Explosives Emboss Holograms

Hear the word "explosives" and images might spring to mind of an old building being demolished, or an avalanche being triggered. But now a more precise, delicate task has been found for explosive materials-- embossing holograms in steel.

Holograms -- colorful, iridescent interference images -- can protect currency, tickets, drivers' licenses and other items from forgery because they take a great deal of effort to produce and are almost impossible to copy. This is because the image is created not only by the interaction of different colors and contrasts, but also by the surface structure. Different pictures can be seen, depending on the direction from which the light is shining.

Holograms are normally produced with the aid of laser beams, starting by creating a prototype from photosensitive material. However, this template is too soft to be able to act as an embossing or injection-molding tool for holograms. Consequently, the filigree relief-pattern is copied onto a harder material, such as nickel, through electroplating. Mounted on a roller, this nickel shim transfers the hologram onto a plastic film for use on items such as credit cards and concert tickets.

But now researchers at the Fraunhofer Institute for Chemical Technology (ICT) in Pfinztal have come up with the more radical method of using sheet explosives to impress holograms into steel. With the right dosage, explosives enable a template to be copied with far greater accuracy than by conventional methods, they said. The "explosive embossing" method achieves a resolution in the two-figure nanometer range.

“Nobody believed such a thing could be possible,” said ICT project manager Günter Helferich. Almost any structure, such as those made from wood, leather and textiles, can be rapidly and accurately impressed on metal in perfect detail with the aid of a sheet explosive, he said.

The scientists are now working with industrial partners to create steel tools with holographic structures as a "stamp" for applying holograms to plastic parts. The challenge is tremendous: The structures that have to be imprinted into the steel are so tiny that they cannot even be discerned under an optical microscope. The experts have optimized the method to the desired image sharpness through numerous series of experiments.

The advantage of this method over electroplating is that it does not produce a soft nickel piece that quickly wears out, but a hard steel stamp. Steel treated in this way is also in demand in the plastics industry, where many plastic parts now must look attractive, particularly if they are placed in elegant surroundings.

For more information, visit: www.fraunhofer.de/EN