Fast-cure Prepreg is Making Inroads

New capability opens the way for high-volume production.

Facing higher Corporate Average Fuel Economy (CAFE) standards of 54.5 miles per gallon in the U.S. and 95g/km CO² emissions limits in Europe, automotive OEMs are evaluating every option for lightweighting vehicles. The increasing stringency creates a need for new approaches. Automotive OEMs and Tier 1 suppliers are well aware of the strong, lightweight properties offered by CFRP prepreg and are eyeing the development of fast-curing prepreg to replace metals for a range of structural and Class A surface automotive parts. The attraction is based on the capability for out-of-autoclave compression molding of CFRP prepreg to reduce cycle times for high-volume production. Until recently, the potential for CFRP prepreg had a ceiling. Production volumes were inhibited by the time needed for thoroughly cross-linked curing during conventional autoclave processing. Significant manual labor for layup is a non-starter, and the steep cost of CFRP prepreg also creates a boundary. “About 14,000 to 40,000 parts would be considered very high volume for CFRP prepreg, but for automotive applications this is considered low- to mid-volume,” says Adam Harms, marketing manager – automotive for Huntsman Advanced Materials. “Around 20,000 to 25,000 parts per year on a single production line such as the Corvette roof has been the sweet spot for prepreg, an application where it makes the most sense. In automotive, you haven’t reached high volume until about 100,000 parts.” Several material suppliers have accepted the challenge from OEMs to develop rapidly curing prepregs that can achieve 2- to 3-minute cycle times through compression molding, qualifying prepreg for high-volume production in the area of 80,000 to 100,000 parts per year. The race between material suppliers, Tier 1 suppliers and automotive OEMs is on. Speed + Quality Control Compression molding of fast-cure prepregs is being evaluated alongside high-pressure resin transfer molding (HP-RTM) and wet compression molding. In HP-RTM and wet compression molding, the end user places dry carbon fiber into a press, wets it out with an appropriate resin system and applies pressure to form the component. There are disadvantages to these related approaches, says Will Ricci, technical services engineer for Gurit. “When wetting out the carbon fiber, the fibers may move, compromising directional strength and aesthetic appeal,” he says. “Shifting fibers may cause distortion and negatively impact the distribution of forces within the part, reducing overall part stability, a critical concern especially for structural parts requiring impact resistance. In structural parts positioned where vehicles emit high heat loads, the maintenance of fiber orientation and the polymer’s thermal stability is critical.” In the curing stage of HP-RTM and wet compression molding, the processor is taking the composite from a high energy state to a low energy state. The difference between the two states is substantial, and, if not handled properly, can lead to high shrinkage, microcracking and severe or uncontrolled exothermic reaction. “Loading fibers correctly, using the right resin mix, wetting correctly – there are a lot of ways to go wrong in HP-RTM and wet compression molding,” Ricci notes. Since the carbon fiber in fast-cure prepreg is fully impregnated, this approach offers reliability advantages by maintaining fiber direction for long-fiber composite parts. Pre-staged, the difference in energy states from the start to the end of the process reduces the risk of microcracking and offers better quality control over tensile strength and visual aesthetics. In a market that is less familiar with CFRP, the compression molding of prepreg can be a less formidable challenge. Most importantly, prepreg compression molding can reduce the forming cycle time from as much as four hours down to five to 10 minutes. Outlife of the prepreg material, frequently a concern for small run parts, poses little risk for these fast-cure, high-volume applications. The material is quickly used once outside of a cooling room. In any case, most fast-cure prepregs exhibit low reactivity, with an outlife of two weeks at 20 C. It may take hours for a standard prepreg cure process, allowing enough time for the resin system to heat to temperatures between 80 and 150 C, reach its gel point, begin to flow and cure so that chemical linkages form and the full mechanical properties of a cured state are attained. With rapid-cure prepreg, production time for all three stages – polymer flow, gel point and final cure – is measured in minutes. Chemically, the difference between standard and fast-curing CFRP prepreg is slight. For example, Gurit is using a host of polymers and catalysts which are staged to allow the total cure process to be condensed. The initial catalyst activates at a lower temperature, while a second catalyst kicks in at a mid-temperature and so on, maintaining precise control over the cure process. Beyond that, Gurit’s formulation and process are confidential, Ricci notes. Huntsman Advanced Materials, already a supplier to global aerospace prepreg manufacturers, is partnering with existing and new automotive customers to develop fast-curing prepregs by supplying resin matrix building blocks, including epoxy resins, hardeners, diluents and accelerators. The accelerators play the role of speeding up the reactivity of slower acting resin formulations. [caption id="attachment_3637" align="alignleft" width="300"] The CFRP hoods for Cadillac’s 2016 ATS-V and CTS-V high-performance vehicles are more than 27 percent lighter than aluminum hoods and 72 percent lighter than steel hoods. Magna International molded the hoods, which represent the auto industry's first volume production of CFRP hoods. Photo Credit: Magna International, Inc. [/caption] “The key is carefully pairing accelerators to allow the use of higher processing temperatures, thereby decreasing the need for epoxy diluents that may decrease thermal and mechanical performance properties and increase cost,” says Harms. “Properly chosen accelerators and their use level in the resin matrix can serve to beneficially influence properties such as modulus, strength and elongation, as well as shorten cure time.” The formulation of the prepreg fibers – unidirectional tape, woven reinforcements, tow, number of stacks and thickness – is part dependent. Typical processing temperatures are consistent with standard prepreg at around 80 to 150 C. Press Makers See Opportunity One of the keys to successfully processing fast-cure CFRP prepreg in high volumes is investing in the efficiency and reliability of a manufacturing work cell, consisting of a laser cutter, high-volume mold, hydraulic press and robotic automation for loading the fabric and picking the final part. “The speed of the hydraulic press contributes to the cost-effectiveness of the switch from metal to prepreg,” says Paul Thom, sales and product manager for Schuler Group, a supplier of hydraulic presses to BMW. Typical pressure required by fast-cure prepreg applications ranges from 2 to 10 megapascal (MPa) or 290 to 1,500 psi. Using an automated work cell not only eliminates time and safety issues related to manual labor, it also offers better part quality control. Hydraulic presses respond to the behavior of both the press and internal processing conditions inside the mold. “Ram parallelism (the ability to keep all four corners of the press in alignment to apply pressure evenly) controls for parts with small wall thicknesses,” says Thom. “Sensors in the four corner columns or ‘slides’ of the press sense when there is a difference in the distance between the top and bottom mold halves that may be the result of resin accumulating in one area of the part over another. The machine operator is able to adjust the pressure in each corner independently to achieve uniform wall thickness. Most parts are not exactly symmetrical. Part loads may vary and the press is able to adjust to load conditions.” Currently, pressure adjustments are made manually by the machine operator via the control, but Schuler Group is working with BMW and the Aachen Center for Lightweighting Technologies at the University of Aachen in North Rhine-Westphalia, Germany, to develop closed-loop control for compression molding and RTM presses. Traditional automotive suppliers exploring fast-cure prepreg applications may find hydraulic presses offer more familiar technology than composites processes such as autoclave production, reducing the learning curve as they convert from stamping of metal parts. It would be convenient if metal stamping presses could be converted for compression molding prepreg, but that is not the case. “The biggest difference is in the control system,” says Thom. “The processing of prepreg requires a more sophisticated control of pressure.” Presses are specified by tonnage range, and the press size is completely dependent on the part dimensions and shape. A large part – such as a 36-square-foot automotive hood – may require a press in the neighborhood of 3,000 tons, while a part measuring 2 square feet can be produced on a 600-ton press. “Since we are in the early stages of developing expertise in the compression molding of fast-cure prepregs, the size and specifications of the press are still customized for each prepreg application,” admits Thom. If a supplier is currently using a hydraulic press for RTM, it may be possible to repurpose the press for compression molding of fast-cure prepregs. “Different features are required for the control system to account for varying process behaviors, but generally, yes, this possible,” says Thom. The Schuler Group is currently building a hydraulic press for placement with the Institute for Advanced Composites Manufacturing Innovation (IACMI) Vehicle Technology Area, where member companies such as Ford and Lockheed Martin will be able to run their own fast-cure prepreg applications tests on the press. The Cost Equation Fast-cure CFRP prepreg carries a higher price point than steel, but using industrial grade carbon fiber (as opposed to aerospace grade with additives for flame, smoke toxicity and functionality requirements) helps to mitigate the cost differential. Purchasing the prepreg in higher volumes lowers the cost even more. An important consideration in the decision to use fast-cure prepreg is the capital expenditure for the compression molding infrastructure. Adding up the cost of high-volume capable tooling, hydraulic presses and sophisticated automation, a hefty capital expenditure is required.  Ricci notes, “A production tool alone that is capable making 50,000 car hoods can run in the hundreds of thousands of dollars.” To achieve high production volumes, automated work cells are needed to cut prepreg, precisely orient and assemble the plies, and transfer the multiple plies (preform) to the heated mold, then after curing, take out the part likely using a 6-axis robot for placement on a cooling fixture or for further in-line or downstream processing such as trimming, painting or bonding. Tier 1 Supplier Magna International has developed an automated work cell that robotically lays multiple plies of standard-width, 0.02 mm-thick prepreg in a 0°/90° orientation, which are then trimmed and robotically transferred to the mold. The mold features a patented design to prevent movement of the stacked plies during the mold close phase and during the application of pressure. According to Magna International, the conversion of hoods for the Cadillac ATS-V and CTS-V Series from steel to CFRP resulted in a cost savings between 20 and 30 percent. Potential savings for processors depends on the part and the grade of steel or aluminum being replaced. It may be five years or so before fast-cure prepreg is used on mass-produced vehicles, notes Dale Brosius, chief commercialization officer for IACMI. “But compression molding of rapid-cure prepreg is creating a successful path. The possible developments down the road are exciting, such as the production of prepreg right in line with the compression molding process. Among composite processing alternatives, rapid-cure prepreg is simpler to achieve and offers the potential to be very, very fast.” [divider]Fast-cure Prepreg Partnerships[/divider] Automotive OEMs, Tier 1 and 2 suppliers and suppliers of CFRP materials are forming cooperative teams to push the development of CFRP applications forward in advance of fuel efficiency standards. Some of the partnerships are listed here: Mitsubishi Rayon Co. Ltd. (MRC): In 2014, mass-produced CFRP parts made their commercial debut for deck lid inner and outer panels for the Nissan Skyline GT-R supercar, reducing the part’s mass by 30 percent. Since then, diffusers for the Skyline GT-R have incorporated MRC’s prepreg compression molding (PCM) technology. MRC claims a 2- to 5-minute cure time is possible with heat and high pressure of its proprietary molding process. In March 2016, MRC signed a memorandum of understanding with Tier 1 supplier Continental Structural Plastics for the development of Class A body panels, as well as non-class A structural automotive applications, including pillars, engine cradles or supports, radiator supports, frames and rails, bumper beams, underbody shields, door inners and intrusion beams using fast-cure prepreg and/or sheet molding compound. Cytec Industrial Materials: Cytec is working in tandem with Jaguar Land Rover to develop fast-cure prepreg applications with 5-minute cure cycles such as floor pans, sidewall panels, crush cans for bumper beams and body panels. Hexcel: One of the most intriguing components of the BMW 7 Series is the B-pillar – the side post between the front and rear doors. The weight-saving sandwich design features an exterior metal pillar reinforced with an inside preform of HexPly® M77 rapid-cure CFRP prepreg. Hexcel supplies BMW with preforms made of unidirectional carbon prepreg set in various orientations and combined with adhesive. The prepreg cures in 1.5 minutes at 160 C. It requires special attention to the baking process because the temperatures needed to cure the CFRP component approach temperatures that weaken the heat-treated metal stampings. Barrday Composite Solutions/Zoltek: These materials suppliers are working with Tier 1 supplier Magna International, using fast-cure prepreg from Barrday and carbon fiber from Zoltek, to run a 10-minute cycle to produce 14,000 composite hoods for the Cadillac ATS-V and CTS-V Series. Toray Carbon Fibers America: Tier 1 supplier Plasan Carbon Composites, a U.S. manufacturer and distributor of auto parts, is leveraging its proprietary high-speed pressure press molding technology to supply CFRP-based exterior body panels (hoods, roofs, etc.) for performance and luxury cars of General Motors. Plasan uses Toray’s rapid curing CFRP prepreg for its high-speed pressure press molding. [divider]Fast-cure Prepreg Process at a Glance[/divider] Here’s a look at the fast-cure prepreg process at one company, Mitsubishi Rayon Co. Ltd. Step 1: Lay up plies of fast-cure prepreg; debulk, trim. Step 2: Preform the prepreg by heating for 1 minute between 60 and 70 C. Step 3: Shape into an air-cylinder press set with a two-sided tool of polyurethane modeling board; place under light pressure (0.3 MPa). Step 4: Cool to room temperature. Total time ≤ 5 minutes. Optional: If an aesthetic part, cover the preform with silicone rubber sheet and pull a vacuum to smooth out wrinkles.