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).
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.
SMC’s goal, says Ulrich, is to meet the challenge posed by thermoplastic solutions. “SMC gives molders the structure, impact resistance, appearance and weatherability without thermoplastics, which,” he maintains, “tend to warp and fade.”
These qualities are in demand in another application pursued by Meridian: mounting platforms for heating, ventilation and air conditioning (HVAC) systems in commercial and residential environments, mounted on a roof or adjacent to an outside wall. The traditional material in this application is steel, which Ulrich says requires a great deal of stamping and riveting, depending on the HVAC system size and type. Further, because of their proximity to ventilation systems, the platforms must pass stringent Underwriters Laboratory (UL) smoke, flame, UV and structural integrity requirements.
Meridian’s replacement platforms, molded of polyester with 32 percent glass, are designed to support residential systems of up to 500 lb/227 kg and commercial systems of up to 1,000 lb/454 kg. They are molded in one piece, include an integrated drain pan, and require no brackets or other attachments. Ulrich says the pads are “structurally as strong as steel, if not stronger. It represented an overall cost savings to the customer and it passed the UL smoke and flame requirements, which to the HVAC industry is critical.”
The industrial market overall, admits Ulrich, is decidedly cost-conscious, which means that materials like SMC have to prove their value over the life of a product. “It allows customers who have a steel mindset to see that there’s a whole other world that provides them design and manufacturing flexibility,” says Ulrich, adding that SMC’s relatively low-cost tooling has become a significant selling point.
This isn’t to say that SMC’s mechanical performance isn’t appealing. Meridian has made major inroads with nonautomotive transportation and heavy truck applications, particularly the manifold cover for the 13-liter Mack Truck diesel engine. This 4-ft by 2-ft (1.2m by 0.6m) vinyl ester cover is in production and has to withstand continuous exposure to 190°C/374°F for 12 hours, and afterward, says Ulrich, “it has to be as flat as a ping-pong table.” It is, and Meridian is now trying to migrate its SMC cover concept to other truck engines for customers such as Cummins and Navistar.
For the future, he says, look for more of the same from SMC: thinner parts with better structural integrity and increased integration of nanomaterials and air-injected “microbubbles” to enhance mechanical strength.
Breaking Into Braking Apps
Premix Inc. (Kingsville, Ohio) has a long history of developing and molding SMC and BMC parts for a variety of end markets, ranging from automotive to industrial to power tools. Two of the more notable applications are a piston housing for an air spring suspension system for large trucks and an electrical enclosure.
The piston components, formerly aluminum, are used in Firestone’s reversible-sleeve air spring suspension systems for large trucks and now are molded with Premi-Glas 1200-50 VE, a glass-reinforced vinyl ester SMC formulated by Premix. The company says the material’s strength and stiffness hold part shape under loads greater than 20 tons (18 metric tonnes), and the piston housing’s inside surface reportedly stays smoother, prolonging the life of the air bag inside the piston. Aluminum, says Premix, oxidizes and holds grit that can wear the air bag and cause failure of the suspension system. Further, the piston was designed to incorporate inserts and studs to cut the cost of assembly.
Premix’s electrical enclosure was designed and manufactured for Square D (Palatine, Ill.), which was looking for a way to maintain rigorous performance standards while reducing the cost of its motor control centers. Motor control centers are used in commercial and industrial facilities to control and distribute electricity. They must have good dielectric strength and arc/track resistance to meet insulation demands and to protect against system failure, fire and potential injury. The biggest need, says Premix, was to reduce enclosure wall thickness using a high-strength, rigid material that could withstand electric magnetic forces generated in short-circuit testing up to 100,000 amps at 600V. A Premix engineering team formulated Premi-Glas 3200-30, a high-strength, low-shrink SMC for the application, and during the design process, the team discovered that 16 individual components could be consolidated into two and that secondary finishing work could be automated. Ultimately the new enclosure design uses less compound, fewer parts, requires fewer fasteners and can be assembled more quickly.
Insulating Transit Third Rails
Daniel Leslie, president and CEO of Penn Compression Molding (Smithfield, N.C.), has his eye on another market: urban transit. One of the products for which Penn is best known is an SMC insulator for the “third rail” in urban transit systems. The third rail, which sits between the two rails that support the railcars, provides electricity for the train and therefore requires a series of insulative supports that prevent the third rail from contacting the ground.
Penn’s insulator is comprised of a cylindrical base with a flat top about 8 inches/203 mm wide on which the rail rests. The base, says Leslie, is typically about 6 inches/152 mm in diameter. The entire insulator ranges in height from 6 inches to 12 inches (152 mm to 305 mm), depending on the rail system, transit type and location (different cities have different requirements, says Leslie). SMC was picked for this application, he says, because it offers the strength and weatherability required for these high-wear parts.
For one particular insulator, the two parts, base and a top, are molded in a 1+1 cavity family mold (both parts, although different, made in the same mold) from 48-inch/1.2m wide SMC sheets. The material, says Leslie, is a polyester SMC provided by Industrial Dielectrics Inc. (Noblesville, Ind.). After molding, the base and top are bonded together to complete assembly. The insulators are designed to last 15 to 20 years.
Shooting SMC Trouble
Two of the most common problems in compression molding are incomplete or inadequate coverage of in-mold coating, cracking on mold open and knitline/flowline problems that create structural weakness in a part. If ever you’ve stared, helplessly, at flawed, out-of-spec parts coming out of your compression molding machine and scratched your head over the cause of the problem, there’s a decent chance the problem is not your SMC formulation but your compression molding press.
“Most people don’t have a clue how their machinery is performing,” says Mark Bohler, president of Plant Engineering Services Inc. (Fort Wayne, Ind.) and a 20-plus-year veteran of compression molding. This is particularly true, says Bohler, of older machines that lack modern control and software systems. Shop personnel modify settings with little understanding of the molding process factors that most influence part quality. These problems, however frustrating, are easily manageable, according to Bohler, who says there is a need “to bring science to the art of molding.”
In his experience, reports Bohler, process optimization in compression molding almost always starts and stops at one basic machine function: “Ninety percent of process improvement comes from the final mold closing and initial opening. It’s as simple as that.” When they evaluate a machine or process, Bohler says he and his engineers start first by verifying that all external variables — temperature, vacuum, tonnage, closing speed — are accurate and consistently held. Next, sensors are attached to each corner of the tool; these sensors interface with Plant’s process analysis software and provide the data needed to evaluate the quality of mold close. The sensors report parallelism, whether the tool is off stops, and provide data on hold consistency, decompression rate, and other variables.
“If we see variation in the closing pattern of a tool, then we know we’re going to have bad parts,” says Bohler. Nonparallel close, he says, is a common ailment of many compression processes, causing uneven or incomplete material flow in the tool. Another is inconsistent open position of the tool before injection of an in-mold coating. This is vital to good paint spread, says Bohler.
After cure, the initial rate of decompression before mold open is the variable that most affects the quality of part release. If decompression is too fast, or if sidewalls aren’t opened straight, lifting and part cracking can result. “Ninety percent of cracking occurs during open — especially if opening unevenly,” he says. “Modern systems have programmable decompression rates because of that.” He adds that up to 25 percent of overall mold pressure capability is required to open a mold, which, if too violent (a “pop” as small as 1 mm/0.04 inch) can introduce cracking. Additionally, oscillation in the closing profile can cause glass in the resin to clump, producing waviness in the part surface.
Bohler also says he often finds that machine variables change depending on who’s running the machine. “You wouldn’t believe the difference between first shift, second shift and third shift,” Bohler quips.
Typical solutions, Bohler contends, are relatively simple, and in most cases can by tackled by “a decent process-and-tool engineer to make the mold close and open nice and clean.” Alternatively, he might recommend a change in the material charge pattern to more evenly distribute mold pressure and maintain parallelism. And, easiest of all is to define machine parameters for a given part and mold and stick to them, no matter which shift’s operators are on duty.
SMC And Weatherability
Ashland Performance Materials, Composite Polymers (Dublin, Ohio) reports that it has completed research that demonstrates that a new sheet molding compound (SMC) formulated to withstand the effects of weather should improve SMC outdoor performance and simplify processing as well. Ashland presented the results from its tests in a paper presented at the American Composite Manufacturers Association’s October trade show in Tampa, Fla.
Of particular interest to molders is the fact that color can be molded into the part, thereby eliminating painting and other secondary finishing operations. “The results from our laboratory and outside weather tests of Ashland’s AROTRAN 805 SMC formulation show improved weatherability of molded-in-color SMC components,” says Cedric Ball, manager, global transportation for Ashland Composite Polymers. “Automobile manufacturers and composite molders can realize significant cost savings using UV-stable, molded-in-color systems. With no need to paint, the process can be more environmentally friendly as well.”
The AROTRAN 805 material is a polyester thermoset resin available with glass loadings of 30 percent to 50 percent. Rob Seats, a principle scientist with Ashland who worked on the material and the study, says the resin is a next-generation SMC based on Ashland’s established EKADURE weatherable SMC formulation. The goal with the new AROTRAN resin, says Seats, was to build on EKADURE’s impressive weatherability while enhancing its dimensional stability. The weathering tests, which exposed the AROTRAN 805 material to a xenon arc for 3,750 hours, proved the SMC’s low-shrink attributes and, says Seats, makes the material suitable for non-Class A surfaces (such as pickup truck beds) and nonautomotive, outdoor applications.