Today’s car designers are confronted with a number of challenges (increasing fuel efficiency, improving safety and enhancing performance) and a number of construction materials to choose from (steel, aluminum, thermoset or thermoplastic).
How to get rid of such difficulties? An important approach here is to replace metallic components with fiber-plastic composites. A growing trend of thermoplastic composites is the use of unidirectional tapes (UD tapes, much lighter than woven or non-crimp fabric reinforcements, because the tapes can be cut and placed precisely, with very little scrap). Let’s see how UD tapes and other thermoplastic composites are used in various car components.
The most notable example in 2018 was the CFRT rear wall for the Audi A8 luxury sedan made in a fully automated, Industry 4.0 production line. Offering a 50% weight reduction vs. 3-5 welded aluminum parts and providing 33% of the drive cell’s torsional stiffness, the rear wall begins with carbon fiber. It is spread into bound, 50-mm-wide, UD tape which is then cut to various lengths and applied at specified angles on a rotary table to form a tailored blank. The blank varies from a base of six plies up to 19 plies where local reinforcement is added, and a thickness of 1.5-3.7 mm. It is then shaped into a 3D preform in a heated press. The completed preform is then injected with resin and press-molded. Although the epoxy resin cures in 90-120 seconds at 120°C, the total part cycle time is 5 minutes.
A kind of bio-based UD tapes was created in a research project. In a film impregnation process, non-directionally aligned regenerated cellulose fibers were combined with the developed plastic film to form UD tapes so that very stable laminates could be pressed. The result: With weight savings in the automobile interior of up to 20% and at the same time a performance increase, for example, with the flexural strength being more than 90% compared to conventional bio-based material systems, these can be used very well as a basis for targeted structural lightweight construction.
Other Application of Fiber Reinforced Thermoplastics
Porsche developed a thermoplastic composite battery tray demonstrator for the Porsche Boxer car. Replacing six welded steel parts, the design saves 1kg (30%) weight for a moderate lightweight cost premium below €10/kg.
The tray consists of an injection molded PA6 reinforced glass fiber reinforced or PA66 platform, over-molded onto a hollow profile made from braided yarn and a cast aluminum fixing console on each side. The yarn is made of continuous hybrid glass/PA6 fiber. The profile is formed by air blown under pressure from a hose while the PA6 fiber melts and cools, forming a rigid profile with variable cross-section. It is filled with a fluid or particle foam to withstand pressure during injection over-molding.
The CFRT composite tray retained the battery without itself showing external signs of damage when submitted to front, side and rear impact tests with acceleration peaks up to 36g to withstand the most critical front impact requirement.