Refining Doctor Blade Technology

The goal of every print process is to meet the needs of a print buyer.

Usually, that involves print quality, turnaround time or delivery, and price. Technical advancements made in flexography over the past 50 years allow it to best meet these needs for many print buyers in 2020. Why? The answer is multi-pronged:

• The quality of a flexographically-printed dot today is competitive with any other print method
• New high-speed, wide web press technology can reliably maintain print quality at 2,000 fpm, with little startup or run waste, making flexographically printed packaging, shrink sleeves and labels less expensive
• Advances in prepress technology make flexography quicker to press, beating out other print methods on delivery—especially when last-minute changes are made

Flexography’s transformation from “second-class” to “premier” print process is the result of thousands of technical advancements spreading out over five decades. Often, improvement in one process element triggered innovation in one or more other areas.

For example: As the highest anilox line counts increased from 360 mechanically engraved cells to well beyond 1,000 laser engraved cells per liner inch:

• Ink films were reduced, allowing thinner plates with smaller, more stable dots to be developed
• Mounting tapes were modified to accommodate new technology plates
• Inks were formulated to deliver density in this new environment
• Process changes prompted press manufacturers to make improvements that monitor all variables and print quality, and when needed, automatically make on-press adjustments at speed—often before a defect can occur

No “one thing” allowed flexography to advance to its present status. Instead, today’s flexography came about as a result of a synergistic evolution. Change begot change over what seemed like—but actually was not—a long period. No element in the flexographic process is the same today as in 1970. Every element has evolved.

Doctor Blade Strip Steel Micro-Structure Advancements

Ink Metering

Let’s take a historical perspective. In the beginning, ink was metered on the surface of a mechanically engraved and chrome plated anilox roll by a rubber roller. Some metering rolls oscillated and turned with the anilox; others, against it. At that time, flexography’s advantage was price. Cost, rather than print quality, drove buyers to flexography.

While flexography could deliver a reasonable solid or large type, two of the biggest shortcomings of the process, then and for years to come, were an inability to hold a clean dot and produce a smooth vignette. While preventing dirty print is still a focus in pressrooms, it has no comparison to the magnitude of the problem in the past.

The culprit—anilox ink film thickness! The raised dot on a plate was easily overwhelmed when it made contact with the anilox roll. The size of the dot grew again when it came into contact with the substrate, often resulting in a wider printed dot with no ink in the center—in essence, a “donut” or “halo” dot. This defect was a telltale sign a job was printed flexographically, and not roto or offset. The dot was bigger, yet less dense.

To alleviate or minimize the problem, a “kiss impression” to the anilox ink film and to the substrate was advocated. In practical terms, a “plus impression” was needed at both transfer points to compensate for tolerance shortcomings in the anilox and plate cylinder’s total indicated runout (TIR) and taper. Plate and mounting tape tolerances were also an issue. If the raised image at one end of a plate was “kissing” the ink film, the other end may not have made contact. To avoid starvation, the entire raised image on a plate was pressed through the anilox ink film to touch the anilox surface. The dot became bigger as ink built up on its sides; it grew again when it was pressed into the substrate and ink squeezed out to the sides.

To address the issue of overly thick anilox ink films, the industry abandoned rubber roll metering in favor of a doctor blade—first a single wiping blade and then a single reverse angle blade. A blade in a wiping position, as used in rotogravure, was prone to hydraulic lift as speeds increased, still allowing too much ink to pass beneath the blade. In the reverse angle position, a blade sheers excess ink off the anilox, dramatically reducing the volume of ink presented to the plate. In the reverse position, the blade is less prone to hydraulic lift.

While reverse angle doctor blade ink metering was a big improvement, it didn’t go far enough. Due to the limits of mechanical engraving, the maximum anilox cell line count was 360 cells per inch. In the 1970s and 1980s, common anilox line counts for line work ranged between 180 and 240, while process counts were 300 to 360. Even when metered using a reverse angled doctor blade, these low line counts left too much ink film to maintain clean print.

Laser Engraved Anilox

Cells of mechanically engraved anilox are formed when a gear-like tool is forced into a thin malleable copper coating on the anilox surface. The teeth of the tool are a reverse of the cell being formed. Because copper is relatively soft, a thin 0.005-in. final coating of hard chrome plate was applied to reduce wear and damage. Unfortunately, the surface was still very easy to damage. Depending on blade pressure, the chrome could wear off in a week of use.

Longer life was the original advantage of the laser engraved anilox. Ceramic could resist both damage and wear. Because a laser can burn an ink-carrying cell straight down without the sloping walls of mechanical engraved cells, a laser cell carries more ink. Gradually, laser engraved line counts increased. Process anilox went from 360 to 600 cells per inch. Increased cells could be metered to deliver required ink density from the cell, rather than the anilox surface. Eventually, line counts surpassed 1,000 per inch. Anilox ink films could be almost entirely removed. It was then possible to have plate to anilox plus impression without overwhelming the dot.