's ability to turn out really big parts cost-effectively.

Some of this cost reduction comes from the material's relatively low specific gravity (1.03), which can result in reduced material usage—as well as weight savings in the final part. The bulk of the cost advantage versus competing composites comes from the RIM process itself and the material's low viscosity. "It flows like milk," Lee says. That flow allows parts as big as 120 sq ft and 700 lbs to be molded with pressures less than 30 psi and cycle times under six minutes, Lee notes.

PDCPD offers some design advantages, too. Features such as molded-in inserts, ribs, and bosses are easily achievable in a RIM tool. Lee also points out that that PDCDP allows parts to have severe thickness transitions—in part because the low-pressure RIM process keeps molded-in stresses to a minimum. As an example, he holds up a container lid in which the solid-section thicknesses range from 9 to 50 mm.

Meanwhile, some of the traditional drawbacks of PDCPD have been lessened through years of development work. PDCPD has always had a reputation for being, to put it bluntly, kind of stinky. Recent improvements in controlling the RIM process have brought these manufacturing-related odors under control, Lee says. More important, the material lacks some of the mechanical and thermal performance of FRP and SMC. But in the right applications, users can design around these trade-offs and reap the cost, weight, and cosmetics benefits of PDCPD.

Keep on trucking. Nowhere are the advantages of PDCPD more evident than in the heavy truck industry, where the material has seen some of its greatest successes. Truck makers have been making exterior parts such as fairings and extenders since the mid-80's. This year, however, one truck maker applied the material to a far more challenging part: the hood. "It's the crown jewel of the truck exterior," says Scott Weatherford, a project engineer at Kenworth Truck Company (Kirkland, WA). Not only do hoods have to meet stringent requirements for mechanical and thermal performance, but they also have to resist damage and have a defect-free appearance.

Reaction injection molded PDCPD makes sense at production volumes too high for time-consuming hand lay-up and too low for expensive injection mold tooling - as this graphic from Kenworth Project Engineer Scott Weatherford shows.

Kenworth managed to achieve all these goals and more when it adopted PDCPD for the hood of its top-of-the-line W900L truck. The new PDCPD hood, which went into full production in March, replaces a 12-year old FRP version produced in an open-mold process. "FRP has been more than adequate in this application but we thought PDCPD could do even better," Weatherford says. And it did do better. PDCPD outperforms alternative hood materials on several scores, including:

• Weight loss. With a density that's 28% less than FRP and RTM, 48% less than SMC, and 62% less than aluminum, PDCPD provided an obvious path to weight savings. Some of the savings were offset by PDCPD's comparatively low tensile modulus, which added weight in the form of extra structural reinforcements. But the PDCPD hood still weighed 84 lbs less than its FRP predecessor while maintaining a comparable stiffness, Weatherford reports: "And every ounce helps us improve fuel economy."

• Sticks and stones. PDCPD has also proved more resistant to bumps and bruises than traditional hood materials. "Because it has a lower tensile modulus, we knew it would be more flexible and more resistant to impact damage," Weatherford explains. As a test, Kenworth in 1995 put a prototype PDCPD hood on one of its logging trucks—which spend much of their time on unpaved roads. "After 500,000 kilometers, the hood still looks fantastic," Weatherford says, noting that the hood has even survived the impact from a dropped log. "FRP would shatter, but the PDCPD just dented and then popped out." PDCPD also fights the "star cracking" that occurs on the top of an FRP part when its underside takes an impact. "PDCPD is a remarkably tough material," Weatherford says.

• Beautiful skin. PDCPD has demonstrated an ability to achieve Class-A surface quality with minimal preparation for painting. What's more, the material produces a B-side that's smoother than open molded FRP. "With FRP, the B-side is typically full of glass splinters, which can be a nuisance for the assembly people," Weatherford says.

• The bottom line. On a part-cost basis, PDCPD at first glance yields some mixed results. It does cost more per pound than FRP, SMC, or aluminum. But the RIM process used to make PDCPD parts supports automation that produces labor cost reduction versus FRP. And the nickel shell tooling for these RIM parts costs far less than the steel molds used for SMC. Life-cycle costs, which Weatherford took into account as part of an ROI analysis, decisively tilt the scales in favor of PDCPD. Because of its damage resistance, for example, PDCPD has a lower expected warranty cost. Taken together, these factors resulted in savings that Weatherford describes as "substantial."

Design for the long haul. For all these advantages, switching to the new material did require some extra engineering attention. "Some of the material characteristics required careful design considerations to prevent problems with the final hood," says Weatherford. To design around PDCPD's shortcomings, Kenworth engineers performed FEA simulations for each iteration of the hood design, looking primarily at stresses and modal frequencies. This CAE work helped them manage PDCPD's low tensile modulus, which required a beefed-up structural framework and a UHMWPE wear strip at the interface with the cab. Weatherford points out that this loss of stiffness could have been particularly vexing since the PDCPD hood had to recreate the shape of the stiffer FRP version to fit on older trucks.

Then there's thermal performance. PDCPD has a relatively high coefficient of linear thermal expansion (CLTE), making dimensional stability a problem when PDCPD parts have to mate with components made from other materials. Worried about joints pulling apart under thermal stresses as different materials expanded at different rates, the Kenworth engineers designed all of the hood's structural components, ten in all, from PDCPD. And with hood surface temperatures approaching 240F as new engine technologies hit the market, the hood lastly pushes the limits of what PDCDP can withstand with any margin of safety. "If surface temperatures were much hotter, we probably would investigate additional thermal insulation packages to protect the hood," Weatherford admits. "But the benefits of using PDCPD far outweigh this minor design consideration." For more information about engineering plastics from Metton America Inc.: Enter 533