EuroAuto: Fast Performance for Performance Vehicles

Columns From: 12/1/2006 Automotive Design & Production, , Editor from Automotive Engineer, London, England

Although Mitsubishi was winning all of the plaudits for taking still another victory in the Lisbon-Dakar Rally—a grueling race covering more than 5,500 miles on some of Africa’s most remote and inhospitable terrain—earlier this year (its 11th victory in 24 years), it was innova-tive Italian company CRP Technology that was quietly congratulating itself on a job well done.

Although Mitsubishi was winning all of the plaudits for taking still another victory in the Lisbon-Dakar Rally—a grueling race covering more than 5,500 miles on some of Africa’s most remote and inhospitable terrain—earlier this year (its 11th victory in 24 years), it was innova-tive Italian company CRP Technology that was quietly congratulating itself on a job well done. It had provided some of the key components that made that win possible.

“CRP is a pioneer in rapid prototyping application, having started in 1996 as a spin-off from Roberto Cevolini & C when the technology was almost unknown in Italy,” says Franco Cevolini, CRP’s chairman and technical director. “It has its roots deep in motorsport, focusing on manufacturing prototypes with the latest rapid prototyping technology. It has supplied Formula One, rally, rally-raid and Moto GP teams amongst others. Overall, though, the business is driven by three main activities. There are our rapid prototyping capabilities, the complete manufacturing of parts using, for example, rapid casting or CNC machining processes, and the production and commercialization of high-performance materials for the laser sintering process we called Windform.”

An example of the complete manufacturing of parts relates to the Mitsubishi performance in the race. “The Mitsubishi victory in the Dakar Rally was not really an unexpected result considering all the effort put in by the technical partners, including CRP, into the extraordinary Mitsubishi Pajero/Montero Evolution,” says Cevolini. “With our task being to optimize the uprights and suspension levers, our specialist technicians dedicated themselves to re-engineering them for rapid casting. Previously, they had been made by the standard process, fabricated and machined. The process and production of mechanical parts through investment casting made from steel allowed us to save weight while at the same time achieving an optimized structural behavior with greater efficiency. The perfect synergy between the technical offices of Mitsubishi and CRP permitted the re-design of the component, obtaining an excellent mechanical answer while respecting the requirements of the foundry. The end quality was much better. As soon as the 3D CAD file was ready, the mechanical structural quality was evaluated and then checked again through different sessions of structural calculation. Tests were made with advanced workstations in order to check reaction at various load cases, simulating real stress from track. After validation from Mitsubishi engineers, we began the production cycle involving our internal rapid prototyping department. After full inspection, the sintered lost models were used for the casting process. As part of the quality process from the inspection department, CRP also undertook various test including non destructive test inspection and 3D tests on the parts before delivering them to Mitsubishi.”

There are a variety of ways of manufactur-ing suspension uprights, usually involving long and complicated forming and cutting of steel sheets. Processes include welding, heat treatment, CNC machining, and finally painting to protect against corrosion. However, where the required properties are lightness as a non-suspended mass, stiffness as any deformation compromises the kinetics of the suspension and braking, the downside of this process is that the uprights are heavier than they should be. Moreover, being a welded piece, it presents a structural anisotropy that shortens its lifespan, and therefore its reliability in time.

While the Mitsubishi uprights were cast from steel, CRP is also very proud of Castform, its rapid casting process for highly reactive materials, the Italian company claiming a world first when it introduced its titanium rapid casting process 10 years ago. Significant advantages of rapid casting technology, very much appreciated by CRP’s customers, were the possibility of best post-stress control of the piece compared to carbon laminated parts, and the durability and reliability of the detail—a casting is naturally isotropic for compensation. Not only are there are fewer design limitations but also the possibility of both lightening and stiffening the part. This is particularly useful for racing car manufacturers and teams during the season. The technique allows complete freedom for shape con-ception without the handicap of undercut and tool path problems during CNC machining.

The rapid casting procedure is composed of various steps, the most important of which are the disposable pattern in Castform which is made through rapid prototyping. The pattern undergoes wax infiltrations—immersion and capillarity—to increase its strength to avoid handling breaks and elimination. The casting structure is formed of an aggregate of grains or polyhedral crystallites which produce isotropy compensation, while in a solid metal they are anisotropic.

CRP has also developed a process that is based on the combination of rapid prototyping technology, to manufacture the disposable pattern, and investment casting technology (lost wax casting), which it claims is superior in every respect, including the quality of the end product. The rapid prototyped pattern is made by a consecutive overlapping of layers, using selective laser sintering technology in a chamber with inert atmosphere and at constant temperature. A roll lays a layer of thin powder on a platform at which a CO2 laser is directed. It does not require any support because the piece is held up by the non sintered powders, therefore giving a complete freedom of shape.

CRP’s Windform laser sintering process family currently comprises Windform GF, Windform PRO and Windform PRO B whose identifying characteristics are the stiffness and the Ultimate Tensile Strength (UTS). The latest addition to the family, though, is Windform XT, a carbon fiber-based material for SLS technology for true rapid manufacturing applications. It is similar to CarbonMide, the carbon fiber filled polyamide material marketed by German company EOS, the two companies getting into something of a squabble earlier in the year over patents rights. CRP’s Windform XT material has a low density (1.1g/cm3) and a high tensile strength (77.85MPa) and tensile modulus (7320.8MPa), which means that it has an exceptional ultimate tensile strength per unit density of 70.71MPacm3/g and a tensile modulus per unit density of 6649.2MPAcm3/g. The surface finish on as-built parts is 6.0microns (Ra), and a finish of 1.8microns (Ra) can be achieved after finishing. For parts up to 150mm the standard tolerance is ±0.3mm, while the tolerance on larger parts is ±0.05mm per 25mm. Its excellent detail definition, stiffness, resistance to vibration and can be used to create thin and intricate parts with high strength.

Its light weight, high UTS characteristics, excellent surface finish, and resistance to wear, compared to standard SLS Duraform and SLS GF Duraform, also lends the technology to the low-volume direct manufacture of parts for race cars and motorcycles without the weight penalty. CRP was already using its Windform GF material to create parts for Formula One cars but things like brake ducts, air intakes, cooling ducts and bodywork flaps are now produced in Windform XT. The material is also being used for components on World Championship motorbikes including the chain pad, head cover, water pump cover, seat, mudguards, windscreens and airbox. It is also the preferred SLS material for wind tunnel models due to its strength and stability under wind load and vibration while it has even found applications on a road car even if it is at the exotic end of the scale and found on the Lamborghini Gallardo.

“The main improvement with Windform XT,” says Cevolini, “is the homogeneity of the material. It is now very easy to achieve good surface quality, reliability during the build process and accuracy. It is also quite simple to attain good mechanical properties without any warpage at all.” Some of the potential applications for Windform XT are also related to vehicle engines, so laboratory tests have been performed to quantify the material’s characteristics at elevated temperatures. After soaking the test pieces in an environmental chamber for at least 90 minutes, the material’s tensile strength, yield strength and tensile modulus (E) have been analyzed from data collected at five temperatures ranging from 60° up to 150°C. All three material properties were found to decrease moderately with an increase in temperature, with values at 150°C being in the region of half those at 60°C. Indeed, the pattern observed is very similar to that for PA6BG-35, a glass-filled polyamide that is typically used to produce injection-molded components for automotive applications.