Steel-skinned plastics for car panels

VW Polo WRC model with Litecor hood.

The Society of Plastics Engineers Central Europe presented applicants, finalists and winners at its 16th biannual automotive innovation awards night in July 2015. The body exterior first place went to a part that does not feel or look like plastic: the hood outer skin of the Volkswagen Polo R WRC (World Rally Car) road version in Litecor from steel producer ThyssenKrupp, cutting overall hood weight by 30% to save 25% CO2 emissions.

Litecor involves two layers of 0.2?0.3mm sheet steel with a 0.3-1.0mm “special thermoplastic blend” film core layer. The patent WO/-2012/126923 (“Composite material and structural component for a motor vehicle”) published on 27 September 2012 refers to “at least one fibre?reinforced plastic layer” with its matrix “based on polypropylene, polyethylene, polyamide, and/or mixtures thereof”.

SPE CE automotive award events are chaired by SPE CE president Dr Klaus-Dieter Johnke (from VW) and commented by Dr Rudolf Fernengel of 2R Kunststofftechnik and Büro für Kunststofftechnik consultancy as jury chairman (earlier at BMW). Fernengel observed, as part of the usual jovial VW/BMW sparring, that BMW had investigated plastic-cored steel body panels much earlier, “but without any success, as the plastic was too thick then – but with its more economical approach, VW has managed [it]”.

Production of 2,500 pre-series Litecor hoods took place at Thyssen-Krupp in Dortmund, where there is also a 30m long x 8m wide x 6m high Litecor sheet pilot line with 10,000 tonnes per year capacity installed in 2013. Volkswagen in Navarra, Spain meanwhile presses Polo R WRC hoods with the Litecor skin in standard VW sheet steel processing tools.

Instead of conventional soft forming steel, Litecor involves easily formable interstitial-free (IF) steel without carbon alloying elements. Cold joining techniques such as punched rivets, screws, Betamate adhesive from Dow or low temperature laser welding ensure the plastic core does not creep. Litecor is however suitable for inline painting, as it withstands exposure to cataphoretic electrodip (“e?paint”) paint coating lines at up to 210°C.

Oliver Kleinschmidt of the ThyssenKrupp car sales department and steel sandwich materials product co-ordinator, who is listed in the above patent as one of the Litecor technology inventors, predicted in March 2013 “we should be able to start supplying Litecor in large quantities as from 2017”.

ThyssenKrupp has identified at least 14 potential Litecor applications as large flat and high?stiffness bodywork components such as roofs, doors, tailgates and hoods. Compared with full-steel design, ThyssenKrupp found Litecor-skinning made a hood 21% more expensive and a door panel 7% more expensive.

In the long term, ThyssenKrupp expects thinner Litecor skins with sprayed polyurethane reinforcement.In July 2015, the organiser of the Composites Europe 2015 fair in September announced on behalf of PU machinery producer Hennecke that it would show a ThyssenKrupp “Incar Plus project” lightweight door on its fair stand with its thin steel/plastic hybrid outer skin (Litecor sheet) selectively back-sprayed with a PU-based plastic. Hennecke’s composite spray technology sales manager Jens Winiarz says this compensates for loss of dent stiffness and resistance associated with use of the very thin steel sheet.

Having also investigated seat shells, ThyssenKrupp says Litecor “is equally ideal for structurally relevant car interior parts, as it is much lighter than monolithic sheet steel and therefore presents a real alternative to aluminium, especially in cost-sensitive volume markets”.

Litecor and Bondal – a similar sandwich product for low noise electric motor stators – are examples of ThyssenKrupp’s 40 different InCar Plus lightweighting solutions announced in October 2014. ThyssenKrupp’s InCar Plus project co?ordinator Dr Axel Grüneklee says Litecor and Bondal “open new potentials for future vehicle generations, so that steel will also be the material of first choice for most vehicle producers”.

In the SPE CE 2015 awards, machinery producer Frimo won the body exterior innovation award for the bionic self?cleaning and aerodynamic (low Cw value) sharkskin-inspired roof and hood surfaces of the “Street Shark” demonstrator used on the BMW Z4 car (European Plastics News July/August 2014). Dominik Schwager of D?style – racing driver son of BMW’s former plastics specialist and i-series project manager Hans Schwager – customised the car.

Eschmann Textures supplied the ceramic?coated textured mould, ISL Berlac the in-mould coating (IMC) paint system for the parts. The parts are produced with a 3D-Core expanded structural composite foam core by low pressure resin transfer moulding (LP-RTM) process with a “snap-curing” Vitrox PU from Huntsman and Puroclear PU from Rühl Polymer.

D-style, Eschmann Textures and Frimo also featured in the body interior innovation award for the BMW Z4 prototype decorative trim strips developed in 2014 with bionic design features and a self-healing surface obtained with Rühl’s Puroclear polyurethane.

The body exterior second place went to Kunststofftechnik Backhaus for an air duct under a charge-air cooler moulded for the Mercedes-Benz E-Class launched in September 2014. Fernengel praised moulding of the two-component part in the 20% talc filled PP grade Hostacom M2UO2 from LyondellBasell and Tefabloc 823, a TPS/SEBS from Feddersen as the soft component, in a Wittmann Battenfeld moulding machine with a highly complex slider system in the mould from Uniteam Italia. The part is first moulded straight for easy release, then bent 90° “like a film hinge” in the mould with an additional slider.

Kunststofftechnik Backhaus also took the power train fifth place for backmoulding to metal of a phenol formaldehyde, Vyncolit W2005 from SBHPP Sumitomo Bakelite High Performance Plastics, in a B&M Formenbau mould by a Wittmann Battenfeld machine to make a brake system pressure component for various vehicles. Although Fernengel said there are no weight or price advantages or disadvantages, this approach reduces heat transmission to the brake fluid.

The body exterior third place went to the Airpanel adaptive grille shutter on the 2014 Mercedes?Benz C-Class, moulded by Montaplast in a TJ Moldes mould. Holger Jakobs of Daimler, Sindelfingen accompanied the part at the SPE CE automotive awards night. Earlier in a 2015 VDI plastics in automotive conference paper, he claimed the actuated plastic louver based shutter application with eight painted louvers behind the C?Class chromed ABS grilles to be “the first time a controllable cooling louvers system has been applied to a car’s visible front area, whereas competing OEMs in general use them behind the vehicle front design area”.

Lanxess supplies PA6GF15 for the louvers and their supporting structure and PA6GF30 as a stiffer, more torsion-resistant material for the crankshaft applying movement from the actuator to the louvers. PPGF30 is used as a low friction material for the sliding bolts. The actuator is made by Mirror Controls International (MCI), applying for the actuator housing its experience in use of PPGF40 as mirror actuation system housing material. This proved to have better acoustic performance than a PBT considered at the prototype stage, Jakobs said at the VDI conference.

Jakobs said that the louvers are painted while mounted on the louver carrier and admits that painting significantly increases the system cost, but this has been compensated with part integration and narrower than usual air slots.

Body interior first place went to the claimed worldwide first all-plastic instrument panel carrier, developed for a BMW M4 (GTS) special model due in March 2016. The IP carrier has been produced in a Siebenwurst mould in a combination of PA66 and 60% glass fibre reinforced PA61/X from EMS-Chemie. IP carrier designs have so far progressed over the years from all-metal, to hybrid PA/metal overmoulded, adhesive bonded or riveted versions.

The new Mercedes-Benz high gloss touchpad produced in the Coverform process (full details: Plastics News Europe July/August 2015) received the electronics and optical parts first place and grand innovation awards. M-B talked at the 2015 VDI plastics in automotive engineering conference about work with mouldmaker AWM Moldtech, which became integrated within Adval Tech group’s Foboha in March 2014 as Foboha (Switzerland).

Among only two other electronics and optical finalists, toggle switches of the 550bhp Ford North America Mustang took second place for use of one injection mould to produce all plated toggles with their red day?and?night symbols. They are moulded in a Schreiber Kunststofftechnik mould on Sumitomo (SHI) Demag machinery, with moulding and plating taking place at BIA Kunststoff- und Galvanotechnik in Schulman’s Polyman ABS Galvano Rot platable material and Alcom PC 740/4 UV WT1257?04LD polycarbonate from Albis, a light-diffusing UV stabilised easy?flow grade containing an unspecified special filler.

BIA also featured with the body interior second place, for use of its BDC “black diamond chrome” (as shown at Fakuma 2014) for the centre console trim in Bayer MaterialScience’s Bayblend T45 PG grade of PC/ABS on the Mercedes-Benz AMG GT C190, launched in August 2014, also moulded on SHI Demag machinery. The console also includes the new touchpad produced in the Coverfom process. A third place in this category went to “piano black” round ventilator housings and lamellae for their high gloss with an impression of depth on 2014 model-year Mercedes-Benz A-, B- and C-Class cars. The parts are moulded by Fischer Automotive Systems on Arburg machinery in Grilamid TR30, a transparent amorphous polyamide from EMS-Chemie.

A lightweight glove box designed by BMW for the 3-series with start-of-project (SOP) November 2018 received the body interior fourth place for substituting conventional flock finishing with a decorated natural fibre reinforced PP film produced by Isowood, applied to Quadrant Plastic Composites (QPC) composite based on a Borealis 30% long glass fibre reinforced PP. BMW has been evaluating the glove box in various moulding machines at its Landshut plant, using a mould from Wisa Werkzeug + Formenbau.

Magna Exteriors & Interiors (Bohemia) submitted and displayed an injection moulded prototype dashboard made in a Minerv grade of PHA (polyhydroxyalkanoate) from Bio-on in Italy, but it did not feature among winners. Bio-On has been making PHA using cane and beet sugar production waste; in April 2015 it announced plans to develop production from potato waste, and in June 2015 it announced that it was ready to grant licences to realise the first PHA bioplastic production plants using glycerol derived from biodiesel by?products.

The electronics and optical third place went to a Continental Automotive head-up-display (HUD) mirror retainer on various vehicles since 2013, on account of precision, dimensional stability and low thermal expansion over a wide temperature range from the Celanese Fortran 6165A6 and 1140L4 grades of PPS that were used. Fernengel says there “was astonishment at integrated clips, but directing glass fibre flow orientation in the clip areas means they don’t break”.

Powertrain first place was received by a water assist (WIT) moulded clutch pedal and associated bearing block on BMW cars as from July 2015. These were developed by compound producer Akro?Plastic, injection moulding machinery maker Engel, mouldmaker Moldetipo PR, fluid assist equipment specialist PME Fluidtec, along with Spanish moulder Batz S. Coop. Use of Akromid B3 ICF15 (pedal) and B3 ICF20 (bearing block) – respectively 15% and 20% short carbon fibre reinforced PA6 grades – cut part weight by 15% and increased stiffness of the pedal by 25% (see also Plastics News Europe, June 2015).

The powertrain grand innovation award went to a gear setting module moulded since 2014 on Arburg machinery by FTE Automotive for all Audi vehicles worldwide using the DKG 7?gear Ultra S-tronic gear system. Fernengel referred to high creep resistance of the EMS?Chemie Grivory HT1VL-50X grade of 50% long glass fibre reinforced polyphthalamide, its withstanding operating temperatures up to 160°C and the need to be processed at over 320°C – “something not all moulders can manage,” he said.