Hot Stamping

By Tom Gray and Mike Wilks | July 20, 2005

Steel-backed flexible plates for hot stamping retain quality and lower costs.

Innovative new products are changing both the practices and financial calculus in several areas of the printing industry. Often a relatively minor change in the way a press is outfitted can make a major change in operating expense, productivity, and profitability. One such change is the use of magnetic hot stamping cylinders.

Hard tooling has historically been a significant expense item for the converting end of the business. In some cases, the skill level required to produce hot stamping or embossing dies equals that involved in fine art, and of course, on a practical level, the time and money involved add up even when much lower levels of talent are required.

In rotary hot stamping, three processes are currently growing in popularity as alternatives to costly hard tooling. These processes use either photopolymer plates for cold foil work, copper dies, or flexible rubber dies. Photopolymer plates can be used with conventional nonmagnetic aluminum cylinders or with magnetic cylinders if provided with flexible steel backing. Magnetic hot stamping cylinders are required when printers use flexible steel-backed copper or steel-backed rubber dies. There are pros and cons to each process.

The cold foil approach requires the use of a print station equipped with nip rollers and UV. This method can waste large amounts of foil and adhesives. The cold foil process is improving, but in our opinion it lacks the vivid color, shine, and clean edges that are produced by other hot stamping methods. Using a high quality magnetic cylinder with the cold foil process will help some of the quality issues and improve up-time, but the cold foil process needs further development before it can be truly competitive for high-quality jobs.

Steel-backed copper plates are typically mounted on steel magnetic cylinders and run as if you were running a hard brass hot stamp die. This process gives a clean hot stamp image, but the image height is very low. So the copper plate method results in more limited relief than is sometimes desired. An economic drawback to this process is that it requires a sacrificial cushion roller, which normally needs to be replaced after several dozen jobs. Another mark against the copper plate method is that it sometimes improperly hot stamps the background body area. If you apply the high pressure needed when running at speed, you are likely to increase the background problem. In addition, using a magnetic cylinder made of steel for hot stamping means that temperature will vary over the cylinder face and heat loss will be relatively high during runs.

Flexible steel-backed stamping plates.

Hot stamping with flexible steel-backed rubber dies is the third and in many ways the most promising alternative to conventional hard-tooled cylinders. Molded rubber hot stamping dies last about as long as flexographic printing plates: Depending on a number of factors, the useful service life of a rubber die ranges from about one-half million to three million impressions. That's well below the life expectancy of nearly indestructible hard-tooled cylinders but more impressions than many jobs require. There's also an obvious price advantage. Molded rubber dies cost only a fraction as much as hard tooling of comparable complexity.

Hot stamping with rubber dies gives crisp, clean impressions and even helps minimize the risk of a meltdown and the time-consuming work of cleaning up after one. Some rubber dies are made from a specially formulated rubber compound that actually repels melted plastic and, in the event that the foil should become severely overheated, helps keep the melted plastic from sticking to the die surface. Hot stamping with this method produces a high relief and will not hot stamp the background, as happens in the copper plate method.

The best results are achieved when steel-backed rubber dies are mounted on magnetic cylinders made of solid brass. Using a brass cylinder gives you exceptionally even temperature distribution across the face of the cylinder. Even heating is critical to hot stamping quality and translates into even temperatures over the surfaces of your dies for superior hot stamping results. Compared to steel cylinders, brass cylinders provide much more consistent heat transfer and reduce heat loss. Properly designed, brass magnetic cylinders are also very heat tolerant. One proprietary design can withstand temperatures up to 1000° F.

Rubber hot stamping dies offer 48-hour turnaround and typically cost from one-tenth to one-half as much as a comparable hard-tooled cylinder. A rough average for a new die is around $530. They are highly adaptable and economical in operation, as well. On some presses, they can run in print stations mounted on a magnetic cylinder. But more commonly, they run in die stations direct to anvil — without the need for a sacrificial cushion roller. So in addition to their low cost and durability, rubber hot stamping dies save printers the expense of replacing the cushion rollers, which are required by other hot stamping methods.

The table above details some of the savings achievable with brass magnetic hot stamping cylinders and steel-backed flexible rubber dies versus solid brass tooled dies and brass segment tooled dies. The figures reflect the financial savings in materials after just one year at various levels of hot stamping activity.

Despite higher initial cost, the brass magnetic cylinder and flexible rubber dies come in significantly below the basic hot stamping expenses of hard-tooled cylinders and segment-tooled dies even in this relatively narrow production range. Using hard-tooled cylinders is more than twice as expensive at the five-job level, over three times as expensive at the 15-job level, and more than four times as expensive at the 50-job level. The costs associated with segment-tooled dies are over $800 higher at the five-job level and more than twice as high at 50 jobs. Even if you assume that cushioned anvil rolls can last for more than 20 jobs, the trend stays headed in the same direction because of the higher costs of the competing dies. The conclusion couldn't be more obvious. Who wouldn't like to cut costs by more than $90,000 a year?

But the financial advantages don't end there. Solid brass magnetic cylinders allow printers to cut changeover times dramatically by changing dies right on the press without waiting for cylinders to cool down and without changing cylinders. Flexible steel-backed rubber dies wrap around the magnetic cylinders and are securely held by the magnetic field. Press operators can mount, register, and change dies in a fraction of the time required for the same operations with conventional cylinders. Faster changeovers and reduced labor not only cut costs but also improve productivity and your capacity to handle more work.

During runs, well designed and precision-machined solid brass magnetic cylinders virtually eliminate die shifting and stretching. Dies as small as 4" x 4" stay securely in place even at high press speeds. So there's reduced waste. Yet even large dies are easy to remove or reposition after lifting a corner with a beveled-edge tool or knife. They provide top-quality foil transfers at speeds up to 400 feet per minute and more — far higher than conventional cylinders normally run.

One major manufacturer has a process to remanufacture hard-tooled brass cylinders that are in good condition into magnetic hot stamping cylinders, saving customers 10 to 30 percent off the price of comparable new cylinders. But even at full price, brass magnetic hot stamping cylinders are a valuable enhancement that will quickly pay for themselves by saving on setup time, die costs, and rejects. One magnetic cylinder combined with flexible hot stamping dies, can replace a warehouse full of solid tooled cylinders and save printers many thousands of dollars annually in hard-tooling, cushion rolls, storage, and other expenses.

Tom Gray is printing products group manager for Bunting Magnetics Co., a manufacturer of magnetic printing cylinders and flat bases. He has years of experience working in the printing industry in the United States and abroad, specializing in applications, and is credited on several patents for narrow web printing and converting equipment. His office is in Tamarac, FL. His e-mail address is tgray@buntingmagnetics.com.

Mike Wilks is director of marketing and sales for Bunting Magnetics, and is based at company headquarters in Newton, KS. Bunting also manufactures a wide range of other magnet-based industrial equipment. His e-mail address is mwilks@buntingmagnetics.com.

The web site is www.buntingmagnetics.com.