SMC Flexes Muscles in “Other” Markets
Often identified with automotive body applications, sheet molding compound shows growing promise in a variety of other temperature- and weather-resistant applications.
By Jeff Sloan, Editor-in-Chief | December 2007
The role that sheet molding compound (SMC) plays in automotive applications is well known and well documented. SMC, a compression-molded blend of polyester or vinyl ester resins, chopped glass and mineral filler, for many years has been part of an ongoing effort to lightweight vehicles — particularly as fuel prices climb (see “Related Content,” at left).
Source: Meridian Automotive Systems
This SMC diesel engine manifold cover for Mack Trucks’ 13-liter engines was developed by Meridian Automotive Systems (Allen Park, Mich.). In tests, the cover withstood sustained temperatures of 190°C/374°F for 12 continuous hours of operation.
What’s less known, however, is SMC’s role outside of the automotive industry. Away from high-profile automobiles, SMC’s strengths are employed in different ways, and the material is chosen as much for its value, high strength and weatherability as it is for its manufacturability. Although most composites manufacturers typically use resin systems and compounds formulated by industry suppliers, many of the molders contacted for this story manufacture their own resins and compound their own SMC, designing them to meet needs in specific applications or end markets. Application parameters revolve around a variety of issues: UV, impact and moisture resistance, and surface-quality demands that drive the need for customized material development. The following examples show that SMC’s adaptability has much to offer in markets where products might lack the visual appeal of a high-gloss car hood but still are relied upon in a variety of performance-critical environments.
More Than an Enclosure
One such environment is in fact an environment in and of itself: the industrial enclosure. While it’s easy to overlook the millions of gray boxes that top utility poles, line industrial shop walls and rise out of the ground along utility rights of way, it’s not a stretch to say that all-purpose, all-weather, impact-resistant enclosures are a critical part of the electrical utility lifeline around the world.
Materials used for these enclosures run the gamut, depending on application, ranging from polymer concrete to metal to thermoplastics and thermosets. SMC is one of the dominant thermoset materials, and one such application is Lightning Switch, a patented, wireless, battery-less, remote-controlled system that sends coded radio signals to a receiver to power lights, appliances and other devices. It can be installed as a new switch or replace existing wired switches. Lightning Switch, marketed by PulseSwitch Systems (Norfolk, Va.), grew out of a NASA piezoelectric research and development program.
The challenge for PulseSwitch was to find an enclosure for receivers in industrial environments. A metal enclosure was eliminated because it would interfere with radio frequency signals. Brad Face, president at PulseSwitch, eventually came across Stahlin Non-Metallic Enclosures (Belding, Mich.) and that company’s Diamond Shield enclosures, a family of SMC-based products that enable enclosures of any size to be mounted at any height or depth, with enclosure access doors hinged in any direction as well. They come in 10 sizes, ranging from 6 inches by 6 inches by 4 inches (152 mm by 152 mm by 102 mm) to 20 inches by 16 inches by 10 inches (508 mm by 406 mm by 254 mm).
Mike Jackson, product marketing manager at Stahlin, says the 14-inch by 12-inch by 8-inch (356-mm by 305-mm by 203-mm) enclosure (with nominal wall thickness of 0.125 inch/3.2 mm) developed for PulseSwitch by Stahlin met several needs, providing radio frequency transparency, high strength and good surface quality. The material is a patented, unsaturated polyester SMC developed in-house by Stahlin. The glass content is 22 percent. Jackson says Stahlin uses SMC for 95 percent of its products and favors the material for its durability, corrosion resistance, application flexibility, surface quality and moldability. All products, he says, have molded-in color.
Core Molding Technologies (Columbus, Ohio) faced a larger challenge when it was approached by a supplier of enclosures for in-ground vault systems for electric and water utilities. This customer, says Core sales engineer Brian Gourley, produces a line of polymer concrete enclosures for which it wanted to assess the viability of using an SMC. The customer had evaluated composites for enclosure use a decade ago but, according to Gourley, it had “never gotten off the ground.” The customer’s perception at the time was that it would be difficult for composites to effectively replace polymer concrete. The “sale” of SMC by Core as a replacement would be as much an intellectual exercise as it would be a technical one.
“When we start talking about composites, it’s a whole different world,” says Jeff Blevins, production development manager at Core. “You have to go through this iterative process.” The process often involves some basic education about composite materials, how they differ from traditional materials like steel and concrete, and discussion of tooling and manufacturability, not to mention a discourse on up-front cost vs. lifecycle costs, to put the customer’s focus on the latter.
The utility vault customer’s initial approach was cautious: Core was asked to convert a single enclosure cover to SMC. Gourley says Core produced a cover design that uses a polyester base resin with glass content of 38 to 40 percent. The 22-lb/10-kg cover, which was produced in a 25- to 45-minute cycle in polymer concrete, was reduced to 7 lb/3.2 kg with SMC in a two- to five-minute cycle.
The success of the cover allowed Core a chance to prove the viability of SMC in the enclosures themselves. The company went to work on two in particular, one 17 inches wide by 24 inches long by 24 inches deep (432 mm by 610 mm by 610 mm), the other 48 inches wide by 60 inches long by 24 inches deep (1,219 mm by 1,524 mm by 610 mm). Initial design work included finite element analysis (FEA) with software from ANSYS Inc. (Canonsburg, Pa.). Each box included an integrated SMC ring molded into the rim of the enclosure; this allowed Core to reduce cycle time and therefore reduce overall costs. Core also was able to integrate several optional knockouts in the enclosures so that installers in the field can pass water lines and electrical cabling through the box wall as needed.
More challenging, however, was the job Core had to do to convince the customer of SMC’s value. Gourley says the initial conversation regarding SMC focused on the material’s unit cost, which easily exceeded the $0.01/lb to $0.04/lb unit cost that the customer had paid for polymer concrete. “The hardest part was getting the customer to look at the overall costs and not get hung up on one particular aspect of cost,” says Gourley. Although Core wouldn’t provide specific figures, in the end it was able to prove SMC’s value through savings in tooling costs and weight. As CT went to press, Core was awaiting a production decision from the customer in early 2008.
Taking the Heat in Engine Applications
As its name implies, Meridian Automotive Systems Inc. (Allen Park, Mich.) has a well-earned reputation as a supplier of composite and plastic parts to the automotive industry. But in recent years the company has branched out into nonautomotive markets, emphasizing consumer and industrial products. David Ulrich, director of sales for this segment, says SMC has been a part of that effort.
Looking back to the 1980s, Ulrich sees much progress in the evolution of SMC materials, design and application, pointing to a marine engine cowl that 20 years ago was fabricated from a polyester-based SMC with a specific gravity of 1.9 and 5-mm/0.2-inch walls. The resulting product was, ultimately, deemed too heavy and too bulky, forcing many boat manufacturers to switch to metal. Today, he says, the SMC engine cowl is back, now with 3-mm/0.12-inch walls and a specific gravity of 1.3 to 1.6, providing a much lighter, more easily handled part.
