by Dale Brosius, Contributing Writer
High-Performance Composites Magazine
Text and Images: ©2004 High-Performance Composites Magazine
March 2004, pg. 33-37
Putting the hood into production proved to be the trickiest part of the equation. After reviewing the alternate methods, such as resin transfer molding and compression molding, neither of which were capable of achieving large, extremely thin profiels nor the required surface quality, GM opted to pursue autoclave process fo rthe outer panel. However, this decision posed new challenges. Autoclave processing is commonly used for European super sports cars priced above $250,000 and built at rates of one or two per day. But GM was looking for volumes approaching 16 parts per day, still relatively low by automotive standards, but far beyond the normal production requirements of the aerospace industry. Following an intensive supplier screening process, GM selected MacLean Quality Composites to build the hood, based on its extensive experience with carbon fiber in high-volume sporting goods and parent company MacLean-Fogg's hisotry as a supplier of functional metal and plastic parts to the automotive industry.
"The demands of the automotive industry really force you to think differently about processing," emphasizes MacLean operations manager Tim Daily. "What you do has to be both fast and repeatable, and small time savings at all process stemps really add up." To ahcieve the required part volume, two molds are processed simultaneously in McLean's 1.8 m/6 ft diameter autoclave, supplied by BondTech Corp. (Somerset, Ky.). Daily explains that MacLean is turning the molds every three hours, or eight times per day on a round-the-clock, five-day-per-week operation. For each cycle, this includes application of mold release, layup and vacuum bagging of the parts, loading the autoclave, pressurization to 100 psi/6.8 bar and ramp up to 150o/302oF, followed by cool down to just above 43oC/110oF, unloading the autoclave and part removal. Daily notes that significant engineering effort was involved in optimizing the pressure/temperature profile of the cure cycle to obtain the best surface finish on the molded part.
A significant constraint on the process time is the heating and cooling of the large Invar molds, each weighing almost 2,500 lb/1,136 kg. Although lightweight carbon/epoxy tooling would provide faster heating and cooling, and might suffice for low-volume production, GM wanted molds durable and accurate enough to produce thousands of parts, including the potential for aftermarket production. An iron alloy with 36 percent nickel content, Invar was selected because of its near-zero coefficient of thermal expansion (CTE) similar to the CTE of carbon fiber composite. Beyond its weight, Invar's thermal conductivity is one-fourth that of steel and one-twentieth that of aluminum, slowing heat transfer. The molds were produced by Visioneering Inc. (Fraser, Mich.) and Models & Tools Inc. (Troy, Mich.).
The production process begins with automated pattern cutting of the unidirectional tape on cutting machinery supplied by Cutting Edge, now part of Gerber Technology (Tolland, Conn., U.S.A.), followed by vacuum preforming of four of the six plies on a fiberglass male tool. Preforming for one set of parts occurs while the previous set is curing in teh autoclave. After application of the mold cavity, followed by placement of the four ply preform, then application of separator film, breather and a reusable vacuum bag, which evacuates the air between the part and the mold. The vacuum bag, supplied by Toor Technologies (Tacoma, Wash.) is a thermoset silicone rubber, formed to near-net shape, and supported by an aluminum frame. Following a vacuum check, the molds are loaded into the autoclave. Through repetition and proper sequencing, Daily says MacLean has reduced the time to complete these steps to only 45 minutes, with separate temas working on each mold.
After molding, the outer panels are trimmed on a 5-axis CNC router. A two-component polyurethane adhesive, supplied by Ashland Speciality Polymers and Adhesives (Dublin, Ohio, U.S.A.). is robotically applied to the outer panel in the areas of the corresponding bond flanges of the inner panel. The adhesive is a room-temperature version of the heat-cured polyurethane used by Meridian on the all-SMC hoods. Ashland did considerable thermal- and water-aging of test panels to prove excellent bond strength, as measured by lap shear, between the carbon prepreg and the carbon SMC. The autoclave-cured outer and Meridian supplied inner panels are mated in a dedicated bonding fixture, designed to achieve accurate location in space of all suraces. After curing at room temperature in the fixture for about 30 minutes, teh hood assembly is inspected for dimensions (with many tolerances under 1.0 mm/0.40 inch), and for surface appearance.
The latter characteristic was a source of frustration and a major development target, Daily points out. "The as-molded surface of the outer panel is excellent by aerospace standards, but the standard for automotive Class A is much more critical," he notes. "A single pinhole or voidon the painted hood is cause for rejection." Daily explains that pinholes, when they occur, are relatively easy to find and fill, yet the more difficult defrects are extremely small surface voids, some smaller than 2 mm/0.080 inch long gthat are aligned with the fiber direction. These "linear voids,"some smaller than 2 mm/0.080 inch long that are aligned with the fiber direction. These "linear voids," as he terms them, are prohibitevely expensive to identify, fill and sand, so either a surface film or a primer capable of filling these is required. After extensive testing, MacLean and GM, working with painting subcontractor ASC Inc. (Southgate, Mich.), identified a primer that solves teh problem. Following receipt of the assembled hoods from MacLean, ASC's Bowling Green, Ky. facility applies a low-temperature curing primer, supplied by Akzo Nobel Coatings (Troy, Mich.). The primed hoods are shipped into the nearby Corvette assembly plant in Bowling Green, KIY., where final painting (a process involving temperatures as high as 143oC/290oF) is completed with the remaining body panels. Teh use of Synskin surfacing film, from Loctite Aerospace (Bay Point, Calif., U.S.A.) has been qualified by GM as an alternate method for addressing linera voids.
GM's Voss says the 2004 Commemorative Edition Z06 is a big hit with customers, and demand easily exceeds GM and MacLean's capacity to supply. Moreover, there are significant implications going forward. "In addition to being the first to implement Class A carbon fiber at these volumes, we have learned a tremendous amount that can be applied to future vehicles, and on a broader scope, " says voss. For obvious competitive reasons, he won't reveal exactly where GM next plans to implement the technology, but confirms the company considers carbon fiber "The right fit for certain niche vehicles." Lower tooling and fixture costs, especially at these volumes, help offset the premium cost of carbon fiber composites.
MacLean's Daily also is enthused about the prospects, especially after surviving the growing pains associated with making the autoclave process more stremlined and economical. He refers to a number of "firsts" with the Corvette program, in the areas of materials, tooling, design, production rates and part quality. "We've proven it is possible to produce large, Class A carbon fiber body panels in these kinds of volumes," he declares. "And the fact that we have moved past theoretical application and into production gives us a unique position in the composites industry."