Doctor Blades
A simple tool can make the difference between a good flexo print job and a great one.
By Jack Kenny
For want of a nail, goes the old verse, the shoe was lost. And because of that one missing nail, the horse, its rider, the battle, and the kingdom were lost. Just for the heck of it, let’s compare that nail to a doctor blade. It’s small, inexpensive, and performs a simple task on press, just as the nail does in the horseshoe. But if there were something wrong with the blade – its condition, the way it was installed, its age – and it went unchecked, then consider the chain of events that could follow: The print job is shoddy, the customer is unhappy, and the printer loses a good client.
The function of a doctor blade is to remove excess ink from the surface of the anilox roll so that the anilox surface can deliver the right amount of ink to the printing plate. Before blades, this job was performed by rubber rolls, known as nip rolls or fountain rolls. They worked for years, though press operators faced challenges connected with the hardness levels of the rolls, their flexibility or lack thereof, the smoothness of the surface, and their susceptibility to damage. These rollers, supported at the ends by journals, could often inflict undue stress on the journals, resulting in costly repairs.
In his book Common Sense Flexography, published in 2007 by PIA/GATF Press, David Lanska writes, “Over 20 years ago, a new approach for excess ink removal was developed. Rather than squeegeeing off ink with pressure from a rubber roll, ink is sheared off with a hard, thin strip of metal, plastic or composite material known as a doctor blade.”
Indeed, today’s blades are manufactured of several types of materials, including metals and nonmetals. Those that are crafted of metal come in various types of steel, including blue, stainless, carbon steel, and ceramic coated steel. Plastic blades are fashioned of ultra-high molecular weight material. Composite blades can be composed of fiberglass, carbon fiber and epoxy.
“Doctor blades have certainly evolved quite a bit, even over the past five or six years,” says Ed Dedman, president of ProtoPrint Technologies, a recently formed company that develops flexo innovations. “The choices one has for styles, materials and tip designs are monumental compared with what it was 10 to 15 years ago.” Dedman is an industry veteran with 30 years of experience in the flexo ink industry.
Plastic blades traditionally are the least expensive, and Lanska says that they are the most gentle on the anilox roll. “Unfortunately, plastic blades have been relatively poor at ink doctoring because they come with blunt, unfinished edges. As the edge rides against the anilox face, it wears in to match the curvature of the roll. Even after the break-in period, plastic blades struggle to maintain a consistent doctoring action because ethey are not rigid enough to keep the edge in contact with the face. As blade pressures increase, the blade tip bends away. The underside of the blade then rides against the anilox roll, and because the tip is not in contact with the anilox, it can’t do its job.”
Steel blades are more expensive, but they perform the doctoring job with precision. They come in various thicknesses that offer degrees of rigidity. The major concern with steel blades is safety: They are extremely sharp. “A momentary lapse in attention, especially when the blades are being removed from the blade holder, can result in a serious cut, which is why special cut-resistant gloves are recommended for blade installation and removal,” says Lanska.
The edge of the blade, also called the tip, comes in a couple of different shapes. Most common are round and beveled. Also available are stepped blades, which provide the rigidity of a thick blade with a fine edge that is more flexible.
“A lot of the variation that we see in doctor blades today has been driven by the issues that blades can typically correct,” observes Dedman. “With UV flexo, for example, you can get ink spitting under the blade. When I was in the ink business, every third call we got was for spitting. People blamed it on the anilox, on the blade, on the ink. We were able to find ways around it, but the blade designs have done away with that. There are a lot of theories of what caused it, but generally it was by an ink that was not cleanly metered. Ink gets under the blade and builds up in volume on the back side, and eventually will let go. They will create ink residue that transfers to the plate and to the print. Blade designs have been developed to fix that.”
In flexography, a few variables stand out. Plates and inks are the two major ones, but doctor blades are certainly another.
“It’s a variable because of its very nature over the course of use and time. As the blade wears, and as the anilox wears, the angle changes. It’s a moving target.”
Dedman offers a simple piece of advice that could save a press operator time, and save the company money as well. “Based on what I’ve seen in the industry, when an operator has issues with print quality on press, the doctor blade is often the last thing that’s looked at or thought about. In actuality it probably should be one of the first things. The blade is one of the quickest, simplest and cheapest things to replace. Look at the blades almost before you look at anything else.”
Allison Systems (allisonblades.com)
Headquartered in Riverside, NJ, USA, Allison Systems has been manufacturing cutting blades for a variety of industries for 40 years. The company has established strong ties with blade material suppliers in Europe over the decades.
President Tom Allison says that the company offers a wide range of blades for the flexographic industry. These include several varieties of steel, plastic and composite blades. “One of my most interesting experiences was at the Providence Journal newspaper in Rhode Island, where we pioneered fiberglass reinforced blades – very corrosion resistant. We use cryogenic grade composites mostly, the material used to make circuit boards for aircraft. It has no bubbles, and it’s also heat resistant. You don’t want them to fail with heat.”
Allison says that blade edge shapes should be carefully tailored to the application. “We’ve been making blades for Mark Andy for years, and at one point in the past we were at their plant watching UV ink build up in lines on what was supposed to be the dry side of the blades,” he recalls. “After it builds up it runs off on the anilox and gets all over the printing. We took a tool steel blade with a laser hard tip and got it to run at a nice sharp angle. Now the ink doesn’t grow as the blade wears.”
The metal blades manufactured by Allison for flexography include Superhoned, a brand name with a 4.5° bevel; Standard, with a 15° bevel, and Rounded. Superhoned blades offer “minimal tip growth as the blade wears, with small tip deflection under applied loading.” The Standard blade offers a more rigid tip for maximum anilox clean-up, and the Rounded blade is for anilox rolls with fragile cell walls or rough ceramic, and where maximum anilox clean-up is not required.
The company’s list of plastic and composite blades is long: glass composite, for flexo doctor and containment blades on ceramic anilox rolls; high-glass composite, for high speed ceramic anilox rolls; graphite composite, for large anilox rolls and long life with abrasive water inks; Mylar polyester, often used as a flexo inker containment blade, hard polyester, for doctoring applications in flexo and some flexo containment blades; acetyl, for doctoring applications in flexo and some flexo containment blades; acetyl with lubricant, for doctoring applications in gravure and flexo and some flexo containment blades; nylon with lubricant, for special applications in gravure and flexo; and low wear polyethylene, for special applications in gravure and flexo containmentblades.
Label converters, says Tom Allison, “tend to run metal blades because a lot of them are printing fine screens. Depending on run length and changeover cycles, they may want to go with conventional carbon steel blades. One thing to try to avoid with water based inks is stainless steel blades.”
Max Daetwyler (daetwyler-usa.com)
Max Daetwyler Corporation (MDC), based in Switzerland with a US plant in Huntersville, NC, manufactures a variety of doctor blades designed for maximum life.
The MDC Longlife doctor blade, says the company, “is the best choice for fighting print defects, such as streaking.” It is commonly used for higher line screens where cleaner doctoring is required. Its hardened coating lengthens blade life, reducing the amount of steel contamination in the ink.Fewer blade changes are needed, resulting in reduced downtime and waste.
The MDC Soft doctor blade is coated with a soft, corrosion-resistant nickel coating. The soft coating provides a more gentle contact point, therefore reducing or eliminating the chance of roll scoring. This coating also heals small nicks in the blade edge, reducing lines and streaking as well as steel contamination of the ink.
The MDC Starlife doctor blade has a hardened coating designed to resist wear when wiping abrasive inks, such as white inks that contain titanium dioxide. The coating also reduces friction coefficients, which lowers blade and anilox wear. The smooth-wearing coating maintains its edge to provide a long-lasting, clean and sharp wipe.
MDC Stableflex is a specialty doctor blade developed to prevent chatter marks. Its tip configuration allows the blade to “ride” with vibration (from older, unstable presses) or the bounce of the anilox roll when running at higher speeds (on newer, high-speed presses), giving a more precise metering of the ink film and resulting in a better quality print.
The tip of the MDC One-Step doctor blade maintains the same quality and characteristics of the MDC lamella edge (a stepped edge), but the design allows thicker ink to meter normally. The strengthened base decreases blade flex, creating “consistent ink film metering” without an ink hydroplaning effect behind the blade and eliminating or substantially lessening UV spitting.
The MDC Standard blade offers a consistent tip thickness. The contact area remains unchanged throughout the life of the blade, therefore no changes in tonal value or gradation occur due to wear.
Flxon (flxon.com)
Flxon, based in Charlotte, NC, USA, offers Swedcut blades manufactured in Sweden by Swedev. According to Flxon President Paul Sharkey, the blades are manufactured from a range of proprietary steel compositions and structures. Sharkey says they offer superior ink metering, fewer print defects like lines, slow and even tip wear, reduced friction, less wear to the anilox roll, smaller and less threatening wear and debris, and fewer reasons to stop during a run.
Flxon offers a range of blades in several series.
The 300 Series M.Flex I, of custom carbon steel, “has excellent wear resistance and uniformity in a variety of flexo and rotogravure applications using water, solvent, UV, and EB inks and coatings requiring resistance to blade-to-anilox pressure and less flex.”
The 400 Series Micronox, custom stainless steel, “has excellent corrosion and wear resistance and uniform edge wear in flexo and gravure applications. Also, it offers a resistance to high or low pH and other chemicals.”
The 500 Series Micronox II, custom alloy steel, “has a longer life with superior, uniform edge wear and corrosion resistance. It is ideal for extended runs in flexo and roto applications when using water, solvent, UV and EB inks and coatings. Plus, it resists silver formation.”
The 600 Series M.Flex II Longer Life Blade, of custom carbon steel, “has superior overall wear and smooth edge wear in a wide range of flexo and roto applications using water, solvent, UV and EB inks and coatings.”
