Designed to conceal an integrated aerial, a new type of roof element for the Mercedes-Benz R-Class is playing a groundbreaking role in the automotive industry’s use of the paint film coating process, which relies on a special paint-coated plastic film for manufacturing body parts.
This new process cuts costs and may well develop into a key technology for the production of lightweight vehicle parts. In the long term, it also opens up new avenues for vehicle designers, who will be able to use the process to integrate aerials and sensors directly into plastic body parts.
Once upon a time, listening to your car radio was a very simple matter - all you needed to tune in to your favorite station was a trusty rod aerial, usually affixed to the fender.
Today, however, a luxury car can be fitted with as many as 18 different aerials - not only to receive radio and TV programs, but also for telecommunications, navigation, telematics, remote-control functions and proximity-control systems.
What’s more, with the advent of vehicle-to-vehicle communications, this number will be expanding even further in the foreseeable future.
Given a choice, car designers actually prefer to hide away all of these aerials beneath the vehicle’s bodyshell. As a rule, however, automotive body parts are made of sheet metal, a material that interferes with radio signals. In searching for a solution to this problem, engineers developed special covers made of plastic, which does not hinder reception, making it possible for the aerials to nonetheless be concealed in or behind a vehicle’s body.
There’s an aerial concealed in the trunk lid of the Maybach, for example, and in the trunk lids of the Mercedes-Benz CL-Class and the CLK convertible as well. Similarly, the roof of the M-Class and R-Class incorporates a plastic aerial cover that’s painted the same color as the vehicle. So where’s the problem? The aerials function flawlessly, and the plastic elements are, to the naked eye, entirely indistinguishable from the rest of the bodyshell.
Actually, there’s no reason at all why developers at the Mercedes Car Group shouldn’t be satisfied with their work. On the other hand, as every engineer knows, a good solution can always be enhanced. Take the aerial cover for the R-Class, for example. At present, this plastic element is spray-painted, just like the rest of the bodyshell.
“But that’s an expensive process with high unit costs,” explains Walter Aichholzer of Production and Materials Technology (PWT) in Sindelfingen. “Today, more and more parts are being produced by different suppliers around the world, which means they all have to invest in their own paint shops. What’s more, we need to guarantee that the colors of these attachments will perfectly match the rest of the bodyshell, regardless of whether the part is made of sheet steel, aluminum or plastic, and whether it has been manufactured in Germany, the United States or South Africa.”
> Perfectly matching colors
In response to this need, Aichholzer and his colleagues have come up with quite an elegant solution: a ready-to-fit body part made of plastic already coated with a paint film. Not only does the new aerial cover allow perfect radio reception; the paint film process that was developed to produce it also guarantees a perfect color match with the rest of the bodyshell - while significantly reducing manufacturing costs.
As a rule, car body parts are first formed from a suitable material, such as sheet steel, and then painted on the outside. With the new roof element for the R-Class, which is approximately half a square meter in size, this process is reversed.
The plastic carrier film is first painted with the base coat to provide the color and then with a protective clear coat, before being heated in an ambient-air oven until it is physically dry and fast to handling but not chemically hardened. The coated film is then placed in a deep-drawing die and formed into the outer “skin” of the aerial cover. Finally, the paint is hardened by means of ultraviolet (UV) light from a special lamp.
Although the coated film skin has now achieved its ultimate three-dimensional shape, it still has a thickness of only one millimeter and therefore requires further mechanical reinforcement. This is accomplished by placing a fiber mat on the back of the coated film and spraying it with polyurethane (PU) foam. Finally, the part is given one last pressing to shape it into the form of a ready-to-fit roof element.
“A crucial feature of the process from a technological point of view is that the drying and hardening of the UV-paint are conducted in two separate stages,” explains Aichholzer, who first led the project at DaimlerChrysler Research Center in Ulm and then moved with it to Production and Materials Technology in Sindelfingen, where he works closely with Matthias Beyer from R-Class Development and paint expert Bernd-Uwe Kettemann of PWT.
> Right up to scratch
The new paint film cover developed by DaimlerChrysler engineers is every bit as good as the conventionally painted plastic aerial cover. “One of the biggest challenges we encountered was to achieve a perfectly matching color tone for all the colors on offer for series-produced Mercedes-Benz vehicles. With metallic and other special effect paints in particular, the human eye is capable of registering even the smallest of differences,” explains Kettemann. However, thanks to a new paint application process developed at the Ulm Research Center and used to coat the thermoplastic film, the specialists are now able to produce paint films even in difficult metallic colors.
The new type of roof element also fulfills all of the other requirements that Mercedes-Benz outer body parts are expected to meet, including chemical stability, hardness, thermal expansion and a high degree of scratch-resistance.
Another decisive advantage of the paint film process is the low unit costs compared to conventionally manufactured parts. Should other outer body parts need to be coated with paint films, in addition to the aerial cover, the process guarantees that any other body parts produced in this way will be absolutely identical in color.
In fact, it would be possible for an individual manufacturer to produce entire rolls of the coated film in the required color and then deliver the rolls to various automotive suppliers at locations around the world. The suppliers would then be able to cut up the film as needed, mold it to the required shape in a deep-drawing press, harden the paint with a UV lamp and back it with PU foam so as to produce a whole variety of parts in an identical color.
> Delivered to the line ready-to-install
The new aerial cover for the R-Class marks the first time that Mercedes-Benz has used paint film components in series production. Produced by a specialist company based in Altbach, near Stuttgart, the roof elements are made as ready-to-install aerial covers, and then shipped to the United States.
In the States, workers merely have to insert a seal, after which the parts are delivered to the assembly line in the R-Class plant in Tuscaloosa, where a robot glues them into the vehicle roof. “For capacity reasons, we’re going to have to limit ourselves initially to just two colors,” explains Beyer. The supplier company is currently ramping up for volume production, though, and by the end of the year it will be in a position to deliver aerial covers in all colors for the R-Class and the M-Class.
> Functional integration
Meanwhile, researchers and developers are already eying the possibility of using the innovative coated film to produce a host of other plastic body parts. Options here include sills for cars, spoiler parts for trucks and, in the medium term, complete trunk lids.
At the same time, Jan Krüger, the leader of the Functional Integration and Modularization team at DaimlerChrysler Corporate Research, is looking even further ahead: “The coated film process is a completely new type of construction, and in the future it will give us all sorts of opportunities to integrate vehicle components in the bodyshell. This includes incorporating aerials or sensors directly into outer skin body parts.”
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