Most auto makers feel the strong pressures to reduce carbon emissions and improve fuel economy for passenger vehicles and commercial trucks throughout the world. The interest and use of composites by transportation OEMs is growing. As a result, composite material suppliers are working hard to improve thermo-mechanical performance, increase processing speeds, reduce part mass and costs and enhance surface aesthetics with fewer post-mold operations. Generally, they want to make it easier to position composites against high-strength steel and aluminum.
Last year, a study conducted by European and American organizations looked at methods to increase stiffness / strength and impact resistance of thermoplastic composites. They accomplished this by employing continuous-strand, unidirectional-glass (UD) tapes to produce woven fabrics as well as laminates. Combinations of the tape fabrics and the tape laminates in various layup patterns were used in conjunction with charges produced in the direct-long-fiber thermoplastic (D-LFT) inline compounding (ILC) process. This compression molded both test plaques and later an actual automotive underbody-shield to test the extent to which impact performance could be improved. (Click here to read the paper.)
Drivers for Composites Growth in Automotive
The weight of the average European compact-car increased 400 kg, or almost 50% from the 1970s through the 2000s. This happened mainly due to the addition of electronic systems, passenger convenience, and safety equipment. North American vehicles gained even more weight and size over the same time period, mostly due to readily available and inexpensive petroleum, and the desire for greater luxury and more power. The high-fuel prices of the mid-to-late 2000s significantly increased consumer demand for alternative-powertrain vehicles. Automakers scrambled to design and produce more hybrid-electric, battery-electric, and even fuel-cell vehicles. Unfortunately, the energy-storage systems commonly used for these platforms tend to increase vehicle mass an average of 100-300 kg, effectively limiting driving range. More demanding fuel-economy as well as greenhouse-gas emissions standards are pending or already in place in North America, Europe, and Asia. Automakers must get weight out of vehicles quickly. These trends have created market conditions that favor great interest in polymer composites as a means to reduce mass and increase a vehicle’s overall energy efficiency.
Carbon fiber-reinforced plastics (CFRP) regularly receive attention in the automotive industry for their high-tech glamour and lightweight stiffness and strength. Current technology constraints in cycle times, fiber availability, and costs, however, continue to price these materials out of reach for most applications on all but the most exclusive performance and prestige vehicles. Most automakers are searching for practical ways to reduce component mass while also managing costs. There is great interest in increasing performance of readily available, workhorse composites like glass-reinforced polypropylene (GR-PP), which is light, cost effective, damage tolerant, offers a good balance of mechanical properties, and has a long history of use on semi-structural and select structural components.
The key to broadening the use of such materials on passenger vehicles is to increase stiffness and strength. This is best accomplished by an increase in retained fiber length and fiber-volume fraction (FVF). By using the fibers in a more intelligent manner (where they can do the most good) – all of which are well-known in the industry to improve mechanical performance of composites.
Development of Tailored D-LFT Composites
An interesting collision of two competing technologies occurred in the late 90s and early 2000s that has led to development of a fairly new form of glass-reinforced (GR)-PP composite, the tailored direct long fiber thermoplastic (D-LFT). The study reiterated results seen in numerous other studies that longer fibers and more fibers increase stiffness / strength and impact resistance (toughness) of parts. The study also showed the benefits of a hybrid molding process like tailored D-LFT. Although pure tape fabrics and pure tape laminates provide better ultimate performance at a given wall thickness, these materials are more costly to use per unit weight than pure D-LFT and can pose filling challenges for parts with complex geometry such as thin ribs if used alone. D-LFTs are best suited for selectively adding performance locally where needed. (Learn more)
Click here to read the entire white paper.