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.
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.
“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.
