Project "Holo-Impact"-explosive hologram embossing.
Until recently the only technique available to form holographic relief patterns into hard metallic surfaces was pixel-by-pixel dot-matrix laser ablation, which is a relatively time consuming process with a limited dot resolution.
New Process Offers Change
The recently developed and patented process of explosive embossing may change this situation considerably. This technique involves sandwiching the holographic relief, such as a shim or the developed photoresist, between a thin explosive sheet and the workpiece to be embossed. The detonative shock then stamps the pattern directly into the (eg. hardened steel) workpiece (see Figure 1).
Alternatively, the relief pattern can be moulded into the pliable explosive sheet, which is then placed on the workpiece surface and exploded. Explosive embossing can be utilised to structure a wide variety of metals, from aluminium to high strength alloys with Vickers hardness up to over HV 1000.
On May 1, 2006 the German Federal Ministry of Education and Research initiated the 'Holo-Project', namely the nano-structuring of metal surfaces using holographic patterns. The project will end on April 30, 2009.
The project is presided over by the Institute of Measurement and Automatic Control (IMR) of the Leibniz University Hannover and the Fraunhofer Institute for Chemical Technology (ICT) in Pfinztal. The commercialization of the developed technology will be realized by project partners Kugler GmbH, Georg Neumann GmbH, topac GmbH, and Rieger GmbH. The company Holo-Support of Gunter Beichert provides additional advice on holographic matters.
The main objective of the Holo-Impact project is the development of an economic and environmentally clean process for moulding of micro- and nano-structured holographic and other surface relief patterns in metal surfaces. Sub-objectives are listed as:
* Structuring workpiece inserts for injection moulding, hot- and cold-embossing;
* Texturing of embossing and printing cylinders and subsequent embossing of foils;
* Direct and unique holographic characterization of metal parts;
* Manufacture of adjustable test explosion chambers for small structured surfaces.
The project is still in an experimental stage. Its first milestone has been realized, namely the explosive embossing of holographic structures in flat hardened steel inserts of a mould in order to visually mark injection moulded plastic parts with irremovable diffractive patterns. Plastics used can be mixed with reflective particles to allow for good diffraction efficiency, so that no reflective coating is required (see Figure 2).
The second milestone, which must be achieved by the end of January 2009, is the explosive embossing of metal casings of any shape (such as an engine block) and cylindrical steel surfaces with holographic structures. The latter can be used as embossing cylinders which have a significantly longer tool life than nickel shims.
The explosive sheet preferably consists of composition B, which consists of castable mixtures of RDX and TNT. The sheet, when detonated, causes a shock wave that travels with a speed of 8,000 m/s, resulting in a high detonation pressure with a pressure rise rapid enough to stamp even soft textures such as plant leaves into the workpiece with high fidelity. Typical thickness of explosive sheets used is 0.8 to 1.2 mm.
An additional effect of the shock pulse impact is increased surface hardening of the workpiece, allowing longer tool life. It has further been found that thinner texture layers render greater embossed texture contrast on the workpiece. An example of the submicron surface detail that must be transferred to the workpiece by the explosive impact is given in Figure 3.
Till Scholz of IMR, who is Holo-Impact project manager together with Gunter Helferich of ICT, told Holography News that, up to now, holographic shims have been exploded into steel. This has the disadvantage of requiring nickel plating as an intermediate step, but only one single shim is necessary. A direct transfer technique to be investigated is the application of photoresist to steel directly; after exposure and development the resist pattern is exploded into steel.
Explosive sheets appear to be expensive (up to US$1,000 per [m.sup.2]) and when asked if this would significantly affect the cost of the process, Scholz told Holography News that, although there are only a few producers and that thin explosive sheets are indeed expensive, the main current problem is to have quality requirements met. An option worth considering may be explosive sheet production at the ICT labs.
Strong Protection Against Copying
Concerning the possibility of mechanical copying of the moulded structures, Scholz explained that even if this were possible, subsequent transfer of the structure to a hard substrate would create a significant hurdle. Additionally, the moulded structure might be protected by an adequate access blocking top coating. Consequently, holograms integrated in hard substrates provide a strong protection against hologram counterfeiting, copying and product piracy.
Scholz further emphasizes the great advantage of the process: it can blast holograms directly into hard materials, thus producing products protected by unique holograms of which a master shim no longer exists, if it existed at all.
As an example of a commercial application Scholz refers to project partner Georg Neumann GmbH (Berlin), producer of high quality professional microphones. Neumann intends to manufacture unique, individualized studio microphones that, for instance, have the holographic portrait of the artist blasted into one of its metal parts as product ennoblement.
A further application is decorative patterning of materials in the printing and packaging industry, which additionally serves as protection against product piracy.
Contacts: Till Scholz, (IMR), email@example.com,
Gunter Helferich, (ICT),
By Ruud van Renesse-VanRenesse Consulting