Hybrid construction: the view from car makers

Photo by Porsche The Porsche 918 Spyder has a CFRP monocoque body shell made using the RTM process.

In this feature, David Vink reports from the IKV Colloquium on developments at Porsche, Opel and BMW and their views on lightweight construction.

Three automotive OEMs presented papers at the 2016 Plastics Technology Colloquium, organised in February by the IKV plastics processing institute at RWTH University in Aachen, Germany.

Dr Werner Tietz, Porsche’s vice president for body engineering, described a non-structural plastic application on the Porsche 911 Carrera (991 generation): the rear spoiler lower section in 35% glass fibre reinforced PA6, produced using Trexel’s MuCell microcellular physical foam moulding process.

Tietz also spoke about the car’s lightweight reinforced thermoplastic underbody panel with integrated glass mat thermoplastic (GMT) air ducts. Porsche and partners HBW Gubesch Thermoforming and Quadrant won a 2013 AVK innovation award for this development (European Plastics News January 2014).

In structural applications, Tietz showed the Porsche 918 monocoque body shell, produced as a single resin transfer moulded (RTM) carbon fibre reinforced plastic (CFRP) part, to which a power unit sub-frame made in autoclaved preform?based CFRP is attached at the rear. Front and rear aluminium profiles applied to the CFRP parts complete the body shell structure.

Porsche’s strategy towards achieving economic lightweight design involves reducing the cost of conventional thermosetting resin based CFRP through faster cycle time while retaining properties, and also introducing thermoplastic matrices. Lightweighting costs should be minimised through “intelligent hybrid design with cycle time retention”, Tietz stated.

Tietz spoke of hybrid design in thermoplastics, with fibre spraying, organic sheet and unidirectional (UD) tapes providing high specific strength and stiffness, along with part integration and high complexity – and combining these techniques with injection moulding. Porsche has applied this approach to the Porsche 918 Spyder’s brake pedal, combining 47% long glass fibre fabric reinforced PA6 organic sheet with injection moulded 60% short glass fibre reinforced PA6.

Battery tray

Together with Munich Technical University, Porsche developed an “e-generation” research project composite battery tray demonstrator for the Porsche Boxer car. Replacing six welded steel parts, the three-part design saves 1kg (30%) weight for a moderate lightweighting cost premium below €10/kg.

The tray consists of an injection moulded glass fibre reinforced PA6 or PA66 platform, overmoulded onto a hollow profile made from braided yarn and a cast aluminium fixing console on each side. The yarn is made of continuous hybrid glass/PA6 fibre. The profile is formed by air blown under pressure from a hose while the PA6 fibre 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 overmoulding.

Flow and mould filling simulation studies ensured knit lines are away from points of high mechanical load. The FRP 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.

Looking at various hybrid design approaches, Tietz said combining long fibre reinforced thermoplastic (LFRT) with organic sheet and a half-open steel profile provides an ideal combination of high cross-sectional stability and specific strength, along with ease of integration with metal body-in-white (BIW) components. Injection moulding into a steel profile does less well in cross-sectional stability, as does FRP patch application. An all?plastic injection overmoulded organic sheet does well in cross-sectional stability and specific strength, but performs less well than steel/plastic hybrids in BIW integration.

Tietz referred to a hybrid organic sheet/steel B-pillar demonstrator designed with the ILK lightweight plastics construction institute at Dresden University, LZS Saxony lightweight centre, Mitras Composites and mouldmaker Siebenwurst. The hybrid plastic/steel part replaces five steel parts, saving 1.3kg weight. He said there are challenges before such hybrid designs achieve higher market penetration: simulated failure prediction quality needs improvement, serial production processes need development, as do mass production joining processes.

Dr. Petra Krammer, advanced manufacturing technology manager at Opel, spoke about challenges and requirements in hybrid construction from the viewpoint of a global volume car manufacturer. She said a “paradigm shift” occurred in 2010 when Opel Corsa BIW structures stopped increasing weight as models changed – helped by, for example, hybrid steel/aluminium BIW structures replacing all-steel ones. In the 1993-2015 period, the Corsa C model’s BIW weight was 16% higher than the Corsa B’s, and the Corsa D’s BIW weight was 19% higher. However, the 2015 Corsa E weighed 3% less.

Krammer said the Chevrolet Corvette C7 Stingray sports car has an aluminium frame to which pre-painted plastic composite panels are bonded by mechanical fastening, with the cockpit model bonded and bolted. The aluminium, glass and carbon fibre composite material mix involves aluminium accounting for 57% of BIW weight, GF and CF composites for 32%, steel for 9% and magnesium 2%. The hood is in CFRP.

This “multi-layer body architecture facilitates lightweighting at affordable cost and enables multiple variants at low investment cost”, said Krammer.

Although not mentioned by Krammer, Opel presented an Astra OPC Extreme limited edition sports car at the Geneva Motor Show in 2014 with extensive CFRP use, cutting roof weight for example from 9.3kg in steel to 2.2kg in CFRP. Opel and the AZL automotive lightweight centre institute showed an OPC Extreme roof at the 2014 IKV Colloquium, as a visible carbon roof “manufacturing study”.

Krammer claimed the organic sheet based lightweight front seat “seat cushion” shell in the Opel Astra OPC car, shown in Geneva in 2012, was an “industry first”. BASF described in March 2012 how unreinforced BASF Ultramid PA6 impregnates TenCate Tepex continuous glass fibre fabric organic sheet, short glass fibre reinforced PA6 injection overmoulding ribs and edges onto the seat shell.

This results in higher rigidity, cutting wall thickness to 1mm. Krammer said weight is 47% lower (down from 1.54kg to 0.82kg) over the preceding seat shell produced from 2008 in a BASF Ultramid short glass fibre reinforced PA6 grade. Reinert Kunststofftechnik moulds the hybrid shell with 70s cycle time in a Maier Formenbau mould. Lanxess announced at K 2013 that its Durethan PA6 and Tepex Dynalite organic sheet from its Bond Laminates subsidiary are also used by Reinert Kunststofftechnik for the seat shell.

The hybrid solution lowered the H-point (pivot point between the torso and upper leg) by 17mm and crash performance improved due to higher strength and rigidity, Krammer said. Calvin Nichols of BASF revealed at the 2013 SPE automotive composites conference in the US how the SediTec subsidiary of the seat producer Seatcon (both now part of Inter Groclin Auto) submitted the seat shell to 25kg crash impact load at 5?9 m/s velocity.

Krammer said appropriate joining technologies are key to hybrid fibre reinforced plastic/metal (FRP/metal) development. FRP/metal bonding methods include self?piercing or blind riveting, flow drill screwing, ultrasonic welding, structural bolting or bonding, and fastener bonding.

She said long-term challenges for high volume production in multi-material FRP/metal bodywork include: design approaches suitable for selected materials, functional integration as an “enabler”, flexible production and integration into existing production systems. FRP composite challenges include: Class A surface quality, avoiding manual material handling, cutting cycle time, waste, and high costs in materials, process and painting, Krammer said. FRP should also withstand electro?deposition coating (e-coat) cure temperature, and both recycling and repair issues need consideration.

Krammer said increased lightweight material use means “increased investment and piece part costs, and that a long-term strategy is essential to ensure the value chain is ready for execution”. However Opel parent GM is well prepared here, Krammer observed.

Dr Jochen Kopp, a former manager of the IKV fibre composites and polyurethane department, who later worked for Airbus Deutschland and as FRP lightweight project manager at Volkswagen, has been meanwhile involved at BMW in developing the use of CFRP in large-scale serial production. His IKV Colloquium presentation covered plastics lightweighting in BMW i?series cars.

Investment

Kopp described BMW i3 painted thermoplastic exterior body skins as an attractive solution for the CFRP bodyshell. This is because it involved “minimum investment in manufacturing facilities and tooling”, he said, as well as 50% saving in energy consumption and 40kg less weight than with production of conventional painted steel body panels. Thermoplastic panels are also corrosion-free and have dent-free robustness against minor damage. He also spoke about multi-coloured clamshell “exterior filigree panels with wafting structures”, with wrap-round corner geometries, minor radii and “striking lines” achieved with thermoplastic.

The area of the BMW i3’s large front door panel was a particular challenge, with a large pressure gradient, and waviness on the panel opposite injection, Kopp revealed. Aside from panels bonded with TPE in the mould, adhesive seals applied inline to certain shells involves infra?red heating, ensuring adhesive cure in step with the moulding cycle.

The thermoplastic roof side layer presented challenges in TPE seal integration for both water and thermal management, allowing for thermal expansion coefficient differences between the thermoplastic and the thermosetting CFRP roof panel.

BMW’s SGI injection-compression integral structural foam moulding process is used for the instrument panel, the initial 1.8mm thick moulding expanding to final 4mm part thickness in a partly opened mould.

There are 42 CFRP components in the i3 car. These involve 14 monolithic RTM parts, including three made from preforms braided over a foam core with Herzog braiding machinery, five in recycled carbon fibre and 23 compression moulded CFRP parts. The fully automatic bonding process for various parts extends over a distance of 150m. The 2,345mm long, 1,100mm high i3 side frame is produced as a single part from 11 preforms, including three braided profiles and two local patches.

The i8 doors combine steel with RTM and compression moulded CFRP. Kopp described a braiding process with a water assist injection moulded “blown” core pulled out after resin-cure to form a hollow RTM profile. Plastics News Europe identified this technique as also used on the 2015 7-series car, for which Maximator supplied water assist equipment (Plastics News Europe, March 2016). With “wet compressing” used for some 7-series parts, Kopp revealed the i8 also has some wet compressed parts.

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