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HOLOGRAPHIC REGISTRATIONAL HOT-STAMPING FOIL
The first high volume production of hot-stamped holograms was a fifty million piece MasterCard credit card order in the early eighties. In the development of the manufacturing process, one of the deciions to be made was how to accurately register the holographic image. Obviously some sort of registration mark was necessary, but what?

he process of producing holographic images in hot stamping foil involves embossing the image into the surface of the metallized layer of the foil, a process which modifies the shape of the surface. Ideally the registration mark should be added when the hologram itself is produced on the holographic master. The only means of forming a registration mark at the hologram producer's disposal is a modification of the shape of the surface, since the mark must be replicated through all the production steps along with the holographic image. We can't print a mark, or punch a hole.

Type:-

The most obvious method turned out to be the best. Since there is already a laser used to produce the hologram itself, it was simple to add a diffraction grating as a registration mark. But what kind? What geometry?

An ideal registration mark would produce a signal with an extremely high signal-to-noise ratio under all manufacturing conditions. Signal-to-noise ratio is a measure of how well a signal stands out above the backgroun, the higher the better. When a diffraction grating is illuminated straight on, light is produced at several different angles, depending on the fineness of the grating. There is also, of course, a direct reflection. It made sense, therefore, to design the grating to produce a strong beam at 45 so that it could not be confused with the direct reflection. The next question was - should the registration mark be placed along the web or across the web? It was decided to make it across the web since this would not be affected by the foil going around rollers. However, it is simple to produce a grating to give strong beams in both cross web and along web directions.

Other techniques have been used to produce marks, most commonly diffuse marks. These marks scatter incoming light in all directions, and while it is possible to detect these, the signal-to-noise ratio is low, necessitating frequent detector sensitivity adjustments. In contrast, the diffraction marks, in conjunction with a well-designed detector, produce a signal so strong that sensitivity adjustment is never required.


Position:-

Each image on the foil should have an associated registration mark. This is true even if many holograms will be stamped simultaneously. Obviously for any given project, the geometric relationship between the image and its associated mark must be constant. In most circumstances, the mark is centered on the image, which is fine. Positioning the mark at either the leading or trailing edge of the image could place it close to one of the embossing seam lines. The mark should be alongside the image, not between images. Most registration systems have a "windowing" feature, which allows them to ignore false signals like die seams. While this feature makes it possible in some circumstances to use a mark between images, setup is difficult and waste is likely to increase.

The marks should be spaced at least 1/8 inch away from the hologram image in the cross web direction, and no closer than 1/8 inch from the foil edge.


Size:-

The minimum recommended size for a registration mark is 1/8 inch square. Making it wider in the cross web direction helps in setting up, but is not necessary for operation. This means that the foil needs to be wider, which is wasteful. In the long web direction, the size is not important. Most registration systems will accommodate 1/16 inch to 1/2 inch or more.



Shape:-

To avoid unnecessary variations in register position, the edges of the marks should be straight, sharp, and uniform in optical properties.



Conclusion:- 

The need for standards in the hologram industry cannot be stressed enough. The industry is now twenty years old, has an annual market well in excess of one hundred million dollars and still there are no firm standards in this crucial area of registration mark design. Even today, holographic foils are manufactured with unusable registration marks, typically from a lack of communication or from individuals who produce marks in isolation without either knowledge or regard of the process of application. Working closely with the holographic foil manufacturer and the manufacturer of the hologram registration equipment can help eliminate these types of problems and create a registration mark that is functional at press speeds the market now demands. 

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