Since the early 1970s, when fuel crises forced automakers to find ways to reduce vehicle weight, sheet molding compound (SMC), a compression-molded blend of polyester or vinyl ester resins, chopped fiberglass and mineral filler, has been part of the lightweighting solution. SMC offers up to 30 percent less weight than equivalent steel panels and has been specified for a number of components on sports cars, luxury vehicles, SUVs and, for the last 15 years or so, heavy-truck cabs. These applications have included painted Class A exterior body panels, underbody shields, and semistructural components. In recent years, SMC has gained popularity in pickup truck cargo beds. According to data supplied by the Automotive Composites Alliance of the ACMA (Arlington, Va.), North American use of SMC in automobiles and heavy trucks totaled almost 270 million lb (123,000 metric tonnes) in the 2006 model year. A 2005 estimate prepared by DSM Composite Resins AG (Schauffhausen, Switzerland) put the European consumption in the range of 220 million lb (100,000 metric tonnes). Globally, the transportation market for SMC is very important to molders and material suppliers.
At the Society of Plastics Engineers' Automotive Composites Conference (Sept. 12-14, 2006, in Troy, Mich.), keynote speaker Dr. Jeff Helms, manager of paint engineering for Ford Motor Co. (Dearborn, Mich.), described Ford's experience with SMC. In 2000, Ford consumed approximately 112 million lb (50,000 metric tonnes) of SMC composites, primarily in mixed material applications, such as an SMC hood or decklid on an otherwise steel vehicle, making the company, that year, the largest automotive user of Class A SMC in North America.
At the time, however, Ford and other SMC users were plagued with numerous quality problems, particularly in the finishes of painted parts. Consumers today expect the painted surface of vehicles to exhibit the same high level of smoothness and gloss, regardless of the material used. High defect rates led Ford and other auto OEMs to reduce the SMC content of vehicles, such that by 2005, Ford was using significantly less of the material. SMC suppliers responded to these quality issues with new developments in sealers that reduced to a large degree the main source of the problem: paint "pops"in the surface as heat from the OEM paint curing ovens — designed to process steel vehicles — caused entrapped solvents in SMC parts to out-gas (see “Related Content,” at left).
The introduction of toughened base polyester resins, notably Atryl TCA (Tough Class-A) from AOC Resins (Collierville, Tenn.) and AROTRAN 700 series resins from Ashland Performance Materials, Composite Polymers (Dublin, Ohio), provided the balance of the solution, at least for assembly lines using solvent-based primers. According to Helms, defect rates related to SMC have been reduced by more than 95 percent, and the material is being specified again on new Ford vehicles.
However, the automotive industry is a moving target, and resin suppliers, SMC compounders and major molders can't breathe easily yet. Continued cost pressures, coupled with new OEM manufacturing techniques and increasing quality demands, are raising the bar for further market penetration. Fortunately, contributors at every level of the SMC supply chain are responding with new materials, techniques and equipment to meet these challenges.
Adapting SMCs for powder priming
Just when SMC suppliers thought they had solved OEM paint line compatibility problems, several automakers, notably General Motors (GM) and DaimlerChrysler in North America, started replacing traditional solvent-based spray primer systems with powder coating systems. Because the electrodeposition coatings are water-based, and many topcoat systems were moving to water-based or low-VOC (volatile organic compound) systems, the primer applied between these two steps had become a target for solvent reduction. While the switch to powder coating was no problem for steel (large appliances have been powder coated for years), SMC parts, even those incorporating the new toughened resins and sealers, did not fare well. During cure, the primer on many SMC parts developed a rough, "leather-like"surface — a clear cause for rejection.
In search of a solution, GM's SMC expert Hamid Kia assembled a team that included resin suppliers AOC and Ashland, paint supplier Red Spot Paint and Varnish (Evansville, Ind.) and molders Continental Structural Plastics (formerly Budd Plastics, Troy, Mich.) and Meridian Automotive Systems (Allen Park, Mich.). GM research identified the cause of the problem: Moisture and air were absorbed into SMC during transport and storage, then released from molded parts in the 350°F/177°C primer ovens. The source of the absorption was found to be the industry standard low-profile additive polyvinyl acetate (PVAC), which formed microvoids as the SMC cured. "We found that all organic materials have this problem to some degree,"explains Mike Dettre, business manager for closed mold resins at AOC.
Both Ashland and AOC reformulated resins with alternate low-profile additives that limit the moisture pickup to less than 0.3 percent — less than half that of SMCs that incorporate PVAC — yet still achieve the same level of molded surface finish. Although the reformulated resins, with existing sealers, stopped the problem when the parts were painted soon after molding, parts still failed if allowed to sit for more than a few days, especially in humid environments. Since then, Red Spot has developed a new sealer that prevents moisture uptake for longer periods. The sealer has, to date, passed tests. A second round of testing in warm, humid weather is planned for summer 2007 at GM's Shreveport, La. assembly plant, after which the new materials can formally enter the approval process.
Reducing density in Class A SMCs
Traditional SMC formulations used in Class A applications have a density of 1.8 g/cm² to 1.9 g/cm² — 30 percent less than aluminum and 75 percent less than steel but relatively high for a plastic material. Because SMC parts must be thicker than steel or aluminum, actual weight savings are smaller than these figures would indicate. The classic method for reducing density in SMC is to add hollow glass microspheres, and formulations are in production for unexposed, lightly loaded components with densities as low as 1.3 g/cm². This method, however, is not acceptable for painted exterior components because some of the spheres crush during molding or are broken during sanding operations, yielding a poor surface finish.
Based on its AROTRAN 720 toughened Class A resin, Ashland has developed a formulation that replaces high-density calcium carbonate filler with lower density materials and introduces one of several clay nanofillers to maintain the strength and stiffness of conventional SMC, yielding a molded density of 1.55 g/cm², explains Cedric Ball, Ashland transportation market development manager. "This formulation can achieve a 35 to 40 percent reduction in weight over steel, without sacrificing surface finish,"he says. The material has been molded into parts currently on test, with considerable interest coming from European automakers and the heavy truck market, where weight savings carry a higher value than they do in the North American car market and, therefore, override concern about the higher formulation costs that result from the use of nanofillers. However, interest in North America may be stimulated if fuel prices rise above $3 per gallon again. Two structural systems also have been developed, with densities as low as 1.15 g/cm².
Designing paint-free SMCs
Molded-in color has long been a goal of the automotive industry, due to the high capital cost of paint lines. Both AOC and Ashland have resin systems designed for molding functional parts, such as truck beds, with sufficient weatherability to stand up to UV rays without the need for painting. "We're very excited about the formulation we have developed,"says AOC's Dettre, explaining that the company reviewed each component of the SMC formulation (resin, low-profile additive, fillers, pigment, mold release, etc.), selecting the most UV-stable option in each case. "Then we worked on getting the cost down to a reasonable level,"he adds. Dettre emphasizes that AOC has conducted extensive accelerated weathering tests to simulate Florida and Arizona sun exposure, with excellent results. While black is the logical first option, Dettre believes other colors could follow later.
Ashland has introduced AROTRAN 800 for paintless applications, accumulating six to seven years' exposure on formulations with 35 and 55 percent glass content, comments Ashland's Ball. "The formulation is a bit more expensive than non-UV resistant systems,"he says, due to the various additives and pigments, so the material is most attractive to suppliers who wish to avoid the expense of paint line installation. Both Dettre and Ball see pickup truck beds as logical initial applications for UV-resistant SMC.
Moving toward eco-friendly SMCs
Several years ago, polyester resins based on soy oils were introduced, and some composite parts made with them entered production, notably for agricultural equipment makers, such as John Deere. As more automakers look for products produced from renewable resources, such materials are getting a fresh look. Meridian has been working with Ashland's ENVIREZ, a polyester incorporating 25 percent soy and corn oils in the resin, to produce a nonappearance structural SMC for applications like underbody heat shields. Jeff Robbins, Meridian's director of R&D, notes that in aging tests (180°C/356°F for 1,000 hours), the ENVIREZ-based SMC performed as well as the traditional vinyl ester systems typically employed in these applications. Ashland's Ball notes that the soy component imparts "natural toughness"to the resin and has excellent paintability, but surface quality is not yet up to Class A automotive standards. A production application (not Class A) is slated for the 2009 model year, and Ball and Robbins see potential for the resin to be used in other high-temperature applications, such as valve covers.
Improving QC for SMC molders
Ashland recently introduced two improved quality control tests for use with SMC. The first is an instrumented flow test, the Flow Analysis Cure Time System (FACTS), which pairs a specially designed spiral flow mold with the SmartLab dielectric cure monitoring system developed by Signature Control Systems (Denver, Colo.). A specially developed software package provides analysis of multiple runs, allowing comparison between materials and lot-to-lot comparisons for a single material. The tool has a 6-inch/150-mm square loading area for placing the SMC charge, with a 2-inch/50-mm wide flow channel for measuring the flow of the material during the molding process. "The advantage to this system is that we can replicate to a great degree what is happening on the shop floor, in terms of process conditions,"notes Ashland's Cedric Ball. "It can be used for troubleshooting problems with existing materials or molds, as well as aid in the development of new SMC formulations."Ashland is offering the FACTS system (mold and cure monitoring system) for sale to compounders and molders.
Ashland developed the Laser Optical Reflected Image Analyzer (LORIA) in the mid-1980s as a way of measuring surface smoothness and waviness in Class A automotive parts. With time, components used in the equipment became obsolete, so a new instrument, the Advanced Laser Surface Analyzer (ALSA), has been introduced by Ashland as a replacement, with much greater capability. The ALSA system employs a diode laser and captures part surface data using a charge-coupled device (CCD) similar to those used in digital cameras. AddiÂtionally, a modern computer interface provides not only the surface waviness results previously available from the LORIA system, but also independently derives values for orange peel and distinctness-of-image (DOI). Ball expects the new ASLA system to replace LORIA as the industry standard within the next few years.
One of the largest concerns for SMC consistency is measuring the viscosity of the matrix resin. The industry's traditional method is to use a Brookfield viscometer, with a T-spindle rotating at high shear rates, to measure the viscosity in a given time window. Continental StructÂural Plastics (CSP, Bingham Farms, Mich.) has performed a correlation with an alternate method that uses a vane-shaped spindle rotating at low speed, measuring the shear stress and shear modulus of the high-viscosity matrix (typically 10 to 25 million centipoise). This alternate "soft solids tester"is typically used with silicone sealants, cements and plaster — materials similar in consistency to thickened SMC resins. Tests show a high correlation between results from the CSP method and the traditional method, with much higher reproducibility. The CSP method, which employs off-the-shelf devices available from lab equipment suppliers, is being used along with the traditional method to gain further support for its potential adoption as an industry standard.
In an effort to better understand the factors that influence molded part quality, CSP implemented multivariate regression analysis on several part molding programs that demonstrated higher-than-acceptable scrap rates. In one study, useful data were retrieved on 83 factors related to the SMC compounding, the manufacturing process and the end product, more than 734 molded pickup boxes. The regression analysis identified 15 factors as significant, which allowed the engineering teams to focus in the appropriate areas, resulting in a significant overall reduction in scrap. "We use this technique whenever we don't have an easy answer and need to take a deep dive into the data,"explains Probir Guha, general manager of materials for CSP. Guha emphasizes that this method is more practical than using a design-of-experiments approach, which forces the production of bad parts, and allows for regular production to continue.
Elevating speed, consistency for SMC compounders
Production of consistent compounds — that is, those in which variation in resin content, filler content, glass content and length, sheet areal weight and thickness, color, viscosity, reactivity and moldability are minimized — is an important prerequisite to the achievement of tighter molding tolerances. Therefore expectations for the newest SMC compounding machines are very high. "Cost and quality are more important than ever,"emphasizes Dr. Jerry Fram, president of Finn & Fram Inc. (San Fernando, Calif.), a major producer of SMC compounding equipment since the early days of the technology. "Our latest machines have a number of features for improved quality control and steadily increasing production speeds.â€
SMC is manufactured in a continuous process by automated, web-based systems. As the process begins, a paste containing the resins, pigments and fillers is metered onto upper and lower carrier films (on the bottom of the upper and the top of the lower), while continuous fiberglass roving is chopped and deposited onto the moving lower film after paste application. Chopper speed, blade height settings on the resin "doctor boxes"and overall line speed control the glass-to-resin ratio in the compound, the most critical contributor to the mechanical performance of the material. Then, the coated upper film is mated to the lower film/glass combination, and the result is passed through a series of serpentine rollers to wet out the glass prior to entering the dual-wire-mesh belt compaction zone where the materials are rolled and squeezed to remove the air and ensure intimate distribution of the glass throughout the compound thickness.
Proper compounding of SMC requires a uniform, well-wet-out sheet with accurate weight per unit area and proper proportions of paste and glass. SMC is made at high production rates in factory environments with commercial, not laboratory-quality, ingredients. The newest machines are ruggedly built, but with modern controls that take away guesswork, are simple to use, and can be adapted quickly to changing conditions. Ease of cleanup is a very important consideration.
Dr. Fram explains that newer machines are now outfitted with gamma backscatter gauges, which provide instantaneous measurement of paste weight and total SMC weight. These are used for closed-loop paste weight control and, with a complete system, closed-loop control of glass content and total areal weight of the compound as well. Film payout is smoother and more precise with tension control and magnetic braking. Ergonomic film splicing stations with accumulators supply film continuously through roll changes at high speeds and permit smooth splicing without shutting down the production line.
The compounded sheet, with both films left in place to keep the volatile styrene from evaporating, is either taken up on rolls or fan-folded into large boxes. The latter method, also called "festooning,"is becoming more popular, notes Al Johnson, Finn & Fram sales manager. "There is less labor involved with packaging the material, as well as lower labor costs at the press, due to fewer material change outs,"Johnson says. Rolls typically weigh 150 lb to 1,000 lb (68 kg to 455 kg), while a box of festooned material can hold 1,000 lb to 3,000 lb (455 kg to 1,365 kg).
Another advantage to bulk packaging is that line speeds are considerably higher today than they were 15 years ago, Johnson points out. "Older machines ran at 20 ft to 25 ft [6m to 7.5m] per minute, while current equipment is running at two to three times those rates, which helps reduce manufacturing costs."He expects rates to reach 100 ft/min (30 m/min) within a few years. In order to maintain sufficient residence time in the compaction zone at these higher line speeds, Finn & Fram has extended the compaction length and increased the number of rollers from a nominal 16 to as many as 24 on the company's largest machine to date.
Molders also are increasingly automating their processes, from the cutting of SMC charges to finishing and inspection. Tier 1 supplier INAPAL PlÃ¡sticos SA (Porto, Portugal) started a new facility in early 2006 with three production lines to make parts specifically for multipurpose vehicles designed by automaker VolksÂwagen (VW). Two of the lines are fully automated for production of Class A finish trunk lids for VW's EOS convertible model. The third line produces underbody shields for both the EOS and VW's Spanish subsidiary's SEAT models. The new plant is located adjacent to VW's Autoeuropa industrial complex in Palmela, Portugal, to ensure JIS (just-in-sequence) delivery to the automaker's production lines.
The INAPAL production lines are the result of a joint venture between INAPAL and SMC supplier Menzolit-Fibron GmbH (Bretten, Germany). SMC sheet is made at the INAPAL facility and spooled onto rolls or coils large enough to hold the material produced in an entire work shift. To minimize scrap and part weight fluctuations, automated, inline CNC trimmers cut the SMC sheet to shape. A three-sheet stack forms the EOS trunk lid charge, which then is weighed to ensure that part-to-part consistency does not vary more than 1 percent by weight. After maturation, SMC charges are robotically loaded into the presses. High-speed, 2,300-ton compression presses (model DC-U 2100/ 1800 AS, capable of 21,000 kN/4.7 million lbf) were supplied by Dieffenbacher GmbH & Co. KG (Eppingen, Germany). All are equipped with active parallel motion control to ensure accurate tool closing and (although the function isn't currently required at INAPAL) can be configured for inmold coating. Molding cycle times are in the range of 90 seconds at 275°F to 325°F (135°C to 163°C).
Downstream of the press, openings in the outer trunk lid skin are robotically cut on high-precision CNC milling machine, while a waterjet cutter trims the trunk's interior surface. After a bar code is attached to each part, detailed inspection identifies out-of-tolerance parts before parts are transferred to VW's Autoeuropa plant for painting.
On the expansion path
As the latest developments gain OEM approval and enter production, what new solutions lie on the horizon? Frank Henning, director of polymer engineering at Fraunhofer Institute Chemische Technologie (Pfintzal, Germany), is leading a collaboration between Volkswagen, Dieffenbacher and a team of suppliers to develop inline compounding and molding of SMC, a processing concept already in widespread use for long fiber-reinforced thermoplastics. According to Henning, a key element will be successful maturation (aging) of the SMC using a microwave technique that will eliminate the several-day wait required with traditional chemical thickening methods. A lab-scale effort has been underway for almost two years, with a full-scale line now under construction at Dieffenbacher's facilities in 2007.
As the auto industry trends toward greater model differentiation with niche vehicles produced in smaller quantities, Ford's Helms believes SMC is the right material for short runs of up to 100,000 units per year, and expects low-density formulations, inherently conductive compounds and molded-in color to enable growth. He sees particularly bright promise for pickup boxes. According to AOC's Dettre, a large market may develop if a suitable coupling system can be developed for polyester resins that will be used with carbon fiber reinforcements in structural applications. He adds that AOC recently developed a resin capable of very high loadings of alumina trihydrate for flame, smoke and toxicity applications, creating opportunities for SMC in train and bus interiors and, eventually, in fire-resistant automobile components. But he also sees a strong upside potential now. "Today, we are only serving one-quarter to one-third of the applications available to us,"he points out. "With the recent successes in powder-prime-capable and UV-resistant systems, SMC is now positioned to capture many more."