by Neil Meador, Permadur Industries, Inc.
Servo-Driven Hot Stamping Machines
What are the advantages of a servo-driven hot stamping machine?
Servomotors can sense the force they apply and “know” where they are. Therefore, servo-driven machines always can apply a set print pressure even if the thickness of the material being printed varies. Another advantage is in set-up. Since stroke lengths and speeds are set to real numbers, a machine can be restored to a previous set-up without special skills or trial and error. Typically, the upstroke speed and foil delay timing are critical in a press set-up. In the most common traditional pneumatic hot stamping press, these speeds and timings are set by adjustable pneumatic flow controls and dial set timers. Putting a repeat set-up in a pneumatic press usually depends on the memory and skill of the set-up person since there are no hard numbers to return to.
A servo-driven press typically has a touchscreen or other type of display that sets press opening and closing speeds in real units of inches per minute. Repeating a set-up is simply a matter of entering the numbers for the job. Some machines even have recipe storage capacity for many different jobs so a set-up can be recalled by a single number or job name. The most advanced servo hot stamping machine can scan a bar code and automatically recall and set itself up for the job indicated by the bar code. Even without advanced bar code or recipe storage features, set-up time is always dramatically shorter with servo-driven machines.
Finally, tremendous energy savings can be realized by using a servo-driven hot stamping machine. These “all electric” presses turn the incoming electricity directly into mechanical motion of the driving screw with mechanical efficiencies of up to 85 percent. By comparison, the electricity used in a pneumatic machine starts at the air compressor. The compressed air is then piped throughout the shop until finally, it is controlled by valves as it expands in the actuating cylinders of the press. The energy loss is huge. Some conservative studies have shown a side-by-side comparison of energy savings that offsets the additional cost of a servo-driven machine in less than two years on energy savings alone.
Heat Transfer v. Foil Stamping
When is a heat transfer a better choice than direct foil stamping?
Even though the application methods are almost identical, there are two main technical differences between hot stamping and heat transfer that govern cost and suitability of purpose. The image on a heat transfer label is not created at the point of application. Heat transfer is an indirect printing process. The clarity and definition of the image is established when the label is made, not when it is applied. In the case of hot stamping, the die defines the image when it strikes the part. If the die contact is too hard, too hot, or dwell time is too long, the image quality will suffer, edges will be ragged, and the print will have an overly broad appearance. Likewise, inadequate heat and pressure can cause an image that is lighter and finer than it should be.
For most single-color and simple two-color designs, direct hot stamping is still the least expensive method. An exception is if fine graphics are needed and the parts are known to have sinks, warps, or other defects, since these will cause print quality problems for direct hot stamping – problems that a heat transfer label may be able to compensate for while still maintaining acceptable quality.
Rainbows and Blushing
How do I get rid of “rainbows” and “blushing” in bright metallic hot stamping jobs?
In all methods of hot stamping, the foil should be laying on the surface to be printed prior to the die contacting the foil. Even though it may appear that the die presses the foil against the part, it is really the foil guide bars that must do this. The placing of the foil onto the part prior to die contact and the subsequent stripping of the foil after die contact is usually referred to as the “drape” of the foil. If the foil contacts the die or even gets too close to the die before it is in intimate contact with the part, the foil will blush and rainbow. The foil must be in contact with the part and then pressed between the hot die and the part to allow the heat to transfer through the foil to the part. If the foil alone contacts the die the heat of the die will cause uncontrolled distortion of the foil. This causes the bright metallic layer to fracture on a microscopic scale, often accompanied by creases and wrinkles. This condition is eliminated by proper position of the foil guide bars and foil advance timing so that the guide bars stretch the foil over the surface to be stamped before the die contacts the part – it’s all in the drape.
Neil Meador started his career in decorating as a process engineer for a large U.S. contract decorator. His first U.S. patents were granted in 1985 and were embodied in the first commercial multi-axis servo-controlled decorating machines ever sold. European and Japanese patents were also granted. Currently, he is the manager of Permadur Industries Machinery and Controls Division and is widely recognized as one of the leading authorities on hot stamping/heat transfer decorating methods and techniques combined with electronic servo-controlled motion. For more information on Permadur Industries, Inc., visit www.permadur.com.