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Rotary Dies



Solid and flexible dies each have advantages in certain applications, though flexible dies are gaining ground on the traditionally more popular solid dies.



Published September 25, 2006
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Rotary dies


Solid and flexible dies each have advantages in certain applications, though flexible dies are gaining ground on the traditionally more popular solid dies.



By Michelle Sartor



Rotary dies and tools are critical to the labeling industry because they cut desired shapes for labels. Rotary dies come in two basic forms: solid and flexible. Solid dies make up the majority of those in use, but flexible dies are growing in popularity.

Although all rotary dies are made out of steel, there are variations among them. Gary Smith, vice president of sales at RotoMetrics, Eureka, MO, USA, explains, “Solid rotary dies are made of various grades of tool steel, which is determined by the application and the process used to engrave the die.”

Making dies is a multi-step process. Bob Potratz, sales manager for Action Rotary Die, Addison, IL, USA, says, “The first step in creating a rotary die is lathing a cut bar of steel. From there, this die blank is machined using special CNC (computer numerical controlled) milling equipment. After the heat treating process, where the steel achieves a targeted hardness, the die is ground to a specific diameter. The exact clearance is put into the die to meet the specifications that are required by the end user.”

In addition to CNC, EDM (electro discharge machining) can be used to make dies. Bill Reichard, president and CEO of Gerhardt USA, Dallastown, PA, USA, says the company uses two machining processes to produce dies. “One is CNC engraving, which is using a machine with cutting tools to remove the steel material, leaving the blades in the desired pattern. Another machining process we use is called EDM, which is the removal of hardened steel by spark erosion. This process is more accurate and allows for more intricate geometries,” he explains.

Sharpening


Companies have different approaches when it comes to sharpening their rotary dies. Potratz says, “Action Rotary Die uses the tried-and-true hand sharpening method using solid carbide sharpening tools. Using stereomicroscopes to magnify the die blade allows the talent of the hand sharpener the best opportunity of creating the proper blade angle for the material that the die is cutting.”

Tim Bussard, product manager at Atlas Chem Milling, Elkhart, IN, USA, says only 5 percent of his company’s dies are hand sharpened, while 95 percent are CNC machine sharpened. He says, “We have an apprentice program to train people how to sharpen by hand. It’s minimal now since everything’s moving toward CNC, but we still have people in training and trainers for hand sharpening.”

Some companies make the decision case by case. “At RotoMetrics, we determine the most effective finishing method based on the application and the material being cut,” says Smith. “We keep an extensive database of material diecutting results to ensure we use the correct steel, proper blade angles and best finishing method for each application.”

Although some companies use machine sharpening, there may be manual components to the process. Karl Schober, president of Schober USA, Cincinnati, OH, USA, says, “Our dies are sharpened by a machine process, numerically controlled by manufacturing them. At the end, manual labor is involved for the reduction of the cutting edge. The final work on the die is by hand.”

Reichard believes that hand sharpening is the most common form for dies. “In the industry I think that fewer than 5 percent of rotary dies are machine sharpened,” he says. “Most manufacturers are unable to hold the tight tolerances that are necessary to machine sharpen a rotary die.” At Gerhardt, Reichard says, all flexible dies are machine sharpened, while just some of the solid dies are.

Sevki Ergun, area sales manager/engineer for American Die Technology, Suwanee, GA, USA, agrees that hand sharpening is the most common. He says, “A much higher percentage of dies are continuing to be hand sharpened especially if the shapes are irregular. The machine and laser sharpening methods are more frequent in standard label shapes and the frequency of their usage should continue to increase with time and improvements in the technology.”

At Wilson Manufacturing, St. Louis, MO, USA, 60 percent of the rotary dies are hand sharpened. Wade Fouts, vice president of sales, says the company is always training people in hand sharpening. He says it takes about 12 months to completely train an individual in the technique. “You have to always be training, because if you lose somebody you can’t go out and hire a new person who’s never done it and have them produce a good product in two weeks.”

All rotary dies become dull and either need to be resharpened or discarded. Flexible dies cannot be resharpened, so once they become dull they need to be replaced. When it comes to solid rotary dies, there isn’t necessarily a set number of times they can be resharpened, but they can’t be resharpened indefinitely. Ergun explains, “The number of times a die can be resharpened is limited simply because resharpening involves shaving off a certain amount of steel from the body of the die, making the circumference smaller each time. Since the dies are most commonly gear driven, there is a mechanical preset limit to how small their diameters can get before experiencing problems with web tension.”

Though a limit exists for the number of times a solid die can be resharpened, the use of the die influences the number. Smith says, “On average, a solid die can be resharpened four to five times under normal usage and assuming that the die is handled properly. Careless handling and abuse can cause damage to the die that may limit the number of times that it can be resharpened.”

Solid vs. flexible dies


Although solid dies remain more popular than flexibles in the US, flexible dies are growing in popularity in the narrow web industry, following the trend in Europe. Reichard explains, “In Europe, the flexible die has been developed over the last 15 years. Flexible dies make up about 80 percent of the dies purchased in Europe today. In the US, it is around 20 percent flexible and 80 percent solid, but the trend is definitely moving toward the flexible dies in the label industry.”

Several reasons exist for the increased use of flexible dies. “Flexible dies are growing in popularity as the technology improves and converters are able to use them for more demanding applications,” says Smith. “It used to be that flexible dies were used primarily to cut pressure sensitive papers, primarily EDP labels to be applied by hand. Now they are used to convert labels constructed of a wide variety of papers and films, which can be applied by high speed labeling machinery.”

In addition to improved technology with flexible dies, cost is also a reason for the popularity. Frank Hasselberg, executive vice president for Kocher + Beck USA, Lenexa, KS, USA, says a flexible die is between 30 and 50 percent of the cost of a solid die, adding that shipping charges and storage costs are lower as well. Smith puts the cost of a flexible die at about 25 to 30 percent of that of a solid rotary die. Reichard believes the price of a flexible die is 75 to 80 percent lower than that of a solid rotary die.

Ergun believes that converters need to take into account more than just initial costs when deciding to use a flexible or a solid die. He explains, “It’s hard to quantify because flexible dies have a shorter life and are thrown away when they become dull. In addition, one must already have the costly magnetic cylinder with the correct repeat for the job. A flexible plate itself might cost only a third less than the cost of a solid die but it might also last a third less. Flexible dies can be a good alternative when converting easy to cut stock and for rather short runs.”

Michael Wilks, director of marketing and sales at Bunting Magnetics Co., Newton, KS, USA, says magnetic cylinders, which are required for using flexible dies, can cost from $700 to $800 for a high quality small one, and up to $5,000 to $7,000 for a large one. He says, “We make magnetic cylinders out of carbon steel, stainless steel, brass (which we have a patent on), and we have different types of magnets, different magnetic circuits, which we custom manufacture based on the application.”

David McEachern of T.D. Wright, St. Paul, MN, USA, says, “Simple narrow web magnetic cylinders for label presses can cost less than $1,000, and large complex cylinders for large repeat male/female applications can cost tens of thousands. Due to their many similarities, the general rule of thumb is that if it would be an expensive conventional rotary die, chances are a like-sized magnetic cylinder would be similarly costly.”

The cost of a magnetic cylinder and a flexible die is higher than a solid die, but over time, using magnetic cylinders and flexible dies cost less. Wilks explains, “The first magnetic cylinder will probably cost as much as two solid tool dies, maybe three. But once you’ve got your cylinder, all you have to do is buy flexible dies and those are very inexpensive. By the time you buy the fourth or fifth die, it starts getting more economical.”

Although flexible dies have been growing in popularity, solid dies are still in heavy use. Smith says, “While there has been, and continues to be, a steady transition from solid rotary dies to flexible dies, the actual number of solid dies being produced remains steady. Not all applications are compatible with flexible dies and some applications run more efficiently with solid dies.”

McEachern believes that solid dies are best suited to certain types of jobs. He explains, “Solid dies still have a big advantage in very high volume applications where the pattern is static and
A Marathon solid die from American DIe Technology
huge quantities can justify the lead time and cost of dedicated die cylinders.”

Paul Young, national sales manager for Lederle Machine Co., Pacific, MO, USA, says, “I think that there may be some decreases in solid dies, but still the magnetic cannot serve all industries.” He concedes that flexible dies are less expensive and are the right choice in certain instances. “I hear from our customers more now than ever the label producer must absorb the tooling costs,” he says. “This makes some solid dies cost prohibitive, especially in some low quantity orders of labels.”

Sometimes, however, price might actually be a reason to stay with solid dies. Fouts explains, “With flexible dies, you have to have a magnetic cylinder for every size you want to run a die on. To go completely flexible, you have to have a magnetic cylinder for every size.” If a company needs dies in many sizes, the additional cost for the magnetic cylinders could be a deterrent for choosing flexible dies.

Solid dies offer other benefits. Smith says, “The biggest advantages of solid dies are that they can be resharpened and, in most cases, will last longer than flexible dies. In addition, solid dies allow for higher blade height for diecutting thicker materials and multi-layer constructions.”

Young explains, “The longevity and the ability to retool are still the main decision factors among solid die purchasers. In many cases even a chipped blade can be repaired on solid dies making this very important when dies get very expensive.” He adds, “I haven’t heard yet of a sheet die being repaired if it gets damaged.”

Though cost is the most recognizable advantage for flexible dies, a few others exist as well. Reichard says, “They are very light and easy to handle. They can be shipped anywhere in the world overnight for a very cheap price. They can be produced in one day compared to the standard lead time of three to four days for a solid rotary die. They don’t take up very much room to store them. You can change from one job to another very quickly. If the flexible die goes dull you simply put another one on the cylinder and you’re back up and running with very little down time.”

McEachern adds, “Lead times are a big issue. Both rotary and magnetic cylinders often have significant lead times, especially in larger sizes. Flexible dies can be made in just days. If a plant has access to magnetics, rush jobs can be turned around fast.”

Challenges


As with any piece of equipment, rotary dies pose challenges to those working with them. One of the greatest challenges is relaying to press operators the fragility of the dies. Fouts says, “They look very rugged, but they’re the opposite. You can damage them with a ring on your finger. A big portion of the dies that come in for resharpening are damaged, not dull.” He adds that at Wilson Manufacturing, “if we see a lot of tools that are damaged from the same company, we contact them and bring it to their attention. We conduct care and handling presentations to help prevent that.”

Hasselberg, of Kocher + Beck, points out that the shapes that need to be cut are getting increasingly difficult. He says, “In the old days, there were only circles or rectangles. Now you can have anything.” Hasselberg also says that the material being cut can pose challenges. Kocher + Beck performs test cuts to determine the best cutting angle when a converter wants to use a new material. He adds that sometimes if the diecutting wasn’t working, they changed the material being used, which fixed the problems.

Markus Marfurt, president of Electro Optic US, Alpharetta, GA, USA, agrees that materials make a difference. He believes that film material is more challenging to diecut than paper.

According to Potratz, of Action Rotary Die, the thickness of the material may be more important than the material itself. He says, “The challenges of rotary diecutting thinner substrates to thinner liners is often more difficult than cutting complex and abrasive substrates.”

Prolonging die life


Several treatments exist to help prolong the life of rotary dies, but certain practices can also help dies last longer. Ergun, of American Die Technology, says, “Cutting at the absolute minimum die pressure will prolong the life of a die.” He adds, “The pressure being applied must be maintained equally at both ends of the die at all times throughout the entire run so that the dies do not wear unevenly.”

Coatings can be applied to rotary dies to make them last longer. Bussard, of Atlas Chem Milling, says that coatings can be used to prevent dies from rusting. He explains, “All dies will wear with time and abrasion. If you surface treat the die or if you just put a light coating on the tip of the knife, that coating wears off.”

Marfurt believes that coatings make a difference in the life of a die. Since the blade material used to make dies is basically the same across the board, he thinks coatings represent an area where companies can improve die life. He says, “With Electro Optic’s coating, we get between three to five times the life of a regular die.”

Schober, however, disagrees. He says, “We do not believe in coatings. Release coatings, yes, but that’s all we do. We don’t coat the die for wear. It’s expensive due to the material we’re using, and the coating doesn’t necessarily improve the lifetime. It can cause problems with sharpening because you have to take the coating off. By choosing different materials for dies, you can avoid problems with coatings.”

Potratz says, “Cryogenically treating tool steels (a controlled process of exposing tool steels to subzero temperatures) can be very effective in prolonging die life. This process is done at the time the die is manufactured and is permanent, unlike a coating that has to be stripped and reapplied when a die is retooled. Increased strength, greater dimensional or micro structural stability, improved wear resistance, and the relief of residual stress are among the most significant benefits of the cryogenic treatment.”

Reichard explains, “Chrome plating is a very common surface treatment in the industry and allows the dies to last a long time, even on the most abrasive materials such as thermal transfer paper.” He adds that since chrome has a hard, wear-resistant surface, it protects dies from corrosion.

Although flexible dies have to be discarded when they become dull, their life spans have increased. McEachern says, “Flexible dies sacrifice longevity for flexibility, although the gap between them has gotten narrower over the years and this trend will likely continue. Many flexible dies are now durable enough to routinely run in the millions of revolutions.”

Magnetic cylinders, on the other hand, last a very long time. McEachern says, “The magnetic field of a magnetic cylinder lasts indefinitely (with a half life of 700,000 years). The end of a magnetic cylinder’s useful life is usually determined by wear or physical damage.”

Other types of rotary tools


Rotary dies are common forms of rotary tools, but others exist as well. Tools & Production Inc., Temple City, CA, USA, produces punching equipment designed to punch holes at press speeds and to remove the waste punched pieces.

Peter Tupman, sales manager, says, “Recently, T&P has witnessed increased requirements for the clean and successful punching of tickets, tags and labels particularly for the tea, transportation and retail industries.”


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