Doctors Do Lots

While not a headline-catching subject, doctoring systems have a big impact on printing quality and overall production costs.

ALTHOUGH NOT A particularly glamorous topic, package printers know that doctoring systems represent a complex challenge. The selection, operation, and maintenance of these systems can have a major impact on consistent print quality, along with longer term cost issues when it comes to roll maintenance and life.

When selecting the right doctor blade for your application, there are any number of places to begin and approaches that can be taken. For starters, Perry Lichon, president of Retroflex, lists blade material composition, tip design, thickness, and width. He recommends having a doctor blade system manufacturer cover the available options, while pointing out the benefits and disadvantages of each type.

Even with a clear understanding of the options and tradeoffs, Lichon says that many times the best way to choose a doctor blade material is by trial and error. "Even small differences in anilox or gravure roll engraving, machine speed, fluid chemistry, blade system design, operator experience, and end-product requirements and expectations can result in a big difference when doctor blade materials are compared," he states.

Johnny Stamey, technical product manager for Max Daetwyler, says there are four basic doctor blade style/tip configurations to choose from for all printing applications. These include a straight steel blade with a square edge or no tip, a radius tip blade or round edge, a bevel blade, and a MDC lamella tip or stepped-edge blade. "These four types of blades range in thickness from .004˝-.012˝," says Stamey. "Nonmetallic or plastic blades have all the same tip configurations, but the blade thickness ranges from .007˝ up to 1/8˝, depending on the type of plastic."

As a starting point in choosing a blade combination for a particular application, Stamey points to the line screen of the anilox roll. "For anilox rollers up to 400 line screen, it is best to use straight steel or a radius blade," he says. "For line screens between 400-800, the radius edge or bevel blade is an appropriate choice, and if you stay to the higher side of 800, a lamella tip also works well."

Above 800 line screen, a cleaner wipe is needed because the smaller cells carry less ink per cell. Because the contact area on the anilox roll must be smaller and more concentrated, Stamey recommends the use of a lamella tip blade. "The lamella tip will provide a consistent tip thickness for the life of the blade because the contact area remains unchanged."

With flexo, both the doctor blade and the containment blade must perform their separate functions without contributing to the potential for anilox roll damage, says Tom Allison, president of Allison Systems Corp. He recommends that doctor blade tips be rounded with a 2-mil minimum tip to avoid tip "dig in" on the anilox roll, while containment blades can be "square" in most cases to reduce costs.

For demanding high-wear, flexo applications, Allison says that tool steel and laser-hardened-tip tool steel are often more cost-effective than either quality carbon steel or plastic options. "Very long life, and metal that won't attach itself to ceramic anilox rolls are the key," he says. "The option of special ceramic coatings on steel blades has shown its merit in some extremely demanding high-speed, high-abrasion flexo applications."

In the ongoing effort to extend the life of doctor blades, Edward Graphics' Anthony Foley cautions to not overlook the potential downside of shortened anilox roll life. "Certain coated blades and specialty alloy blades have been experimented with in an effort to achieve longer blade life. However, due to their higher tensile strength and abrasive coatings they often dramatically reduce the life expectancy of the anilox roll."

Foley believes that high-quality carbon steel doctor blades have proven themselves to be the best overall solution for flexo printing requirements. "In addition, the radius edge (rounded) wiping tip has gained wide acceptance in flexo in the past few years," says Foley. "This tip configuration seats quickly, and due to the fact that it is not ground, causes the least amount of trauma to the anilox surface during the seating process."

For Allison, the objective of a good doctoring system boils down to the ability to produce predictable and repeatable print results that the customer requires. "The most fundamental issue is the capability to position and load doctor blades properly, and the capability to efficiently and reliably repeat proper settings," he says.

Loading pressure on the doctor blade is a key variable that impacts printing quality, along with doctor blade and anilox roll life. It is always the objective of the doctor blade system to provide proper ink metering at the lowest possible loading pressure.

Loading pressure is even more of a concern when a printer is dealing with higher line screens. "Higher line screen anilox rolls have resulted in much thinner cell walls which have contributed to the scoring of anilox rolls," says Nick Bruno, president of Harris & Bruno International. "It is imperative that chamber loading pressures be kept to an absolute minimum."

According to FLXON's Paul Sharkey, some printers are turning to thicker blades to help deal with the problem of controlling the blade-to-anilox pressure. Thicker blades offer more rigidity to help maintain the narrowest contact area possible. He reports that many printers have gone from .006˝ to .008˝ or .010˝ with great success.

With all the different design options to consider, suppliers still recognize that initial price may be a primary factor in the purchasing decision. If so, Lichon says to share this with the supplier so they can offer a system that meets any budget constraints. "However, regardless of your initial purchasing capabilities, always consider the long-term financial impact of your purchase," he cautions. "Even an inexpensive piece of doctoring blade can have a huge impact on your long- and short-term bottomline. Work with your supplier to help obtain the highest return for your dollar."

by Tom Polischuk

Editor-in-Chief