Polypropylene foam parts for automotive applications

EPP-Processing and Applications
EPP (Expanded Polypropylene) is already established in the automotive market. In bumper cores, side impact panels, head and knee cushions, sun visors, headrests, tool boxes and even instrument panels, EPP combines a very good energy absorbing capability with the requirements of full recyclability, temperature and chemical resistance, as well as excellent structural stability.

Some important developments in the last 5 years have improved the performance of EPP dramatically. In contrast to the conventional autoclave batch process, the more flexible extrusion process allows the production of coloured foam particles (beads) for moulded parts with higher temperature resistance and less squeaking problems in a cost effective way. A new tool surface technology and the back foaming process (skin moulding) have improved surface properties and new composite structures have increased energy absorption.

Polypropylene monomaterial composite with TPO skin

Monomaterial TPO skin / PP foam composites
A simple and effective cost saving method for producing decorative and light weight shaped parts out of polypropylene is achieved by the combination of TPO skin (thermoplastic olefin based on PP) and a foam backing of XPP (foamed PP sheets). The semi finished product is achieved by direct or indirect lamination of a TPO skin onto XPP, which can be thermoformed in a second step or inline directly after the lamination process. This is the most economic way of producing decorative parts such as sun visors, pillar, and door trims.

 

Thermoformable PP foam composite

The TPO skin / XPP foam composite can also be used for back pressing or back foaming with EPP parts. During forming, the surface of the TPO skin can be grained inside the surface treated mould cavity. Limited undercuts are also possible. The soft touch polyolefin foam sheet generally used today, is a crosslinked PE /PP. With the non crosslinked XPP foam a higher thermal formability is achieved, as well as an improved fusion between EPP and the TPO skin. The final part offers a soft touch and full recyclability.

Another successful product development example is a patented thermoformable semi-finished composite called FAWOTOP, made out of extruded, foamed PP films (FAWO^®LIT) with EPP (FAWO^®CEL or VESTO^®CELL) as the middle layer. High stiffness, thermoforming capability and extremely low weight of the EPP core offer an excellent product with tailor-made properties. With other appropriate materials for the top layer like textiles, fibre reinforced or wood sheets, the variety of products can be extended from boot liners and roof trims to e.g. lightweight furniture systems etc.

FAWO^®TOP is already used with a decorative PES textile for rooftrim since 1998. The development for a monomaterial roof trim made out of FAWO^®TOP with a decorative PP-textile and energy absorbers made out of EPP is already finished. The SOP will happen this year.

Vacuum-formed FAWO^®LIT itself can be used as shields located inside car doors providing thermal insulation, noise deflection and sealing against water. FAWOLIT has a cost advantage against physically crosslinked PP shields and the advantage of full recyclability.

Stress-strain curves for EPP, one single pin and FAWOPIN

Developments for increased energy absorption
Energy absorption can be improved tremendously in the EPP field by a patented composite of EPP and injection moulded PP pins. Due to fast increasing uniaxial pressure at the beginning of the pin deformation, this composite enables the excellent energy absorption of the EPP to be optimised from the beginning of deformation along with a large amount of compression. This mono-material composite has already been incorporated in riding helmets and it is on its way to passenger safety applications like bumper cores and interior crashpads.

New EPP technology eliminates friction noise and provides enhanced appearance
A problem in automotive interior applications is squeaking caused by sliding friction between two EPP or EPS parts or between foam parts and other surfaces. The so called 'stick slip' effect produces high-frequency periodic relative movements caused by elastic deformations of the surface structure. The squeak potential can be measured by pulling a foamed sample under a defined vertical force over foam or other surface. The measurement of the gliding force versus distance is an objective method for measuring sound behaviour.

Friction test

With extruded particle foam and a new surface technology for both extruded and autoclave particle foam, the squeak effect can be eliminated permanently. The lower slip force amplitude affects a reduced acoustic pressure. Specifically, the lower frequency of the stick slip effect is in an inaudible range. In addition, the low increase of slip force amplitude with growing friction distance results in a permanent anti-squeak effect.

Beside the positive acoustic effect, the fine porous mould technology (FAWOPOR) surface provides enhanced optical appearance of the moulded part as steam nozzles or gussets are non existent and offers less or no attraction to contamination.

Author
Dr Maik Ziegler is senior manager of the R&D centre with Fagerdala World Foams. Dr Ziegler graduated from the University of Stuttgart, Germany, in mechanical engineering. He worked for four years at the Fraunhofer Institute for Chemical Technology and supervised an engineering office. He lectures in new materials and processes as well as innovation management and patents at the Berufsakademie in Karlsruhe, Germany.

Fagerdala Deutschland GmbH
Herrenhöfer Landstr. 6
D-99885 Ohrdruf
Phone +49 (0) 3624 / 339-90
Fax +49 (0) 3624 / 339-975
E-mail vertrieb@fagerdala.de
Web site - Fagerdala.com

 

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