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.”
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.
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.
