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Peguform France’s new technology based on the thermocompression of thermoplastic composites – Sandwiform® – is used in automotive floor applications. Two of these are presented in this article: one is for the floor of the Microcar MC1, made in France; the other, for the cargo load floor on the Jeep Grand Cherokee, manufactured in the United States by parent company Venture Industries.
(Published on August-September 2005 – JEC Magazine #19)
BY THIERRY RENAULT, RESEARCH & INNOVATION MANAGER, PEGUFORM FRANCE
Sandwiform® is a recyclable thermoplastic composite that consists of a honeycombed cellular core placed between two reinforced thermoplastic skins. A thermocompression production process transforms a stack of plies into a thermoplastic sandwich structure with very high stiffness-to-weight ratio, impact strength and corrosion resistance. Sandwiform® can be covered on one or both sides with carpeting or other type of decorative finish. During the thermocompression process, both skins and the core are all stacked together and heated in an infrared oven. At this stage, the core is not yet bonded to the skins. The stack is transferred to a cold mould, where it is press-moulded at about 10 bars of pressure to form both the sandwich structure (all three layers bonded together) and the part in a single step.
The moulds can be designed so that part edges can be cut during the press-moulding step to obtain the finished part straight from the mould. Cycle time for moulding a finished part is about a minute, although it varies slightly depending on the type of materials used. The function of a sandwich panel is to separate, insulate, and support, at lower weight. Current Sandwiform® applications include large interior parts like cargo load floors, rear shelves, and even interior floor pans. Other potential applications are for engine shields, aerodynamic skis, or spare-wheel pans.
Semi-structural floor for the Microcar MC1
Since 2004, the Microcar Company in Montaigu, France has been manufacturing a licence-free vehicle called the Microcar MC1. The car is built using innovative spaceframe architecture, with aluminium profiles and a Sandwiform® floor. The floor consists of four carpet-clad parts from 5 to 7mm thick and a TPO-clad cover. The materials used in the application include Twintex co-mingled glass/polypropylene woven fibre fabric (770g/m2) for the skins, and PP honeycomb (density: 110g/l) for the core. Sandwiform® was chosen for its high stiffness-to-weight ratio compared to RTM; for the possibility for integrating carpet cladding into the moulding process; and for its good mechanical behaviour in terms of crashworthiness.
Microcar worked hard on the crash behaviour of its vehicle, and was able to demonstrate that the vehicle’s structure remained intact in EuroNCAP-requirement impact conditions against a deformable barrier at 45km/h, which is the vehicle’s maximum speed. The weight constraints for the MC1 car are very stiff: the vehicle’s maximum weight is 350kg.
Microcar’s previous vehicle, the Virgo, had an RTM-manufactured structure, with the floor weighing 29.4 kg. The floor on the MC1 in Sandwiform weighs only 17.3 kg, or 12.3 kg less – a drop of 42%! Peguform France moulds the entire five-piece floor at Pouancé in France’s Maine et Loire department. To cut down on the cost of moulds, parts cutting is a “compromise”, using cutter blades integrated into the moulds, with the more complex cutting done on demoulded parts using water-jet cutting. The moulds are made either of aluminium or of resin concrete.
Microcar requires an installed production capacity of 10,000 vehicles per year. When it receives the parts from Peguform, they are bonded to the vehicle’s aluminium structure. At the bonding station, the surfaces of the PP skins are activated with a flame treatment, and a cooling jig is used to hold the parts in place on the body during the heat-accelerated bonding step.
The new Jeep Grand Cherokee’s cargo load floor
Peguform France has transferred its Sandwiform technology to its parent company, Venture Industries in the United States. Venture started out on several short-run parts, including the rear shelf for the Ford T-Bird, then started in 2004 to work on a massproduced part – specifically, the cargo load floor for the new Jeep Grand Cherokee (230,000 parts per year). The material used in this application is a polypropylene (PP) honeycomb between two glassmat- reinforced PP skins. The skins are produced by the Eleison Company from recycled PP mixed with chopped glass fibres into a thin sheet of glass-mat thermoplastic (GMT).
Using the Sandwiform® process it was possible to meet the builder’s price objectives and still integrate multiple functions into the moulding process. These included carpeting, living hinges, and the possibility for welding injected PP parts onto the Sandwiform® structure. It also became possible to develop a complex part to be produced at the rate of 1,500 units per day. The functional integration increases product quality and helps to cut down on inventory costs, rework operations, and risk of operator error, as well. Venture Industries is developing several new products for SUV cargo floor applications, to be marketed in the coming months. These take full advantage of Sandwiform® properties.