The work that goes into building a physical model of a concept car is usually hidden behind closed doors, known only to engineers sworn to secrecy and similarly tight-lipped subcontractors.

Ask anyone involved exactly which companies use 3D printing technology to create concepts and the answer will invariably be, “Many of them.”

One company, however, has opened up its secret vault. Remember the Citroën Hypnos? We took great delight in teasing the automaker for their outlandish interior, but never imagined it could actually be built. Of course i.materialise proved us wrong by printing up some pod-like seats (shown above) one can actually rest in — if you dare. The same goes for the futuristic interior of the Renault Ondelios, another i.materialise creation.

For the 2008 Paris Motor Show, the folks over at 3D printing service i.materialise built parts of a concept car for Citroën based on the GT supercar from the Gran Turismo 5 video game. The only reason they’re able to talk about it is that the car is three years old, and they got the OK from all parties involved.

“Concept cars are meant to stretch the imagination,” said Joris Peels of i.materlialise. “This means that on the engineering front they often stretch what is currently possible in manufacturing. The methods and concepts that lead a car manufacturer towards new insights in design and manufacturing is something that the car manufacturer would rather keep close to its chest.”

Most people don’t even realize that the parts and panels of concept cars gracing turntables from LA to Geneva are often the work of a 3D printer. Those that do, but aren’t closely familiar with the process, might imagine a giant version of a desktop printer that spits out a final product, ready for installation. The actual process involves just as much art as science.

“What a lot of people don’t know is that there is actually a lot of skilled artisanal work involved in 3D printing,” said Peels. “We have the best finishers in the world. These production people are artists and in some cases spend hours sanding down particular parts by hand. Because each part is unique they have to have a thorough understanding of 3D printing and understand what the customer wants.”

Citroen GT interior: The finished product

3D printing — also known as stereolithography — cuts a design up into “slices” that are printed from the bottom up. A laser hardens liquid resin into a solid as it “draws” the slices, layering one directly onto the next to build a 3D object. The resin is then finished, painted or coated with another material. “The touch does depend on the finishing of the part and the material,” Peels said. “The tops of parts are also very smooth while you can feel layers in the sides. The touch of the resin feels a lot like the back of an iPhone 3. But, again this does depend a lot on the finishing and if there is paint on the part.”

For the GT, resin pieces were coated with copper and nickel. As a test, i.materialise also printed and plated a grille that’s traveled over 7,500 miles on the front of a car. One staffer even printed an out-of-production part for his own personal car.

Aside from one-off concepts, Peels sees a future in 3D printing. Already, the Urbee concept is to be printed by Stratasys, and i.materialise is looking to high-end aftermarket shoppers who want to create unique parts for their custom creations. “We are already 3D printing parts for production cars,” Peels said. “We for example have been called on to make the bodywork for one off sports cars for enthusiasts. There are a number of 3D printed parts on some small batch automobiles.”

Of course, he can’t tell us which ones they are.

Photos: i.materialise

Post updated at 3:45PM EST to reflect new information from i.materialise

See Also:

The unfinished Citroen GT interior

Citroen GT exterior

The Renault Ondelios' printed interior

Citroen Hypnos interior: The final, terrifying product