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Question and Answer

Foil Stamping Image Holograms

by Christopher Van Pelt

February-March, 1999
Foil stamping holographic foil images has continued to become a growing market for both promotional and security applications. Where standard hot stamping foil relies mostly on its promotional benefits for market share, holograms have the additional security benefit that has created a large increase in image hologram usage throughout the world in recent years. If you have considered adding the ability to foil stamp image holograms in the services you offer, there are several things to keep in mind before taking the plunge.

Holographic Foil Considerations

The holographic foil quality is a critical factor in achieving registration accuracy and minimizing waste. Most holographic foils are manufactured with equipment which creates "blocks" of images on the foil roll - commonly 6 inches long. The number of holographic images within each block depends on the size of the holographic image. Spacing between individual images within each block is usually perfect and never a challenge. However, the spacing between each block is many times not uniform. Often, spacing between blocks can vary as much as plus or minus .05", giving a total error of a tenth of an inch.

Another side effect of inaccurate spacing of the image blocks is the creation of gap and overlap seams. These seams can create problems with optical sensors which may confuse seams with the registration marks on the foil. Splices within a roll of holographic foil can also cause unforeseen problems with waste and registration accuracy. Always ask your foil manufacturer to supply your holographic foil with no splices to decrease the chances of any production problems.

Next to each holographic image there is a small registration mark that is used by the holographic registration equipment to detect each image before stamping There are two basic types of marks-diffraction and matte. A matte registration mark is generated mechanically or chemically, while a diffraction mark is generated optically on the laser table through a similar method as the mastering of the holographic image itself. Make sure that the holographic registration equipment you are using is compatible with the registration mark used in the manufacturing of the foil. This is critical to achieve a quality foil stamped hologram

Registration Equipment Choices

There are several types of registration systems currently in operation. These systems fall into three main categories.

Sense and register the specific image to be printed. This system requires that the optical sensor be mounted so the registration mark can be detected just before the stamping die. This requires that the sensor be mounted in the impression area of the press which is a very hostile environment (heat and possible crash damage). The advantage of this system is that spacing errors between blocks of images will not effect registration and set-up procedures are relatively simple. In spite of the advantages this system offers, the problems of positioning the sensor within the impression area are considerable.

Offset sense and register, without compensation. This system puts the optical sensor outside the impression area to protect it from possible heat and crash damage. This system is easy to set-up, with minimal operator skill and waste. When using good quality foil, with all images equally spaced, this system will do a good job registering the images. However, without the ability to compensate for spacing errors this system may have a registration error which is unacceptable.

Offset sense and register, with compensation. Today, this is probably the most common system in use. This system also moves the optical sensor out of the impression area. Through special programming, inaccuracies between each block of images is sensed, calculated and compensated for automatically. This is a complex system which is continuously discarding the information from the image just foil stamped and entering the information for the next image as it moves into the stamping area. The down side to this system is the complexity of the setup procedures and greater set-up waste; although, this continues to become less and less of a problem with technology improvements on the newest registration equipment

Other Considerations

Most optical sensors in use today use a fiber optic cable and a sensor control module. Sensor control modules which have sensitivity adjustment and some type of sensitivity display many times can help simplify the set-up time and offer superior performance and control. The actual sensor end of the fiber optic cable must be mounted securely. This mounting must be adjustable so the position of the sensor can be properly aligned. Directly beneath the sensor the foil should be supported so that a uniform sensing surface is maintained. Unsupported foil can bow or flex, which can affect the performance of the optical sensor and registration.

Typically, electronic foil drives are either servo or stepper motor type. Both of these drives are controlled by a computer and software. The software programming is what allows the sensor and foil drive to interface in such a way as to compensate for image spacing errors on the foil. Acceleration control and "seam ignore" options are two additional features which should be incorporated within the foil drive software. Gap and overlap seams (or other unwanted marks on the foil) can sometimes send a false signal through the optical sensor. Using a "seam ignore" feature, the operator can enter data which lets the foil drive know the distance between registration marks on the foil. This feature will not allow any seam or mark on the foil to be detected between registration marks.

Foil stampers often focus on the electronics when they detect registration problems. However, mechanical factors are a common cause of registration inaccuracy. Gear belts and gear belt pulleys, which connect the servo/stepper motor to the foil draw system, should be checked to ensure that they are secured and correctly adjusted. A loose set screw or gear belt will affect registration accuracy.

Concentricity of all foil support shafts and metering rollers should be carefully checked as well. A few thousands of an inch eccentricity in one of these shafts or rollers can cause registration errors. Foil support shafts and metering rollers should be strong enough not to bow or deflect during foil advance. In addition, the foil tension must be controlled and adjustable. Too much tension can stretch the foil and, in extreme cases, cause a stall condition in the servo or stepper motor. Too little tension and the foil advance may over travel due to kinetic energy. The amount of tension required will vary based on the size of the foil roll, thickness of the foil carrier (polyester film), and acceleration settings.

Finally, as with any machine, the operator is perhaps the most critical performance element. The operator must be responsible for correctly maintaining, adjusting, and operating the complete registration system. Each type of registration system requires specific set-up procedures. Operator error during any part of these procedures will result in unnecessary registration inaccuracy and waste.

Holographic foil stamping is a strong growth industry. Every foil stamper that is in this market or is considering entering this market needs to understand the complex combination of dynamics which affect holographic foil registration. Through improved foil quality, advanced registration equipment and software, and improved education, holographic foil imprint quality and accuracy will continue to improve.