By: James Snodgrass
EUROPEAN PLASTICS NEWS
March 21, 2014
It has been more than three years since Paris-based Total SA introduced a concept car showcasing the potential of using rotational molding to build a lightweight monocoque shell.
Unlike a traditional automotive monocoque, an internal skeleton onto which an exoskeleton of body panels is added, the concept car from the petrochemical giant showed the possibilities of using multi-layered rotational molding to make a structure that is both chassis and skin, internal and external.
Highlighting the properties of its Bio TP Seal polymer, the Total Concept Car had a multi-layered rotational molded body with a paintable outer shell, foamed inner structure (providing stiffness and insulation, whilst keeping weight down) and interior surface.
Although Total still has no plans to manufacture the vehicle, the concept, shown at the Paris and Frankfurt motor shows, wowed the traditional car makers – some of whom showed significant interest in the concept.
Even if the concept has not made it to auto showrooms, though, some of the technology is moving on to an industrial application — providing both structure and body of the cab of an agricultural vehicle.
The VSV (Viability Service Vehicle), made by French manufacturer, Noremat, is an adaptable vehicle for agricultural and highway maintenance functions designed to be fitted with one, two or three pieces of operational equipment. Depending on specification, it can be used for mowing, pruning, collecting off-cuts, grinding up-branches, gritting and various other uses.
Originally Noremat, based in Ludres, France, had considered using a non-recyclable grade of polyester for the VSV’s chassis, bodywork, fuel tanks and access panels, produced through thermoforming. However, after consulting with Total and project designers Evok, the France-based design consultants, the entire project was reimagined with rotational molding in mind.
Bio TP Seal is a modified polyethylene , created using metallocenic catalysts, which is blended with polylactic acid. The Total Concept Car’s structure was made in single molding cycle wherein a foamed layer is sandwiched between two outer skins.
The rotomolding process used to manufacture the VSV was developed — as with the Total Concept Car — in conjunction with the Italian rotational molding equipment manufacturer, Persico. Using Leonardo machines from Persico SpA, the temperature of the mold, and the pressure inside the cavity of the mold was precisely controlled to deal with variable wall thicknesses, the foaming process and to prevent against bubbles forming on the inside and outside of parts.
The manufacturing process takes place in a single stage. As the cab of the VSV is elevated above the ground, it doesn’t have the same crash-protection requirements of the Concept Car, a small, electric city car in which the occupant is situated close to ground level. It is therefore made from a single layer of rotational molded Bio TP Seal, rather than having a sandwich construction.
The fully automatic Leonardo machines ensure that rotational molding is a repeatable process, producing consistent molds time and again without human intervention. When conceived as a part made from thermoforming, the VSV’s cab structure would have had seven components. By switching to rotational molding, Noremat is now able to make the cab structure in a single piece.
Likewise the side wings of the cabin are made out of a single part, rather than the proposed three-part manufacture; the rear cabin roof is made from a single part, rather than two thermoformed parts and the water tank, radiator cover, fuel tank and engine hood are all made from single, rotational molded parts. Only the wheel covers and mudguards (with integrated housing for the headlamps) are made using thermoforming.
The advantages of using rotomolded Bio TP Seal, rather than metal, are many-fold. Bio TP Seal is 100% recyclable, a growing concern for vehicle producers keen to demonstrate the sustainability of their products from manufacture to end-of-life. Structures made from Bio TP Seal result in a chassis that is three times lighter than the metal equivalent. And the ability to make a large component, such as a cab, from a single piece, in a single manufacturing process, thus contributes to greater efficiency, reduced production costs and a quicker production cycle.