There’s a lot of materials science and fluid dynamics to process control on a modern flexo press and every element plays an important part. As we visit converters’ shops around the country, we hear the same requirements over and again: Everyone is after consistent and repeatable results. This means not only throughout a run, but from one run to another, even when those runs may be a few months apart. Controlling the processes in your plant and on your press is essential, and the lowly doctor blade—especially steel doctor blades—plays a crucial role.
The Moving Parts
Consider a 10-deck flexo press that’s putting down a white flood coat, a normal CMYK inkset, two spot colors, a metallic and a clear coat on top of everything. The six decks with CMYK and spot colors can use the same doctor blades because those inks have similar characteristics. But to ensure the best possible work, the white, metallic and clear coat requiredoctor blades that will work well with those inks and last as long as the blades for the CMYK and spot colors.
Take the white flood coat for instance. The ink contains titanium dioxide, which is a bit like liquid sandpaper. It can seriously degrade the leading edge of a doctor blade, leading to a poor wipe and other problems, sometimes partway through a long run. To avoid this, you either have to stop the press and change out those blades before they are too badly worn, or use blades designed to withstand the abrasive nature of the white ink if you want to maintain print quality and ink control.
There’s a real cost to this: Add the cost of the replacement blade, the downtime required—when stopping the press, changing the blade, restarting the press and getting color back up—and then add in the material wasted. If you can avoid changing that blade, the costs decrease and the math changes significantly—in your favor.
As ink control specialists, we think of doctor blades as critical parts of process control. Their job is to control the amount of ink on the anilox roll. The roll collects ink from the ink chamber and the doctor blade is there to wipe excess ink off the roll so it remains only in the anilox cells. To do this on a press running at or near full rated speed, the blade must have a fine edge that consistently cleans the ink off the roll and is rigid so it is immune to flexing along its length.
Think of it like a wood working chisel. A chisel’s tip is thin and sharp so it can peel up a slender shaving of wood, while its shaft is much thicker so it won’t deflect under pressure. Doctor blades work much the same way. Although they may be long enough to span a 58-in. press, the blade still must be thick enough just above its leading edge so it doesn’t flex.
Three Problems
A blade that flexes causes three problems. The first is flooding, which is similar to the tires on your car hydroplaning when hitting deep water at speed. On a tire, the water can’t be displaced quickly enough, causing the tire to ride up onto the water. On a flexo press, a doctor blade that isn’t fine enough rides up onto the film of ink that coats the anilox roller. Inconsistent wiping is taking place, and you can see this up close and personal if you’re watching a running press with worn or poorly chosen doctor blades. Instead of seeing a clean wipe with ink retained in the cells of the roller, the ink coats the roller, giving it a glossy appearance.
The second problem is flutter. This can begin as a pumping issue where the ink pumps may be putting more pressure on the ink than a blade can handle. A blade that isn’t stiff enough can flex due to the pressure the pumps apply to the ink. A pump that is supplying the ink in pulses, for example, can cause a blade to flex momentarily, not wiping as completely as it does when there is less pressure from the pump. Standing by the press, you can see this as bands of glossy ink contrasting against clean or partially wiped sections of the anilox roll.
Related to these is back doctoring. When a blade is not wiping the roll clean because it is no longer metering properly, ink comes back around the roll. Excess ink can even be flung off by the rapidly rotating anilox roll, necessitating additional cleanup. While the containment blade is designed to allow some of this extra ink back into the chamber, it can be overwhelmed if the doctor blade is not doing its job. This results in the extra ink dripping down the chamber into the catch pan, wasting time and money.
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Flooding, flutter and back doctoring are all symptoms of inefficient wipes. This can be due to choosing the wrong blade or edge profile, worn blades or improper setup of the deck itself. If the blade is the problem, it can be addressed with a finer, stiffer doctor blade that will properly wipe ink from the roll. This would seem to be a pretty straightforward job, but a flexo press running at full speed is a demanding environment and process control of all components is essential.
Steel Doctor Blades & Their Benefits
Steel doctor blades provide a trifecta of benefits. First, they provide a finer edge (in a variety of profiles) that cleanly wipes ink from the anilox roll. Second, they have the stiffness needed to eliminate flutter. Third, they have the durability needed to maintain image quality over long runs.
Plastic, fiberglass and composite blades initially provide a sharp edge, but the softer materials can wear faster, which degrades image quality and often results in the need to stop a press mid run to change blades. Plus, synthetics are less able to provide consistent results when used with inks that cause greater blade wear, such as metallics and white flood coats. Additionally, synthetic blades may not be effective on presses being run at full rated speed because the materials cannot meet the demands of pressurized inks and fast turning rollers.
Planning for Performance
One best practice we encourage is performing regularly scheduled blade changes on all presses. By pre-planning this downtime, other tasks can be scheduled concurrently, reducing overall downtime. This includes cleaning, needed lubrication and other light maintenance. For example, a three-shift, seven-day operation might do its preventive maintenance on Monday and Thursday mornings, devoting the first hour or two of those days. As part of this, blades are changed twice a week. To support process control, we might recommend a blade that could possibly be run an extra day because it would let you work with less concern, knowing your process control issues are optimized and that you are delivering consistent quality. It’s a little like the oil in your car: You change it at 5,000 miles even though you know it could go longer. But you don’t analyze it; you just change it.
Delivering the best quality labels and flexible packaging requires tight process control involving every element in each deck on a press. While the condition of anilox rolls can be suspect when print quality slips, even the best maintained rolls can be rendered ineffective if the doctor blades aren’t right for the application. But because doctor blades are consumables, it can be easy to underestimate their importance. Yet their impact on providing consistent, repeatable print quality is key to being able to work without worry, no matter how long the run or the inks being used.
About the Author: Randy Carter provides technical sales and service support for The Provident Group, which includes introduci
ng new and innovative solutions to the company’s customers throughout the southeastern U.S. Randy offers extensive flexographic knowledge with more than 18 years of experience in the flexo industry. He most recently served as a technologies specialist for Anderson & Vreeland, where he assisted customers with technical support for software, digital plates, color management and print.
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