Innovative composite materials reduce the quality, cost and noise of high-capacity mid-engine sports cars. #discontinuousfiber #generalmotors #precursor
Earlier this year, General Motors (General Motors, Detroit, Michigan, USA)'s first eighth-generation Chevrolet Corvette sports car (C8s) rolled off the assembly line in Bowling Green, Kentucky, USA. Described as the "fastest and most powerful entry-level Corvette" in the 67-year history of the model, it is also the Corvette with the densest composite material, and the first Corvette with a mid-engine configuration. 2020 Corvette Stingray is not only beautiful and fast, but also has many luxurious features. But don't let the beautiful appearance and fast track time fool you: there are many composite innovations on this car. CW’s two-part report on the composite materials used in this vehicle began in July 2020. This is part 2.
The feature of the new Corvette is not one, but two suitcases, which together can hold 12.6 cubic feet/0.36 cubic meters of cargo. Both tree trunks are made of 42 wt% chopped glass fiber/vinyl ester unsaturated polyester (VE-UP) resin, but different processes are used according to geometrical shape and mechanical requirements. The front luggage case (frunk) is compression molded from 0.95 specific gravity (SG) "float" sheet molding compound (SMC), while the rear luggage case is molded through a proprietary PRiME (preset reinforcement to ensure manufacturing excellence) process, which is a Liquid Compression Molding (LCM) variant. The float SMC and PRiME processes were developed by the processor Molded Fiberglass Company (MFG, Ashtabula, Ohio, US). MFG produces all structural SMC and LCM parts for cars.
"Although these two spaces are characterized by shallow ventilation and long walls, the trunk is smaller than the trunk and can be compressed and molded," explained Chris Barcela, chief engineer of Corvette body structure. "The trunk needs higher mechanical properties and is a solid geometry that can be filled with SMC charges. Because the PRiME process allows us to change the fiber length, we can use longer presets in our preforms Reinforcement. It turns out that letting resin (rather than glass) flow is the best way."
The 2020 Corvette is the first car to use a body structure cavity (integrated with the body frame) for an air intake duct system (above), which is powered by multiple main cooling paths (below). The naturally aspirated V8 engine of a car requires a lot of air, so it is important to ensure a good, unobstructed air flow to keep the engine cool. Source (two pictures) | General Motors
Ingenious engineering design and new materials are the key to cooling the C8 engine. Multiple main cooling paths (rear air inlets, front wheel housing vents and outside cooling inlets behind each door) transport highly complex air intake ducts, which transport air through the vehicle and engine, and then through the aluminum mesh vents on both sides And SMC decals are discharged to display the glass partition of the Corvette engine. The applique is made of MFG's float (0.95-SG) SMC (chopped glass/UP-VE resin). The material's low volatile organic compound (VOC) content can reduce emissions and eliminate the need for resonators on the rear induction pipe, while reducing cost and mass (2.4 kg) compared to alternative technologies.
The huge, customer-visible rear surround frame-64 x 69 x 24 inches (163 x 175 x 61 cm)-surrounds the rear half of the passenger compartment and is compression molded with a reinforced 1.2-SG SMC. This hybrid reinforced material uses 15% fiber volume fraction (FVF) carbon fiber and 30% FVF glass fiber, as well as low VOC UP resin, and is formulated to withstand engine compartment heat. The frame forms the dimensional basis for all rear exterior and interior panels, and the flexible design allows it to be used in a variety of vehicle variants. Thanks to the integration of important parts, auxiliary accessories are eliminated, internal packaging space is increased, noise/vibration/roughness (NVH) is reduced, a better body structure and sealing performance are provided, and the visibility of the rear hatch is improved and Reduced quality (15%) and cost and outgoing frames. "Considering the size of this part, which was almost 6 x 6 feet high and 2 feet deep, we actually had to let the material flow upward in the mold, so we were a little worried during the first injection," Basela recalled.
The very large, customer-visible rear surround frame provides the dimensional basis for all rear exterior and interior panels. Each half of the mold that forms the part weighs approximately 35,000 pounds/15,900 kilograms and is produced by Century Tool, a division of Tooling Tech Group (Fenton, Michigan, USA). The press itself has a clamping pressure of 2,800 tons and a 108 x 68 inch (274 x 173 cm) platen. Despite its size, the cycle time is nominally 3 minutes. | SPE Automotive Division
Also interesting is the bulkhead (middle window frame), which was customized by MFG to solve the high heat and noise challenges, because this part of the nacelle is directly above the V8 engine. The combination of glass fiber reinforced materials and the beryllium graphite filler in the low-VOC VE-UP resin can effectively suppress the transmission of sound to the passenger compartment, thereby eliminating the need for secondary sound insulation countermeasures. This in turn reduces costs, increases internal packaging space and passenger comfort, and improves body sealing and NVH. "With a specific gravity of 2.2, this is the first time I have been asked to increase mass rather than remove mass composite parts," Basela laughed.
The exterior body panels are all used by Teijin Group Corporation (Auburn Hills, Michigan, USA). By bolting to the composite cover, GM achieved the C8's aggressive styling, aerodynamics, and functional cooling integration, and achieved cost-effective weight reduction on multiple model variants using common components. All body panels are in-line sprayed on the "skuk system" at the vehicle position, using Bowling Green's innovative robotic wet polishing process.
The rear access door is molded from toughened SMC and has a high-temperature, foam-in-place silicone gasket that provides a durable seal and excellent resistance to compression deformation, while coping with long-term exposure to engine compartment heat. Another composite material—heat-resistant and wear-resistant aluminized aramid fabric—is used as a heat shield to provide additional thermal protection inside the door. | SPE Automotive Division
Another innovation involves the use of a one-piece thixotropic silicone elastic foam gasket on the back of the SMC service door, which is located in the trunk (one on a coupe and two on a convertible) and allows the customer to be exposed to air . filtering system. Due to its proximity to the engine compartment, a high-performance foam-in-place (FIP) elastomer (Silastic 3-8186 from The Dow Company in Midland, Michigan, USA) is specified for continuous use at temperatures up to 392 °F/200 °C, and at the same time Provides durable seals with excellent resistance to compression deformation, even after repeated opening/closing cycles. General Motors reports that most other die-cut foams and gaskets will melt or decompose when exposed to such temperatures. After dispensing, the applied gasket is heat treated at 167°F/75°C for 10 minutes to expand the foam and eliminate die cutting costs and waste. The door itself is tempered SMC (42% FVF glass in the VE-UP matrix). Produced by Gentex Corp. (Carbondale, Pennsylvania, USA) using heat-resistant and wear-resistant aluminized aramid aramid fabric (fiber from DuPont de Nemours, Inc., Wilmington, Delaware, USA) The heat shield provides additional thermal protection for the door interior.
General Motors' new mid-engine 2020 Chevrolet Corvette Stingray is said to be the fastest and most powerful entry-level Corvette in the 67-year history of the model. It is also the densest composite material and has many innovative technologies. | General Motors
General Motors also used its second-generation fully automatic precision wheel balancing system on the C8. Developed by The 3M Co. (St. Paul, Minnesota, U.S.) and ESYS Automation (Auburn Hills, Michigan, U.S.), a high-density (5.8-SG) composite wheel counterweight with customized magnetism replaces the traditional stamped metal counterweight painted steel , Zinc or lead, which has a specific quality and must be applied to the wheel by hand. The new system uses a large roll of extruded tape with an adhesive backing, which contains 67% post-industrial, corrosion-resistant, fully recyclable steel alloy, using fluoropolymer as the base material. The automated system checks each wheel, then cuts and applies custom-weight tape segments in smaller, more precise increments to improve driving performance and reduce tire wear. It also reduces assembly time and cost, simplifies inventory, eliminates waste and labor, and provides more color than metal weight.
Coupé roofs are available in three trim levels: painted, low-density toughened SMC (from CSP); clear hard-coated polycarbonate (PC); and clear-coated/exposed woven carbon fiber composite with painted edges ( From deBotech Inc., Mooresville, NC). CSP also provides a variety of grade A toughened 1.2-SG SMC panels for use in open-top model retractable roof systems.
Additional photos of Corvette beauty. Source | General Motors
Other exterior trim panels include painted and underpainted thermoplastic polyolefin (TPO) front trim panels with integrated ducts to direct air to the brakes (Z51 kit only) and outboard heat exchangers. The outer grille and brake cooling vents are coated with acrylonitrile butadiene styrene (ABS).
The upper rear panel is Color Molded (MIC) TPO, but the lower rear panel assembly is A-painted, 1.2-SG SMC (Glass/UP resin) because this part is very close to the hot exhaust tip. The outstanding mechanical properties of SMC enable GM to design short unsupported rear overhangs and use larger spacing between attachments without sagging. Compared with thermoplastics, SMC can also effectively distribute the load to a larger area during a low-speed rear collision. The bracket and the rear parking assist sensor are connected to the SMC. It is said that this is the first time that SMC bumper panels have been used in large-capacity vehicles.
The upper panel of the rear hatch decals is painted PC/ABS, and the side vents are equipped with aluminum mesh SMC and SMC lower decals to adapt to the thermal load directly above the engine. These SMC panels are produced by LyondellBasell's Quantum Composites Inc. (Bay City, Michigan, USA). The door handle release switch is PC/ABS, and the rear air inlet is SMC. The base car uses blow-molded painted ABS spoiler, and the rear air outlet grille uses injection-molded painted ABS. The A-pillar and ceiling decals are coated with ABS, as are the exterior side mirror covers and radiator grille. Depending on the option package, toughened SMC front wing and lower wing and blow-molded TPO or carbon fiber composite front splitter/diffuser — deBotech uses Solvay Composite Materials (Alpharetta, Ga., US) — provides additional aerodynamic stability sex. The rear diffuser of the tempered SMC is matched with the lower rear panel of the SMC of MFG's Class A painted SMC.
The high-quality carbon fiber decoration kit adopts various transparent coatings/visible woven carbon fiber decoration inside and outside, including rearview mirror cover, front grille insert, front splitter, accessory wings, engine compartment cover, inner door switch panel, rocker moldings, instrumentation -Panel group frame, door handle cover, quarter decals and driving control panel. These components are produced by deBotech, SMI Composites LLC (Commer, Georgia, USA) and Plasan Carbon Composites (Wixom, Michigan, USA).
Corvette is not only fast and beautiful, but also contains many material comforts. For composite enthusiasts, the advanced carbon decoration upgrade adds a lot of transparent coating/visible woven carbon fiber composite materials to the inner and outer surfaces, such as the driving control panel shown here. | General Motors
The 2020 Corvette has won many prestigious industry awards, including 2020 MotorTrend Car of the Year, 2020 Automotive News PACE (Premier Automotive Supplier Contributions to Excellence) Partner Award, and multiple awards from the Society of Plastics Engineers (SPE), including 2019 Vehicle Engineering team award. "Although the new Stingray's mid-engine architecture has dominated the headlines, Corvettes has always been ahead of the most advanced automotive material technology for eight generations, regardless of whether the engine is located in front of or behind the driver," Tadge Juechter added. , Executive Chief Engineer-Global Corvette. "Propulsion technology is at the core of our work."
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